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Thermodynamics

Session8

Standard Heat of reaction:

It is defined as the enthalpy change accompanying a reaction when both reactants and

products are at their standard states at 298 K and is denoted by 2980

H .

)g(CO)g(O)s(C 222

kJ.H 22212980

If the stoichiometry of the above reaction is represented as

Then,

kJ.kJ.

H 61102

2221298

0

Standard Heat of Combustion:

The standard heat of combustion of a substance is defined as the enthalpy change

accompanying the reaction when one mol of the substance undergoes combustion when

both reactants and products are at 298K.

The heat of combustion is1168.12 kJ.

The standard heat of formation:

Standard heat of formation of a substance is defined as the change in enthalpy

accompanying the formation of 1 mol of the substance from the constituent elements

when the reactants and products are at 298K.

)g(CO)g(O)s(C 222

kJ.H 22212980

Standard Heat of Formation of Carbon monoxide is kJ.H f 61100

.

)g(CO)g(O)s(C 2

2

1

;OH)g(CO)g(O)l(CHOCH2223

4452 2336.24kJH 2980

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Hesss Law of constant heat summation:

The net enthalpy involved in a reaction is the same whether the reaction takes place in a

single step or in a series of steps.

CBA

Let A and B react to form C with heat of reaction H. Let there be one more method of

carrying out this reaction in two steps as given below.

ABBA 1

H

CAB 2H

Treating arrow as an equal sign, adding these two reactions gives,

CBA , 21 HH

It has been found that 21 HH is same as .H

This idea is used to determine heat of reaction from heat of formation or heat of

combustion data.

For example if we want to determine heat of formation of sulfuric acid, we must

determine this from the reaction,

No matter what we do, the above reaction cannot be carried out. Let us carry out other

possible reactions such that the over-all reaction is the formation reaction as below.

H

42222 SOHOSH

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22SOOS

1H ----------(1)

32222 SOOSO

2H ----------(2)

4223SOHOHSO

3H -----------(3)

OHOH222

2

1 4

H ----------(4)

2(1), 2(3) and 2(4) gives,

22222 SOOS

12 H ----------(1)

32222 SOOSO

2H ----------(2)

4223222 SOHOHSO

32 H -----------(3)

OHOH222

22 42 H ----------(4)

Adding these equations along with heat of reactions,

and heat of this reaction will be 24312 H)HHH(

Therefore heat of formation of sulfuric acid is2

2

431

HHHH

This is how Hesss law can be used to determine the heat of reaction of a reaction from

heat of reaction of other reactions.

4222 2422 SOHOSH