Q.v. State whether $\Delta S$ is positive, negative or zero for the reaction $\mathrm{2H(g) \rightarrow H_2(g)}$. Explain.

Entropy (S) is a state function that measures the degree of molecular disorder or randomness in a system -- the greater the disorder, the larger the entropy. This is illustrated by water's three states: highly-ordered solid ice, less-ordered liquid water, and highly-disordered gaseous steam, so entropy increases (ΔS > 0) on melting and on vaporization; similarly, dissolving solid iodine in water (I2(s) + aq to I2(aq)) or breaking H2 molecules apart into free atoms (H2(g) to 2H(g)) both increase disorder and so have ΔS > 0, while a process that brings particles together into a more ordered arrangement (such as 2H(g) combining into H2(g), or a gas condensing to a liquid) decreases disorder and has ΔS < 0. Quantitatively, the entropy change of a system equals the heat transferred to it in a REVERSIBLE manner (Qrev) divided by the absolute temperature T at which the transfer occur …