# Thermochemical equations

A thermochemical equation is an equation that includes enthalpy change. There are four different ways of writing them:

energy term

Include $\displaystyle \Delta{} H$ as a term in the equation on the left side for endothermic reactions and on the right side for exothermic reactions.

C6H12O6(g) + 6 O2(g) → 6 CO2(g) + 6 H2O(g) + 2820 kJ

enthalpy change value

Add the value of $\displaystyle \Delta{} H$ to the end of the equation (negative if exothermic, positive if endothermic).

C6H12O6(g) + 6 O2(g) → 6 CO2(g) + 6 H2O(g), $\displaystyle \Delta{} H = - 2820 \, \text{kJ}$

molar enthalpy of reaction

Write only the value of $\displaystyle \Delta{} H_{x}$ (no equation), including the chemical formula of the substance with the unit.

$\displaystyle \Delta{} H_{\text{comb}} = - 2820 \, \text{kJ/mol}$ C6H12O6

potential energy diagram

A potential energy diagram is a graphical representation of energy transfer, with $\displaystyle E_{p}$ on the vertical axis and “Reaction Progress” on the horizontal axis, reactants on one line and products on another, and an arrow joining the two labelled with the enthalpy change. See page 318 of the textbook for examples.