1st year chemistry- 4th Chapter exercise short questions answers
Q.5. Explain the following with reasons.
Q.5. Explain the following with reasons.
a) In
the hydrogen bonded structure of HF, which is the stronger bond: the shorter
covalent bond
or
the longer hydrogen bond between different molecules.
ans:
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.
ans:
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.
b) In
a very cold winter the fish in garden ponds owe their lives to hydrogen
bonding?
ans:
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.
ans:
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.
c) Water
and ethanol can mix easily and in all proportions.
ans
Both water and Ethanol (C2H5OH) show H-bonding with each other. It is the reason that they can mix easily in all proportions.
ans
Both water and Ethanol (C2H5OH) show H-bonding with each other. It is the reason that they can mix easily in all proportions.
d) The
origin of the intermolecular forces in water.
Answer:
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.14. Explain the following with reasons.
(i) Evaporation causes cooling.
ans : 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.
ans : 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 takes place at all
temperatures.
ans: 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.
ans: 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) Boiling needs a constant supply of heat.
ans: 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.
ans: 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 keep water cool.
ans: 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.
ans: 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.
ans: 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.
ans: 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) 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.
ans: 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.
ans: 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) Vacuum distillation can be used to avoid
decomposition of sensitive liquid.
ans: The distillation which is carried out at low pressure is called Vacuum distillation. It has many advantages.
ans: 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.
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) Heat of sublimation of a substance is
greater than that of heat of vaporization.
ans : 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.
ans : 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) Heat of sublimation of iodine is very
high as compared to other halogens.
ans: 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.
ans: 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.
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