If you're searching for how to calculate the enthalpy of a reaction, this calculator is for you! This tool has two functionalities:

  • Calculating the enthalpy change from a reaction scheme; and
  • Calculating delta H with the enthalpy change formula.

Read on if you still don't know what is and how to calculate the delta H of a reaction.

Different forms of energy

The molecules of a system possess four types of energy:

  • Sensible energy: it is associated with the kinetic energy of the molecules. For gases, for example, molecular kinetic energy increases with temperature and pressure.
  • Latent energy: this type of energy is associated with the phase of a system. For example, when we heat a liquid and cause it to change to gas, part of the energy doesn't go to the increase in the temperature but to achieve the phase change by overcoming the molecular forces that make the liquid phase possible.
  • Chemical energy: Different molecules will have different amounts of energy associated with the atomic bonds that compose their specific molecular structure. The formation of CO2 from carbon (C) and dioxygen (O2) combustion is an example of a change in chemical energy. The CO2 chemical energy is lower than the energy of the original constituents, as part of it was released in the combustion process.
  • Nuclear energy: it's associated with the atomic structure of the elements that compose the molecules.

What is enthalpy?

By definition, the enthalpy of a system (H) is the sum of its internal energy (U) and the product of its volume (V) and pressure (P):

H = U + pV

The enthalpy change of a reaction refers to the difference between the enthalpy of the products and the enthalpy of the reactants. A negative change indicates the reaction is exothermic, while a positive value means it is endothermic.

This tool calculates the change of the reaction in two ways: 1. Using the enthalpy equation, or 2. Using the standard enthalpies of formation of the components from a reaction scheme.

How do I calculate delta H from the enthalpy change formula?

From the enthalpy formula, and assuming a constant pressure, we can state the enthalpy change formula:

ΔH = ΔU + pΔV = (U2 - U1) + p(V2 - V1)


  • ΔH — Enthalpy change;
  • ΔU — Internal energy change;
  • U1 — Internal energy of the reactant;
  • U2 — Internal energy of the product;
  • V1 — Volume of the reactant;
  • V2 — Volume of the product;

Now, let's see how to calculate delta H from a reaction scheme.

How do I calculate enthalpy change from a reaction scheme?

This calculator uses the enthalpy of formation of the compounds to calculate the enthalpy change from a reaction scheme. We can define the enthalpy of formation as the enthalpy of a substance at a specified state due to its chemical composition. In other words, it represents the energy required to take that substance to a specified state.

In processes involving chemical energy changes, all substances must have the same reference state to be able to use the enthalpy of formation consistently. This reference state corresponds to 25°C (77°F) and 10⁵ Pa = 1 bar. Having defined a universal reference state, we can discuss a new term called standard enthalpy of formation.

The standard enthalpy of formation is simply the enthalpy of formation with standard conditions as the specified state. By definition, it is the change in enthalpy, ΔH, during the formation of one mole of the substance in its standard state (1 bar and 25°C), from its pure elements, f.

The standard enthalpy of formation of all stable elements (i.e., O2, N2, C, and H2) is assumed as zero because we need no energy to take them to that stable state under our atmospheric conditions.

For a reaction, the enthalpy change formula is:

ΔH°reaction = ∑ΔH°f(products) - ∑ΔH°f(reactants)


  • ΔH°reaction — Standard enthalpy change of formation expressed in kJ;
  • ∑ΔH°f(products) — Sum of the standard enthalpies of formation of the products multiplied by their corresponding numbers of moles, expressed in kJ; and
  • ∑ΔH°f(reactants) — Sum of the standard enthalpies of formation of the reactants multiplied by their corresponding numbers of moles, expressed in kJ.

Apart from the enthalpy equation, you need to know the standard enthalpies of formation of the compounds. The following table contains some of the most important ones, but you can look at the rest in the enthalpy calculator:


Δh°f (kJ/mol)













As an example, let's suppose we want to know the enthalpy change of the following reaction:

2 SO₃(g) → 2 SO₂(g) + O₂(g)

Considering the number of moles of the compounds and the enthalpies of the table, we can use the enthalpy change formula:

ΔH°reaction = ∑ΔH°f(products) - ∑ΔH°f(reactants)
= (2 mol)(−395.72 kJ/mol) - [(2 mol)(−296.83 kJ/mol) + (1 mol)(0)]
= -197.87 kJ

Check the result with the calculator, and you'll figure out it's the same.

Now that you know how to calculate the enthalpy change with the formula, you can use the calculator more confidently!

Luis Hoyos
Calculate enthalpy change from
reaction scheme
anA + bnB + cnC → dnD + enE + fnF

By default, you can only calculate for two reactants/products. Click the advanced mode button to include more compounds in the reaction.
aₙ coefficient
Reactant A
bₙ coefficient
Reactant B
dₙ coefficient
Product D
eₙ coefficient
Product E
Change in enthalpy
Your reaction:
2 SO₃(g) → 2 SO₂(g) + O₂(g)
Standard enthalpies of formation:
SO₃(g): Hf = -395.72 kJ
SO₂(g): Hf = -296.83 kJ
O₂(g): Hf = 0 kJ
Please note that we don't check if your reaction scheme makes chemical sense.
People also viewed…

Gay-Lussacs's law

Learn how to find the missing variable in an isochoric process with our Gay-Lussac's law calculator.

Schwarzschild radius

Discover the fundamental of black hole physics with our Schwarzschild radius calculator.

Thermal conductivity

The thermal conductivity calculator finds the one-dimensional heat flux using temperature difference and distance between material's ends.
main background