When `4g` of iron is burnt to ferric oxide at constant pressure, `29.28KJ` of heat is evolved. What is the enthalpy of formation of ferric oxide (At wt. of `Fe=56`)

Options

A. `-81.98KJ`
B. `-819.8KJ`
C. `-40.99KJ`
D. `+819.8KJ`

Answer

A

Explanation

Given: Weight of iron burnt `=4g` , Heat liberated `-29.28KJ` and atomic weight of iron `(Fe)=56` . We know that in ferric oxide `(Fe_(2)O)` , 2 moles of iron or `2xx56=112g` of iron are burnt. We also know that when `4g` of iron are burnt, then burnt, liberated `=29.28KJ` , therefore when `112g` of the iron are burnt, then heat liberated
`=(29.28xx112)/(4)=-819.8KJ`
(Minus sign due to liberation of heat).