Enthalpy of Reaction Calculator: Hess's Law from Enthalpies of Formation or Bond Energies

Work out the enthalpy change of a reaction two ways.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose enthalpies of formation or bond energies.

Enter the coefficients and the enthalpies of formation, or the number and energy of each type of bond broken and formed.

The calculator gives the sums and the enthalpy change, and says if the reaction is exothermic or endothermic.

Formulas used

  • \Delta H = \sum\Delta H_f(\text{products}) - \sum\Delta H_f(\text{reactants})
  • \Delta H = \sum E_{broken} - \sum E_{formed}

Worked example

With the default numbers in the calculator above:

  1. Sum for the products: coefficient times enthalpy of formation: -965.1 kJ [1 x (-393.5) + 2 x (-285.8)]
  2. Sum for the reactants: -74.8 kJ [1 x (-74.8) + 2 x (0)]
  3. Enthalpy change of the reaction (Hess's law: products minus reactants): -890.3 kJ per mole of reaction
  4. Heat for the number of moles of reaction you gave (negative means heat given out): -890.3 kJ

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Reactant 1: coefficient in the equation 1; Reactant 1: enthalpy of formation (methane is -74.8) 0 kJ/mol; Reactant 2: coefficient 1; Reactant 2: enthalpy of formation (an element is 0) 0 kJ/mol; Product 1: coefficient 2; Product 1: enthalpy of formation (CO2 is -393.5) 90.3 kJ/mol; Product 2: coefficient 0; Product 2: enthalpy of formation (liquid water is -285.8) 0 kJ/mol.

  1. Sum for the products: coefficient times enthalpy of formation: 180.6 kJ [2 x (90.3)]
  2. Sum for the reactants: 0 kJ [1 x (0) + 1 x (0)]
  3. Enthalpy change of the reaction (Hess's law: products minus reactants): 180.6 kJ per mole of reaction
  4. Heat for the number of moles of reaction you gave (negative means heat given out): 180.6 kJ

Common questions

What is Hess's law?

The enthalpy change of a reaction is the same whichever route it takes, so you can add the enthalpy changes of steps.

Why is the enthalpy of formation of an element zero?

It is defined as zero for an element in its standard state, as the starting point of the scale.

Why are bond energy answers only estimates?

Bond energies are averages over many compounds, so they are not exact for a particular molecule.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any first-year chemistry text (thermochemistry). Check enthalpy and bond energy values against your own data book.

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