Write net equations for the spontaneous redox reactions that occur during the following or NONE if there is no extensive reaction. Use the lowest possible coefficients. Include states-of-matter under the given conditions in your answer.

My effort:

Chromium metal is added to a solution of $\ce{Mg^2+}$

\begin{align} \ce{2Cr^3+ (aq) + 6e- &-> 2Cr(s)}\\ \ce{3Mg(s) &-> 3Mg^2+ (aq) + 6e-} \end{align}

The net equation is $$\ce{2Cr^3+ (aq) + 3Mg(s) -> 2Cr(s) + 3Mg^2+ (aq)}$$

Standard reduction potential of $\ce{Cr}$ is $\pu{-0.74 V}$ and the standard reduction potential of $\ce{Mg}$ is $\pu{-2.36 V}$.

$\ce{Cr}$ reaction is cathode reaction and therefore it is $\ce{Cr^3+}$ getting reduced, $\ce{Mg}$ is anode reaction and therefore it is $$\ce{Mg(s) -> Mg^2+ (aq) + 2e^-}.$$

$$E^\circ_\mathrm{cell} = \pu{-0.74 V} - (\pu{-2.36 V}) = \pu{1.62 V}$$

Yet apparently my homework here says that that is wrong. Does anyone see anything wrong with how I am going about this problem?

  • $\begingroup$ It's the magnesium ion that is being converted into magnesium metal. $\ce{Mg^{2+}}$ is being reduced at the cathode. $\endgroup$ – Yunfei Ma Apr 10 '16 at 15:17
  • $\begingroup$ In other words, all your equations are written backwards. $\endgroup$ – Ivan Neretin Mar 14 at 15:24

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