Reaction Enthalpy and the Conservation of Energy

Reaction Enthalpy ΔrH

Enthalpy H is a quantity that describes the energy content of a system at constant pressure. What is measurable is its variation over the course of a reaction, the reaction enthalpy:

ΔrH = H(products) - H(reactants)

• If ΔrH < 0 (negative), the system loses enthalpy: the reaction is exothermic, it releases energy to the surroundings.
• If ΔrH > 0 (positive), the system gains enthalpy: the reaction is endothermic, it draws energy from the surroundings.

Conservation of Energy

The first law of thermodynamics requires that energy is neither created nor destroyed: it is only transformed, or transferred from one system to another. When a chemical reaction releases energy, that energy does not disappear: it is fully transferred to the surroundings, most often as heat (it warms the air, the water in a calorimeter...), sometimes in another form (light in a flame, electrical energy in a battery).

Conversely, the energy absorbed by an endothermic reaction does not come from nowhere: it is drawn from the surroundings, which cool down accordingly.

Why This Matters

This conservation allows precise accounting: the total energy released by burning a fuel can be calculated, measured by calorimetry, and compared from one fuel to another. This accounting is what allows an efficient fuel to be chosen, or the heating of a building to be sized.

Common Mistake

Do not think that an endothermic reaction is "impossible" or "abnormal" because it absorbs energy rather than releasing it: it happens as soon as the surroundings can supply it with the necessary energy, which is exactly the principle behind a heat pack, or an instant cold pack.