Question

4. Classify each of the following reactions as one of these four types: • spontaneous at all temperatures not spontaneous at any temperature • spontaneous below a certain temperature but not above • spontaneous above a certain temperature but not below See Table 17.1 in Tro, Fridgen and Shaw. (a) CaH, (s) + 2 H20 (1) - Ca(OH)2 (s) + 2 H, (g): AH -230 kJ; (b) 2 NO(g) + Cl (g) → 2 NOCI (g); AH = -78.4 kJ; (c) C.H. (g) - CH: (g) + 2 H2(g); AH-311.4 kJ; (d) AgCl (s) +2 H20 (g) + CO (g) → AgClo, (s) + CH. (g); AS= 163 J/K AS=-121.0 J/K AS-232.7 J/K AH-616 kJ; AS --343 J/K

Answer 1

Reaction	Classification
(a) CaH2 (s) + 2 H2O(g) $\rightarrow$ Ca(OH)2 (s) + 2 H2(g)	spontaneous at all temperatures
(b) 2 NO (g) + Cl2 (g) $\rightarrow$ 2 NOCl (g)	spontaneous below a certain temperature but not above
(c) C2H6 (g) $\rightarrow$ C2H2 (g) + 2 H2 (g)	spontaneous above a certain temperature but not below
(d) AgCl (s) + 2 H2O (g) + CO (g) $\rightarrow$ AgClO3 (s) + CH4 (g)	not spontaneous at any temperature

Explanation

A reaction is spontaneous when Gibbs free energy change, $\Delta$ G is negative.

But, $\Delta$ G = $\Delta$ H - (T) * ($\Delta$S)

where $\Delta$ H is enthalpy change

$\Delta$S is entropy change

T is temperature of reaction

If $\Delta$ G is negative, then, $\Delta$ H - (T) * ($\Delta$S) will be negative

This means if a reaction is always spontaneous, then, $\Delta$ H will always be negative and $\Delta$ S will always be positive. This will make the whole expression $\Delta$ H - (T) * ($\Delta$S) negative always.