Chemistry XI Chapter 4

Question No. 4.8: Water is a liquid at room temperature but H2S is gas. Give the reason.

Answer: Water has hydrogen bonds between its molecule due to high electronegativity of oxygen (only second to F), while H2S has weak vender Waal forces between its molecules. So, water molecules are closely packed as compared to that of hydrogen sulphide. Thus, H2O is a liquid and H2S is a gas.

Question 4.9: How hydrogen bonding helps exceptionally to lower the acidic strength of HF molecule?

Answer: The exceptional, low acidic strength of HF molecule as compared to HCl, HBr, and HI is due to strong hydrogen bonding between HF molecules. Because of strong hydrogen bonding the partial positively charged hydrogen is entrapped between two highly electronegative F atoms so it dissociates into water very weakly.

Question 4.10: Lower alcohols are water soluble but corresponding alkanes are not soluble in water. Why?

Answer: Alcohols have OH group which is polar and makes hydrogen bonds with water molecules. This hydrogen bonding develops solubility. Hydrocarbons are non –polar molecule. They do not have such functional group. So no hydrogen bonding and hence no solubility in water.

Question 4.11: How can you justify that structure of ice is just like that of diamond?
Answer: The oxygen of water molecule is SP3 hybridized just like the carbon of diamond. Oxygen of water molecule makes two covalent bonds and two hydrogen bonds. So the hydrogen bonded water molecule in ice is just like the carbon of diamond.

Question 4.12: H-bonding is present in chloroform and acetone. Justify it.

Answer: It is not advisable to limit the hydrogen bonding to O, N and F. H-bonding bonding is also present between chloroform and acetone molecules. Reason is that the three chlorine atoms attached with carbon in chloroform molecule are responsible for H-bonding with other molecules. Three Cl atoms withdraw sufficient electron density from directly bonded C atom which in turn attracts electron from H atom. So, partial positive H atom can form hydrogen bond with O of acetone.

Question 4.13: Ice floats on water. Justify it.

Answer: When temperature of water is decreased, and ice is formed, then water molecules become more regular due to hydrogen bonding and this regularity extends throughout the structure. Thus more empty spaces are created. That is the reason when water freezes, it occupies 9% more space and its density is decreased. The result is that ice floats on water.

Question 4.14: Why are boiling points of hydrides of second period in group IVA, VA,VIA and VII greater than the B.P. of hydrides of third period for these groups?

Answer: Reason is that elements of IV–A ,V–A , VI–A and VII–A groups are relatively more electronegative than elements of III –A in their respective series. So that polarities of the bonds with hydrogen are greater for IV–A ,V–A , VI–A and VII–A groups than those of third period elements . So greater the polarities of bonds, greater are the boiling points.

Question 4.15: Why do NH3, HF and H2O show maximum boiling points in their respective group hydrides?

Answer: This is because of enhanced electronegative character of N, O and F in their respective groups. Due to this enhanced electronegative character hydrides of N, O and F are able to possess hydrogen bonding. So they show maximum boiling points.

Question 4.16: Why are boiling point of HF lower as compared to boiling point of H2O, although the hydrogen bonding in HF are stronger than that of water?

Answer: Fluorine is more electronegative that oxygen. So we should expect H-bonding in HF to be stronger than that in water and as a result the boiling point of HF should be higher than that of H2O. However, it is lower and the reason is that fluorine atom can make only one hydrogen bond hydrogen of neighbouring molecule. In water oxygen atom can make two hydrogen bonds with neighbouring molecules, as it has two lone pair lone pairs on it.

Question 4.17: Why the alcohols and carboxylic acids of low molar masses are soluble in water but those of higher molar masses are insoluble?

Answer: Both alcohol and carboxylic acid contain polar (OH) and non polar (alkyl) part. Due to presence of O–H group both alcohol and carboxylic acid are soluble in water. However , when the length of non polar part exceed a certain limit then non polar part can not be solvated by water molecule. Hence CH3OH and C2H5OH are water soluble , but C5H11OH are not soluble.

Question 4.18: What is role of hydrogen bonding in cleansing action?

Answer: Soaps and detergents are sodium salts of long chain of carboxylic acids and benzene sulphonates respectively. The polar parts i.e COONa group of soaps and SO3Na group of detergents have a negative charge on oxygen atom and create hydrogen bonding with water. The non-polar part of detergent remains outside water. In this way water do cleansing action. Next

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