Redox couples and electrode processes

Electrons that can be made to work

Earlier in this chapter you saw zinc push copper out of a copper sulphate solution. The electrons jump straight from zinc atoms to copper ions, and the energy leaves as heat.

Could we make the same electrons run through a wire first? If they did, we could light a bulb or turn a motor with them. This is exactly what a battery does.

Two half-reactions, two beakers

The reaction has two halves:

• Oxidation: $\text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^-$
• Reduction: $\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s)$

Add them and the electrons cancel. You get $\text{Zn}(s) + \text{Cu}^{2+}(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{Cu}(s)$.

A metal rod dipped in a solution of its own ions is a half-cell. The oxidised form and the reduced form together make a redox couple. We write the oxidised form first: $\text{Zn}^{2+}/\text{Zn}$ and $\text{Cu}^{2+}/\text{Cu}$.

Now keep the two half-cells in separate beakers. Copper ions can no longer reach the zinc rod. The electrons have only one way to go: through a wire outside the beakers.

The Daniell cell

The zinc rod stands in zinc sulphate solution. The copper rod stands in copper sulphate solution. Both solutions are $1\ \text{mol/L}$. A wire joins the rods, and a voltmeter in the wire reads $1.10\ \text{V}$ at $298\ \text{K}$.

At the zinc rod, zinc atoms give up electrons and become $\text{Zn}^{2+}$ ions. The ions enter the solution, so the rod slowly gets thinner. The electrons travel along the wire to the copper rod.

At the copper rod, $\text{Cu}^{2+}$ ions take the electrons and become copper atoms. They settle on the rod, so it slowly gets thicker. The blue colour of the copper solution fades.

Why the cell needs a salt bridge

Look at what the two beakers would do without any link. The zinc beaker fills with positive $\text{Zn}^{2+}$ ions. The copper beaker loses positive $\text{Cu}^{2+}$ ions and is left with extra negative sulphate ions. Both solutions would stop being neutral, and the electrons would stop flowing almost at once.

A salt bridge prevents this. It is a U-shaped tube filled with a gel that holds a salt such as potassium nitrate. The gel stops the liquid spilling. The ions of the salt move into the beakers:

• Negative ions drift towards the zinc beaker.
• Positive ions drift towards the copper beaker.

Each solution stays neutral, and the circuit is complete. Electrons carry the current in the wire. Ions carry it in the solutions and in the bridge. No electrons travel through the bridge.

The salt must not react with anything in the cell. Potassium chloride would be a poor choice for a cell that holds silver ions, because silver chloride would form.

Anode and cathode

Two words name the electrodes by what happens there:

• The anode is where oxidation happens.
• The cathode is where reduction happens.

A memory aid: anode and oxidation both begin with a vowel. Cathode and reduction both begin with a consonant.

In the Daniell cell, zinc is the anode. It sends electrons into the wire, so it is the negative electrode. Copper is the cathode. It takes electrons from the wire, so it is the positive electrode.

The electrons flow in the wire from the anode to the cathode, which is from zinc to copper. The conventional current is the other way round, from copper to zinc.

The cell diagram

Chemists write the whole cell in one line:

• $\text{Zn}(s) \mid \text{Zn}^{2+}(aq) \parallel \text{Cu}^{2+}(aq) \mid \text{Cu}(s)$

The rules are short:

• The anode goes on the left and the cathode on the right.
• A single bar $\mid$ is a boundary between two phases, such as a rod and its solution.
• A double bar $\parallel$ is the salt bridge.

So you can read the diagram from left to right: oxidation on the left, reduction on the right.

The sim uses $Q = It$ and the Faraday constant $F = 96500\ \text{C/mol}$, the charge on one mole of electrons. Each zinc atom gives up two electrons, so the moles of zinc are half the moles of electrons.

Practise with new numbers

Try it in Python

The program follows the steps of the sim: charge, electrons, then the mass of each rod.

Common mistakes

• Sending electrons through the salt bridge. Only ions move there.
• Calling the anode positive in every cell. In a cell that makes electricity, such as the Daniell cell, the anode is the negative electrode.
• Writing the cathode on the left of the diagram. The anode comes first.
• Using one electron per zinc atom. Zinc forms $\text{Zn}^{2+}$, so each atom gives two.

Remember these

• A half-cell is a metal in a solution of its ions. A redox couple is the oxidised and reduced forms together.
• Anode: oxidation, negative. Cathode: reduction, positive.
• Electrons go from anode to cathode in the wire. Ions carry the charge through the salt bridge.
• Cell diagram: anode $\mid$ its ions $\parallel$ ions of the cathode $\mid$ cathode.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In the Daniell cell, which rod gets thinner, and why?

2. What flows through the salt bridge?

3. Write the cell diagram for a cell with a magnesium anode and a silver cathode.

4. Why does the current stop within seconds if the salt bridge is taken out?