Question

1. For each of the following reactions, indicate whether you would expect the entropy of the system to increase or decrease, and explain why. If you cannot tell just by inspecting the equation, explain why.

a.CH₃OH(l) CH₃OH(g)

b.N₂O₄(g) 2NO₂(g)

c. CO(g) + H₂O(g) CO₂(g) + H₂(g)

d.2KClO₃(s) 2KCl(s) + 3O₂ (g)

e.2NH₃(g) + H₂SO₄(aq) (NH₄)₂SO₄(aq)

2. Predict which of the following reactions has a positive entropy change.

I. 2 N₂(g) + O₂(g) à 2 N₂O(g)

II. CaCO₃(s) à CaO(s) + CO₂(g)

III. Zn(s) + 2 HCl(aq) à ZnCl₂(aq) + H₂(g)

Answer 1

When the number of gaseous molecules increases from reactant to product, entropy increases. Also entropy increases when a solid is converted to liquid and when a liquid is converted to gas.

(a): CH3OH(l) ------- > CH3OH(g)

Here entropy of the system increases, because the substance is converted from less randomly arranged liquid to more randomly arranged gas.

(b): N2O4(g) ----> 2NO2(g)

Here the entropy of the system increases because number of gaseous molecules increases from the reactant to product.

(c):  CO(g) + H2O(g) ----- > CO2(g) + H2(g)

Here the entropy of the system remains same because number of gaseous molecules remain same in both reactant and product side.

(d): 2KClO3(s) ------ > 2KCl(s) + 3O2 (g)

Here the entropy of the system increases because number of gaseous molecules increases from the reactant(0 gaseous molecules) to product(3 gaseous molecules)

(e): 2NH3(g) + H2SO4(aq) ------ > (NH4)2SO4(aq)

Here the entropy of the system decreases because number of gaseous molecules decreases from the reactant(1 gaseous molecules) to product(0 gaseous molecules)

Q.2: Entropy change will be positive when the entropy of the product is more than the entropy of the reactant.

(i):  2 N2(g) + O2(g) ----> 2 N2O(g)

Entropy change is negative, entropy decreases from reactant to product.

(ii) CaCO3(s) ----> CaO(s) + CO2(g)

Entropy change is positive, because entropy increases from reactant to product.

(iii) Zn(s) + 2 HCl(aq) --- > ZnCl2(aq) + H2(g)

Entropy change is positive, because entropy increases from reactant to product.