Are all exothermic reactions spontaneous? $\ce{GaAs (s)}$ is used in solar cells deployed on satellites found in the orbit of Earth. The best way to make these materials is through a technique called &quot;molecular beam epitaxy&quot;, in which the following spontaneous chemical reaction takes place:
$$\ce{Ga(g) + As(g) -&gt; GaAs(s)}$$
Pick the statement which best describes this process:
(A) The process is endothermic.
(B) The process is exothermic.
(C) The process leads to a decrease in the entropy of the universe.
(D) The process leads to an increase in the entropy of the system.
(E) The process leads to a decrease in the temperature of the system.

The key says the correct answer is B. I am wondering if this is because all exothermic reactions are spontaneous?
No, it is not the case that all exothermic reactions are spontaneous.  More generally, it doesn't even make sense to make a broad statement that a reaction is or is not spontaneous without also specifying the concentrations of reactants and products, since for a certain relative concentration of reactants and products, the reaction might be spontaneous in the forward direction, while for another set of relative concentrations it might be spontaneous in the reverse direction. [See illustration below.]

One could, however, make the general statement that chemicals will react spontaneously if, in so doing, the entropy of the universe increases.  This, however, is not one of the options you are given.

Why, then, is "B" the correct choice?  Answer: It's because this Ga and As are forming a chemical bond, and bond formation is exothermic. In addition, there is a phase change here from a gas to a solid, and that is also exothermic.  Furthermore, since Ga and As are both monatomic gases in this reaction, no chemical bonds need to be broken to allow the Ga and As to combine. Thus, since we have chemical bonds being formed but not broken, and a phase change from gas to solid, the reaction is exothermic.

Here's a graphical illustration of the general concept articulated in the first paragraph: