NCERT Solutions for Class 11 Chemistry Chapter 6 Long Answer Type Questions
Question 1. (a)What is a spontaneous process? Mention the conditions for a reaction to be spontaneous at constant temperature and pressure.
(b) Discuss the effect of temperature on the spontaneity of an exothermic reaction.
Answer: (a) A process is said to be spontaneous if it takes place by itself by own or under some condition.
∆G gives a criteria for spontaneity at constant temperature and pressure.
(b) If the temperature is so high that T∆S > ∆H in magnitude, ∆G will be positive and the process will be non-spontaneous.
If the temperature is made low so that T∆S < ∆H in magnitude, ∆G will be negative and the process will be spontaneous.
Question 2. Predict in which of the following, entropy increases/decreases.
(i) A liquid crystallizes into a solid
(ii) Temperature of a crystallize solid is raised from OK to 115 K
(iii) 2NaHCO3 (s) ———-> Na2 C03 (s) + C02 (g) + H2O (g)
Answer: (i) After freezing, the molecules attain an ordered state and therefore, entropy decreases.
(ii) At 0 K the constituent particles are in static form therefore, entropy is minimum. If the temperature is raised to 115 K particles begin to move and entropy increases.
(iii) Reactant, NaHCO3 is solid. Thus, its entropy is less in comparison to product which has high entropy.
(iv) Here, one molecule gives two atoms. Thus, number of particles increases and this leads to more disordered form.
Question 3. Why standard entropy of an elementary substance is not zero whereas standard enthalpy of formation is taken as zero?
Under what conditions will the reaction occur, if
(i) both ∆H and ∆S are positive
(ii) both ∆H and ∆S are negative
Answer: (a) A substance has perfectly ordered arrangement of its constituent particles only at absolute zero. When the element formed from itself, this means no heat change.
Thus,∆f H = 0
(i) If both AH and AS are positive ∆G can be – ve only if T∆S > ∆H in magnitude. Thus, the temperature should be high.
(ii) If both AH and AS are negative ∆G can be negative only if T∆S < ∆H in magnitude. Thus, the value of T should be low.