Given:
i. `2Fe(s) +(3)/(2)O_(2)(g) rarr Fe_(2)O_(3)(s), DeltaH^(Theta) =- 193.4 kJ`
ii. `Mg(s) +(1)/(2)O_(2)(g) rarr MgO(s), DeltaH^(Theta) =- 140.2kJ`
What is `DeltaH^(Theta)`of the reaction?
`3Mg +Fe_(2)O_(3)rarr 3MgO +2Fe`

Options

A. `- 1802 kJ`
B. `+1802 kJ`
C. `- 800 kJ`
D. `- 228 kJ`

Answer

D

Explanation

`2Fe+ (3)/(2) O_(2) rarr Fe_(2)O_(3), Delta H= -193.4kJ` ...(i)
`Mg + (1)/(2) O_(2) rarr MgO, Delta H= -140.2 kJ` ..(ii)
On multiplying eq. (ii) by 3
`3Mg + (3)/(2) O_(2) rarr 3MgO, Delta H= -420.6kJ` ...(iii)
Resulting equation can be obtained by subtracting eq. (i) from (iii)
`3Mg + (3)/(2) O_(2) rarr 3MgO, Delta H = -420kJ`
`2Fe + (3)/(2) O_(2) rarr Fe_(2)O_(3), Delta H= -193.4 kJ`
`bar("Subtraction" 3Mg+ Fe_(2)O_(3) rarr 2Fe+ 3MgO, Delta H= -227.2kJ)`