$\mathrm{Zn}+2{\mathrm{H}}^{+}\to {\mathrm{Zn}}^{2+}+{\mathrm{H}}_2$
Half-life period is independent of concentration of zinc at constant pH. For the constant concentration of Zn, rate becomes 100 times when pH is decreased from 3 to 2. Hence

Rate becomes four times if concentration of H⁺ ion is doubled at constant Zn concentration.

$\left(\frac{\mathrm{dx}}{\mathrm{dt}}\right)=\mathrm{k}\left[\mathrm{Zn}\right]{\left[{\mathrm{H}}^{+}\right]}^2$

Rate is not affected if concentration of zinc is made four times and that of H⁺ ion is halved.

$\frac{\mathrm{dx}}{\mathrm{dt}}=\mathrm{k}[\mathrm{Zn}{]}^0{\left[{\mathrm{H}}^{+}\right]}^2$