General Chemistry

Question

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 atom. Chlorine was previously identified as a catalyst in the breakdown of ozone into oxygen gas.

Using enthalpy of reaction for two reactions with ozone, determine the enthalpy of reaction for the reaction of chlorine with ozone.

1) ClO(g) +O3(g) ----> Cl(g) +2O2(g) Hrxn = -122.8 kJ/mol

2) 2O3(g) ---> 3O2(g) Hrxn = -285.3 kJ/mol

3) O3(g) + Cl(g) -----> ClO(g) + O2(g) Hrxn = ????

_______ kJ/mol

Answer 1

Review Hess' Law

Reverse equation (1): Cl(g) + 2O2(g) ==> Cl)(g) + O3(g) ∆H = +122.8 kJ/mol (note change in sign of ∆H)

Copy equation (2): 2O3(g) ===> 3O2(g) ∆H = -285.3 kJ/mol

Add the 2 equations: 2O3(g) + Cl(g) + 2O2(g) ===> Cl))g) + O3(g) + 3O2(g) and cancel common species

When you do this, you end up with the desired equation (3):

Cl(g) + O3(g) ===> ClO(g) + O2(g) ∆H = -285.3 + 122.8 = -162.5 kJ/mole