By “the overall order of a reaction”,we mean

What is the unit of the rate constant for a $4^{th}$ order reaction?

For the reaction ${H_{2(g)} + I_{2(g)} \to 2HI_{(g)}}$,the possible mechanism is given below:
$I_2 \underset{k_{-1}}{\stackrel{k_1}{\rightleftharpoons}} 2 I$ (fast step)
${2I + H_2 \xrightarrow{K_2} 2HI}$ (slow step)
Determine the rate law for the reaction.

For the reaction:
$2NO_{2(g)} + O_{3(g)} \to N_2O_{5(g)} + O_{2(g)}$
The rate law is $R = K[NO_2]^1 [O_3]^1$.
Which of these possible reaction mechanisms is consistent with the rate law?
Mechanism $I$:
$NO_{2(g)} + O_{3(g)} \to NO_{3(g)} + O_{2(g)}$ (slow)
$NO_{3(g)} + NO_{2(g)} \to N_2O_{5(g)}$ (fast)
Mechanism $II$:
$O_{3(g)} \rightleftharpoons O_{2(g)} + [O]$ (fast)
$NO_{2(g)} + [O] \to NO_3$ (slow)
$NO_{3(g)} + NO_{2(g)} \to N_2O_5$ (fast)

The mechanism of the reaction $2NO_{(g)} + Br_{2(g)} \rightarrow 2NOBr_{(g)}$ is given by:
$1) NO_{(g)} + Br_{2(g)} \rightleftharpoons NOBr_{2(g)}$ (fast)
$2) NOBr_{2(g)} + NO_{(g)} \rightarrow 2NOBr_{(g)}$ (slow)
If the second step is the rate-determining step,what is the order of the reaction with respect to $NO_{(g)}$?

The rate law for the decomposition of hydrogen iodide is $-\frac{d[HI]}{dt}=k[HI]^2$. The units of rate constant $k$ are