Question

Consider the following equation.

Fe2O3(s) + 3H2(g) Right arrow. 2Fe(s) + 3H2O(g)

Delta.H = 98.8 kJ, and Delta.S = 141.5 J/K. Is this reaction spontaneous or nonspontaneous at high and low temperatures?
spontaneous at high temperatures, nonspontaneous at low temperatures
nonspontaneous at high and low temperatures
spontaneous at low temperatures, nonspontaneous at high temperatures
spontaneous at high and low temperatures

Answer 1

ΔG = ΔH − TΔS. Here ΔH = +98.8 kJ (+98,800 J) and ΔS = +141.5 J/K, so ΔG becomes negative only when T > ΔH/ΔS.

T = 98,800 J / 141.5 J·K−1 ≈ 699 K.

Therefore the reaction is nonspontaneous at low temperatures and spontaneous at high temperatures (spontaneous above ≈699 K).