Chapter 4rth

on Saturday, 24 December 2011
Chapter 4rth
                             LIQUIDS AND SOLIDS  
                                                MCQs


Q.1      Ionic solids are characterized by
(a)        low melting points
(b)        good conductivity in solid state
(c)        high vapour pressure
(d)        solubility in polar solvents
Q.2      Amorphous solids.
(a)        have sharp melting points
(b)        undergo clean cleavage when cut with knife
(c)        have perfect arrangements of atoms
(d)        can presses small regions of orderly arrangements of atoms
Q.3      The force of attraction between the atoms of helium is
(a)        hydrogen bonding
(b)        coordinate covalent bond
(c)        covalent bond              
(d)        london dispersion force
Q.4      Which of the following is a pseudo-solid
(a)        CaF2                           (b)        Glass
(c)        NaCl                            (d)        All
Q.5      Diamond is a bad conductor because
 (a)        It has a tight structure   (b)        It has a high density
(c)        There is no free electron present in the crystal of diamond to conduct electricity
(d)        None of the above
Q.6      The weakest intermolecular for a
(a)        dipole–dipole force
(b)        electrostatic force between ions
(c)        ion–dipole force
(d)        dipole–induced dipole force
Q.7      In liquids intermolecular forces are
(a)        very weak                    (b)        very strong
(c)        reasonably strong         (d)        ion–dipole force
Q.8      Values of heat of vaporization for liquids, with strong dipole–dipole forces will be
(a)        very high                     (b)        very low
(c)        reasonably high           (d)        negligible
Q.9      Instantaneous dipole–induced dipole force is also called
(a)        dipole force                 (b)        london dispersion
(c)        hydrogen bonding        (d)        none of the above
Q.10    Down the group polarizability generally
(a)        increases                      (b)        decreases
(c)        remains constant          (d)        do not follow a regular trend
Q.11    Trend of boiling points of halogens from fluorine to iodine       is that it.
(a)        decreases                     (b)        increases
(c)        remains constant           (d)        negligible
Q.12    Molecules of hydro carbons with large chain lengths experience
(a)        repulsive forces                       (b)        strong attractive force
(c)        weaker attractive forces
(d)        no attractive force
Q.13    Hydrocarbons which generally have high molecular masses exist is.
(a)        solid form                    (b)        liquid form
(c)        vapour form                (d)        gaseous form
Q.14    Exceptionally low acidic strength of HF is due to
(a)        strong polar bond                (b)        small size of fluorine
(c)        strong hydrogen bonding     (d)        Vander Waal’s forces
Q.15    Long chain of amino acids are coiled about one another into spiral by.
(a)        covalent bond              (b)        ionic bond
(c)        hydrogen bond             (d)        Vander Waal’s forces
Q.16    Evaporation of water is possible at
(a)        100oC                          (b)        0oC
(c)        at all temperatures        (d)        above 100oC
Q.17    Boiling point is low for liquid with
(a)        high vapour pressure at given temperature
(b)        low vapour pressure at a given temperature
(c)        very high vapour pressure
(d)        very low vapour pressure
Q.18    At equilibrium rate of evaporation and rate of condensation
(a)        become very high        (b)        become very low
(c)        can never be equal       (d)        become equal
Q.19    In an open system vapour pressure of water at 100oC at sea level is
(a)        700 mm of Hg             (b)        760 mm of Hg
(c)        670 mm of Hg             (d)        1000 mm of Hg
Q.20    Molar heat of vaporization of water is
(a)        140.6 kJ/mol                (b)        14.06 kJ/mol
(c)        18 kJ/mol                     (d)        40.6 kJ/mol
Q.21    When external pressure is 23.7 torr boiling point of water is
(a)        100oC                          (b)        200oC
(c)        98oC                            (d)        25oC
Q.22    Distillation under very reduced pressure is called
(a)        fractional distillation    (b)        distillation
(c)        vacuum destructive distillation
(d)        destructive distillation
Q.23    Water may boil at 120oC when external pressure is
(a)        760 torr                                    (b)        100 torr
(c)        1489 torr                      (d)        700 torr
Q.24    Amount of heat absorbed when one mole of solid melts into liquid form at its melting point is called
(a)        molar heat of sublimation
(b)        heat of vaporization
(c)        latent heat of fusion
(d)        molar heat of fusion
Q.25    Ethanol is much more soluble in water than ethyl ethanuate which one of the following statement correctly account for this
(a)        ethanol is polar molecule but ethyl ethanoate is non–polar
(b)        ethanol is non polar molecule but ethyl ethanoate is polar
(c)        a hydrogen bond is formed between H–atom of the OH group in ethanol and O–atom of water molecule
(d)        a hydrogen bond is formed between the H–atom of the OH group in ethanol and hydrogen of the water molecule
Q.26    The boiling point of a liquid will be
(a)        lower at high altitude
(b)        higher at high altitude
(c)        same at sea level and high altitudes
(d)        equal to atmospheric pressure
Q.27    The process in which liquids can be made to boil at low temperature is called
(a)        vacuum distillation
(b)        destructive distillation
(c)        distillation
(d)        vacuum destructive distillation
Q.28    Why is the boiling point of methane greater than that of neon
(a)        a molecule of methane has a greater mass
(b)        a molecule of methane has more electrons than a molecule of neon
(c)        the molecules of methane have stronger intermolecular forces than those of neon
(d)        the molecule of methane is polar but that of neon is not
Q.29    The amount of heat required to vaporize one mole of a liquid at its boiling point is called
(a)        molar heat of vaporization
(b)        molar heat of fusion
(c)        latent heat of fusion
(d)        molar heat of sublimation
Q.30    Which of the elements in its crystalline form will have the lowest enthalpy change of vaporizations
(a)        chlorine                       (b)        argon
(c)        phosphorous               (d)        silicon
Q.31    Crystals show variation in physical properties depending upon the direction. The property is called
(a)        isomorphism                (b)        polymorphism
(c)        anisotropy                    (d)        isotropy
Q.32    Certain melt to a turbid liquid phase with properties of liquids as well as some degree of order like solid. Such turbid liquids are called
(a)        anorphous solid                       (b)        vitreous solid
(c)        crystalline solid                        (d)        liquid crystal
Q.33    Isomorphous crystals show
(a)        same chemical properties
(b)        same physical properties
(c)        same crystalline form
(d)        same melting point
Q.34    Existence of an element in more than form is known as
(a)        allotropy                      (b)        isomorphism
(c)        isotropy                       (d)        none of these
Q.35    Crystalline forms of the same, substance can coexist in equilibrium with each other at its
(a)        melting point                (b)        transition temperature
(c)        boiling point                (d)        none of these
Q.36    Crystal lattice of substance can be catagorised into
(a)        five types                     (b)        seven types
(c)        six types                      (d)        none of these
Q.37    Covalent solids are composed of
(a)        ions                              (b)        different molecules
(c)        neutral atoms               (d)        any of the above
Q.38    Carbon atoms of diamond are
(a)        sp hybridized               (b)        sp2 hybridized
(c)        sp3 hybridized            (d)        unhybridized
Q.39    Molecular crystals are generally
(a)        hard                             (b)        soft
(c)        unstable                       (d)        stable
Q.40    Ionic crystals are
(a)        hard                             (b)        soft
(c)        brittle                           (d)        amorphous
                                 ANSWER
Questions
1
2
3
4
5
Answers
d
d
d
b
c
Questions
6
7
8
9
10
Answers
d
c
c
b
a
Questions
11
12
13
14
15
Answers
b
b
a
c
c
Questions
16
17
18
19
20
Answers
a
c
d
b
d
Questions
21
22
23
24
25
Answers
d
c
c
d
c
Questions
26
27
28
29
30
Answers
a
a
c
a
b
Questions
31
32
33
34
35
Answers
c
d
c
a
b
Questions
36
37
38
39
40
Answers
b
c
c
b
a


                                SHORT QUESTION WITH ANSWERS
Q.1      what is difference between
(i)         Intermolecular forces and intermolecular forces
(ii)        Polar molecules and non–polar molecules
(iii)       Induce and dipole and instantaneous dipole
(iv)       Dipole

Ans.    (i) Intermolecular Forces:
            The forces of attraction between two different atoms ions and molecules are called intermolecular forces.
            For example H—Cl……H--Cl…….H--Cl
Intermolecular Forces:
            The forces of attraction between two atoms or group of atoms present with in the same molecule, are called intermolecular forces.e.g covalent bond ,ionic bond etc
(ii)        Polar molecules:
            A molecule which has partial +ve and partial –ve charges on it due to difference of electro negativity between bonded atoms is called polar molecules. For example H8+ – Cl8–
Non–polar molecules:
            A molecule in which bonded atoms have zero or negligible electro negativity difference is called non–polar molecules.
            For example
                                     H
                                      |
            Cl–Cl, H–H, H–C–H
                                      |
                                     H
Important point to remember: All molecules having same atoms(Homoatomic) are non polar
(iii)       Induced Dipole:
            A molecule in which polarity is created due to other polar molecule is called induced dipole.
Instantaneous Dipole:
            The temporary dipole (polarity) produced in a non–polar molecule at a certain instant is called instantaneous dipole.
(iv)       Dipole
            A molecule which has two poles i.e. two charges partial +ve and partial –ve is known as dipole.e.g H8+ – Cl8–
Q.2      Define intermolecular forces, and the types of intermolecular forces?
Ans.    Intermolecular Forces:
            The forces of attraction that exist between all kinds of atoms, molecules, when they are sufficiently close to each other are called intermolecular forces.
Types of intermolecular forces:
            There are four types of intermolecular forces.
(a)        Dipole–dipole forces
(b)        Ion–dipole forces
(c)        Dipole–induced dipole forces (Debye forces)
(d)        Instantaneous dipole–induced dipole forces or (London dispersion forces).
(a)        Dipole–dipole forces:
            The forces of attraction between the positive end of one polar molecule and the negative end of other polar molecule are known as dipole–dipole forces.
            Example is of HCl.
            H8+ ® Cl8– ® H8+ ® Cl8– ® H8+ ® Cl8–
(b)       Ion–Dipole forces:
            The forces of attraction in which the negative ends of polar molecules are attracted towards the cation (+ve ion) and positive ends towards anion (– ion) are called ion–dipole forces. Ionic compounds like Mx are normally soluble in polar solvent like water. Water molecules break the crystal lattice and the ions are set free. These positive and negative ions are then surrounded by water molecules. The negative ends of the dipole of the water are attracted towards the cation (M+) while the positive ends are attracted towards the anion (X). The dissolution of most of the ionic compounds in water is due to this reason. The forces of attraction between ions and water molecules are known as Ion–dipole forces.
(c)     The forces of attraction that exist between already polar molecules and the molecule having induced dipole forces. The forces are also called Debye forces.
(d)      Instantaneous Dipole–Induced Dipole forces. (London dispersion forces). 
  
          The momentary forces of attraction that exist between instantaneous dipole and the induced dipole are called instantaneous dipole–induced dipole forces.
         The momentary force of attraction between instantaneous dipole and the induced dipole is known as instantaneous–induced dipole forces.
Q.3      Explain the factors affecting the London forces.
Ans.
        The strength of these forces depends upon the following two factors.
1.         Size of electronic cloud:
            As the size of electronic cloud of atoms or molecules increases, dispersion becomes easier and these forces are more permanent.
            The elements of zero group are monoatomic gases due to their complete outermost shells, they do not form covalent bonds. Their boiling points increase from top to bottom in a group.
2.         Polarizability:
         The quantitative measurement of the extent to which the electronic cloud can be polarized or distortedis called polarizability.
         The boiling points of halogens increase from top to bottom i.e. from fluorine to iodine.
3.      Number of atoms:
         As the number of atoms in non–polar molecule increases polarizability of the molecule
        increases and London forces become stronger.
       The boiling points of saturated hydrocarbons increase as the number of atoms increases.
Q.4      Define and explain hydrogen bonding./ What is the origin of intermolecular forces in water?
Ans.    Hydrogen bonding:
          “The electrostatic force of attraction between electronegative atom and partial positive hydrogen atom is called hydrogen bonding.”
Explanation:
          Consider water molecules to understand hydrogen bonding oxygen is more electronegative than
          hydrogen. So water is polar molecule. There will be dipole–dipole forces of attraction between water
          molecules. The electrostatic force of attraction between electronegative oxygen of one molecule and
          partial positive hydrogen of other molecule is called hydrogen bonding.
Strength of H–Bonding:
            Hydrogen bonding is stronger than simple dipole–dipole forces.
            This is due to the following reasons.
1.                  There are two lone pairs on oxygen atom. Oxygen forms coordinate covalent bond with hydrogen.
2.         There is sufficient partial positive charge on hydrogen. Both positively charged hydrogen of water molecules produce strong electric field due to their small size.
            Hydrogen bonding in water molecules acts like a bridge between two electronegative oxygen atoms.
            Generally, the strength of H–bonding is twenty times less than that of covalent bond.
Q.5      Give the properties of compounds containing hydrogen bonding.
Ans.
1.         There are dynamic properties of covalent compounds.
2.         Solubility of Hydrogen bonded molecules.
3.         Cleansing action.
4.         Application of hydrogen bonding in biological compounds.
5.         Surface tension.
6.         Effect of hydrogen bonding on viscosity.
7.         Hydrogen bonding in paints and dyes.
8.         Clothing.
9.         Food material.
10.       Structure of ice.
Q.6      Explain the following with reasons.
(a)        In the hydrogen bonded structure of H–F, which is stronger bond, the shorter covalent bond or the longer hydrogen between different molecules?
(b)       In a very cold winter fish in garden ponds owe their lives to hydrogen bonding.
(c)        Water and ethanol can mix easily and in all proportions.
Ans.
(a)                There is sufficient hydrogen bonding in H–F molecules and it gives zig zag structure. Fluorine atom is present at the end while H atoms are entrapped between two strong electronegative atoms. The covalent bond between H and F is stronger because it is produced by the overlapping of orbital’s and two electrons have been shared to give sigma bond.
            The bond which is shown by the dotted line is the hydrogen bond due to electrostatic forces of attraction so, it is a weaker bond.
(b)       When water is frozen at 0oC, then it expands. This is due to the fact that due to H-bonding in ice the
           molecules become arranged density of ice is decreased. That’s why ice floats on water.
(c)       Water (H-OH) and ethanol (C2H5OH) have both are polar solvents and having OH groups. So, they
            can do the hydrogen bonding extensively. That is they can mix with each other in all proportions.
Q.7      Why H2S is a gas while H2O is liquid at room temperature?
Ans:

This is due to high electro negativity of oxygen as compared to sulphur. Water has hydrogen bonding, but H2S does not have. Due to absence of hydrogen bonding in H2S at room temperature, it is a gas.
Q.8      Earthen ware vessels keep water cool?
Ans:
Earthen were vessels are porous- water molecules come out from these pores and evaporate. Heat of the atmosphere can not enter into the liquid. So temperature of the liquid in earthenware's remains less.
Q.9      one feels sense of cooling under the fan after bath?
Ans:
          When one takes bath and sits in front of a fan, water on the surface of body evaporates with greater rate.The high energy molecules escape from surface of the body and one feels sense of cooling.
Q.10    Why the heat of vapourization of water is greater than that of CH4?
Ans:
        Water is a polar liquid and due to strong hydrogen bonding high energy is required to separate the molecules from each other at its boiling point. CH4 is a non-polar and has weak London dispersion forces.
Q.11    Define and explain evaporation is a cooling process. Give reason.
Ans.    Evaporation: The spontaneous change of liquid into its vapours is called evaporation. It continues at all temperature. Evaporation increases with the increase of temperature.
         Explanation: The molecules of liquid are not motionless. The energy of the molecules is not equally distributed.The molecules which have low kinetic energy move slowly while others with high kinetic energy move faster. If one of the higher speed molecules reaches the surface, it may escape the attractions of its neighboring molecules and leaves the bulk of the liquid. This spontaneous change of liquid into its vapours is called evaporation
Evaporation causes cooling:
                    The reason is that when high energy molecules leave the liquid and low energy molecules are left  
                  behind, the temperature of the liquid falls and heat moves from the surrounding to  the liquid and     
                  the temperature of the surrounding also falls. So evaporation is a cooling process.
Q.12    H-bonding is present in chloroform and acetone-justify it?
Ans:
           Chloroform is a polar compound. Acetone is also a polar compound. When chloroform and acetone are mixed with each other, than they create the forces of attractions due to hydrogen bonding.
Q.13    Evaporation of a liquid takes place at all temperatures give reason?
Ans:
            Evaporation takes place due to the K.E of the molecules since the K.E of the molecules can not be zero at any temperature therefore evaporation takes place at all temperatures.  
Q.14    What are the factors that affect the rate of evaporation?
Ans.
1.  Surface Area:
       Evaporation takes place from liquid surface. If area of the surface of liquid increases the rate of evaporation will also increase.
2.         Temperature:
            Temperature also affects rate of evaporation Higher the temperature faster will be the rate of              evaporation.
3.         Intermolecular forces:
            Stronger the intermolecular attractive forces slower is the value of evaporation and vice versa.
Q.15    Define and explain the vapour pressure.
Ans.    Vapour Pressure:
           The pressure exerted by the vapours on the surface of liquid at equilibrium state at a given temperature is called vapour pressure.
Explanation:
             Consider a liquid closed in container at a certain temperature. High energy molecules leave the surface of liquid and gather above the surface in the empty space in the form of vapours. These molecules collide with the walls of container as well as with the surface of liquid. In this way they lose some their kinetic energy and there is a chance that these molecules are recaptured by the liquid surface. This process is known as condensation. Both the process i.e. condensation and evaporation continue, till rates of both processes become equal. This state is called dynamic equilibrium, and the pressure exerted by the vapours at this state on the liquid surface at particular temperature is called vapour pressure. Vapour pressure does not depend upon amount or volume of liquid and surface area.
Q.16    What are the factors affecting vapour pressure.
Ans.
1.         Nature of liquid
2.         Strength of intermolecular forces
3.         Size of molecules
4.         Temperature.
Q.17    Define boiling point.
Ans.
        The temperature at which the vapour pressure of liquid becomes equal to the external atmospheric pressure is called boiling point of liquid.
Q.18    Give variation of vapour pressure and boiling point.
Ans.
        Vapour pressure is closely related to boiling point. Variation in vapour pressure depends upon the following factors.
1.         Temperature:
                 vapour pressure of a liquid increases by increasing temperature. Higher the temperature more will be the vapour pressure and vice versa. Liquids boil at that temperature when their vapour pressures are equal to 760 torr at sea level. By increasing external pressure boiling point can be increased.
2.         Strength of intermolecular forces:
                Stronger the intermolecular forces lower will be vapour pressure and higher will be the boiling point.
Q.19    What is the effect of external pressure on boiling point?
Ans.
         A liquid boils when its internal pressure becomes equal to external atmospheric pressure so, by changing external pressure, a liquid can be boiled at any temperature. If external pressure is greater, the liquid needs more heat to equalize the internal pressure to external atmospheric pressure. Similarly if external pressure is lower, liquid needs less amount of heat to equalize its vapour pressure, the external pressure. under 700 torr (at Murree hills) water boils at 98oC.
Q.20    Why boiling point of water is 980C at Murree?
Ans:
             At high altitudes the atmospheric pressure becomes low therefore B.P of water at Murree is 980C.
Q.21    Why boiling point of water is 1200C at 1489 torr why?
Ans:
             The normal B.P of H2O is 1000C at 760 torr since B.P increases by increasing pressure therefore B.P of H2O is 1200C at 1489 torr.
Q.22    Why the boiling points of the hydrides of second period  in group IV-A,V-A,VI-A and VII-A are greater than the B.P of hydrides of third period?
Ans: 
            The elements of second period are more electronegative than the respective element third period. So,the polarities of the bonds with hydrogen are greater than the third period elements.
           H2 O > H2 S; NH> P H3 ;HF > HCI; CH4 < SiH4
Q.23    Define molar heat of vapourization?
Ans:
           The amount of heat required to vapourize one mole of liquid at its boiling point is called molar heat of vapourization.
Q.24    What is vacuum distillation? Explain.
Ans.    Definition:
              The process in which liquid is heated under reduced pressure, to convert it into its vapours at low temperature and then to condense these vapours into liquid is known as vacuum distillation.
           Explanation:
                 In vacuum distillation boiling point of liquid decreases by reducing the pressure. This is done by connecting the distillation apparatus to the vacuum pump. In this way liquids with high boiling points can be boiled at low temperature.
Q.25    Define enthalpy change.
Ans.
             If physical or chemical change occurs at constant pressure then it is known as enthalpy change.
Q.26    What are types of enthalpy changes?
Ans.
              There are three types of enthalpy changes.
1.        Molar Heat of Fusion (DHf):
              The amount of heat absorbed by one mole of a solid to melt it into liquids at its melting point at atmospheric pressure is called molar heat of fusion. It is denoted by  D Hf.
2.        Molar Heat of vapourization (D Hv):
         The amount of heat absorbed by one mole of a liquid to convert it into one mole of vapours at its boiling point at 1 atmospheric pressure is called molar heat of vapourization. It is denoted by D Hv.
3.        Molar Heat of sublimation (D Hs):
              The amount of heat absorbed by one mole of a solid to convert it directly into one mole of its vapours at particular temperature at 1 atmospheric pressure is called molar heat of sublimation. It is denoted by D Hs.
Q.27    What are liquid crystals? Give their types.
Ans.
      The molecules which are large somewhat rigid and linear having some of structures of solids showing optical properties and some of the freedom of motion of liquids are called liquid crystals.
Types of liquid crystals:
(a)        Smectic liquid crystals.
(b)        Nematic liquid crystals.
(c)        Cholesteric liquid crystals.
Q.28    What are solids?
Ans.
            Solids are those substances which are rigid, hard, have definite shape and definite volume. The atoms, ions, and molecules, that make up a solid are close packed. They are held together by strong cohesive forces.
Q.29    Crystals have their own habits justify it?
Ans:
         The shape of a crystal in which it usually grows called habit of a crystal. The shape of the crystal remains same if its conditions remain same. For example When 10%urea is added in NaCl then needle like crystals are formed instead of cubic crystals
Q.30    Justify that solids are rigid?
Ans:
        The solids are very rigid. This rigidity is due to the fixed positions of the particles. The presence of strong cohesive forces makes particles unable to change their positions. This rigidity of solids can be changed under stress
Q.31    Give types of solids?
Ans.
There are two types of solids:
(i)         Crystalline solids
(ii)        Amorphous solids
Crystalline solids:
                 Those solids in which atoms, ions or molecules are arranged to a definite three dimensional pattern,are called crystalline solids.
Amorphous solids:
             Those solids whose constituent atoms, ions or molecules do not possess a regular orderly arrangement are called amorphous solids.The best examples are glass, plaster and rubber, glue, etc.
Q.32    Define the following:
(i)         Cleavage planes.
(ii)        Anisotropy
(iii)       Symmetry
(iv)       Habit of a crystal
Ans.
(i)         Cleavage planes:
                 whenever the crystalline solids are broken they do so along definite planes. These planes are called the cleavage planes.
(ii)        Anisotropy:
                Some of the crystals show variation in physical properties depending upon the direction; such properties are called anisotrophic properties and the phenomenon is called anisotropy.
(iii)       Symmetry:
                The repetition of faces angles or edges when a crystal is rotated by 360o along its axis is called symmetry.
(v)        Habit of a crystal:
          The shape of a crystal in which it usually grows is called habit of crystal.
Q.32    Define the following:
(i)         Isomorphism
(ii)        Polymorphism
(iii)       Allotropy
(iv)       Transition temperature
(v)        Crystal lattice
(vi)       Unit cell
Ans.
(i)         Isomorphism:
            Isomorphism is the phenomenon in which two different substances exist in the same crystalline form. These different substance are called isomorphs of each other.
                  Examples of ismorphs are    NaNO3, CaCO3,K2SO4, K2CrO4.
(ii)        Polymorphism:
           Polymorphism is a phenomenon in which a substance exists in more than one crystalline forms. The substance which exists in more than one crystalline forms is called polymorphic, and these forms are called polymorphs of each other. Polymorphs have same chemical properties but they differ in the physical propertiesAgNO3, CaCO3   are polymorphs.
(iii)       Allotropy
           The existence of an element in more than one crystalline form is known as allotropy and these forms of the element are called allotropes or allotropic forms.
              Element                       Crystalline forms
             Carbon                         Cubic (Diamond)
Hexagonal (Graphite).
(iv)       Transition Temperature:
                    It is that temperature at which two crystalline forms of the same substance can coexist in equilibrium with each other. At this temperature one crystalline form of substance changes to one another.95.50
              Sulphur S8 (rhombic)     sulphur S8   (monoclinic)
(v)        Crystal lattice:
                A crystal lattice is defined as an array of points representing atoms, ions or molecules of a crystal arranged at different sites in three dimensional space.
(vi)       Unit cell:
                The smallest part of crystal lattice has all the characteristic features of the entire crystal is called unit cell.
                  The simplest unit cell is a cubic unit cell.
Q.33    Name the crystal systems.
Ans.    (Cu T Or T He Mo Tri)
1.         Cubic system
2.         Tetragonal system
3.         Orthorhombic or Rhombic system
4.         Monoclinic system
5.         Hexagonal system
6.         Trigonal system
7.         Triclinic system
Q.34    Define lattice energy.
Ans.
            The energy released when one mole of the ionic crystal is formed from the gaseous ions.It is also defined as the energy required to break one mole of solid into isolated ions in the gas phase. It is expressed in  kJ mol–1.
                   Na+(g) + Cl(g)  ®  NaCl(s)    D H – 792 kJ mol–1
                  or
                  NaCl(3)  ®  Na+(g) + Cl(g).
Q.35    Describe the types of crystalline solids.
Ans.
There are four types of crystalline solids, depending upon the type of bond present in them.
1.         Ionic solids.
2.         Covalent solids.
3.         Metallic solids.
4.         Molecular solids.
1.         Ionic Solids:
            Crystalline solids in which the particles forming the crystals are positively and negatively charged ions are called ionic solids.These ions are held together by strong electrostatic forces of attraction. These attractive forces are also called ionic bonds. The crystals of NaCl, KBr  etc. are ionic solids.
2.         Covalent solids:
               The crystalline solids in which atoms of similar or different elements are held together by covalent bonds are known as covalent solids. They are also called atomic solids.
There are two types of covalent solids.
            Type 1:
                 When covalent bonds give joint molecules like diamond, silicon carbide or Aluminum nitride.
           Type 2:
                When atoms join to form the covalent bonds and separate layers are produced like that of graphite, cadmium
                 iodide and boron nitride.
3.         Molecular solids:
              The solid substance in which the particles forming the crystals are polar or non–polar molecules or atoms, are called molecular solids. In solidified noble gases, there are non–polar atoms. Two types of intermolecular forces hold them together.
1.         Dipole–dipole interactions
2.         Vander Waal’s forces
           These intermolecular forces are much weaker then the forces of attraction between the cations and the anions in ionic crystals and between the atoms in the covalent crystals. Ice and the sugar are the best example of crystals having polar molecules, whereas iodine sulphur and carbon dioxide form crystals containing non–polar molecules.
4.         Metallic solids:
              The crystalline solids in which the metal atoms are held together by metallic bonds are known as metallic solids.
Metallic Bond:
                     The force of attraction that binds positive metal ion to the number of electrons with in its sphere of influence is called metallic bond.
Theories of metallic bond:
1.         Electron gas theory
2.         Valence bond theory
3.         Molecular orbital theory
Q.36    Iodine dissolves readily in tetrachloromethane. Give reason.
Ans.
            We know that “like dissolve like”. Iodine is a non–polar substance. So it becomes solvable in non–polar solvent  CCl4.
Q.37    Justify molecular solids are soft and compressible?
Ans:
            The forces which hold the molecules together in molecular structure are weak so, they are soft and compressible
Q.38    What is crystallite?
Ans:
The small regions in amorphous solids where particles have a regular arrangement are called crystallites.
Q.39    Why diamond is bad conductor of electricity?
Ans:
In diamond each carbon is SP3 hybridized there is no free electron to conduct electricity therefore it is bad conductor.
Q.40    Why metals have shiny surface?
Ans:
When light falls on the surface of metals then the electrons are excited after de-exictation they emit energy in the form of light therefore they show shiny surface.
Q.41    Why Na is soft while Cu is hard?
Ans:
In sodium only one mobile electron is present while in copper two mobile electrons are present due to strong metallic bond in copper it is hard.
Q.42    Why ionic crystals are brittle?
Ans:
            Because ionic solids are composed of parallel layers which contain cations & anions in alternate positions, so that the opposite ions in the various parallel layers lie over each other. When an external force is applied one layer of the ions slide pass over other layer. In this way due to repulsion of similar ions the crystals show brittleness.
Q.43    Electrical conductivity of metals decreases by increasing temperature?
Ans:
             With the increases in temperature the positive ions of metals also vibrate which hinders the motion of mobile electrons due to this hindrance electrical conductivity also decreases.
Q.44    What is coordination no. of an ion? What is the coordination no of the cation in (a)NaCl, and(b)CsCl?
Ans:
The no. of positive ions which surround the anion called coordination no. of anion   (a) Coordination no. Na in NaCl is 6 (b) Coordination no of Cs in CsCl is 8 (due to the greater size of Cs)

                                  TEXT BOOK EXERCISE

Q.1.     Multiple choice questions.
London dispersion forces are the only forces present among the:
(a)        molecules of water in liquid state
(b)        atoms of helium in gaseous state at high temperature
(c)        molecules of solid iodine
(d)       molecules of hydrogen chloride gas
Acetone and chloroform are soluble in each other due to
(a)        intermolecular hydrogen bonding
(b)        ion-dipole interaction
(c)        instantaneous dipole
(d)       all of the above
NH3 shows a maximum boiling point among the hydrides of Vth group elements due to
(a)        very small size of nitrogen
(b)        I one pair of electrons present on nitrogen.
(c)        enhanced electronegative character of nitrogen
(d)       pyramidal structure of NH3
When water freezes at 0oC, its density decreases due to
(a)        cubic structure of ice
(b)        empty spaces present in the structure of ice
(c)        change of bond lengths
(d)       change of bond angles
In order to mention the boiling point of water at 110oC, the external pressure should be
(a)        between 760 torr and 1200 torr
(b)        between 200 torr and 760 torr
(c)        765 torr
(d)       any value of pressure

Ans:    (i) c      (ii) a     (iii) c    (iv) b   (v) a

Q.2.     Fill in the blanks.
i             The polarizability of noble gases ________ down the group and results in the increase in their
ii           ________boiling points is developed in acetone and chloroform when they are mixed together.
iii          Exceptionally weak ________ of HF is due to strong hydrogen bonding present in it.
iv          The concept of dynamic equilibrium is the ultimate ________ of all reversible systems.
v           HV of C6H14 should be ________ than that of C2H6.
vi         During the formation of ice from liquid water there is a ________ % increase in volume.
vii         The rate of increase of vapour pressure of water ________ at high temperatures
viii        A layer of ice on the surface of water ________ the water underneath for further heat loss.
ix          Evaporation is a ________ process.
x          Liquid crystals are used in the display of ________ devices.

Ans:    (i) increases   (ii) H-bonding   (iii) acidic strength   (iv) result / goal   (v) greater   (vi) 9   (vii) increases   (viii) insulates or prevents   (ix) cooling   (x) electrical.

Q.4.     (a) What type of intermolecular forces will dominate in the following liquids.
(i) Ammonia, NH3   (ii) Octane, C8H18   (iii) Argon, Ar   (iv) Propanone, CH3COCH3   (v) Methanol, CH3OH
(b) Propanone (CH3COCH3), propanol (C3CH2CH2OH) and butane have very similar relative molecular masses. List them in the expected order of increasing boiling point. Explain your answer.
Answers:
           In ammonia (NH3) hydrogen bonding will dominate. It is due to high E.N of Nitrogen
          Octane (C8H18) has weak Vander Waal’s Forces but no hydrogen bonding.
          Argon has wear London dispersion forces.
          Propanone (acetone, CH3COCH3) has dipole-dipole interactions. 
         Methanol (CH3OH) has hydrogen bonding. It is due to high E.N of oxygen
(a)           The increasing order of boiling points is Butane < Propanone < Propanol
        The propanol (CH3-CH2-CH2-OH) has hydrogen bonding. So it has maximum boiling point.Propanone has dipole-dipole interaction while butane has no such forces.
 Q.5.     Explain the following with reasons.
               In the hydrogen bonded structure of HF, which is the stronger bond: the shorter covalent bond
               or the longer hydrogen bond between different molecules.
               In a very cold winter the fish in garden ponds owe their lives to hydrogen bonding?
               Water and ethanol can mix easily and in all proportions.
               The origin of the intermolecular forces in water.
Answer:
          The hydrogen bonded structure of HF is shown in figure. The fluorine has maximum electro negativity.The shorter covalent bond. H-F is shorter than longer hydrogen bond H---F.When winter comes, then temperature falls down. The water at 4oC has maximum density. So it goes to the bottom of pond. The surface water freezes into ice. During ice formation the water molecules get regular arrangements due to H-bonds. In these arrangements there are many empty spaces and ice occupies 9% more volume than liquid water. Thus ice has less density and floats on water. Moreover ice is an insulator of heat. So it prevents underneath water from freezing. Therefore fish and other aquatic animals live (survive) under thick blanket of ice.Both water and Ethanol (C2H5OH) show H-bonding with each other. It is the reason that they can mix easily in all proportions.The oxygen atom has small size and high electro negativity. There is high electro negativity difference between oxygen and hydrogen. This high E.N difference between O-atom and H-atom is the origin of intermolecular forces in water.
Q.6.     (a) Briefly consider some of the effects on our lives if water has only a very weak hydrogen bonding present among its molecules.
            (b) All gases have a characteristic critical temperature. Above the critical temperature it is impossible to liquefy a gas. The critical temperatures of carbon dioxide and methane are 31.14oC and -81.9oC, respectively. Which gas has the stronger intermolecular forces? Briefly explain your choice?
Answer:
(a) If water had only very weak hydrogen bonding, then its freezing point would be less than 0oC of ice would not have been less than liquid water. These facts would create problems for human beings and aquatic animals.
(b) Because critical temperature of CO2 is higher than that of CH4, SO, SO2 has stronger intermolecular forces and CH4 has very weak intermolecular forces.
Q.7.     Three liquids have the properties mentioned against their names.
            NO                                                                              water     propanone    pentane

i          Molecular formul                                                        H2O      C3H6O          C5H12 
ii          Relative molecular mass (amu)                                       18           58                  72
iii         Enthalpy change of vaporization (KJ mol-1)             14.1        31.9                27.7
iv        Boiling Point (oC)                                                        100        56                   36

(a)  What type of intermolecular force predominates in each liquid ?
(i) In water      (ii) In propanone         (iii) In Pentane
(b)  What do you deduce about the relative strength of these forces in the liquids? Justify your conclusions.
(c)  If the liquids are shaken together in pairs,
(i)   Which pair would be unlikely to mix ?
(ii)  Explain this immiscibility in terms of the forces between the molecules.
(iii) Choose one of the pairs that mix and say whether the enthalpy change on mixing would be positive or negative.
Answer:
(a)        Water has hydrogen bonding, propanone has dipole-dipole interactions and pentane (C5H12) has London dispersion fores.
(b)       From boiling points and enthalpy of vaporizations of three liquids we deduce that
           H-bonding in water is very strong
           Dipole-dipole interactions in propanone are weak
           London dispersion forces in pentane are very weak.
(c)
  1. The pair (water + pentane) would be unlikely to mix.
  2. Water is polar molecule and pentane is non-polar molecule or water has H-bonding and pentane has very weak London dispersion forces so this pair of liquids is immiscible.
  3. Water and Propanone mix into each other because both liquids are polar. The process of mixing is an exothermic process and some heat will evolve. Therefore enthalpy change will be negative.
Q.8.     Describe the various forces responsible for keeping the particles together in the following elements and compounds and their effect on physical properties making use of the data below:
Substances
Formula
Molar Mass (a.m.u)
M.P (oC)
Neon
Ne
20
-248
Argon
Ar
40
-189
Water
H2O
18
0
Sodium fluoride
NaF
42
993
Diamond
C
12
3350

Answer:
Neon:
Neon is a noble gas. It has very weak London dispersion Forces. Thus it has the least melting point (-248oC).
Argon:
                Argon is also a noble gas. It has very weak London dispersion Forces. But it has high polarizability
                 than that of Neon. So its melting point is higher (-189oC) than neon (-248oC).
Water:
                Water has hydrogen bonding. Due to strong intermolecular forces melting point of ice (water) is high.
Sodium Fluoride:
                    Sodium fluoride has ionic bond. The positive (Na+) and negative (F-) ions are strongly held by electrostatic forces. It is the reason that melting point of NaF is very high.
Diamond:
              In diamond there is SP3 hybridization. Each carbon atom is bonded with four other carbon atoms. A huge network structure is formed. From this network structure a single unit can not be removed. It is the reason diamond has very high melting point (3350oC).
Q.9.     The boiling and molar masses of hydrides of some first row elements are tabulated below:

Substance
Boiling Point (K)
Molar Mass (g mol-1)
CH4
109
16
NH3
240
17
H2O
373
18
            Suggest reasons for the difference in their boiling points in terms of the type of molecules involved and the nature of the forces present between them.
Answer:
CH4:
            CH4 is non polar molecule. It has only weak London dispersion Forces. It the reason that CH4 has very low boiling point (109 K or -164oC).
NH3:
            NH3 is a polar molecule. It has hydrogen bonding and dipole-dipole interactions. It is the reason that boiling point of NH3 is high (240 K or -33oC).
H2O:
            H2O is a polar molecule. It has hydrogen bonding and dipole-dipole interaction. Because oxygen is more electronegative than Nitrogen. So wager has greater hydrogen bonding than ammonia. It is the reason H2O has higher boiling point (373 K or 100oC).
Q.10.   Explain the term saturated vapour pressure. Arrange in order of increasing vapour pressure: 1dm3 water, 1dm3 ethanol, 50cm3 water, 50cm3 ethanol and 50cm3 of ether.
Answer:
            See page No.
            50cm3 ether > 50cm3 ethanol = 1dm3 ethanol > 1dm3 H2O = 50cm3 H2O
Q.11.   While a volatile liquid standing in a breaker evaporates, the temperature of the liquid remains the same as that of its surrounding. It the same liquid is allowed to vapourize into atmosphere in an insulated vessel, its temperature falls below that of its surrounding. Explain the difference in behaviour.
Answer:
            When a liquid evaporates in a beaker, it causes cooling. Now heat from surrounding enters into the liquid. It is the reason temperature remains same. But when a liquid is present in an insulated vessel, then evaporation of liquid causes cooling. But heat can not enter into liquid from surrounding due to insulated vessel. It is the reason that the temperature of liquid falls below that of surrounding.
Q.12.   How does hydrogen bonding explain the following indicated properties of the substances?
(i)         Structure of DNA
(ii)        Hydrogen bonding in proteins
(iii)       Formation of ice and its lesser density than liquid water
(iv)       Solubilities of compounds
Ans:    ////////// ??????????

Q.13.   What are liquid crystals? Give their uses in daily life.
Ans:    The turbid liquids with some degree of order are called liquid crystals. The liquid like crystals which exist between melting temperature and clearing temperature are called liquid crystals.First of all liquid crystals were discovered in 1888 by Frederick Reinitzer an Austrain botanist. He was studying an organic compound cholesteryl benzoate. This compound turns milky liquid at 145oC and becomes a clear liquid at 179oC. By cooling, the reverse process occurs and liquid crystals are formed. The liquid crystals are composed of long rod like molecules. They have some degree of order. On the basis of ordering, the liquid crystals are of three types.
                         (i) Nematic   (ii) Smectic   (iii) Cholestric
Properties:
(i)                 Liquid crystals are always isotropic.
(ii)               They have optical properties.
(iii)             They have fluidity, surface tension, viscosity etc.
(iv)             Their properties are in between liquids and crystals.

Uses of Liquid Crystals:
(i)                 Liquid crystals are used as solvents in chromatography.
(ii)               Liquid crystal screens are used in oscillographic and TV displays.
(iii)             Liquid crystals are used in thermometers.
(iv)             Liquid crystals are used in electrical circuits to find out potential failure.
(v)               Liquid crystals are used in the display of digital watches, calculators and laptop computers.
(vi)             Liquid crystals are used as temperature sensors. When white light is reflected from liquid crystal then its colour changes. When temperature changes, the colour of reflected light also changes. Thus temperature is accurately measured.
(vii)           Liquid crystals are used to locate infection, tumors and breast cancer. They are also used to detect blockage of veins and arteries. These parts are warmer than other tissues. When liquid crystal is painted on them, they produce blue coloured spots. Thus indication of any problem is easy.
Q.14.   Explain the following with reasons.
(i)         Evaporation causes cooling.
(ii)        Evaporation takes place at all temperatures.
(iii)       Boiling needs a constant supply of heat.
(iv)       Earthenware vessels keep water cool.
(v)        One feels sense of cooling under the fan after bath.
(vi)       Dynamic equilibrium is established during evaporation of a liquid in a closed vessel at constant temperature.
(vii)      The boiling point of water is different at Murree hills and at Mount Everest.
(viii)     Vacuum distillation can be used to avoid decomposition of sensitive liquid.
(ix)       Heat of sublimation of a substance is greater than that of heat of vaporization.
(x)        Heat of sublimation of iodine is very high as compared to other halogens.
Answer:
(i)      The reason is that during evaporation first of all high energy molecules leave the liquid and low energy molecules are left behind. So temperature of the liquid falls and cooling is produced. To continue the evaporation heat moves from surrounding to the liquid. Thus temperature of surrounding also falls. For example when we put spirit on our hands then spirit evaporates and hand feels cooling.
 (ii)    Evaporation is a surface process. It takes place at all temperatures. When high energy molecules come at the surface of liquid, they escape out of surface. However by increasing temperature, the average K.E of molecules increases. Hence rate of evaporation increases by increasing the temperature.
(iii)    When we supply heat to a liquid, then K.E of molecules and rate of evaporation increase. At boiling point the K.E of molecules becomes maximum. Thus heat supplied at boiling point is used to break intermolecular forces. So there is no increase in K.E of molecules. Hence temperature remains constant at boiling point. It is the reason that boiling needs a constant supply of heat.
(iv)    Earthenware vessels have pores in them. Water evaporates from these pores and causes cooling. During evaporation, the escaping molecules get energy from neighbours to overcome intermolecular forces. Thus temperature of remaining water decreases. In the old earthenware’s pores are blocked with dust. So water can not evaporate. Thus they do not keep water cool.
(v)     One feels sense of cooling under the fan after bath. It is due to evaporation process. After a bath some water molecules are present on the body. These molecules get heat from the body and evaporate. Thus body feels cooling.
(vi)    See page No. 139
(vii)   Atmospheric pressure is different at Murree hills and at Mount Everest. Due to different atmospheric pressure boiling point of water is different at two places. At Murree hills atmospheric pressure is 700 torr and boiling point of water is 98oC. At Mount Everest the atmospheric pressure is further reduced and boiling point of water is 69oC.
(viii)  The distillation which is carried out at low pressure is called Vacuum distillation. It has many advantages.
(i)         It decreases the time for distillation.
(ii)        It decreases fuel cost for distillation.
(iii)       It prevents decomposition of compounds
         For example  B.P of glycerine is 290oC. Pressure of 760 torr but at this temperature glycerine decomposes. Hence distillation of glycerine is impossible at 290oC. Therefore its vacuum distillation is done at 120oC at reduced pressure of 50 torr.
(ix)    In sublimation a substance directly changes into vapours. It is two steps process (Solidliquidvapours) on other hand vaporization is single step process. It is the reason heat of sublimation of a substance is greater than that of heat of vaporization.
(x)        Iodine has biggest atomic size than other halogens (F2, Cl2, Br2). So I2 has high polarizability. Due to high polarizability Iodine has greater London dispersion forces. It is the reason that heat of sublimation of Iodine is very high


3 comments:

rizwan ahmed said...
This comment has been removed by the author.
rizwan ahmed said...

thankyou so very much for that great work

filza ahmad said...

Thanks a bunch for such a great help. It means alot!

Post a Comment