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 ΔHf. As for example, the heat of formation of ferrous sulphide from its elements is given below:
Fe(s) + S(s) → FeS(s) ΔHf = -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 ΔHc. As for example, the heat of combustion of methane is given below:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔHc = -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:
HNO3(aq) + NaOH(aq) → NaNO3(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:
MgSO4(s) + H2O(l) → MgSO4(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 = Hproducts – Hreactants
ΔH = Hp – Hr ………………(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
Change in number of moles = no.of moles of products – no. of moles of reactants.
= (c + d) – (a + b)
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 = Hp – Hr
ΔH = (HC + HD) – (HA + HB)
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|
In case ΔH is positive that means Hp is greater than Hr, then heat is absorbed or the reaction is endothermic. And when ΔH is negative means Hp is less than Hr then heat is evolved or the reaction is exothermic.