# The enthalpies of combustion of carbon and carbon monoxide are –393.5 and –283 kJ mol-1  respectively. The enthalpy of formation of carbon monoxide per mole is   Option 1) 110.5 kJ Option 2) 676.5 kJ Option 3) –676.5 kJ Option 4) –110.5 kJ.

As we learnt in

Hess's Law -

The total amount of heat change in a chemical reaction is the same whether the reaction is carried  out in one or several steps by one or more methods.

- wherein

$C_{(s)}+O_{2(g)}\rightarrow CO_{2(g)}$

$\Delta H= 393.5\, kj$

$C_{(s)}+\frac{1}{2}O_{2(g)}\rightarrow CO_{(g)}$

$\Delta H_{1}= 110.5\, kj$

$CO_{(g)}+\frac{1}{2}O_{2(g)}\rightarrow CO_{2(g)}$

$\Delta H_{2}= 283.0\, kj$

$\Delta H= \Delta H_{1}+\Delta H_{2}$

i) C + O2$\rightleftharpoons$ CO2  $\Delta$H = -393.5KJ mol-1

ii) CO + $\frac{1}{2}$O2$\rightleftharpoons CO, \Delta H$ = - 283.0 kJ mol-1

operating (i) and (ii)

$C +\frac{1}{2}O_{2}\rightarrow CO\Delta H=-110.5 kJ mol^{-1}$

Option 1)

110.5 kJ

this is incorrect option

Option 2)

676.5 kJ

this is incorrect option

Option 3)

–676.5 kJ

this is incorrect option

Option 4)

–110.5 kJ.

this is correct option

1

_110.5 kJ

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