This question is about the development of sodium-based cells. The value of E⊖ E^\ominus\,E⊖ for sodium suggests that a sodium cell could have a large electromotive force (EMF).
The table below shows some standard electrode potential data:
| Electrode half-reaction | E⊖/VE^\ominus / \text{V}E⊖/V |
|---|---|
| Na+(aq)+e−→Na(s)\text{Na}^+(\text{aq}) + \text{e}^- \rightarrow \text{Na}(\text{s})Na+(aq)+e−→Na(s) | -2.71 |
| 2H2O(l)+2e−→H2g+2OH−(aq)2\text{H}_2\text{O}(\text{l}) + 2\text{e}^- \rightarrow \text{H}_2\text{g} + 2\text{OH}^-(\text{aq})2H2O(l)+2e−→H2g+2OH−(aq) | -0.83 |
| 12Br2(l)+e−→Br−(aq)\displaystyle \frac{1}{2}\text{Br}_2(\text{l}) + \text{e}^- \rightarrow \text{Br}^-(\text{aq})21Br2(l)+e−→Br−(aq) | +1.07 |
Use data in the table to explain why an aqueous electrolyte is not used for a sodium cell.
Sodium-bromine reactant cells have been proposed as energy storage systems. Sodium bromide is the final product of the cell reaction. Use the data in the table to calculate the standard cell EMF of a standard sodium-bromine cell.
An EMF value measured for a non-standard commercial sodium-bromine cell is 3.20 V. Suggest why this value is different from the value calculated in part 2.
In some sodium cells, sodium hexafluorophosphate (NaPF6\text{NaPF}_6NaPF6) is used as the electrolyte. Deduce the oxidation state of phosphorus in NaPF6\text{NaPF}_6NaPF6.
In other sodium-ion cells, sodium cobalt oxide (NaCoO2\text{NaCoO}_2NaCoO2) electrodes and sodium electrodes are used. Give an equation for the reaction that occurs at the positive sodium cobalt oxide electrode during discharge.
Give an equation for the reaction that occurs at the negative sodium electrode during discharge.
