Which of the following is true?
a. a reaction in which the entropy of the system increases can be spontaneous only if it is endothermic.
b. a reaction in which the entropy of the system decreases can be spontaneous only if it is endothermic.
c. a reaction in which the entropy of the system decreases can be spontaneous only if it is exothermic.
d. a reaction in which the entropy of the system increases can be spontaneous only if it is exothermic.

a. A reaction in which the entropy of the system increases can be spontaneous only if it is endothermic.

Explanation:

The change in free energy (ΔG) that is, the energy available to do work, of a system for a constant-temperature process is:

spontaneous in the opposite direction.

If both ΔH and ΔS are positive, then ΔG will be negative only when the TΔS  term is greater in magnitude than ΔH. This condition is met when T is large.

hcl + naoh → h2o + nacl

explanation:

hcl + naoh → h2o + nacl   is the reaction that is correctly balanced.

however, pcl3 + h2 → ph3 +3hcl, 2ca+ 3cl2 → cacl3, and c5h12 + 5o2 → 5co2 + 6h2o are not correctly balanced.