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Question

For the following reaction,
C(diamond,s)+O2(g)CO2(g);H=94.3 kcal
C(graphite,s)+O2(g)CO2(g);H=97.6 kcal
The heat required to change 1 g of C(diamond,s)C(graphite,s) is:

A
3.3 kcal
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B
1.65 kcal
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C
1.29 kcal
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D
0.275 kcal
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Solution

The correct option is D 0.275 kcal
C(diamond,s)+O2(g)CO2(g); H=94.3 kcal
C(graphite,s)+O2(g)CO2(g); H=97.6 kcal
using hess law,
C(diamond,s)C(Graphite,s) H=3.3 kcal

Heat required to convert 12 g diamond to graphite = 3.3
Heat required to convert 1 g diamond to graphite =3.312=0.275kcal




Theory :

Lavoisier - Laplace Law:
If the direction of a chemical process is reversed then magnitude of enthalpy change remains the same but the sign is reversed.

This is because the enthalpy is a state function that means it is independent of path followed. So when we reverse the initial and final state its magnitude remains the same but the sign is reversed.

Reaction - 1 :
AB Δrxn1H

Reaction - 2:
BA Δrxn2H

Δrxn1H=Δrxn2H

For example

H2(g)+12O2(g)H2O(l) ΔH1=286 kJ/mol

H2O(l)H2(g)+12O2(g) ΔH2=286 kJ/mol

Since here direction of the chemical process is reversed,

ΔH1=ΔH2

Scaling of the reaction ΔrxnH
AB x

2A2B 2x

12A12B 1/2x

nAnB nx


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