Electrochemistry

4.1 Electrolysis

An ionic compound only conducts when its ions are free to move, which is when it is molten or dissolved in water. A solid ionic compound does not conduct.

What happens during electrolysis

• In the external circuit, electrons flow through the wires from the negative terminal of the power supply to the cathode, and from the anode back to the positive terminal.
• In the electrolyte, ions carry the charge. Positive ions (cations) move to the cathode and negative ions (anions) move to the anode.
• At the cathode, positive ions gain electrons. This is reduction.
• At the anode, negative ions lose electrons. This is oxidation.

Metals or hydrogen are formed at the cathode, and non-metals other than hydrogen are formed at the anode.

Predicting the products

Molten compounds (a binary compound, made of two elements). The metal forms at the cathode and the non-metal forms at the anode.

Aqueous solutions also contain $\ce{H+}$ and $\ce{OH-}$ ions from the water, so there is a choice of ion at each electrode.

The cases to know

Notes on these cases.

• In concentrated sodium chloride, sodium is more reactive than hydrogen, so hydrogen forms, not sodium. The solution left behind contains $\ce{Na+}$ and $\ce{OH-}$ ions: it is sodium hydroxide, an alkali.
• In dilute sulfuric acid, the net effect is that water is decomposed. About twice as much hydrogen as oxygen is made, by volume ($2$ volumes of $\ce{H2}$ to $1$ of $\ce{O2}$), and the acid becomes more concentrated.
• In copper(II) sulfate with carbon electrodes, the blue colour fades as copper ions are removed from the solution, and the solution becomes acidic.

Copper(II) sulfate with copper electrodes. Copper is not inert, so the anode takes part:

• Anode: $\ce{Cu -> Cu^2+ + 2e-}$. The copper anode dissolves and gets smaller.
• Cathode: $\ce{Cu^2+ + 2e- -> Cu}$. Copper is deposited and the cathode gets bigger.
• The solution stays blue because the concentration of $\ce{Cu^2+}$ does not change.

This is also how copper is purified: impure copper is the anode and pure copper builds up on the cathode.

Zinc chloride is a binary compound. Zinc forms at the cathode and chlorine forms at the anode.

$\text{cathode: } \ce{Zn^2+ + 2e- -> Zn} \qquad \text{anode: } \ce{2Cl- -> Cl2 + 2e-}$

At the cathode, hydrogen is formed. The solution is dilute, so no halogen forms at the anode, and oxygen is formed from the hydroxide ions.

Half-equations

A half-equation shows what happens at one electrode. It must balance in atoms and charge.

• At the cathode, electrons are on the left (they are gained): $\ce{Cu^2+ + 2e- -> Cu}$.
• At the anode, electrons are on the right (they are lost): $\ce{2Cl- -> Cl2 + 2e-}$.

The number of electrons lost at the anode equals the number gained at the cathode.

Electroplating

Electroplating coats a metal object with a thin layer of another metal. It is done to improve the appearance of the object, and to give resistance to corrosion, for example chromium plating on steel or tin plating on steel cans.

• The object to be plated is the cathode (the negative electrode).
• The anode is a bar of the plating metal.
• The electrolyte is a solution containing ions of the plating metal.

For silver plating: the electrolyte is silver nitrate solution. At the cathode, $\ce{Ag+ + e- -> Ag}$ (silver is deposited on the object). At the anode, $\ce{Ag -> Ag+ + e-}$ (the silver dissolves, so the solution keeps its concentration).

4.2 Hydrogen–oxygen fuel cells

$\ce{2H2(g) + O2(g) -> 2H2O(l)}$

The hydrogen is supplied continuously, and so is the oxygen (from the air). Unlike a battery, the cell does not run down while the fuel is supplied. The reaction directly makes electrical energy, which powers an electric motor in a vehicle.

Fuel cell compared with a petrol engine in a vehicle

Advantage: the only product is water, so it does not produce carbon dioxide or other pollutants. Disadvantage: hydrogen is flammable and difficult to store, and there are few places to refuel.