Answer:
1) qrev/ reversible/ has only one particular value/ regardless of;
2) 1 mol of H₂(g) at 0ºC
3) a phase change/ remains constant/ increase
Explanation:
1) The change can occur in a reversible or an irreversible process. When the change is reversible, the system and the surroundings can be restored to their original state by exactly reversing the change. In an irreversible process, that is not possible.
The entropy variation (ΔS) is calculated only in the reversible process because, in the irreversible one, it's difficult to determine it. So, ΔS depends on qrev, which is the heat for the reversible process.
There is only one reversible isothermal path between two states, that's why it's easy to calculate ΔS for it.
The value of ΔS doesn't depend on the path, but only on the initial and the final states, so ΔS has only one particular value regardless of the path taken between states.
2) The entropy is the measure of the disorganization of a system, so when the molecules are more distant and vibrating, the entropy is higher (Sgas > Sliquid > Ssolid), and when the temperature increases, the entropy decreases (ΔS = qrev/T), so 1 mol of H₂(g) at 0ºC has the greatest entropy.
3) During a phase change, for a pure substance, the temperature remains constant, until all the substance changes for the other phase. The entropy is the measure of the disorganization of a system, so when the degrees of freedom and motion of the molecules increase, the system becomes more disorganized, so the entropy increases.
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