Question

Indicate whether ΔS° is large and positive, large and negative, or nearly zero in each of the following reactions. (a) NH3(g) + HCl(g) → NH4Cl(s) large and positive large and negative nearly zero (b) N2H4(g) → N2(g) + 2 H2(g) large and positive large and negative nearly zero (c) H2(g) + I2(g) → 2 HI(g) large and positive large and negative nearly zero (d) HF(aq) + NO2− (aq) → F − (aq) + HNO2(aq) large and positive large and negative nearly zero

Answer 1

The logic for answering this question is explained first:

Entropy is a measure of dis-orderness of the system. Higher the entropy, higher the dis-orderness and vice-versa .

Gases are more dis-ordered than liquids and liquids are more dis-ordered than solids . Hence gases have more entropy than liquids and liquids have more entropy than solids. That is :

entropy of gases > entropy of liquids > entropy of solids

Hence in a reaction where more gas is produced, the entropy increases.

(a)

NH3(g) + HCl(g) + NH4Cl(s)

Answer:

entropy change is large and negative

Reason : 2 moles of gas is producing 1 mol of solid. Hence entropy decreases. Entropy before the reaction is much less than the entropy after reaction. Hence entropy change is large and negative.

(b)

N2H4(g) + N2(g) + 2H2(g)

Answer:

entropy change is large and positive

Reason : 1 mole of gas is producing 3 moles of gas. Hence entropy increases. Entropy before the reaction is much higher than the entropy after reaction. Hence entropy change is large and positive.

(c)

H2(g) +1269) +2H109)

Answer:

entropy change is nearly zero

Reason : 2 moles of gas is producing 2 moles of gas.Entropy before the reaction is almost equal to the entropy after the reaction. Hence entropy change is nearly zero.

(d)

H F(aq) + NO2 (aq) + Flag) + HNO2(aq)

Answer:

entropy change is nearly zero

Reason : 2 moles of aqueous compounds are producing 2 moles of aqueous compounds .Entropy before the reaction is almost equal to the entropy after the reaction. Hence entropy change is nearly zero.