The rate equation for a first-order reaction is given by $\left[\mathrm{R}\right]={\left[\mathrm{R}\right]}_0{\mathrm{e}}^{-\mathrm{kt}}$. A straight line with positive slope is obtained by plotting (${\left[\mathrm{R}\right]}_0=$ Initial concentration of the reactant, $\left[\mathrm{R}\right]=$ concentration of the reactant at time $\mathrm{t}$)