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Types of Enthalpy of Reaction

Standard definitions for Formation, Combustion, Neutralization, and more.

By chemca Team • Updated Jan 2026

Enthalpy change ($\Delta H$) accompanies almost every chemical and physical process. To standardize calculations, chemists define specific types of enthalpy changes based on the nature of the reaction. All standard values are measured at $298K$ ($25^\circ C$) and $1 \, bar$ pressure.

1. Standard Enthalpy of Formation ($\Delta_f H^\circ$)

Definition: The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states.

Key Rules:

• The $\Delta_f H^\circ$ of an element in its standard state (e.g., $O_{2(g)}$, $C_{(graphite)}$, $Fe_{(s)}$) is taken as Zero.
• Example: $C_{(graphite)} + O_{2(g)} \rightarrow CO_{2(g)} \quad (\Delta_f H^\circ = -393.5 \, kJ)$

2. Standard Enthalpy of Combustion ($\Delta_c H^\circ$)

Definition: The enthalpy change when one mole of a substance undergoes complete combustion in the presence of excess oxygen.

Combustion reactions are always Exothermic ($\Delta H$ is negative).
Example: $CH_{4(g)} + 2O_{2(g)} \rightarrow CO_{2(g)} + 2H_2O_{(l)}$

3. Standard Enthalpy of Neutralization ($\Delta_{neut} H^\circ$)

Definition: The enthalpy change when one gram equivalent of an acid reacts with one gram equivalent of a base to form one mole of water in dilute solution.

For a Strong Acid and a Strong Base, this value is constant because the net reaction is always: $$ H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)} \quad [\Delta H = -57.1 \, kJ/mol] $$

If a weak acid or base is involved, the heat released is less than 57.1 kJ because some energy is consumed in ionizing the weak electrolyte.

4. Enthalpy of Solution ($\Delta_{sol} H^\circ$)

Definition: The enthalpy change when one mole of a substance is dissolved in a specified amount of solvent.

For ionic compounds, $\Delta_{sol} H = \Delta_{lattice}H + \Delta_{hyd}H$.

5. Enthalpy of Atomization ($\Delta_a H^\circ$)

Definition: The enthalpy change on breaking one mole of bonds completely to obtain atoms in the gas phase.

Example: $CH_{4(g)} \rightarrow C_{(g)} + 4H_{(g)}$

6. Phase Transition Enthalpies

• Fusion ($\Delta_{fus} H$): 1 mole Solid $\rightarrow$ Liquid.
• Vaporization ($\Delta_{vap} H$): 1 mole Liquid $\rightarrow$ Gas.
• Sublimation ($\Delta_{sub} H$): 1 mole Solid $\rightarrow$ Gas.

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