The standard reduction potentials for ${\mathrm{Zn}}^{2+}\left|\mathrm{Zn},{\mathrm{Ni}}^{2+}\right|\mathrm{Ni}$ and ${\mathrm{Fe}}^{2+}\mid \mathrm{Fe}$ are -0.76 V, - 0.23 V and - 0.44 V, respectively. The reaction $\mathrm{X}+{\mathrm{Y}}^{2+}\to {\mathrm{X}}^{2+}+\mathrm{Y}$ will be spontaneous when

For the reaction $\mathrm{X}+{\mathrm{Y}}^{2+}\to {\mathrm{X}}^{2+}+\mathrm{Y}$ to occur spontaneously, the reduction potential of ${\mathrm{Y}}^{2+}$ should be larger than that of ${\mathrm{X}}^{2+}$. This holds good provided ${\mathrm{Y}}^{2+}$ is ${\mathrm{Ni}}^{2+}$ and  ${\mathrm{X}}^{2+}$ is ${\mathrm{Zn}}^{2+}$. Thus, X and Y are Zn and Ni, respectively.