Q1. In the hydrogen-bonded structure of HF, which is the stronger bond: the shorter covalent or the longer hydrogen bond?
The hydrogen bond is 1% stronger as compared to a covalent bond. So in HF shorter covalent bond is much stronger than hydrogen bonds.
Q2. In very cold winter fishes in garden ponds owe their lives to hydrogen bonding?
In winter as the temperature of water decreases its density increases. Water has a maximum density at 4oC. As the atmosphere further cools ice is formed on the surface water. Ice is an insulator and can not allow heat exchange with the surrounding. In this way water in the depth of ponds and lakes remains warm and fishes remain alive.
Q3. Water and ethanol can mix in all proportions. Explain.
Water and ethanol can mix in all proportions due to hydrogen bonding.
4. What is the origin of intermolecular forces in the water?
The origin of intermolecular forces in water is the difference of electronegativity between oxygen and hydrogen. Oxygen is high electronegative and shows partial negative charge and hydrogen shows partial positive charge. Due to these charges H-bonding forms between water molecules.
05. Briefly consider some effects on our lives if water has only very weak hydrogen bonging present among its molecule.
If weak hydrogen bonding is present in water its boiling point should be less than 100oC and melting point must be less than 0oC. It should affect climates, weather, aquatic life, animal life, plant life, and all other things.
06. Evaporation causes cooling. Explain.
In a sample of liquid, all the molecules do not have the same energy. When high energy molecules come on the surface, they evaporate. The remaining molecules of liquid have low energy. So the liquid gets cooled.
07. Evaporation takes place at all temperatures. Explain.
Evaporation is a surface phenomenon. At any temperature, the molecules have a difference in energy. When high energy molecules come on the surface, they evaporate. However, an increase in temperature increases the rate of evaporation.
08. Boiling needs a constant supply of heat. Explain.
Heat supplied to the system consumed in increasing the kinetic energy and in increasing the rate of evaporation. As the liquid starts boiling the temperature of the liquid becomes constant. Now the heat given to the system consumes in evaporation. So at the boiling point, if we disconnect the supply of heat the boiling stops.
09. Earthenware vessels keep water cool. Explain.
Earthenware vessels have very small pores in them. Water evaporates through these pores and kinetic energy of the remaining molecules decreases. Hence water becomes cool. The process of evaporation reduces in old Earthenware because no. of pores decreases due to dust and they become less effective for cooling.
10. Dynamic equilibrium is established during the evaporation of a liquid in a closed vessel at a constant temperature. Explain.
When evaporation of a liquid takes place in a closed vessel, the vapors can’t go out. Condensation of liquid also starts when some vapors collected on the surface. Initially, the rate of evaporation is high than the rate of condensation. After some time, the rate of evaporation becomes equal to the rate of condensation and a dynamic equilibrium is established.
11. The boiling point of water is different at Murree and at Mount Everest. Explain.
The boiling point of any liquid depends upon external atmospheric pressure. The temperature at which vapor pressure of a liquid becomes equal to its external pressure the liquid starts boiling. At Murree and at Mount Everest the external pressure is low so boiling point of water also reduced.
12. Vacuum distillation can be used to avoid decomposition of a sensitive liquid. Explain.
OR Describe the importance of Vacuum distillation?
Boiling point of any liquid depends upon external atmospheric pressure. If the external pressure is decreased the boiling point also decreases. By applying vacuum distillation process we can boil the liquid at low temperature to avoid its decomposition. For example, boiling point of glycerin is 290oC but it decomposes at this temperature. Glycerin is purified at 120oC temperature by reducing external pressure to 50 torr.
13.Heat of sublimation of a substance is greater than its heat of vaporization. Explain.
The conversion of a substance from solid to directly vapors (sublimation) needs more amount of heat as compared to the conversion of a substance from liquid to vapors (vaporization). Actually; ∆Hsub = ∆Hfus + ∆Hvap
14.Heat of vaporization of iodine is very high.
In a group of periodic table atomic size increases from top to bottom. Polarizability of iodine is greater among other halogens. So due to strong London dispersion forces its heat of vaporization is very high.
15.What is the principle of working of a pressure cooker? OR Why food cooks rapidly in pressure cooker?
Boiling point of any liquid depends upon external atmospheric pressure. In pressure cooker the external pressure is increased. The boiling point of water increases in pressure cooker. In this way water absorbs more heat and meet and vegetables cooked readily.
16.Steam causes more severe burns than boiling water. Explain.
Boiling water has 100oC temperature but steam has high temperature than boiling water. That is why steam causes more severe burns than boiling water.
17.Sodium is softer than copper but both are good conductors of electricity. Explain.
Both are metals hence are good conductors. But sodium has less no. of delocalized electrons as compared to copper. Metallic bond in copper is strong than sodium which makes it hard.
18.Diamond is hard and electrically insulator. Explain.
Diamond has a network of covalent bonds. Each carbon is sp3 hybridized and attached with four other carbon atoms through covalent bonds. Carbon atoms have no free electron so it is insulator.
19.Sodium chloride and Cesium chloride have different structures. Explain.
Co-ordination no. of Na+ is 6 while of Cs+ is 8. Around each Na+ ,6Clions are present while in CsCl around each Cs+ , 8Clions are present. That is why NaCl has face centered cubic structure while CsCl has body centered structure.
20.Iodine dissolves readily in tetrachloromethane. Explain.
Both iodine and tetrachloromethane are non-polar so iodine dissolves more readily in tetrachloromethan as compared to water.
21.The vapour pressures of solids are far less than those of liquids. Explain.
Vapour pressure depends upon intermolecular forces. Solids have greater intermolecular forces than liquids.
So the vapour pressures of solids are far less than those of liquids.
22.Amorphous solid like glass is also called supper cooled liquids. Explain.
The temperature of many liquids may be lowered, below freezing points before crystallization begins. A liquid existing at a temperature below its freezing point is said to be supper cooled or under cooled liquid.
Glass is an example of such liquids.
23.Cleavage of a crystal is itself anisotropic in nature. Explain.
Crystals having different intensity of a property in different directions is called anisotropy. Cleavage is an isotropic property because many crystals have a specific cleavage plane and can be cut only in that plane. For example, mica sheets can be cleaved only parallel to length of sheet and not easily in any other direction.
24.The crystals showing isomorphism mostly have same atomic ratio. Explain.
This is due to the special arrangement of different atoms in isomorphous substance. For example,.
Isomorphous Atomic ratio Crystal shape
NaF and MgO 1 : 1 Cubic
NaNO3 and CaCO3 1 : 1 : 3 Rhombohedral
25.The transition temperature is given by elements having allotropic forms and by compounds showing polymorphism. Explain.
If a substance has only one form, there is no concept of transition temperature. It is only for those elements having allotropic forms and by compounds showing polymorphism.
26.One of the unit cell angles of hexagonal crystal is 120o . Explain.
Two angle α and β are 90o but γ is of 120o. At the top view of each hexagon has angle of 120o at corner. It can be calculated by following formula:
Magnitude of γ angle in hexagon = 180o – 360o /6
Magnitude of γ angle in hexagon = 120o
27.The electrical conductivity of metals decreases by increasing temperature. Explain.
When we increase the temperature the positive ions of metals also begin to vibrate. These vibrations act as a resistance in motion of electrons. So the electrical conductance decreases.
28.In the closest packing of atoms of metals, only 74% space is occupied. Explain.
When sphere is packed according to cubic or hexagonal closest packing, there is always unoccupied space left between spheres. This empty space is called hole. In this way only 74% volume is filled by spheres and 26% space remains unoccupied as hole.
29.Ionic crystals do not conduct electricity in solid state. Explain. OR NaCl is non-conductor of electricity in solid state but conductor in molten or in solution form. Explain.
In solid state the atoms are strongly held together and there is no free electron in them. They can conduct electricity only in molten state or in aqueous solution. In molten or solution form ions are free to conduct electricity. e.g., NaCl is not conductor in solid state but conductor in molten state or in solution form.
30.Ionic crystals are highly brittle. Explain.
When some stress is applied on ionic crystals the layers of ions move and similar ions come in front of each other. These same charged ions repel each other which cause brittleness. Stress applied Repulsion between Broken crystals Same charges
31.The no. of positive ions surrounding the negative ions in the ionic crystal lattice depends upon the size of the two ions. Explain.
The no. of positive ions which surround the negative ion is called its coordination number. Coordination no. depends upon the radius ratio of the two ions. Greater will be the ratio greater will be the coordination number. e.g. in NaCl the coordination no. of Na+
is 6 while in CsCl the coordination no. of Cs+ is 8.
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