ThermochemistryMedium
Question
How much energy is released when 6 moles of octane is burnt in air ? Given ᐃHfo for CO2(g), H2O(g) and C8H18(1) respectively are - 490, - 240 and + 160J/mol
Options
A.-6.2 kJ
B.- 37.4kJ
C.- 35.5kJ
D.- 20.0kJ
Solution
C + O2 = CO2 ᐃH = - 490 (i)
H2 +
O2 = H2O ᐃH = - 240 (ii)
8C + 9H2 = C8H18 ᐃH = + 160 (iii)
2C8H18 + 25O2 = 16CO2 + 18H2O
The required reaction can be obtained by
2 × (iii) - 16(i) - 18(ii)
16C + 18H2 - 16C - 16O2 - 18H2 - 9O2
= 2C8H18 - 16CO2 - 18H2O
or - 25O2 = 2C8H18 - 16CO2 - 18H2O 2C8H18 + 25O2 = 16CO2 + 18H2O
[ᐃH = 2 × 160 - 16 × - 490 - 18 × - 240]
ᐃH = - 11840J = - 11.840 kJ for 2 moles of
Octance. Energy releced for 6 moles of
Octance. = - 11840 × 3 = - 35.5kJ
H2 +
O2 = H2O ᐃH = - 240 (ii)8C + 9H2 = C8H18 ᐃH = + 160 (iii)
2C8H18 + 25O2 = 16CO2 + 18H2O
The required reaction can be obtained by
2 × (iii) - 16(i) - 18(ii)
16C + 18H2 - 16C - 16O2 - 18H2 - 9O2
= 2C8H18 - 16CO2 - 18H2O
or - 25O2 = 2C8H18 - 16CO2 - 18H2O 2C8H18 + 25O2 = 16CO2 + 18H2O
[ᐃH = 2 × 160 - 16 × - 490 - 18 × - 240]
ᐃH = - 11840J = - 11.840 kJ for 2 moles of
Octance. Energy releced for 6 moles of
Octance. = - 11840 × 3 = - 35.5kJ
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