Solid fuel used in rocket is a mixture of Fe$$_2$$O$$_3$$ and Al (in ratio 1 : 2). The heat evolved (kJ) per gram of the mixture is _______
Given $$\Delta H_f^0$$ Al$$_2$$O$$_3$$ = -1700 kJ mol$$^{-1}$$
$$\Delta H_f^0$$ Fe$$_2$$O$$_3$$ = -840 kJ mol$$^{-1}$$
Molar mass of Fe, Al and O are 56, 27 and 16 g mol$$^{-1}$$ respectively

The thermite reaction is:

$$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$$

The heat of reaction is:

$$\Delta H_{rxn} = \Delta H_f^0(\text{Al}_2\text{O}_3) - \Delta H_f^0(\text{Fe}_2\text{O}_3) = -1700 - (-840) = -860$$ kJ

Now, the mass of the reacting mixture (as per the balanced equation) is:

$$M(\text{Fe}_2\text{O}_3) + 2 \times M(\text{Al}) = (112 + 48) + 54 = 214$$ g

So the heat evolved per gram of the mixture is:

$$\frac{860}{214} = 4.02 \approx 4$$ kJ/g

Hence, the correct answer is $$4$$ kJ/g.