The following is a two-step mechanism for how chlorine atoms in the upper atmosphere react with and decompose ozone. \(\mathrm{Cl}+\mathrm{O}_{3} \rightarrow \mathrm{ClO}+\mathrm{O}_{2}\) \(\mathrm{O}+\mathrm{ClO} \rightarrow \mathrm{Cl}+\mathrm{O}_{2}\) According to the mechanism, which is the intermediate and which is the catalyst? Justify your choices. (Hint: Write the overall reaction.)

Understanding chemical reaction mechanisms is essential to grasp how various substances interact to form new products. A reaction mechanism details the step-by-step sequence of elementary reactions by which overall chemical change occurs.

Imagine each chemical reaction as a story. In this story, the molecules are characters, and the reaction mechanism is the plot outlining their interactions. Just as a storyline comprises individual scenes leading to the conclusion, a chemical reaction mechanism consists of multiple steps that result in the final product.

Each step is a straightforward process that involves breaking and forming of bonds between atoms. Some reactions occur in a single step, but many require several stages, each with its own transition state and activation energy. It's the activation energy that determines the speed of the reaction at each phase, hence impacting the reaction rate. Knowing the mechanism offers insights into the speed and conditions necessary for a reaction to take place, and can also point out which catalysts may be useful.

Ozone decomposition plays a pivotal role in atmospheric chemistry, especially when discussing the depletion of the ozone layer in the upper atmosphere. Ozone, or \(\mathrm{O}_{3}\), is a molecule composed of three oxygen atoms. It absorbs harmful ultraviolet light from the sun, preventing it from reaching the Earth's surface.

However, ozone can be decomposed by various substances, including chlorine atoms, which act as a catalyst in the upper atmosphere. In the context of the exercise, the decomposition of ozone involves chlorine atoms in a two-step mechanism, ultimately leading to the production of oxygen molecules \(\mathrm{O}_{2}\).

This decomposition not only affects the amount of ozone in the upper atmosphere but also has implications for climate change and environmental health since the ozone layer protects life on Earth from harmful UV radiation.

In a chemical reaction mechanism, certain species called reaction intermediates are produced in one step and consumed in the next. They play a transient role and do not appear in the overall chemical reaction.

An analogy to understand intermediates is to think of them as 'behind-the-scenes' workers in a play—they play a critical role in transitioning the plot forward but are not seen by the audience in the final act. In the exercise provided, \(\mathrm{ClO}\) is formed when chlorine atoms react with ozone and then it reacts further to produce the final products. Because \(\mathrm{ClO}\) isn't present in the starting materials or the final products, but only in the intermediate steps, it's a perfect example of a reaction intermediate.

Studying intermediates is vital for understanding how reactions proceed and for designing ways to control or optimize these reactions for industrial and environmental purposes.