Enthalpy calculator for reactions

The enthalpy change of a reaction, ΔH, is the heat it gives out or takes in at constant pressure, in kJ/mol. A negative ΔH means energy is released (exothermic); a positive ΔH means it is absorbed (endothermic). With enthalpies of formation, ΔH = Σ products − Σ reactants: burning methane gives −890.3 kJ/mol. This enthalpy calculator works out ΔH four ways: from enthalpies of formation, from enthalpies of combustion, by combining equations with Hess's law, or from bond enthalpies.

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Enthalpy Calculator

Use Hess's law and bond enthalpies to find energy changes

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What the enthalpy calculator does

Pick a Method:

• Formation: type a reaction and it balances it, fills in each substance's standard enthalpy of formation from its data table, and applies Hess's law. It draws the Hess cycle through the elements.
• Combustion: the same, with enthalpies of combustion, and the cycle goes through the combustion products.
• Equations: type a target equation and the equations you've been given with their ΔH. It works out which to reverse and what to multiply each by, then adds them.
• Bonds: list the bonds broken and formed, with how many of each. Mean bond enthalpies fill in from a table of 37 bonds, and an energy level diagram shows the energy in and out.

Every answer shows ΔH, whether the reaction is exothermic or endothermic, and the working step by step.

How to use the enthalpy calculator

1. Open the Enthalpy Calculator and choose Formation, Combustion, Equations or Bonds.
2. For Formation or Combustion, type the reaction in Reaction, such as C4H10(g) + O2(g) -> CO2(g) + H2O(l), or pick a preset. Add state symbols: they decide between H₂O(l) and H₂O(g).
3. Check the values it fills in. You can type over any of them to match your data book, and type in any it leaves blank.
4. For Equations, type the Target equation, then each given equation with its ΔH, using Add an equation for more. Every equation must be balanced.
5. For Bonds, pick each bond under Bonds broken and Bonds formed and set its count, using Add a bond for more rows.
6. Read ΔH and the working. For practice questions with worked answers, press Random problem.

The formulas behind it

n is each substance's number in the balanced equation. All three come from Hess's law: the enthalpy change of a reaction is the same whatever route it takes, so you can go via the elements, via the combustion products or via separate atoms and get the same ΔH. The enthalpy of formation of an element in its standard state, such as O₂(g) or C(s), is 0.

Worked examples you can check in the calculator

Example 1: burning butane, and why the equation matters

Choose Formation and type C4H10(g) + O2(g) -> CO2(g) + H2O(l). The calculator balances it with whole numbers, 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O, and uses ΔfH = −125.6 for butane, −393.5 for CO₂ and −285.8 for H₂O(l):

That is for 2 moles of butane, as the equation is written. For the enthalpy of combustion per mole, type the equation with one butane: C4H10(g) + 6.5O2(g) -> 4CO2(g) + 5H2O(l). The calculator keeps your coefficients when they balance, and gives −2877.4 kJ/mol, half of the first answer.

Example 2: liquid or gas water

Change H2O(l) to H2O(g) in the same equation. Water vapour has ΔfH = −241.8 kJ/mol instead of −285.8, so ΔH becomes (4 × −393.5 + 5 × −241.8) − (−125.6) = −2657.4 kJ/mol. Less heat comes out because some of it is still stored in the vapour, which hasn't condensed.

Example 3: an endothermic reaction

Heating ammonium chloride splits it into ammonia and hydrogen chloride. Type NH4Cl(s) -> NH3(g) + HCl(g). The calculator gives (−46.1 + −92.3) − (−314.4) = +176.0 kJ/mol, and labels it endothermic: energy is absorbed.

Example 4: from enthalpies of combustion

You can't measure the enthalpy of formation of butane directly, because carbon and hydrogen don't react cleanly to make it. Combustion data gets round this. Choose Combustion and type C(s) + H2(g) -> C4H10(g), which balances to 4C + 5H₂ → C₄H₁₀:

The data book value is −125.6 kJ/mol. The small gap comes from rounding in the published values.

Example 5: Hess's law with given equations

Find ΔH for 2C(s) + 2H₂(g) → C₂H₄(g), the formation of ethene, from these three equations. Choose Equations, type the target and the three equations with their ΔH.

Adding them: ΔH = −787 − 571.6 + 1410.9 = +52.3 kJ/mol. Ethene's formation is endothermic. Reversing an equation flips the sign of its ΔH, and multiplying it multiplies ΔH too. If the equations can't make the target, the calculator tells you that a species they need is missing.

Example 6: bond enthalpies

Estimate ΔH for 2H₂(g) + O₂(g) → 2H₂O(g). Choose Bonds. Broken: 2 H-H (436 each) and 1 O=O (498). Formed: 4 O-H (463 each), two in each water molecule.

From enthalpies of formation, the same reaction gives 2 × −241.8 = −483.6 kJ. Bond enthalpies are averages over many molecules, so answers from them are close but not exact. The energy level diagram shows 1,370 kJ going in to make separate atoms and 1,852 kJ coming out as the new bonds form.

What the answer means

ΔH is the energy of the products minus the energy of the reactants. If the products sit lower, energy has been given out to the surroundings, which warm up, and ΔH is negative. If they sit higher, energy has been taken in, the surroundings cool, and ΔH is positive. On an energy profile, ΔH is the gap between the reactant and product levels, and the activation energy is the climb from the reactants to the top of the hump. Read both from a profile, then press Mark to check your answer in the Reaction Kinetics course.

What the calculator doesn't do

• Calorimetry. It doesn't turn a temperature change into ΔH. Do it by hand: q = mcΔT, then ΔH = −q ÷ n. If 0.025 mol reacts and warms 100 g of water by 7.5 °C, q = 100 × 4.18 × 7.5 = 3,135 J, so ΔH = −3,135 ÷ 0.025 = −125,400 J/mol, or −125.4 kJ/mol. The minus sign is there because the water gained the heat the reaction lost. The specific heat calculator guide covers q = mcΔT.
• H = U + PV. It works with reaction enthalpies, not the enthalpy of a gas from its internal energy, pressure and volume.
• Born-Haber cycles and lattice enthalpies. Add the steps by hand: the Born-Haber cycle guide shows how.
• Substances missing from its table. The box is left empty for you to type the value from your data book.

Common mistakes

• Forgetting to multiply by the coefficients. 10H₂O counts 10 × −285.8, not once.
• Mixing up the orders. Formation is products minus reactants; combustion and bond enthalpies are reactants minus products (broken minus formed).
• Leaving out elements. They count, but as 0 for formation. For combustion they don't: C(s) and H₂(g) have combustion enthalpies.
• The wrong state of water. H₂O(l) and H₂O(g) differ by 44 kJ/mol each.
• Not flipping the sign when you reverse an equation. The reverse of an exothermic reaction is endothermic by the same amount.
• Giving the answer for the wrong amount. ΔH is for the equation as written. Divide by the coefficient to get per mole of one substance.

Read next

Chemistry · EnergeticsEnthalpy change formula: ΔH = −mcΔT ÷ n, with examplesThe enthalpy change formula ΔH = −mcΔT ÷ n from calorimetry: q = mcΔT, the sign of ΔH, neutralisation, combustion and dissolving examples, and Hess's law.