In a chemical reaction, bonds are broken. Energy is required for this making this process endothermic. New bonds are then formed. Energy is released when this occurs making the process exothermic.

Enthalpy change takes both of these energies into consideration and can be calculated using the following equation:

?H = energy require to break reactant bonds – energy required to break product bonds

This can be used for reactions which do not include ionic bonds. When ionic bonds are broken and formed a more complicated energetic process sequence takes place which cannot be considered through this equation.

Molecules in gaseous state

The bonds broken and formed in gas phase reactions are covalent. The energy required to completely separate the atoms in one mole of covalent bonds is called bond dissociation enthalpy of the bond: ?H_b.

A – B(g) ? A(g) + B(g)

The exact strength of a bond is dependent on its environment. It is even possible for strengths of the same type of bond to vary in the same molecule. Intermolecular forces are also not taken into account. Therefore, an average should be taken. This means, however, than only an approximate value for enthalpy changes can be taken.

Solids and liquids

Before free gaseous atoms can be formed in giant covalent substances all covalent bonds must be broken. The energy needed to completely separate atoms in one mole of a substance is known as the atomisation energy: ?h_at.

M(s) ? M(g) or A_xB_y(s) ? xA(g) + yB(g)

Determining mean bond enthalpies from data

It is possible to calculate approximate enthalpy changes using mean bond enthalpies. If you know the enthalpy change for a reaction and the majority of bond enthalpies then you can calculate the mean bond enthalpy for a bond.

Hess’ Law

Under Hess’ Law: the enthalpy change for a chemical reaction depends only on the initial and final states and is independent of the path followed.

This means that whichever route a reaction takes, whether direct or indirect, the overall enthalpy change for this reaction will be the same. This is due to the principle of the conservation of energy.

A lot of enthalpy changes that cannot be calculated directly can be measured using Hess’ Law.

Many enthalpy changes can be calculated by using enthalpies of formation and combustion and then applying Hess’ Law.

Using enthalpies of formation

If all the enthalpies of formation for the reactants and products are known then the enthalpy change of the reaction can be calculated. The enthalpy of formation of elements in their standard state is always zero.

?H = ? [?H_f (products)] – ? [?H_f (reactants)]

Using enthalpies of combustion

If all the enthalpies of combustion for the reactants and products are known then the enthalpy change of the reaction can be calculated.

?H = ? [?H_c (reactants)] – ? [?H_c (products)]