The 1995 Nobel Prize in Chemistry was shared by Paul Crutzen, F. Sherwood Rowland, and Mario Molina for their work concerning the formation and decomposition of ozone in the stratosphere. Rowland and Molina hypothesized that chlorofluorocarbons (CFCs) in the stratosphere break down upon exposure to UV radiation, producing chlorine atoms. Chlorine was previously identified as a catalyst in the breakdown of ozone into oxygen gas. Using the enthalpy of reaction for two reactions with ozone, determine the enthalpy of reaction for the reaction of chlorine with ozone. ClO(g)+O3(g)â¶Cl(g)+2O2(g) ÎHârxn=â122.8 kJ
2O3(g)â¶3O2(g) ÎHârxn=â285.3 kJ
O3(g)+Cl(g)â¶ClO(g)+O2(g) ÎHârxn= ?

ΔH = -162.5 kJ.

Explanation:

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In this case, we first rearrange the reactions:

ClO(g) + O₃(g) ⇒ Cl(g) + 2O₂(g);  ΔH =-122.8 kJ

2O₃(g) ⇒ 3O₂(g);  ΔH=-285.3 kJ

O₃(g) + Cl(g) ⇒ ClO(g) + O₂(g);  ΔH= ?

Thus, we are going to use the Hess law, as an strategy to rearrange the known chemical reactions and thereby compute the enthalpy of reaction of the unknown one.

1. The first reaction must be inverted in order to obtain chlorine as a reactant in the third one, therefore, the enthalpy of reaction becomes positive:

Cl(g) + 2O₂(g) ⇒ ClO(g) + O₃(g);   ΔH = 122.8 kJ

2. Second reaction remains the same:

2O₃(g) ⇒ 3O₂(g);  ΔH=-285.3 kJ

Then, we add them to obtain:

Cl(g) + 2O₂(g) + 2O₃(g) ⇒ ClO(g) + O₃(g) + 3O₂(g)

Whereas we can subtract both oxygen and ozone to obtain the third one:

O₃(g) + Cl(g) ⇒ ClO(g) + O₂(g)

Therefore, the enthalpy of reaction turns out:

ΔH = 122.8 kJ + (-285.3 kJ )

ΔH = -162.5 kJ.

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