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