Enthalpy change- wps.prenhall.com

## Enthalpy change-

It is defined as the change in enthalpy or change in internal energy or amount of heat absorbed or released during the complete reaction as represented by a balanced thermochemical equation.

Ex- C + O2 —> CO2    ΔH = -94.3 Kcal

It means 12 gm of carbon reacts with 32 gm of oxygen to form 44 gm of carbon dioxide with the evolution of 94.3 Kcal of heat.

## Factors influencing the Enthalpy change-

### i) Physical nature of reactant & product-

For a given reaction the Heat of reaction varies with the change in the physical nature of reactants & products.

Ex- C(diamond) + O2. ———> CO2          ΔH = -94.3 Kcal

C(amorphous) + O2. ———> CO2          ΔH = -97.7 Kcal

It is clear from the example that the heat of reactions are different when two forms of carbon are different.

When product formed has a different physical state, then heats of reaction are also different.

H2 + 1/2O2 —-> H2O(l)          ΔH = -68.3 Kcal

H2 + 1/2O2 —-> H2O(g)          ΔH = -57.0 Kcal

### ii) The Reaction carried out at constant pressure or constant volume-

Heat changes at constant pressure are expressed in terms of ΔH.

From the first law of thermodynamics,

At constant pressure, Qp = ΔH

Heat changes at constant volume are expressed in terms of ΔU.

From the first law of thermodynamics,

At constant pressure, Qv = ΔU

ΔH & ΔU are related by the formula,

ΔH= ΔU + PΔV

P is the pressure at which reaction is carried out & ΔV is the change in volume during the reaction.

Because, PΔV = Δn RT

Δn= no. of moles of gaseous product – no. of moles of gaseous reactant

Thus,

ΔH= ΔU + Δn RT

### iii) Temperature-

The Heat of reaction also depends upon the temperature. ΔH varies with temperature due to variation in heat capacity of the system with temperature.

ΔH2 – ΔH1= ΔCp( T2-T1)

ΔH= ΔCp.ΔT

where H2 & H1 are change in enthalpy at temperature T2 & T1.

ΔCp = Cp of oroducts – Cp of reactants

ΔU2 – ΔU1= ΔCv( T2-T1)

ΔU= ΔCv.ΔT

where U2 & U1 are change in internal energy at temperature T2 & T1.

ΔCv = Cv of products – Cv of reactants