Question

The electrode with reaction
$C{r}_2{O}_7^{2-}\left(aq\right)+14H^{+}\left(aq\right)+6e^{-}\to 2C{r}^{3+}\left(aq\right)+7H_{2}O\left(l\right)$
can be represented as:
[If needed, use platinum as an inert electrode]

• A. $Pt\left(s\right)|H^{+}\left(aq\right),C{r}_2{O}_7^{2-}\left(aq\right)$
• B. $H^{+}\left(aq\right),C{r}_2{O}_7^{2-}\left(aq\right),C{r}^{3+}\left(aq\right)|Pt\left(s\right)$
• C. $Pt\left(s\right)|H^{+}\left(aq\right)||C{r}_2{O}_7^{2-}\left(aq\right),C{r}^{3+}\left(aq\right)$
• D. $Pt\left(s\right),H_2\left(g\right)|H^{+}\left(aq\right),C{r}_2{O}_7^{2-}$

Answer 1

The correct option is B $H^{+}\left(aq\right),C{r}_2{O}_7^{2-}\left(aq\right),C{r}^{3+}\left(aq\right)|Pt\left(s\right)$

It is a redox electrode.
Here,
$C{r}_2{O}_7^{-2}$ is reduce to $2C{r}^{3+}$, so it must be represented at cathode.
It is a single electrode. Thus, there is no salt bridge.

Rules for Cell representation:
1. Anode half cell should be on left side and cathode half cell should be on right side.

2. Separation of two phase should be shown by vertical line $(\mid )$. This line indicates phase boundary.

3. If various material present in the same phase, it should be shown together with commas (,).

4. The salt bridge is represented by double slash $(\mid \mid )$ between the two half cells.

5. The significant features of the substance like pressure of a gas, concentration of ions, etc. should be indicated in brackets immediately after writing the substance.

6. For gas electrode, gas should be indicated after electrode for anode and should be indicated before electrode for cathode.

So, the cell representation is:
$H^{+}\left(aq\right),C{r}_2{O}_7^{-2}\left(aq\right),C{r}^{3+}\left(aq\right)|Pt\left(s\right)$