Question

We can also use the equation for enthalpy change for physical phase changes. Consider the phase change H2O(l) → H2O(g). Calculate ΔHrxn.

Use ΔHf values from the table above.

ΔHrxn = _____

Answer 1

Standard molar enthalpy:
H2O ( liquid water ) : - 285.8 KJ/mole
H2O ( water vapor ) : - 241.8 KJ / mole.
ΔHrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0
Answer: B ) 44.0 KJ  

Answer 2

Answer: The value of enthalpy of the given reaction is 44.000 kJ/mol , that is energy is supplied to water to change into water vapors

Explanation:

$H_2O(l)\rightarrow H_2O(g),\Delta H_{rxn}=?$

Enthalpy of formation of water in liquid state,$\Delta H_{f,H_2O(l)}=-285.820 kJ/mol$

Enthalpy of formation of water in gaseous state,$\Delta H_{f,H_2O(g)}=-241.820 kJ/mol$

$\Delta H_{rxn}=\sum \Delta H_f \text{of products}-\sum \Delta H_f \text{of reactants}$

$\Delta H_{rxn}=(-241.820 kJ/mol)-(-285.820 kJ/mol)=44.000 kJ/mol$

The value of enthalpy of the given reaction is 44.000 kJ/mol, that is energy is supplied to water to change into water vapors.