The heat of reaction (ΔHrxn) is the enthalpy change for a chemical reaction at constant pressure and isothermal conditions.

Enthalpy values are measured as differences, not absolute values; only changes in enthalpy can be determined.

A reference scale for enthalpy is defined: the standard enthalpy of formation (ΔHf) of any element in its most stable form at 1 bar and 298.15 K is set to zero.

The standard enthalpy of formation (ΔHf) of a compound is the enthalpy change when 1 mole of the compound forms from its constituent elements in their most stable forms under standard conditions.

To calculate the enthalpy change for a reaction:

• Decompose reactants into their elements.

• Combine elements to form products.

• Apply Hess's Law: the total enthalpy change is the sum of enthalpy changes for each step, since enthalpy is a state function.

General equation for reaction enthalpy: ΔHrxn = ∑νpΔHf,products - ∑νrΔHf,reactants where ν is the stoichiometric coefficient.

The sign of ΔHrxn indicates heat flow:

• If ΔHrxn < 0, the reaction is exothermic (heat flows to surroundings).

• If ΔHrxn > 0, the reaction is endothermic (heat flows into the reaction from surroundings).

At constant pressure and for reversible processes, ΔHrxn = q_p (heat at constant pressure).

The temperature dependence of enthalpy is given by: ∂H∂T = C_p , where C_p is the heat capacity at constant pressure.

For reactions, the change in heat capacity is: ΔCp = ∑νpCp,products - ∑νrCp,reactants

The enthalpy change at a new temperature can be calculated by: ΔHT_2 = ΔHT_1 + ∫ΔCpdT