The energy stored in a substance that can be available in the form of heat is called enthalpy or heat constant. It indicated by the symbol H.
H = E + PV ………….(i)
Where E = internal energy
P = Pressure
V = Volume
Enthalpy change is the heat evolved or absorbed at constant pressure. (ΔH = Qp)
Since E, V and P are state function so H is also a state function. The change in enthalpy is calculated by using relation
ΔH = H2 – H1 ……………(ii)
Where H2 = enthalpy of product
H1 = enthalpy of reactant
At constant pressure
ΔH = E2 + PV2 – E1 – PV1
Or, ΔH = E2 – E1 + PV2 – PV1
ΔH = ΔE + PΔV
Relation between ΔH, ΔE and Δn
ΔH = ΔE + PΔV
From ideal gas equation.
PV1 = n1Rt ……(i) (at constant temperature and pressure)
PV2 = n2Rt ……(ii)
Subtracting equation (ii) from (i)
PV2 – PV1 = ΔnRT…….(iii)
∴ΔH = ΔE + ΔnRT
Enthalpy of physical transformation
1. Enthalpy of fusion:
The enthalpy of fusion of substance is defined as the amount of het required to convert one mole of solid substance into liquid at its melting point. If is indicated by symbol ΔHf.
H2O(s) H2O(l) ΔHfus = 6.6KJ mol-1
2. Enthalpy of vaporization
Enthalpy of vaporization is defined as the amount of heat required to convert one mole of the liquid into vapor at the boiling point. It is indicated by the symbol ΔHv
H2O(l) H2O(g) ΔHv = 40.6KJmol-1
3. Enthalpy of sublimation
Enthalpy of sublimation is defined ads the amount of heat required to convert one mole of solid directly into vapour. (It is equal to heat of fusion + heat of vaporization). It is indicated by symbol ΔHs.
I2(s) I2(g) ΔHsub = 62.39KJmol-1
4. Enthalpy of transition
Enthalpy of transition is defined as the change in enthalpy when one mole of substance changes from one allotropic form to another.
Enthalpy of chemical transformation