Answer
a) This redox reaction does not occur spontaneously in the forward direction.
b) This redox reaction occurs spontaneously in the forward direction.
c)This redox reaction does not occur spontaneously in the forward direction.
d) This redox reaction occurs spontaneously in the forward direction.
Work Step by Step
Metal Activity series:
$Li>K>Ca>Na>Mg>Al>Mn>Zn>Cr>Fe>Ni>Sn>Pb>H_{2}>Cu>Ag>Au$
Activity decreases from left to right. More active metal will be oxidized when the reaction occurs spontaneously.
Now, see each reaction.
a) From Metal activity series we can see that 'Ca' is more active than 'Zn'. So, Ca will be oxidized into $Ca^{2+}$. On the other hand, Zn is less active than Ca, so $Zn^{2+}$ will be reduced to Zn. But in the reaction we see that $Ca_{2+}$ is reduced to Ca and Zn is oxidized to $Zn^{2+}$. So, this reaction does not occur spontaneously in the forward direction.
b) From metal activity series it is seen that Ni is more active than Ag. So, Ni will be oxidized to $Ni^{2+}$ and $Ag^{+}$ will reduce to Ag. So, the written reaction occurs spontaneously in the forward reaction.
c) From Metal activity series we can see that 'Mn' is more active than 'Fe'. So, Mn will be oxidized into $Mn^{2+}$. On the other hand, Fe is less active than Mn, so $Fe^{2+}$ will be reduced to Fe. But in the reaction we see that $Mn_{2+}$ is reduced to Mn and Fe is oxidized to $Fe^{2+}$. So, this reaction does not occur spontaneously in the forward direction.
d) From metal activity series it is seen that Al is more active than Pb. So, Al will be oxidized to $Al^{3+}$ and $Pb^{2+}$ will reduce to Pb. So, the written reaction occurs spontaneously in the forward reaction.
