Question

Given that $E_{Ni^{+2}/Ni}^{\circ}$=-0.25 V, $E_{Zn^{+2}/Zn}^{\circ}$=-0.76 V, $E_{Cu^{+2}/Cu}^{\circ}$=0.34 V, $E_{Ag^{+}/Ag}^{\circ}$=0.80 V Which of the following redox processes will not take place in specified direction?

• A. $Ni^{2+}$(aq) + Cu(s)$\rightarrow$ Ni(s) + $Cu^{2+}$ (aq)
• B. Cu(s) + 2$Ag^{+}$(aq) $\rightarrow$ $Cu^{2+}$(aq) + 2Ag(s)
• C. Cu(s) + 2$H^{+}$(aq) $\rightarrow$ $Cu^{2+}$(aq) + $H_2$(g)
• D. Zn(s) + 2$H^{+}$(aq) $\rightarrow$ $Zn^{2+}$(aq) + $H_2$(g)

Answer 1

Answer: (A), (C)

A) $Ni^{2+}$(aq) + Cu(s)$\rightarrow$ Ni(s) + $Cu^{2+}$ (aq) C) Cu(s) + 2$H^{+}$(aq) $\rightarrow$ $Cu^{2+}$(aq) + $H_2$(g)

Answer 2

To determine if a redox reaction is spontaneous, calculate the standard cell potential ($E^\circ_{cell}$) using $E^\circ_{cell} = E^\circ_{reduction, cathode} - E^\circ_{reduction, anode}$. A positive $E^\circ_{cell}$ indicates a spontaneous reaction.

For A) $Ni^{2+}$ + Cu $\rightarrow$ Ni + $Cu^{2+}$, $E^\circ_{cell} = E^\circ_{Ni^{2+}/Ni} - E^\circ_{Cu^{2+}/Cu} = -0.25 \text{ V} - 0.34 \text{ V} = -0.59 \text{ V}$. (Not spontaneous)

For B) Cu + 2$Ag^{+}$ $\rightarrow$ $Cu^{2+}$ + 2Ag, $E^\circ_{cell} = E^\circ_{Ag^{+}/Ag} - E^\circ_{Cu^{2+}/Cu} = 0.80 \text{ V} - 0.34 \text{ V} = +0.46 \text{ V}$. (Spontaneous)

For C) Cu + 2$H^{+}$ $\rightarrow$ $Cu^{2+}$ + $H_2$, $E^\circ_{cell} = E^\circ_{H^{+}/H_2} - E^\circ_{Cu^{2+}/Cu} = 0.00 \text{ V} - 0.34 \text{ V} = -0.34 \text{ V}$. (Not spontaneous)

For D) Zn + 2$H^{+}$ $\rightarrow$ $Zn^{2+}$ + $H_2$, $E^\circ_{cell} = E^\circ_{H^{+}/H_2} - E^\circ_{Zn^{2+}/Zn} = 0.00 \text{ V} - (-0.76 \text{ V}) = +0.76 \text{ V}$. (Spontaneous)

Reactions A and C have negative $E^\circ_{cell}$ and thus will not take place.