How much energy is released when 6 mole of octane is burnt in air ? Given `DeltaH_(f)^(@)` for `CO_(2)(g),H_(2)O(g)` and `C_(8)H_(18)(l)` respectively are `-490,-240` and `+160KJ//mol`

Options

A. `-6.2` kJ
B. `- 37.4` kJ
C. `-35.5` kJ
D. `- 20.0` kJ

Answer

B

Explanation

`C+ O_(2) rarr CO_(2), Delta H_(F)= -490` kJ/mol...(i)
`H_(2)+1//2O_(2) rarr H_(2)O, Delta H_(F)= -240` kJ/mol...(ii)
`8C + 9H_(2) rarr C_(8)H_(18), Delta H_(F)= +160` kJ/mol ....(iii)
Applying
(i) `xx 8+` (ii) `xx 9-` (iii)
`C_(8)H_(18) + (25)/(2) O_(2) rarr 8CO_(2) + 9H_(2)O`
`Delta H^(@)= -3920- 2160 -160= -6240`kJ/mol
`Delta H^(@)`= for 6 moles of octane `=6240 xx 6`= 37440kJ/mol= -37.4kJ