Enthalpy and heat

The energy change during a reaction depends on

the amount of material undergoing chemical change.
whether the volume occupied by the reaction mixture or the external pressure on the reaction mixture changes during reaction.

Most reactions are carried out at constant pressure in containers open to the atmosphere.
To discuss the thermal changes for these, it is useful to define a quantity enthalpy (H) where the change in enthalpy (ΔH) equals q_p, the heat transferred at constant pressure.

For reactions where both the volume occupied and the external pressure are constant, ΔH is also equal to the difference between the energy of the products and the reactants.

The unit for ΔH is the same as the unit for q (J or kJ).

Δ_rH =	q_p
	n

The enthalpy of reaction (Δ_rH) is calculated by measuring the heat transfer at constant pressure (q_p) for a specific amount in moles of reaction (n). Enthalpy changes are generally reported in kJ mol^–1.

The mol^–1 is the amount in moles of reaction

Consider the reaction: Na₂CO₃ + 2HCl → Na₂SO₄ + 2NaCl
The amount in moles of reaction is equal to n(Na₂CO₃) reacting or n(Na₂SO₄) formed.
The amount in moles of reaction is also equal to ½ × n(HCl) reacting or ½ × n(NaCl) formed

The heat released or absorbed by reaction of a particular amount of substance can then be calculated by

calculating the amount in moles of substance involved
relating this amount to the amount in moles of reaction
multiplying the amount in moles of reaction by the enthalpy change for the reaction.