Enthalpy of reaction, the enthalpy change for a chemical reaction is expressed by the equation:

ΔH = H_products − H_reactants  

Here is an example of a thermochemical equation:

  2 H_2(g) + O_2(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:

1. The magnitude of ΔH is directly proportional to the amount of reactant consumed in the process.
    
2. The enthalpy change for a reaction is equal in magnitude, but opposite in sign, to ΔH for the reverse reaction.
    
3. The enthalpy change for a reaction depends on the state of the reactants and products.