# Question #53869

Dec 28, 2017

This requires a fairly good understanding of isothermal processes. I am assuming the solid/liquid is already at its phase change temperature.

When a system is undergoing a phase change, it's an isothermal process,

$\Delta S = \frac{{q}_{\text{rev}}}{T}$, and

it's said that its entropy is generally constant until it's complete.

In more practical terms (since the above assumes this is a reversible process),

$\Delta S = \frac{\Delta H}{T}$

Hence, for melting that mass of ice, the entropy change would be positive since liquid will be more "disorganized" than the solid.

$\frac{900 g \cdot \left(\text{mol")/(18g) * (5.99kJ)/("mol}\right)}{273 K} \cdot \frac{{10}^{3} J}{k J} \approx \frac{1.10 \cdot {10}^{3} J}{K} = \Delta {S}_{1}$

Likewise, the entropy change will be negative for the second change since the solid is more organized than the liquid phase.

$- \Delta {S}_{1} \approx \frac{- 1.10 \cdot {10}^{3} J}{K}$