A chemical reaction can be assigned an enthalpy change of reaction, written $\Delta H_{\mathrm{rxn}}$, by comparing the enthalpy of the products with that of the reactants:

$$\boxed{\Delta H_{\mathrm{rxn}}=H_{\mathrm{products}}-H_{\mathrm{reactants}}}.$$

At constant pressure, when pressure-volume work is the only mechanical work mode,

$$q_p=\Delta H_{\mathrm{rxn}}.$$

A reaction with

$$\Delta H_{\mathrm{rxn}}<0$$

is exothermic: the reacting system releases heat to the surroundings. A reaction with

$$\Delta H_{\mathrm{rxn}}>0$$

is endothermic: the system absorbs heat.

A thermochemical equation is a balanced chemical equation accompanied by its enthalpy change. For example,

$$\mathrm{2H_2(g)+O_2(g)\rightarrow2H_2O(l)}\qquad \Delta H=-571.6,\mathrm{kJ}.$$

The enthalpy value belongs to the reaction as written.

If every stoichiometric coefficient is divided by two,

$$\mathrm{H_2(g)+\tfrac12O_2(g)\rightarrow H_2O(l)},$$

then

$$\Delta H=-285.8,\mathrm{kJ}.$$

If the reaction is reversed,

$$\mathrm{2H_2O(l)\rightarrow2H_2(g)+O_2(g)},$$

then the sign reverses:

$$\Delta H=+571.6,\mathrm{kJ}.$$

Physical states also matter. Forming liquid water and forming water vapor do not have identical enthalpy changes because the liquid and gas are different thermodynamic states.

Reaction enthalpy therefore scales with the reaction amount represented by the balanced equation. The sign tells the direction of heat flow at constant pressure, while the magnitude tells how much energy is exchanged for that specified stoichiometric reaction.