Question

Consider the formation reactions below and pick the most stable species from the answer choices.

2C_graphite(s) + 3H₂(g) → C₂H₆(g) 

∆G◦ _f = −7.86 kcal · mol⁻¹

3Cgraphite(s) + 4H₂(g) → C₃H₈(g)

∆G◦ f = −5.614 kcal · mol⁻¹

8Cgraphite(s) + 9H₂(g) → C₈H₁₈(g)

∆G◦ _f = 4.14 kcal · mol⁻¹

10Cgraphite(s) + 11H₂(g) → C₁₀H₂₂(g)

∆G◦ _f = 8.23 kcal · mol⁻¹

1. The change in entropy is large. 

2. The change in entropy is small. 

3. The reaction is endothermic. 

4. The reaction is exothermic

Answer 1

Correct option 4. The reaction is exothermic

Explanation:

Since the reaction becomes more and more spontaneous as the temperature is lowered, it must be spontaneous at T = 0 K. Since ∆G = ∆H at T = 0 K, ∆H must be negative and the reaction is exothermic.