By Ari Reid Updated Aug 30, 2022

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In a chemical reaction, ΔH represents the enthalpy change and is calculated as the sum of the standard heats of formation (ΔH°f) of the products minus the sum of those of the reactants. The unit is kilojoules per mole (kJ/mol).

Enthalpy (H) is a thermodynamic property equal to the system’s internal energy plus the product of its pressure and volume (H = U + PV). The symbol Δ denotes a change, so ΔH is the difference in enthalpy between products and reactants under the same conditions.

To determine ΔH you need a balanced chemical equation, accurate ΔH°f values, and the assumption that the reaction occurs at constant pressure.

Step 1

Balance the equation so that the number of atoms of each element is identical on both sides. For example, the reaction between water and carbon to form carbon monoxide and hydrogen gas is written as:

H₂O + C → CO + H₂

Step 2

Look up the standard heats of formation for every species in the reaction. Reliable sources include most chemistry textbooks, peer‑reviewed databases, or the NIST Chemistry WebBook. For the example:

• ΔH°f (H₂O, liquid) = –285.83 kJ/mol
• ΔH°f (CO, gas) = –110.53 kJ/mol
• ΔH°f (H₂, gas) = 0 kJ/mol
• ΔH°f (C, solid) = 0 kJ/mol

When a species appears more than once in the balanced equation, multiply its ΔH°f by its stoichiometric coefficient.

Step 3

Calculate the total ΔH°f for the reactants:

(–285.83 kJ/mol) + (0 kJ/mol) = –285.83 kJ/mol

Step 4

Calculate the total ΔH°f for the products:

(–110.53 kJ/mol) + (0 kJ/mol) = –110.53 kJ/mol

Step 5

Determine ΔH by subtracting the reactant sum from the product sum:

ΔH = –110.53 kJ/mol – (–285.83 kJ/mol) = +175.3 kJ/mol

A positive ΔH indicates an endothermic process, meaning the reaction absorbs heat from its surroundings.