Calculating Enthalpy Change

Question

Using the standard formation enthalpies that follow, calculate the standard enthalpy change for this reaction.

2Na(s) + 2H₂O(l)  2NaOH(aq) + H₂(g)     _rH° =  kJ

Species	fH° (kJ/mol)
H2O(l)	-285.8
NaOH(aq)	-470.1

Answer 1

The standard enthalpy change is obtained from the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants.

2Na(s) + 2H₂O(l)  2NaOH(aq) + H₂(g)   In this reaction NaOH is formed by the reaction of Na with H2O.

The standard enthalpies of formation for elements in their standard states are assigned a value of zero.

ΔH_r = 2 x ΔH_{f NaOH} - 2 x ΔH_{f H2O} = 2 x -470.1kj/mpl - 2 x -285.8 kj/mol = -368.6 kj for the reaction as written. If you want the result in kj/mol, you divide the result by 2. (I left out the superscript o for the ΔH.)