Problem: Calculate the standard entropy change for the reaction 2H2(g)+O2(g)→2H2O(l) using the data from the following table:SubstanceΔH°f (kJ/mol)ΔG°f (kJ/mol)S° [J/(K•mol)]H2O(g)0.000.00130.6O2(g)0.000.00205.0H2O(l)-285.8-237.269.90

We can use the following equation to solve for ΔS˚_rxn:

$\boxed{{\mathbf{∆}\mathbf{S}\mathbf{°}}_{\mathbf{rxn}\mathbf{ }}\mathbf{=}\mathbf{ }{\mathbf{S}\mathbf{°}}_{\mathbf{prod}}\mathbf{-}{\mathbf{S}\mathbf{°}}_{\mathbf{react}\mathbf{ }}}$

Note that we need to multiply each S˚ by the stoichiometric coefficient since S˚ is in J/mol • K. 

The given reaction is 2H₂(g)+O₂(g)→2H₂O(l)

Given: 

S° H₂O(g) = 130.6 J/(K•mol)

S° O₂(g) = 205.0 J/(K•mol)

S° H₂O(l) = 69.90 J/(K•mol)