Calculate the change in internal energy when 1 mole of water vapor, treated as an ideal gas, at 100°C and 1 bar pressure is transformed into ice at 0°C. 
(The enthalpy of fusion of ice = 6.00 kJ mol^-1 , heat capacity of water = 4.2 J/g°C)

1. 13.56 kJ mol^-1

2. –12.16 kJ mol^-1

3. –13.56 kJ mol^-1

4. 12.16 kJ mol^-1