Enthalpy of Reaction Calculator
ΔH°rxn from standard enthalpies of formation.
Work out Enthalpy of Reaction. ΔH°rxn from standard enthalpies of formation. Written for the problem set you are stuck on.
Write each as value:coefficient. Elements in their standard state are 0.
O₂, N₂, graphite and the other elemental standard states are all exactly 0.
Standard enthalpy of reaction
-890.5 kJ/mol
Exothermic — releases heat
ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants). Products first, and the subtraction that way round — reversing it gives the right magnitude with the wrong sign, which turns an exothermic reaction endothermic on paper. An element in its standard state has ΔH°f of exactly zero. That is not an approximation or a rounding: it is the definition that the entire table is measured against, so O₂ gas, N₂ gas and graphite all contribute nothing. Watch the physical states. Liquid water's ΔH°f is −285.8 kJ/mol and gaseous water's is −241.8, a 44 kJ/mol difference that is exactly the enthalpy of vaporisation.
How the Enthalpy of Reaction Calculator works
Enter each product and reactant as its enthalpy of formation and coefficient, and this returns the standard enthalpy of reaction. Elements in their standard state contribute exactly zero, which is a definition rather than an approximation.
Also known as: delta h calculator · heat of reaction calculator · standard enthalpy of formation calculator · is a reaction exothermic or endothermic
Frequently asked questions
How do I calculate enthalpy of reaction?
ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants). Products first, and that way round — reversing the subtraction turns an exothermic reaction endothermic on paper.
Why is the enthalpy of formation of oxygen zero?
Because ΔH°f is defined as the enthalpy of forming a substance from its elements in their standard states, and forming oxygen from oxygen requires nothing. Every element in its standard state is exactly zero by construction.
Does physical state matter?
Very much. Liquid water is −285.8 kJ/mol and gaseous water is −241.8. The 44 kJ/mol gap is precisely the enthalpy of vaporisation, and mixing the two states is a common and substantial error.
What makes a reaction exothermic?
Products that are collectively more stable — lower in enthalpy — than the reactants, giving a negative ΔH. Combustion of methane releases 890.5 kJ/mol because CO₂ and water sit far below CH₄ and O₂.
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