Enthalpy of Reaction

A thermochemical equation reports delta H for the reaction as written, tied to its coefficients. Burning one mole of methane, CH₄(g) + 2O₂(g) → CO₂(g) + 2H_2O(l), releases 890 kJ. That number is fixed to one mole of CH₄ reacting with two moles of O₂. Burn two moles and you get twice the heat, because you are running the same reaction twice over. The energy change per mole never changes; only how many moles you run changes the total heat.

This is why delta H scales linearly with the coefficients. If you double every coefficient in the equation, delta H doubles too, because you have doubled the amount of reaction occurring. Multiply the whole equation by three, and delta H also multiplies by three, keeping the ratio of heat to moles constant. This scaling is what lets you convert between grams, moles, and kJ: convert mass to moles using molar mass, then multiply by delta H per mole to get total heat.

This idea sets up Hess's law, where you add thermochemical equations together and their delta H values add the same way, because each one still scales properly with its own moles. It also connects to calorimetry, where measured heat lets you work backward to find delta H per mole of a reaction actually run in a lab.