From the reaction data below, determine whether the reaction is first order or second order and calculate the rate constant.
$$\begin{array}{cc}
\hline \text { Time (s) } & \text { Reactant (mM) } \\
\hline 0 & 6.2 \\
\hline 1 & 3.1 \\
\hline 2 & 2.1 \\
\hline 3 & 1.6 \\
\hline 4 & 1.3 \\
\hline 5 & 1.1 \\
\hline
\end{array}$$