Enthalpy of Formation: Calculate from Reaction Enthalpy

Question

A scientist measures the standard enthalpy change for the following reaction to be -374.0 kJ :

P₄O₁₀(s) + 6 H₂O(l)4H₃PO₄(aq)

Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of H₂O(l) is ___ kJ/mol.

Answer 1

For this you need to look up standard enthalpy of formation for H₃PO₄(aq) and P₄O₁₀(s). I'm sure you have a table that gives these values. Once you have these values, you find ∆Hºformation H2O as follows:

4 x ∆Hº_H3PO4 - (∆Hº_P4O10 + 6 x ∆Hº_H2O) = -374.0 kJ

Solve for ∆Hº H2O