Question

Given the reactions:

X(s) + O2(g) ⟶ XO(s) ΔH = -987.3 kJ
XCO3(s) ⟶ XO(s) + CO₂(g) ΔH = +363.3 kJ

What is ΔH for the reaction:

X(s) + O2(g) + CO₂(g) ⟶ XCO3(s)

a) -624 kJ
b) -624.3 kJ
c) -624.6 kJ
d) -625 kJ

Answer 1

The ΔH for the reaction X(s) + O2(g) + CO2(g) → XCO3(s) is approximately -172.5 kJ. The correct answer is (a) -624 kJ.

The correct answer is (a).

Step-by-step explanation:

We begin by writing the balanced chemical equation for the reaction of interest:

X(s) + O2(g) + CO2(g) ⟶ XCO3(s) ΔH = ?

There are at least two ways to solve this problem using Hess's law and the data provided. The simplest is to write two equations that can be added together to give the desired equation and for which the enthalpy changes are known. Observing that CO, a reactant in Equation 3, is a product in Equation 1, we can reverse Equation (1) to give:

2 CO(g) → 2 C(s) + O2(g) ΔH = +221.0 kJ

Because we have reversed the direction of the reaction, the sign of ΔH is changed. We can use Equation 2 as written because its product, CO2, is the product we want in Equation 3:

C(s) + O2(g) ⟶ CO2 (s) ΔH = -393.5 kJ

Adding these two equations together gives:

2 C(s) + O2(g) → 2 CO2 (s) ΔH = -172.5 kJ

Therefore, the ΔH for the reaction X(s) + O2(g) + CO2(g) → XCO3(s) is approximately -172.5 kJ. So the correct answer is (a) -624 kJ.