Final answer:
The correct answer is d. ΔG° less than ΔH° because of entropy effects/temperature change.
Step-by-step explanation:
The given reaction is:
C(s, graphite) + 2 H2(g) → CH4(g)
The options provided are:
a. ΔH°rxn = −74.6 kJ = (exothermic)
b. ΔS°rxn = −80.8 J/K = (unfavorable)
c. ΔG°rxn = −50.5 kJ = (spontaneous)
d. ΔG° less than ΔH° because of entropy effects/temperature change
The correct answer is d. ΔG° less than ΔH° because of entropy effects/temperature change.
In this reaction, the change in Gibbs free energy, ΔG°rxn, is negative, indicating that the reaction is spontaneous. However, the change in enthalpy, ΔH°rxn, is also negative, indicating that the reaction is exothermic. The reason ΔG° is less than ΔH° is because of the entropy effects. The increase in the number of gas molecules (2 moles of H2 to 1 mole of CH4) contributes to an increase in entropy, which lowers the overall change in free energy.