The $E^{0}_{Red}$ values for $P, Q, R$ and $S$ are $-2.90 \, V, +0.34 \, V, +1.20 \, V$ and $-0.76 \, V$ respectively. The decreasing order of their reactivity is:

When does a cell reaction occur spontaneously?

The standard reduction potential for $Mg^{2+}/Mg$ is $-2.37 \ V$ and for $Cu^{2+}/Cu$ is $0.337 \ V$. The $E^{\circ}_{cell}$ for the following reaction is $Mg + Cu^{2+} \longrightarrow Mg^{2+} + Cu$

$A$ cell is constructed by dipping a copper rod in $1\,M\,CuSO_4$ solution and a nickel rod in $1\,M\,NiSO_4$ solution. The standard reduction potentials of copper and nickel electrodes are $+0.34\,V$ and $-0.25\,V$ respectively. Calculate the $EMF$ of the cell. (in $,V$)

The standard electrode potential $(E^o)$ of $Cu^{2+}/Cu$ is $+0.34 \, V$,while that of $Zn^{2+}/Zn$ is $-0.76 \, V$. Explain the reason for this difference.

Standard electrode potential values,$E^{\Theta}$ for $Al^{3+}/Al$ is $-1.66 \ V$ and that of $Tl^{3+}/Tl$ is $+1.26 \ V$. Predict the formation of $M^{3+}$ ion in solution and compare the electropositive character of the two metals.