We’re asked to calculate the standard enthalpy of formation (ΔH°f) for naphthalene (C10H8) based on the standard enthalpy of combustion (ΔH°rxn) and the chemical equation.
Recall that ΔH°rxn can be calculated from the reactants and products involved:
We’re given the ΔH°rxn for the combustion of 1 mole of C10H8. The chemical equation for that reaction is:
C10H8(s) + 12 O2(g) → 10CO2(g) + 4 H2O(l)
First, we make sure the equation is balanced by checking that the number of elements on both sides is equal.
Balance C: we have 10 C on the reactant side and 10 C on the products side.
Balance H: we have 8 H on the reactant side and 4(2) = 8 H on the products side.
Balance O: we have 12(2) = 24 O on the reactant side and 10(2) + 4(1) = 24 O on the products side.
The standard enthalpy of combustion of naphthalene is -5157 kJ/mol. Calculate its standard enthalpy of formation.
The reaction for the combustion of naphthalene is:
C10H8 (s) + 12 O2 (g) → 10CO2 (g) + 4 H2O (l)
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