Enthalpy change in terms of making and breaking bonds

A chemical reaction is any change that involves the breaking of old bonds and the formation of new bonds. Breaking bonds takes in energy. Making bonds releases energy. For a chemical reaction to occur:

  • chemical bonds in the reactants must first be broken by the application of energy.
  • new bonds then form as the products are produced and energy is released.

Here are the bond energies for some of the common covalent bonds.

BondEnergy (kJ / mole)

For example, this means that:

  • breaking an H-H bond requires 436 kJ per mole
  • formation of an H-H bond releases 436 kJ per mole

Enthalpy change in an exothermic reaction between hydrogen and chlorine

Let us look at the reaction between hydrogen and chlorine in sunshine, to form hydrogen chloride:

H2(g) + Cl2(g) → 2HCl(g)

Here is how the reaction proceeds:

  • first, the H2 and Cl2 bonds are broken.
    • according to the values in the table above, breaking the bonds in 1 mole of H2 bonds requires 436 kJ of energy and breaking the bonds in 1 mole of Cl2 requires 242 kJ of energy. So the total energy needed to break the bonds in the reactants is 678 kJ. In this reaction, the energy can be absorbed from sunlight.
  • 2 moles of HCl are then formed.
    • according to the values in the table, the formation of 1 mole of HCl releases 431 kJ of energy. Since 2 moles are formed, 862 kJ of energy are released as heat.

Enthalpy change = Energy input – energy output

ΔH = 678 kJ – 862 kJ = -184 kJ

Therefore, the reaction is exothermic because it gives out a net of 184 kJ of energy.

When calculating enthalpy changes in a chemical reaction it is easier more presentable to tabulate the process.

Energy required to break the bonds.

Energy absorbed
H—H436 kJ
Cl—Cl242 kJ
Total energy input678 kJ

Energy released when bonds are formed.

Energy released
H—Cl431 kJ
H—Cl431 kJ
Total energy output862 kJ

Enthalpy change is then calculated as usual.

ΔH = 678 kJ – 862 kJ = -184 kJ

Enthalpy change in the endothermic decomposition of ammonia

2NH3(g) → 2H2(g) + N2(g)

From the structural formula of ammonia, you can see that each ammonia molecule is made up of three N-H bonds. So 2 molecules of ammonia requires enough energy to break six N-H bonds.

Energy absorbed
6 × N—H = 6 × 391 kJ2346 kJ
Total energy input2346 kJ

After the N-H bonds are broken, one N≡N and three H-H bonds are formed.

Energy released
N≡N946 kJ
3 × H—H = 3 × 436 kJ1308 kJ
Total energy output2254 kJ

ΔH 2346 kJ – 2254 kJ = +92 kJ

Therefore, the reaction takes in 92 kJ of energy. Which means the reaction is endothermic, since the energy is taken in.

Activation energy of a chemical reaction

As we have seen in the previous section, every reaction (exothermic or endothermic) requires a certain amount of energy to break the bonds in the reactants before the reaction can proceed. That amount of energy is known as the activation energy.


Activation energy is the minimum amount of energy required for reactants to form products.


If reactant particles have energy that is equal to or greater than the activation energy they will be able to react.