Copper electrodes change what happens at each electrode
Definition
Inert electrode
An electrode that conducts but does not react with the electrolyte or with the products, such as graphite or platinum.
- Here both electrodes are made of copper rather than graphite.
- Copper atoms at the anode lose electrons and enter the solution as Cu2+\text{Cu}^{2+}Cu2+.
- Copper ions at the cathode gain electrons and are deposited as solid copper.
- The anode therefore loses mass while the cathode gains very nearly the same amount.
- The blue colour barely changes, because copper ions removed at the cathode are replaced by those dissolving from the anode.
- No oxygen is produced, because the copper anode reacts in place of the sulfate or hydroxide ions.
Example
- At the anode: copper atoms lose electrons and dissolve, so the anode gets thinner.
- At the cathode: copper ions gain electrons and are deposited, so the cathode gets thicker.
Practical
- Method: electrolyse copper sulfate solution first with inert graphite electrodes, then with copper electrodes, and compare what happens.
- With inert electrodes copper is deposited on the cathode as a pink-brown layer, and oxygen bubbles off at the anode because sulfate ions are not discharged.
- The blue fades in that run, as copper ions leave the solution and are not replaced.
- With copper electrodes the anode loses mass as copper atoms dissolve, and the cathode gains very nearly the same mass as copper is deposited on it.
- Weighing: clean both electrodes with emery paper, dry and weigh them before and after, then compare the anode's loss with the cathode's gain.
- The blue stays in that run, because the copper ions removed at the cathode are replaced by those dissolving from the anode.
Purifying copper uses an impure anode and a pure cathode
- The anode is a block of impure copper and the cathode a thin sheet of pure copper.
- The electrolyte is copper sulfate solution.
- Copper atoms leave the impure anode as ions and travel through the solution.
- Those ions are deposited as pure copper on the cathode.
- The mass lost by the anode is close to the mass gained by the cathode, with the impurities accounting for the difference.
- Impurities less reactive than copper, such as silver and gold, do not dissolve and collect below the anode as anode sludge.
- Impurities more reactive than copper, such as iron and zinc, do dissolve but stay in solution rather than being deposited.
- The copper on the cathode is therefore far purer than the anode it came from.
Common Mistake
- Charge travels the external circuit as electrons, while the copper itself crosses the cell as ions.
- Sulfate ions form nothing here, because the copper anode is easier to oxidise than they are.
Half equations and the electron bookkeeping
Definition
Oxidation
The loss of electrons by a substance.
Definition
Reduction
The gain of electrons by a substance.
- Anode with copper electrodes: Cu(s)→Cu2+(aq)+2e−\text{Cu}(s) \rightarrow \text{Cu}^{2+}(aq) + 2\text{e}^{-}Cu(s)→Cu2+(aq)+2e−, an oxidation.
- Cathode: Cu2+(aq)+2e−→Cu(s)\text{Cu}^{2+}(aq) + 2\text{e}^{-} \rightarrow \text{Cu}(s)Cu2+(aq)+2e−→Cu(s), a reduction.
- Anode with inert electrodes: 4OH−→O2+2H2O+4e−4\text{OH}^{-} \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^{-}4OH−→O2+2H2O+4e−.
- Two electrons leave each copper atom at the anode, and two arrive at each copper ion at the cathode.
- The electrons released at the anode are precisely the ones supplied at the cathode.
- That balance is why the concentration of the solution stays almost constant in the copper-electrode cell.
Exam technique
- Naming the electrode alongside each half equation is what shows which is oxidation and which is reduction.
- The copper-electrode cell transfers copper rather than decomposing the electrolyte, so the solution ends up unchanged.
- Inert and copper electrodes give different anode products, so the electrode material is worth reading carefully.
Self review
- Which ion forms the copper deposited at the cathode?
- What happens to the copper atoms at a copper anode?
- Why does the solution's concentration stay nearly constant with copper electrodes?
- Where do the less reactive impurities collect?
- What forms at the anode when inert electrodes are used instead?
