Enthalpy Change

Enthalpy Change Definition

Enthalpy change is the heat change accompanying a chemical reaction at constant volume or constant pressure. The enthalpy change tells the amount of heat absorbed or evolved during the reaction. It is denoted by ΔH.

Types of Enthalpy Change

Enthalpy change of a reaction expressed in different ways depending on the nature of the reaction. Some are discussed below:

Heat of formation

The enthalpy change that takes place when one mole of compound is formed from its elements. It is denoted by ΔH_f. As for example, the heat of formation of ferrous sulphide from its elements is given below:

Fe_(s) + S_(s) → FeS_(s)              ΔH_f = -24.0 kcal

Heat of combustion

The enthalpy change that takes place when one mole of compound is completely burnt in excess of air or oxygen. It is denoted by ΔH_c. As for example, the heat of combustion of methane is given below:

CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(l)              ΔH_c = -21.0 kcal

Heat of neutralization

The enthalpy change that takes place when one gram equivalent of an acid is completely neutralized with one gram equivalent of base in dilute solution. As for example, the heat of neutralization of nitric acid and sodium hydroxide is given below:

HNO_3(aq) + NaOH_(aq) → NaNO_3(aq) + H₂O_(l)              ΔH = -13.69 kcal

Heat of solution

The enthalpy change that takes place when one mole of substance is dissolved in specified quantity of solvent in given temperature. As for example, the heat of solution of magnesium sulphate is given below:

MgSO_4(s) + H₂O_(l) → MgSO_4(aq)              ΔH = -20.28 kcal

Calculating enthalpy change

The enthalpy of a system is measured by the sum of the internal energy and the product at constant pressure and volume.

H = E + PV ………………….. (i)

Here E = internal energy, P = Pressure and V = volume of the system. It is also called heat content. Thus the enthalpy change is

ΔH = H_products – H_reactants

                                                 ΔH = H_p – H_r ………………(ii)

If ΔV = the change in volume at constant temperature and pressure, ΔE = the sum of the change in internal energy at constant volume and ΔH = the change in enthalpy at constant pressure. Then from the equation (i) we can write,

ΔH = ΔE + PΔV ……………………(iii)

Let us consider a reaction

aA + bB → cC + dD

Here,

Change in number of moles = no.of moles of products – no. of moles of reactants.

= (c + d) – (a + b)

= Δn

If V = the volume occupied by one mole of gas, the change in volume,

ΔV = change in no. of moles X volume occupied by one mole of gas.

ΔV = Δn X V

PΔV = P(Δn X V)

                                                          PΔV = PV X Δn …………………(iv)

But for one mole of gas PV = RT. By placing this value in equation (iv) we get

PΔV = RT X Δn

Substituting the value of PΔV in equation (iii) we get

ΔH = ΔE + Δn RT

Here it should be noted that to calculate Δn, the no. of moles of reactants and products in only gaseous state are considered. The value of gas constant

R = 8.314 J/mol.K or 1.987 cal/mol.K.

Exothermic and endothermic reactions

Let us consider a reaction at constant pressure,

A + B → C+ D

Rewritting equation (ii) we get

ΔH = H_p – H_r

ΔH = (H_C + H_D) – (H_A + H_B)

The value of ΔH can be positive, negative or zero. If ΔH is zero that means there is no heat change during the reaction, neither heat is absorbed or evolved.

Sign of ∆H	Energy	Reaction type
+	Absorbed	Endothermic
–	Evolved	Exothermic

In case ΔH is positive that means H_p is greater than H_r, then heat is absorbed or the reaction is endothermic. And when ΔH is negative means H_p is less than H_r then heat is evolved or the reaction is exothermic.

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