During any chemical reaction carried out at constant pressure, heat evolved or absorbed is most conveniently expressed in terms of Enthalpy (H) which is defined as

and change in Enthalpy (DH) may be written as,

Initial H₁ = E₁ + P₁V₁ Final H₂ = E₂ + P₂V₂

Example : Calculate the enthalpy change at 300 K for the reaction.

C₂ H6 (g) + Cl₂ (g) ® 2CH₃Cl (g)

Given the heat of formation of CH₃Cl (g) and C₂ H6 (g)
as - 84.68 KJ and - 90.29 KJ mol^-1 respectively.

\ D H_reactions = S H _products - S H _reactants

= ( 2 ´ -84.68 ) - ( -90.29 ) + 0

Enthalpy change of reaction = -79.07 KJ.

Example : Calculate the heat of combustion of acetic acid at 298 K if the enthalpies of formation of CH₃COOH ( E ),
CO₂ ( g ) and water ( l ) are - 487, - 3833 &
- 285.8 KJ mol ^-1 respectively.

The combustion of acetic acid can be given as

CH3COOH (l) + 2O₂ (g) ® 2 CO₂ (g) + 2H₂ O (l)

DH_combustion = [ 2H (H₂ O) + 2H (CO₂) ]
        - [ H (CH₃COOH) + 2H (O₂) ]

= [ 2 ´ -393.3 + 2 ´ -285.8 ] - [ - 487 + 0 ]

\ DH_combustion= - 871.2 KJ

Index

10.1 Introduction
10.2 First Law of Thermodynamics
10.3 Enthalpy
10.4 Second Law of Thermodynamics
10.5 Gibb's Free Energy
10.6 Absolute Entropies

Chapter 11