Question

Given the following information:

2 H2 (g) + O2 (g) → 2 H2O (g)	ΔH = −438.6 kJ
3 O2 (g) → 2 O3 (g)	ΔH = +284.6 kJ

Which is a true statement about the reaction below?

3 H₂ (g) + O₃ (g) → 3 H₂O (g)

A) The reaction is exothermic.
B) The reaction will not proceed as written.
C) Multiplying both sides of the reaction by a factor of 2 will have no effect on the value of ΔH_rxn
D) The reaction will absorb heat from its surroundings.
E) None of the above answers are true statements.

Answer 1

We know

2 H2 (g) + O2 (g) → 2 H2O (g) ΔH = −438.6 kJ -(reaction 1)

and

3 O2 (g) → 2 O3 (g) ΔH = +284.6 kJ - (reaction 2)

If we have to construct the new equation

3 H2 (g) + O3 (g) → 3 H2O (g)

we can do that by reversing reaction 2 and dividing it by 2

O3 (g) → 3/2 O2 (g) ΔH = - 142.3 kJ - (reaction 3)

Then we multiply react 1 by 3/2

3 H2 (g) + 3/2O2 (g) → 3 H2O (g) ΔH = −657.9 kJ - (reaction 4)

If we now add reaction 3 and 4 we get

3 H2 (g) + O3 (g) → 3 H2O (g) for which the ΔH = -800 kJ

So the True statement among the ones given is

A) The reaction is exothermic.

All other statements are wrong