Question

1. Prove H₂O_(l) is more stable than ​​H₂O_(g) at 373K and 200kPa.

2. Prove H₂O_(s) is more stable than H₂O_(l) at 263K and 100kPa.

Answer 1

(1) At 373 K and 200kPa, H2O (l) is more stable because in the liquid state the molecules are more organized and water molecules form inter-molecular hydrogen bonding between H atom of one molecule and oxygen atom of another water molecule. For this reason, the standard enthalpy of vaporization of water is +44KJ/mol. This means, 44 kJ/mol of heat energy should be applied to liquid water to convert it to gaseous state.In gaseous state, the randomness of the molecules increases and as a result entropy increases. For this reason, H2O (g) is less stable and H2O(l) is more stable at 373 K and 200 kPa.

(2) At 263 K and 100kPa, H2O (s) is more stable than H2O(l) because in liquid state, the molecules are spaced further apart than in the solid state and this increases randomness of the system. For this reason, entropy of H2O(l) is more than H2O(s). Therefore, H2O(s) is more stable and ice spontaneosly forms at temperatures below 273 K.