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10.3 Enthalpy

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

 H = E + PV

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

 (DH) = H2 - H1

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

 \ DHreactions = SH products - SH reactants

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 Hreactions = S H products - S H reactants

= ( 2 ´ -84.68 ) - ( -90.29 ) + 0

Enthalpy change of reaction = -79.07 KJ.

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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)

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

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

\ DHcombustion= - 871.2 KJ

Index

Chapter 11