Question

One of the components of smog is ozone,O₃, a colorless, toxic gas. It is a very strong oxidizing agent and causes respiratory illnesses. Ozone is produced in the lower atmosphere due to the catalytic effect of nitric oxide, NO, a byproduct of the combustion process in car engines.

However, in the upper atmosphere ozone is a very useful gas, because it absorbs ultraviolet radiation and protects the earth from this high energy, damaging radiation. It is referred to as the ozone layer. This ozone layer is depleted by the catalytic action of gases like Freon, which are commonly used in refrigerators and air conditioners. Chemically these gases are unreactive and rise into the upper atmosphere. The high energy radiation in the upper atmosphere produces very reactive chlorine atoms according to the reaction below:

CCl₂F₂(g) → CClF₂(g) + Cl(g) (catalyzed by uv radiation)

The following questions refer to the depletion of ozone in the upper atmosphere according to the mechanism shown below:

step 1:	Cl(g) + O3(g)	→	ClO(g) + O2(g)	(slow)
step 2:	O(g) + ClO(g)	→	Cl(g) + O2(g)	(fast)

Mark each of the following statements as either True or False:

True False  ClO(g) is an intermediate formed in this reaction mechanism.

True False  Cl(g) is a catalyst in this reaction mechanism.

True False  Step 2 of the mechanism is a bimolecular.

True False  The rate law is deduced directly from the coefficients of the overall reaction.

True False  The rate equation for this mechanism is rate = k [O][ClO].

True False  The sum of the two steps is equal to the overall reaction: O(g) + O₃(g) → 2O₂(g).

Answer 1

1) ClO is an intermediate because it is produced in first step and consumed in second step. ........ True

2) Cl is catalyst because it is used at very beginning of the reaction. .... True

3) step 2 is bimolecular because there are total of two molecules O and ClO. ...... True

4) Statement 4 is false as rate law cannot be deduced directly.

5) Statement 5 is false as this is a fast step.

6) Statement 6 is true. By adding both the steps, we get

O + O3 ----------> 2O2