Explain why the average rate of reaction decreases with the concentration of the reactant using an example.

(N/A) The rate of a chemical reaction depends on the concentration of the reactants. As the reaction proceeds,the concentration of reactants decreases,which leads to a decrease in the frequency of effective collisions between reactant molecules,thereby decreasing the rate of reaction.
Example: The reaction of $C_{4}H_{9}Cl$ with $OH^{-}$ ions:
$C_{4}H_{9}Cl + OH^{-} \rightarrow C_{4}H_{9}OH + Cl^{-}$
The following table shows the average rate of reaction at different time intervals:

$t_{1(s)} - t_{2(s)}$	$r_{av} = -\frac{\Delta [R]}{\Delta t} \ (mol \ L^{-1} \ s^{-1})$
$0 - 50 \ s$	$1.9 \times 10^{-4}$
$50 - 100 \ s$	$1.7 \times 10^{-4}$
$100 - 150 \ s$	$1.58 \times 10^{-4}$
$200 - 300 \ s$	$1.22 \times 10^{-4}$

As time increases,the concentration of the reactant decreases,causing the average rate of reaction to decrease.