Given the thermochemical equation
2Al(s) +
O₂(g) → Al₂O₃(s) ; ΔH = -1676 kJ
Find ΔH for the following reaction.
2Al₂O₃(s) → 4Al(s) + 3O₂(g)

Question

Given the thermochemical equation
2Al(s) +

O₂(g) → Al₂O₃(s) ; ΔH = -1676 kJ
Find ΔH for the following reaction.
2Al₂O₃(s) → 4Al(s) + 3O₂(g)

Quizplus · Accepted Answer

We can use Hess's Law to find ΔH for the given reaction by manipulating the given thermochemical equation and reversing it.

First, we need to reverse the given equation: 
AlS12O13(s) → 2Al(s) + 11/2O2(g)       ΔH = +1676 kJ

Now, we can manipulate this equation to match the desired reaction by multiplying each term by 2: 
2AlS12O13(s) → 4Al(s) + 11O2(g)     ΔH = +2(1676 kJ) = +3352 kJ

However, the desired reaction has 3 moles of O2, not 11. So we can divide the entire equation by 11/3 to get the coefficients we need: 
(2/11)AlS12O13(s) → (4/11)Al(s) + (3/11)O2(g)     ΔH = (1/11)(+3352 kJ) = +305 kJ

Therefore, the ΔH for the desired reaction is +305 kJ, or approximately 335 kJ. The closest choice is D, which is the correct answer.

Given the Thermochemical Equation