For each phase change, determine the sign of Delta H and Delta S

You can think of ΔH as being heat energy added ( + ), or heat removed ( - ) .

Entropy refers to the degree of disorder or randomness.   Gases are more disordered (more entropy) than liquids, and liquids are more disordered (more entropy) than solids. Solids have the least entropy.

1. Sublimation is a change from solid directly to a gas.   Heat energy has to be added for this process to occur, so ΔH is +.     Gases are more disordered, so ΔS is + also.

2. Freezing is a change from liquid to a solid. Heat energy has to be removed, so ΔH is - .   Solids have less entropy so ΔS is - .

3. Condensation is a change from a gas to a liquid.     Heat energy has to be removed, so ΔH is - .

     Liquid entropy is lower, so ΔS is - .  

4. Deposition is a change from a gas directly to a solid. Heat energy must be removed, so ΔH is -   .

     ΔS is -   also.

5. Boiling is a change from liquid to a gas.   Heat energy must be added, so ΔH is +.   ΔS is + since gases have higher entropy.

6. Melting is a change from a solid to a liquid.   Heat energy must be added , so ΔH is + .   ΔS is + since liquids have more disorder than solids.