when free cl(g) atoms encounter o3(g) molecules in the upper atmosphere, the following reaction mechanism is proposed to occur. three equations are shown. the first equation is c l gas o 3 gas gives c l o gas o2 gas. the text displayed to the right of this equation reads slow step. the second equation is c l o gas o3 gas gives c l gas 2 o2 gas, which is placed aligned with the first equation. the text displayed to the right of this equation reads fast step. below this second equation, there is a horizontal line. below this line, the third equation is placed by aligning the first two equations. the third equation is 2 o 3 gas gives 3 o 2 gas. the text displayed to the right of this equation reads overall reaction, delta h