What happens to the entropy during the melting of ice to water?

Which functions are required to understand the second law of thermodynamics?

Given the standard entropy values at $298 \ K$ and $1 \ atm$ as $H_2(g) : 130.6 \ J \ K^{-1} \ mol^{-1}$,$Cl_2(g) : 223.0 \ J \ K^{-1} \ mol^{-1}$,and $HCl(g) : 186.7 \ J \ K^{-1} \ mol^{-1}$,calculate the entropy change $(\Delta S^{\circ})$ in $J \ K^{-1} \ mol^{-1}$ for the reaction: $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$

Identify the reaction/process in which the entropy decreases.

At $25\,^oC$ and $1\,atm$ pressure,mercury reacts with chlorine to form mercurous chloride: $2Hg + Cl_2 \to Hg_2Cl_2; \Delta H = -31.3\,kcal$. What is the entropy change of the reaction at $25\,^oC$?

Provide the mathematical equation for entropy change.