The changes accompanied by the heat content of the materials which react, where the correct term for heat content is enthalpy(H). Usually this change in the heat content or enthalpy is shown by a change in temperature.
Every chemical reaction involves a change in energy – in the form of heat. The chemical reaction may be between elements and/or compounds. Either a reaction proceeds by supplying this energy or the reactions are spontaneous, with release of energy.
Chemical reactions are generally run on constant pressure of 1 atmosphere and not on constant volume and hence the heat absorbed or evolved is not equal to the intrinsic energy change (ΔE).
The difference of this energy from the start of a reaction to the completion of the same is called enthalpy change (Δ H). H can be thus related to E + PV.
During the reaction there is change in the heat of the reaction. The enthalpy of products is H2
and is less than the heat content of reactants H1
. The enthalpy change for the reaction is denoted by $\Delta$H (sometimes called the heat of reaction).Enthalpy Formula
is given by
The enthalpy of reaction equals the heat of reaction at constant pressure.