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Question

When 10g of Al is used for reduction in each of the following alumino thermic reactions, which reaction would generate more heat and by how much ?
a. 2Al+Cr2O3Al2O3+2Cr
b. 2Al+Fe2O3Al2O3+2Fe

Standard heat of formation of Al2O3, Cr2O3 and Fe2O3 are 1676kJ, 1141kJ and 822.2kJ, respectively.

A
Reaction (b) produces more heat by 59.04kJ
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B
Reaction (a) produces more heat by 59.04kJ
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C
Reaction(a) and (b) produces equal heat
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D
None of these
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Solution

The correct option is A Reaction (b) produces more heat by 59.04kJ
Given,
2Al+32O2Al2O3;ΔH1=1676kJ......(i)

2Cr+32O2Cr2O3;ΔH2=1141kJ....(ii)

2Fe+32O2Fe2O3;ΔH3=822.2kJ....(iii)

To calculate
ΔHa=?
For 2Al+Cr2O3Al2O3+2Cr....(iv)
Subtract ΔH2 from ΔH1 to obtain ΔHa
ΔHa=ΔH1ΔH2
ΔHa=1676(1141)
ΔHa=535kJ
ΔHb=?
For 2Al+Fe2O3Al2O3+2Fe......(v)
Subtract ΔH3 from ΔH1 to obtain ΔHb
ΔHb=ΔH1ΔH3
ΔHb=1676(822.2)
ΔHb=853.8kJ
For reaction (iv)
2×27g of Al produces 535 kJ of heat
10g of Al produces 535×102×27=99.07kJ
For reaction (v)
2×27g of Al produces 853.8 kJ of heat
10g of Al produces 853.8×1054
=158.11kJ
Reaction (ii) produces more heat by =158.1199.07
=59.04kJ

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