Q.1. What are dipole-forces?
Ans. The electrostatic forces of attraction between the positive end of one polar molecule and negative end of other polar molecule are called dipole-dipole forces. The strength of these forces depends upon the electronegativity difference between the bonded atoms and the distance between the molecules. The shorter the distance and greater the electronegativity difference between the bonded atoms, the stronger will be the dipole-dipole forces. A dipole-dipole force exists between neutral polar molecules. For example, HCl and CHCl3.
Q.2. What are dipole-induced dipole forces? (Or Debye forces)?
Ans. In a mixture of substance containing polar and non-polar molecules, the positive end of the polar molecule attracts the mobile electrons of the nearby non-polar molecule. Thus polarity is induced in non-polar molecule, and both molecules become dipoles. These attractive forces that exist between a molecule having a dipole and that molecule in which dipole has been induced are called dipole-induced forces or as Debye forces.
Q.3. What are London dispersion forces?
Ans. The momentary force of attraction created between instantaneous dipole and the induced dipole is called dipole-induced dipole interaction or London dispersion force. These forces can exist between non-polar atoms or molecules like H2, Cl2 and noble gases (He, Ne, etc). London recognized that the motion of electrons is an atom or molecules can create and instantaneous dipole moment. Because electrons repel one another, the motions of electrons on one atom influence the motions of electrons on its neighbours. Thus, the instantaneous (temporary) dipole can induce a similar dipole on an adjacent atom, causing the atoms to be attracted to each other. This interaction is called the London dispersion force (or merely the dispersion force).
London forces are present in all types of molecules whether polar or non-polar, but they are very significant for non-polar molecules.
Q.4. What is meant by polarizability?
Ans. The ease with which the charge distribution in a molecule can be distorted by an external electric field is called its polarizability. Polarizability of a molecule is the quantitative measurement of the extent to which its electronic cloud can be distorted or polarized.
Greater the number of atoms in a molecule, greater its polarizability. The greater the polarizability of a molecule, the more easily its electron cloud can be distorted to give a momentary dipole. Therefore, more polarizable molecules have stronger London dispersion forces.
Q.5. Describe briefly the factors affecting the London forces.
Ans. 1. Molecular mass of the atom or molecule. London forces tend to increase with molecular mass. This is because molecules with larger molecular mass have more electrons, and London forces increase in strength with the number of electrons. Also larger atoms are more polarizable.
2. Shapes of molecules. The shapes of molecules can also play a role in the magnitude of London forest. Consider Pentane, Isopentane and Neopentane (Alkanes), all have the same molecular formula, C5H12, and thus the same molecular mass. Pentane a long chain of carbon atoms experiences stronger attractive forces than Isopentane and Neopentane which have increasingly more compact arrangement of atoms. The relative strengths of the London forces depend upon the polarity, polarizability, size and shape the molecule.
Q.6. What are ion-dipole forces?
Ans. An ion-dipole force exists between anion and a partial charge on the end of a polar molecule. Polar molecules are dipoles; they have a positive end and a negative end. Positive ions are attracted to the negative end of a dipole whereas negative ions are attracted to the positive end. Ion-dipole forces are especially important for solutions of ionic substances in polar liquids.
Q.7. Why the melting and boiling points of halogens increase down the group?
Ans. In general, larger molecules tend to have greater polarizability because they have greater number of electrons. Moreover, because molecular size and mass generally parallel each other, London dispersion forces tend to increase in strength with increasing molecular mass. Thus the melting and boiling points of the halogens increase with increasing molecular mass down the group.
Q.8. What is hydrogen bonding?
Ans. Hydrogen bonding is a special type of intermolecular attraction that exists between the hydrogen atom in a polar bond (Particularly In H – F , H – O , or H – N bond) and an unshared electron pair on a nearby small, electronegative ion or atom (usually on F,O, or N atom on another molecule). For example, a hydrogen bond exists between the H atom in an H – F molecule and the F atom of an adjacent HF molecule, F – H —–F – H ; where the dots represent the hydrogen bond between the molecules.
Q.9. In a very cold winter fish in garden ponds owe their lives to hydrogen bonding. Explain.
Ans. The low density of ice compared to that of water is due to hydrogen bonding interactions between water molecules. Because ice is less dense than water, ice floats on top of water and acts as a blanket protect the aquatic life below from the extremes of cold that occur above the ice. Thus in a very cold winter, fish in garden ponds owe their lives to hydrogen bonding.
Q.10. What kinds of intermolecular forces are expected in the following substances? (a) CH4 (b) Chloroform, CHCl3 (c) Ethanol, C2H5OH?
Ans. A) methane, CH4 is a non-polar molecule. Hence the only intermolecular attractions are London forces.
b) Chloroform, CHCl3 is an unsymmetrical molecule with polar bonds. Thus we expect dipole-dipole forces, in addition to London forces.
c) Ethanol, CH3CH2OH has a hydrogen atom attached to an oxygen atom. Therefore, we expect hydrogen bonding. Because the molecule is polar, we expect dipole-dipole forces. London forces exist too, because such forces exist between all molecules.
Q.11. What type of intermolecular forces will dominate in the following liquids. (i) ammonia, NH3 (ii) octane, C8H18?
(iii) Argon, Ar (vi) Propanone, (v) methanol, CH3OH.
Ans. (i) An NH3 hydrogen bonding (ii) in octane, C8H18, London forces (iii) in AR, London forces (iv) in propanone, CH3COCH3, dipole-dipole interaction (v) in CH3OH, hydrogen bonding.
Q.12. Which is the weakest acid in halogen acids? Explain.
Ans. In halogen acids, HF is the weakest acid due to hydrogen bonding in HF, because the partial positive charge is entrapped between two highly electronegative atoms. Therefore, HF does not release proton H+ easily as compared to other halogen acids.
Q.13. Which type of intermolecular attraction force operates between (a) all molecules (b) polar molecules (c) non-polar molecules (d) the hydrogen atom of a polar bond and a nearly small, electronegative atom?
Ans. (a) London dispersion (b) dipole-dipole forces (c) London dispersion forces (d) dipole-dipole forces and in certain cases hydrogen bonding.
Q.14. Water and ethanol can mix easily and in all proportion. Explain.
Ans. Water (H – O – H ) and ethanol (C2H5 – O – H ) both have O – H groups and have an unshared electron pair on a small, electronegative O – atom, so they can form hydrogen bonding extensively. Thus water and ethanol can mix easily and in all proportions because of intermolecular hydrogen bonding between them.
Q.15. Water is a liquid at room temperature while H2S is a gas. Explain.
Ans. Oxygen atom in H2O is small and more electronegative as compared to sulphur is H2S. Because O is so electronegative, a bond between hydrogen and O atom is quite polar with hydrogen at the positive end, thus, H2O has hydrogen bonding. Therefore, H2O Is a liquid because of hydrogen bonding, while H2S has non hydrogen bonding and thus it is a gas at room temperature.
Q.16. How does hydrogen bonding explain the structure of proteins?
Ans. One of the most important and common secondary structures of proteins is the helix. The helix is held in positive by hydrogen – bond interactions between N –H bond and the oxygen of nearby carbonyl groups, C = O. the pith of the helix be such that the N – H and C = O functional groups on adjacent turns are in proper position for hydrogen bonding. In the right handed helix the NH and C = O groups are vertically adjacent to one another r and they are linked together by hydrogen bonds. These hydrogen bonds link one spiral to the other.
Q.17. How does hydrogen bonding explain the structure of DNA?
Ans. DNA molecules consist of two deoxyribonucleic acid spiral chains that are wound together in the from of a double helix. The two chains or strands are held together by hydrogen bonding between their subunits.
Q.18. How does the soaps and detergents perform the cleaning action?
Ans. Soaps and detergents perform the cleaning action because the polar part of their molecular are water soluble due to hydrogen – bonding and the non-polar parts remain outside water because they are alkyl or benzyl portions and are insoluble in water. The cleaning action of soap and detergents occurs because oil and grease can be absorbed into the alkyl or benzyl portions and washed away.
Q.19. HF is a weaker acid, while HI is strong acid. Explain it.
Ans. HF is a weaker acid as compared to HI is due to strong hydrogen bonding in HF, because the partial positively charged hydrogen is entrapped between two highly electronegative F atoms and cannot be easily unionized.
Q.20. Why does the density of ice is lower than water?
Ans. Ice compared to that of water can be explained I terms of hydrogen bonding. The interactions in water are random. However, when water freezes, the molecules assume the ordered arrangements, creating empty spaces in ice. Thus, when water freezes, it occupies more space and density decrease. The result is that the ice has low density compared to that of water and ice floats on water. The structure of ice permits the maximum number of hydrogen – bonding between the H2O molecules.
Q.21. Briefly consider some of the effects on our lives, If water has only a very weak hydrogen bonding present among its molecules.
Ans. The density of ice compared to water profoundly affects our lives. If water has only a very weak hydrogen bonding among its molecules, then ice would be more dense than water. If ice were more dense than water, ice forming at the top of a lake would sink to the bottom, and the lake could freeze solid. Most aquatic life could not survive under these conditions. The expansion of water upon freezing causes water pipes to break in freezing weather. So, the pattern of our lives would have been totally different in the presence of very weak hydrogen bonding in water.
Q.22. Why does the boiling point of HF is lower than that of H2O.
Ans. The boiling point of HF is lower than that of H2O, because fluorine atom can make only one hydrogen bond with electropositive hydrogen of a neighboring molecule, while water can form two hydrogen bonds per molecule, as it has
two hydrogen atoms and two lone pairs on oxygen atom
Q.23. What is evaporation?
Ans. The spontaneous change of a liquid into its vapours is called evaporation and it occurs at all temperatures. The molecules of a liquid are in a constant motion and possess kinetic energy which is not equally distributed. When high kinetic energy molecules reach the surface of a liquid, they may escape and leave the bulk of the liquid.
Q.24. Evaporation causes cooling?
Ans. Temperature is a measure of average kinetic energy of the molecules of a liquid. As the liquid evaporates, the escapes of high – energy molecules from the liquid lowers the average kinetic energy of the remaining molecules in the liquid. As a result; the temperature of the liquid falls down and heat moves from the surrounding to the liquid, so the temperature of surrounding also decreases. Thus evaporation causes cooling.
Q.25. Why does evaporation take place at all temperatures?
Ans. The molecules of a liquid are in a constant state of motion and possess kinetic energy at all temperatures, therefore, evaporation takes places at all temperatures. However, if the temperature is increased, the rate of evaporation also increases.
Q.26. Earthenware vessels keep water cool. Explain.
Ans. Earthenware vessels are porous, when water is kept in earthenware vessels, water evaporates through these pores. The more energetic water molecules leave the vessel. As a result, the average kinetic energy of the remaining water molecules decreases and hence the temperature falls. Therefore, earthenware vessel keeps water cool.
Q.27. What are the factors, which control the rate of evaporation of a liquid?
Ans. 1. Surface area. Since evaporation occurs from liquid surface, so greater the surface area, the greater will be rate of evaporation.
2. Temperature. Since kinetic energy increases with increases in temperature, so rate of evaporation increases.
3. Intermolecular forces. The weaker the intermolecular forces, the faster the rate of evaporation.
Q.28. One feels sense of cooling under the fan after bath.
Ans. When one comes under the fan after both, the more energetic water molecules on the surface of body evaporates with greater rate. When high energy molecules escape from the surface of the body, one feels sense of cooling.
Q.29. What is vapour pressure?
Ans. The pressure exerted by the vapours in equilibrium with its liquid at a given temperature is called vapour pressure of the liquid. When a liquid is allowed to evaporate inside a closed vessel, a state of dynamic equilibrium is established in which the rate fo evaporation is equal to the rate of condensation. The pressure exerted by the vapours at this stage is called vapour pressures and it increases with increasing temperature. The stronger the intermolecular forces, the lower the vapour pressure. Liquids that evaporate readily are said to be volatile.
Q.30. Dynamic equilibrium is established during evaporation of a liquid in a closed vessel at constant temperature. Explain.
Ans. When a liquid is allowed to evaporate inside a closed vessel, high-energy molecules leave the liquid and start gathering above the surface of the liquid. These molecules collide with the walls of the vessel as well as with the surface of the liquid. The two opposing processes, evaporation and condensation continue till a stage is reached at which the rate of evaporation equals to the rate of condensation. The condition in which two opposing processes are occurring simultaneously at equal rates is called a dynamic equilibrium. All equilibrium between different states of matter possess this dynamic character.
Q.31. Describe briefly the factors affecting the vapours pressure.
Ans. The values of vapour pressure of various liquids depend upon the nature of liquids and intermolecular forces, because the molecular masses of different liquids are different. Stronger the intermolecular forces, lower is the vapour pressure of the liquid. Vapour pressure of the liquid increases with increasing temperature, since with increase in temperature, the kinetic energy of molecules is increased and capability to leave the surface increases which causes the increase of vapour pressure.
Q.32. Define boiling point. Is it related with the external pressure?
Ans. The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure or to any other external pressure on the liquid is called the boiling point of the liquid. The boiling point is related with the external pressure. When the external pressure is changed, the boiling point of the liquid is also changed. A liquid can be made to boil at any temperature by changing the external pressure. The boiling point of the liquid is increased by increasing the external pressure. The atmospheric pressure is lower at higher altitudes, so water boils at a lower temperature.
Q.33. Liquid boils at that temperature, when its vapour pressure becomes equal to the external pressure. Why?
Ans. When the temperature of a liquid is raised, the escaping tendency of tis molecules may be increased to the point that boiling occurs. Boiling is the formation of bubbles of vapour (gas) within the body of the liquid. The bubbles of vapour formed with the body of liquid have greater internal pressure than the external pressure on the surface of the liquid. This makes the bubble to come out of the liquid and burst upon the surface, when the vapour pressure becomes equal to the external pressure on the liquid. This bursting of the bubbles is called boiling.
Q.34. The boiling point of a liquid remains constant although heat is continuously supplied to the liquid at its boiling point. Explain.
Ans. At the boiling point, the kinetic energy of the molecules becomes maximum and any further heating at this stage will not increase the temperature, rather the supplied heat will only be utilized to break the intermolecular forces and convert the liquid into its vapours. Therefore, the boiling point of a liquid remains constant although heat is continuously supplied to the boiling point.
Q.35. Steam can cause sever burns. Explain.
Ans. When steam comes in contact with skin it condenses, releasing considerable heat, which causes severs burns.
Q.36. What is vacuum distillation and what are its advantages.
Ans. The process of boiling a liquid and condensing its vapour to the liquid under reduced pressure is called vacuum distillation.
Advantages: It decreases the time for the distillation process and is economical because less fuel is required. It can be used to avoid decomposition of a sensitive liquid like glycerine.
Q.37. Why does the heat of sublimation of a substance is greater than that of heat of vapourization?
Ans. In sublimation, solid directly changes into vapour phase, while in vapourization, liquid changes into vapour. The values of Hs are larger the Hv because attractive forces in solids are stronger than those in liquids. Therefore, heat of sublimation of a substance is greater than that of heat of vapourization.
Q.38. Why heat of sublimation of iodine is very high?
Ans. The value of Hv for I2 is very high due to its greater polarizability and van der Waals forces, which are sufficient strong.
Q.39. Differentiate between sublimation and condensation.
Ans. The change of a solid directly to the vapour is called sublimation, while condensation is the change of a gas to either the liquid or the solid state(the change of vapour to a solid is sometimes called deposition).
Q.40. Define molar heat of fusion, molar heat of vapourization and molar hat of sublimation.
Ans. The amount of heat absorbed by one mole of a solid when it melts into liquid at its melting point at one atmosphere is called molar heat of fusion, Hf.
The amount of heat absorbed when one mole of a liquid is changed into vapours at its boiling point at one atmosphere is called molar heat of vapourization, Hv.
The amount of heat absorbed when one mole of a solid sublimes to give one mole of vapours at a particular temperature and one atmospheric pressure is called molar heat of sublimation Hs.
Q.41. What are liquid crystals?
Ans. A substance that exhibits one or more partially ordered turbid liquid phases above the melting point of the solid form and resemble crystals in certain properties is called a liquid srystal. A crystalline solid may be isotropic or anisotropic, but liquid crystals are always isotropic
Crystal liquid crystal liquid
Because of the partial ordering, liquid crystals may be very viscous and possess properties between those of the solid and liquid phases. Substance that form liquid crystals are often composed of long, rod like molecules. Depending on the nature of ordering, liquid crystal can be divided into Nematic, Smectic and Cholesteric.
Q.42. Give four uses of liquid crystals.
Ans. (1) Liquid crystals can be used as temperature sensors.
(2) Liquid crystals are used in the display of electrical devices such as digital watches, calculators,
(3) In chromatographic separations, liquid crystals are used as solvent.
(4) Oscillographic and TV displays use liquid crystal screen.
(5) Liquid crystals are used to find the point of potential failure in electrical circuits.
(6) Liquid crystalline substance are used to locate the veins, arteries infections and tumors.
Q.43. State the biological applications of liquid crystals.
Ans. Liquid crystalline substance are used to located the veins, arteries, infections and tumour, since there parts of the body are warmer than the surrounding tissues. Specialist use the techniques of skin thermography to detect blockages in veins and arteries. When a layer of liquid crystal is pointed one surface of the breast, a tumour shows up as a hot area, which is, coloured blue. This technique is useful in the early diagnosis of breast cancer.
Q.44. How does a crystalline solid differ from an amorphous solid?
Ans. A solid in which atoms, ions or molecules are arranged in a definite three dimensional pattern is called a crystalline solid. These solids have flat surfaces or faces that make definite angles with one another. These solids have sharp melting points point. E.g., diamond, SiO2, sucrose.
A solid whose atoms, ions or molecules have no orderly structure is called an amorphous solid (amorphous means shapeless). The se solids lack well-defined faces and shapes. They do not have sharp melting point e.g., glass, rubber.
Q.45. What are cleavage planes?
Ans. Crystalline solids are broken easily along definite planes, called the cleavage planes and they are inclined to one another at a particular angle for a given crystalline solid. The value of this angel varies from one solid to another solid. Cleavage itself is an anisotropic behaviour.
Q.46. What is anisotropy? Why graphite is anisotropic in electrical behaviour?
Ans. The physical properties that differ in different directions through crystalline solid are called anisotropic properties and the phenomenon is called anisotropy. The physical properties like refractive index, coefficient of thermal expansion, electrical and thermal conductivities are sometimes anisotropic in nature for some crystalline solids. The variation in these properties with direction is due to the orderly arrangement of the particles in crystalline solid is different in different directions.
Graphite is anisotropic in electrical conductivity, because electrons is graphite are mobile for electrical conduction parallel to the layers only. Therefore its electrical conductivity in this direction is greater than perpendicular to the other direction.
Q.47. Why Cleavage of crystals is can isotropic in behaviour?
Ans. The physical properties that differ in different directions through the crystalline solid are called anisotropic properties. Cleavage is the breakage of a crystal along definite planes. Since cleavage of crystals can take place only in particular directions, so, it exhibits anisotropic behaviour.
Q.48. What is symmetry?
Ans. The repetition of faces, angles or edges when a crystal is rotated by 360o along its axis is called symmetry. It is an important property of the crystal. There are various types of symmetry element found in crystals. The more important symmetry elements are center of symmetry, axis of symmetry and plane of symmetry.
Q.49. What is meant by habit of crystal?
Ans. The shape of a crystal in which it usually grows is called habit of a crystal. Crystals are usually obtained by cooling the saturated solution or by slow cooling of the liquid substances. Crystals grow in various directions.
If the condition for growing a crystal are maintained then the shape of the crystal always remains the same. If the conditions for growing a crystal are changed, the shape of the crystal may change. For example, a Cubic crystal of NaCl becomes needle like when 10% urea is present in its solution as impurity.
Q.50. What is isomorphism?
Ans. A phenomenon in which two different substances exist in the same crystalline form is called isomorphism and these different substances are called isomorphs of each other. Mostly the ratios of atoms is various compounds are such that isomorphism is possible. The isomorphs substance are quite different from each in their physical and chemical properties. A crystalline form depends only on the number of atoms and their way of combination and is independent of the nature of atoms. For example, NaNO3 and KNO3 both exhibit rhombohedral crystals and the atomic ratio is 1:1:3.
Q.51. What is polymorphism?
Ans. A compound which exists in more than one crystalline form is called a polymorphic; these forms are called polymorphs of each other and the phenomenon is called polymorphism. Polymorphs have same chemical properties but they differ in physical properties. The difference in physical properties is due to different structural arrangement of their particles. For example, CaCO3, exists in trigonal and orthorhombic forms. AgNO3 exists in rhombohedral and orthorhombic forms.
Q.52. What is allotropy?
Ans. The existence of an element in more than one crystalline forms is called allotropy and these forms of the element are called allotropes or allotropic forms. Sulphur, carbon and tin elements show allotropy.
Q.53. What is transition temperature?
Ans. The temperature at which two crystalline forms of the same substance coexist in equilibrium with each other is called transition temperature. Above and below this temperature, only one form exists. Some examples are shown below:
Grey tin (Cubic) White tin (Tetragonal)
Sulphur; S8 (rhombic) Sulphur S8 (monoclinic)
Q.54. Define unit cell?
Ans. The smallest part of the crystal lattice which has all the characteristics features of the entire crystal is called a unit cell. The complete information about the crystalline structure is present within a unit cell which repeats itself in three dimensions to form a crystal.
Q.55. How unit cell is defined by unit cell dimension?
Ans. There are three unit cell lengths a, b, c and three unit cell angles , . The six parameters are called crystallographic elements or unit cell dimension. The angle ‘ α’ is between the lengths ‘b’ and ‘c’ the angle ‘β’ is between the sides ‘a’ and ‘c’ and angle ‘γ ’ is between sides ‘a’ and ‘b’. the unit cell lengths a, b, c may be assigned along x, y and z-axis, respectively but angles , have to be decided accordingly.
Q.56. What is crystal lattice?
Ans. All crystals consist of atoms, ions or molecules. In crystalline solids, these atoms, ions or molecules are located at definite positions in space. These position are represented by points in a crystal. These points are called lattice points or lattice sites. The overall arrangement of points in a crystal is called crystal lattice or space lattice. A crystal lattice is an array of points which represent atom. Ions or molecules of a crystal, arranged at different sites in three dimensional space.
Q.57. How a crystal system is identified. Write the names of crystal systems.
Ans. A cry stem system is identified by the dimensions of its unit cell along its three edges or axes, a, b, c and three angles between the axes , . There are seven crystal systems. These seven crystal systems are : cubic system, tetragonal system orthorhombic system, monoclinic system, hexagonal system, trigonal system and triclinic system.
Q.58. What are ionic solids?
Ans. Ionic solids consists of positive and negative ions held together by strong electrostatic forces of attractions i.e., by ionic bonds. These forces are non-directional . the strength of an ionic bond depends greatly on the charges of the ions. The structure of the ionic crystals depends upon the radius ratio of cations and anions. Ionic solids do not conduct electricity in solid state, but conduct electricity when they are solution or molten state. They are highly brittle and have density.
Q.59. Ionic solids do not conduct electricity in the solid state. Why?
Ans. Ionic solids do not conduct electricity in the solid state because the cations and anions are tightly held by strong electrostatic forces of attraction, hence ions occupy fixed positions and cannot freely move.
Q.60. Ionic crystals are highly brittle why?
Ans. Ionic crystals are highly brittle because ionic crystals consists of parallel layers of cations and 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 slides a bit over the other layer along a plane and so, a slight shifts brings the like ions in front of each other and thus interionic repulsions brittleness.
Q.61. Why the ionic crystalline solids have high melting and boiling points?
Ans. In ionic crystals, the cations and anions are held together by ionic bonds. Very high energy is required to separate the cations and anions from each other against the forces of attractions. That is why ionic crystals are very hard, have low volatility and high melting and boiling points.
Q.62. How does the number of positive ions surrounding the negative ion in the ionic crystal lattice depend upon the sizes of two ions.
Ans. In anionic solid, the coordination number of a particular ion depends upon the radius ratio. The greater the radius ratio, greater is the coordination number of the ion. Thus the number of positive ions surrounding the negative ion in the crystal lattice depends upon the sizes of the two atoms.
Q.63. Why NaCl and CsCl have different structure?
Ans. The structure of the ionic crystals depends upon the radius ratio of cations and anions. Since the radius ration of cations and anions in both NaCl and CsCl are not the same i.e., are different, therefore NaCl and CsCl have different structures.
Q.64. Why NaCl and CsF have the same geometry?
Ans. The structure of the ionic crystals depends upon the radius ratio of cations and anions. NaCl and CsF have the same geometry because the radius ratio in both the cases are the same.
Q.65. What is lattice energy?
Ans. The energy released when one mole of the ionic crystal id formed form the gaseous ions under standard conditions is called lattice energy. It is also defined as the energy required to break one mole of the solid into isolated ions in the gas phase. It is expressed in kJ mol-1.
Na+(g) + Cl–(g) → NaCl (s) -1
NaCl (s) → Na+(g) + Cl- (g) -1
Lattice energy gives us some idea of the force of attraction between opposite ions in crystalline solid.
Q.66. Why the lattice energy of NaCl is greater than of KCl which in turn is greater than KBr?
Ans. Lattice energy depends upon the size of ions. In case of NaCl and KCl, the size of Na+ ion is smaller than that of K+ ions, so in KCl the packing of oppositely charged ions is less tight than that of NaCl. Therefore, the lattice energy of NaCl is greater than KCl.
In case of KCL and KBr, the size of Cl– ion is smaller than that of Br–, therefore the lattice energy of KCl is greater than KBr, because the packing of opposite charged ion in KBr is less tight than that of KCl.
Q.67. What are covalent solids?
Ans. Covalent solids are composed of neutral atoms held together by covalent bonds. These are of two types:
- When the covalent bonds join to form giant molecules like diamond or SiC and AlN.
- When atoms join to form the covalent bonds and separate layers are produced, e.g., graphite, cadmium iodide and boron nitride.
They contain a network or chains of atoms. Covalent crystals are hard, mostly insoluble in polar solvent but soluble in non-polar solvents.
Q.68. Diamond is hard and an electrical insulator. Why?
Ans. The hardness of diamond is accounted for by its closely interlocked, three dimensional structure. The four valecnce electrons of each carbon are shared with electrons of four adjacent carbon atoms in sp3 (tetrahedral) hybrid orbitals to form covalent bonds which run through the crystal to give a three-dimensional covalent net-work. Diamond is an electrical insulator since all valence electrons are fully involved in single (sigma) bond formation, thus no free electrons are available.
Q.69. Why graphite is a good conductor of electricity along the layers.
Ans. In graphite the carbon atoms are arranged in layers of interconnected hexagonal ring and thus graphite has a layered structure. Each carbon atom is bonded to three others in the layer and thus the electrons are available in between the layers. These electrons are delocalized which makes graphite and good conductor of electricity along the layers. Graphite Is not a conductor perpendicular to the layers.
Q.70. What are molecular solids?
Ans. Molecular solids consist of atoms or molecules held together by intermolecular forces (dipole-dipole forces, London dispersion forces, and hydrogen bonds). These crystals are either polar or non-polar molecules or atoms. Because these forces are walk, molecular solids are soft, have low melting and boiling points, low density and are bad conductor of electricity. Examples include He, Ar, CO2 and H2O.
Q.71. Molecular solids are soft and easily compressible. Why?
Ans. Molecular solids are composed of atoms or molecules held by weak intermolecular forces i.e., dipole-depole forces, London dispersion forces. Therefore, molecular solids are weak and easily compressible.
Q.72. Dionne dissolves readily in tetrahloromethane. Why?
Ans. Iodine being non-polar in nature, readily dissolves in teterachoro-methane, which is also non-polar. Like dissolves like.
Q.73. What are metallic solids?
Ans. Metallic solids consist of entirely of metal atoms. Metallic solids usually have hexagonal closed-packed, cubic-close-packed or body-centered cubic structures. The bonding in metals is due to valence electrons that are delocalized throughout the entire solids. In fact, we can visualize the metal as an array of positive ions immersed in a sea of delocalized valence electrons. Metals are good conductor of electricity and thermal conductivity. Metals are malleable and ductile whenever stress is applied on the them.
Q.74. Why the metals are malleable and ductile?
Ans. Metals are malleable, which means they can be hammered into thin sheets, and ductile, which means they can be drawn into wires. When a stress is applied on the surface of a metal, its layers slip past each other, changing the structure of the metal without fracturing. Due to this reason metals are malleable an d ductile.
Q.75. Why do most of the metals when freshly cut show metallic luster?
Ans. The shining appearance of a metal, its luster, is caused by the mobile electrons. Most of the metals possess luster, whenever are freshly cut. When light falls on the metallic surface, the incident light collides with the mobile electrons and they are excited. These electrons when deexcited give off some energy in the form of light. This light is reflected from the metal surface at all angles giving metal its peculiar luster.
Q.76. Metals are good conductor of electricity. The electrical conductivity of metals decreases with the increase in temperature. Why?
Ans. The delocalized electrons in a metal give rise to electrical and thermal conductivity. When electric field is applied between two ends of a metal then the mobile electrons begin to move towards the positive pole and the new electrons form the negative pole take their place. Passage of electrons from one end to the other constitutes electrical conduction.
The electrical conductivity of metals decrease with the increase in temperature because the positive metal ions also begin to oscillate and the motion hinders the free movement of mobile electrons. This hindrance decrease the electrical conductivity.
Q.77. Why the metals are good conductors of heat?
Ans. The delocalized electrons in a metal give rise to thermal conductivity. Heating one end of a piece of metal increases the average kinetic energy of both the ions, which vibrate more violently, and the electrons, which move more rapidly throughout the metal. The freedom of electrons to transfer energy rapidly from one end of the metal to the other is responsible for the high thermal conductivity.
Q.78. in the closest packing of atoms of metals, only 74% space is occupied. Explain?
Ans. In any close-packed arrangement, each interior atom is surrounded by 12 nearest neighbours atoms: 6 in one plane, 3 above that plane and 3 below. Thinking of atoms as hard spheres, one can calculate that the spheres occupy 74% of the space of the crystal; 26% is empty space between the spheres. There is no way of packing identical spheres so that an atom has the spheres occupy more than 74% of the space of the crystal. Therefore, in the closest packing of atoms of metals, only 74% space is occupied.
Q.79. Why the vapour pressure of water, ethyl alcohol and diethyl ether are different from each other at 0oC?
Ans. The vapour pressure depends in part on the strength of intermolecular forces in the liquid. Water can form two hydrogen bonds per molecule, ethyl alcohol can form one hydrogen bond per molecule and diethyl ether has no hydrogen bonding. The strengths of intermolecular forces increased with increased number of hydrogen bonds per molecule. Since the strengths of intermolecular forces are different therefore, the vapour pressure of water, ethyl alcohol and diethyl ether are different from each other at 0oC.
Q.80. lower density of ice than water has got significance. Comment.
Ans. The low density of ice than water greatly affects our lives. If ice were more dense than water, ice forming at the top of a lake would sink to the bottom and the lake could freeze solid. Most aquatic life could not survive under these conditions. The expansion of water upon freezing causes water pipes to break in freezing weather. So, pattern of our lives would have been totally different. Therefore, lower density of ice than water has got significance.