Enthalpy of reaction, the enthalpy change for a chemical reaction is expressed by the equation:
ΔH = Hproducts − Hreactants
Here is an example of a thermochemical equation:
2 H2(g) + O2(g) → 2 H2O(g) ΔH = − 483.6 kJ
This equation indicates two moles of hydrogen gas burn to form two moles of water at a constant pressure, releasing 483.6 kJ of heat.
Enthalpy change during a chemical reaction can also be represented in an enthalpy diagram, showing the reactants at the top and the products at the bottom.
Guidelines for using thermochemical equations and enthalpy diagrams:
- The magnitude of ΔH is directly proportional to the amount of reactant consumed in the process.
- The enthalpy change for a reaction is equal in magnitude, but opposite in sign, to ΔH for the reverse reaction.
- The enthalpy change for a reaction depends on the state of the reactants and products.