Question

Predict the sign of entropy change for each of the following change of state
A.Hg (l) to Hg (s)
B.$AgN{O_3}$ (s) to $A{g^ + }$(aq) + $N{O_3}^ -$(aq)

Answer 1

First of all, we should know the definition of entropy, Entropy is a measure of the lack of thermal energy of any mechanical work system. It also represents the system's randomness. As randomness increases the system entropy. Entropy of the system is directly proportional to the randomness.

Complete answer:
For a given substance, the entropy of the liquid state is greater than the entropy of the solid state and likewise, the entropy of the gas is greater than the entropy of the liquid. Therefore, the entropy increases in processes in which solid or liquid reactants form gaseous products. Entropy also increases when solid reactants form liquid products.
Now, let us predict the sign of entropy change in the following:
A.Hg (l) to Hg (s)
Here, liquid changes to a less disordered solid state. Entropy decreases, i.e., changes into negative. Hence, ΔS = -ve.
B.$AgN{O_3}$ (s) to $A{g^ + }$(aq) + $N{O_3}^ -$(aq)
Here, aqueous solution has more disorder than solid. Entropy increases, i.e., changes into positive. Hence$\Delta S = + ve$.

Note:
Entropy increases when a substance is broken up into multiple parts. The process of dissolving increases entropy because the solute particles become separated from one another when a solution is formed. Entropy increases as temperature increases. An increase in temperature means that the particles of the substance have greater kinetic energy. The faster moving particles have more disorder than particles that are moving more slowly at a lower temperature. Entropy generally increases in reactions in which the total number of product molecules is greater than the total number of reactant molecules. An exception to this rule is when a gas is being produced from nongaseous reactants.