Introduction to Reaction Mechanisms Practice Test

A student proposes the following mechanism for the overall reaction $\text{Cl}_2(g)+\text{CH}_4(g)\rightarrow \text{CH}_3\text{Cl}(g)+\text{HCl}(g)$:

Step 1: $\text{Cl}_2(g)\rightarrow 2\text{Cl}(g)$

Step 2: $\text{Cl}(g)+\text{CH}_4(g)\rightarrow \text{HCl}(g)+\text{CH}_3(g)$

Step 3: $\text{CH}_3(g)+\text{Cl}_2(g)\rightarrow \text{CH}_3\text{Cl}(g)+\text{Cl}(g)$

Which species is an intermediate in the mechanism?

A proposed mechanism for the reaction $\text{H}_2(g)+\text{Br}_2(g)\rightarrow 2\text{HBr}(g)$ is:

Step 1: $\text{Br}_2(g)\rightarrow 2\text{Br}(g)$

Step 2: $\text{Br}(g)+\text{H}_2(g)\rightarrow \text{HBr}(g)+\text{H}(g)$

Step 3: $\text{H}(g)+\text{Br}_2(g)\rightarrow \text{HBr}(g)+\text{Br}(g)$

Which species is an intermediate in the mechanism?