Electrolysis – SS2 Chemistry Lesson Note (First Term)

Electrolysis is the chemical decomposition of a compound (an electrolyte) by passing direct current through its solution or its molten form.

Behavioural objectives

1. Define electrolysis, electrolyte, electrode, anode, cathode and electrolytic cell.
2. State what happens at the anode and at the cathode of an electrolytic cell.
3. Describe the electrolysis of a molten electrolyte such as NaCl.
4. List the ions present when an electrolyte is dissolved in water.
5. Explain why ions compete to be discharged during the electrolysis of a solution.
6. Name three uses of electrolysis.

What is electrolysis?

Electrolysis is the breaking down of a compound by an electric current passed through it in liquid form.

• Decomposition by heat: some substances break down chemically when heated.
• Decomposition by electricity: an electric current can also break down some compounds.
• Ion movement: positive ions go to the cathode and negative ions go to the anode, where they are discharged.
• Current used: direct current (d.c.) from a battery or d.c. supply.

What are the key terms in electrolysis?

The key terms name the substance, the electrodes and the changes that take place in an electrolytic cell.

• Electrolysis: the chemical decomposition of a compound (electrolyte) by direct current passed through its solution or its molten form.
• Electrolyte: a compound that conducts electricity and is decomposed in the process.
• Electrolyte condition: it must be liquid, either molten or as an aqueous solution.
• Non-electrolyte: a substance that does not conduct electricity in this way.
• Electrodes: conductors (wires, rods or plates) through which current enters or leaves the electrolyte.
• Anode: the positive electrode, joined to the positive terminal of the supply. Electrons leave the electrolyte here.
• Cathode: the negative electrode, joined to the negative terminal of the supply. Electrons enter the electrolyte here.
• Electrolytic cell: two electrodes placed in an electrolyte, used for electrolysis.
• Discharge: the change of an ion into a neutral atom or molecule by gaining or losing electrons.

What are electrolytes and non-electrolytes?

An electrolyte conducts electricity and is decomposed, while a non-electrolyte does not conduct in this way.

• Metals and graphite: they conduct electricity but are not electrolytes, because they are not decomposed.

What are strong and weak electrolytes?

Electrolytes are of two kinds, strong and weak, depending on how fully they form ions in solution.

• Strong electrolyte: it forms many ions and conducts well, as in sodium chloride solution, hydrochloric acid and sodium hydroxide solution.
• Weak electrolyte: it forms few ions and conducts poorly, as in ethanoic acid and aqueous ammonia.

What are inert and active electrodes?

Electrodes are either inert or active, depending on whether they take part in the reaction.

• Inert electrode: it does not react, as in carbon (graphite) and platinum.
• Active electrode: it takes part in the reaction, as in copper in copper(II) sulphate solution.

What happens at the anode and the cathode?

Oxidation (loss of electrons) occurs at the anode, and reduction (gain of electrons) occurs at the cathode.

• Memory aid: “An Ox” (anode, oxidation) and “Red Cat” (reduction, cathode).

How does electrolysis take place?

Electrolysis takes place in two forms: the electrolyte is either molten or dissolved in water.

• Molten form: the electrolyte is melted (fused) and placed in the cell.
• Solution form: the electrolyte is dissolved in water and placed in the cell.

How is a molten (fused) electrolyte electrolysed?

A molten electrolyte contains only one kind of positive ion and one kind of negative ion, so no ions compete.

• Ions present: only one kind of positive ion and one kind of negative ion.
• Example: molten NaCl contains Na⁺ and Cl⁻ ions only.
• Competition: there is none, because each electrode attracts only one kind of ion.

1. Na⁺ ions move to the cathode.
2. Each Na⁺ accepts one electron and becomes a neutral Na atom: Na⁺(l) + e⁻ → Na(l).
3. Cl⁻ ions move to the anode.
4. Each Cl⁻ gives up one electron and becomes a Cl atom: Cl⁻ → Cl + e⁻.
5. Two Cl atoms pair up to form chlorine gas: Cl + Cl → Cl₂(g).

• Overall change: 2NaCl(l) → 2Na(l) + Cl₂(g).

How is a solution of an electrolyte electrolysed?

A solution contains ions from the electrolyte and ions from water, so ions compete to be discharged.

• Ions from the electrolyte: for NaCl solution, Na⁺ and Cl⁻.
• Ions from the solvent: water gives H⁺ and OH⁻ in small amounts.
• Total ions in NaCl solution: four, namely Na⁺, H⁺, Cl⁻ and OH⁻.
• Movement: Na⁺ and H⁺ go to the cathode. Cl⁻ and OH⁻ go to the anode.
• Competition: the ions at each electrode compete to be discharged.
• Products: they depend on which ion is discharged, the one from the electrolyte or the one from water.

Which ion is discharged first?

Preferential discharge is the discharge of one ion in place of another at the same electrode.

What factors decide preferential discharge?

Three factors decide which ion is discharged: its position in the electrochemical series, its concentration and the nature of the electrode.

• Position in the electrochemical series: the ion lower in the series is discharged first.
• Concentration: an ion in high concentration is more likely to be discharged.
• Nature of the electrode: an active electrode such as copper may itself take part in the reaction.

What is the order of discharge?

• Cathode rule: H⁺ is discharged in preference to Na⁺, K⁺, Ca²⁺, Mg²⁺ and Al³⁺.
• Anode rule: OH⁻ is discharged in preference to SO₄²⁻ and NO₃⁻.
• Concentration effect: in concentrated chloride solution, Cl⁻ is discharged ahead of OH⁻.

What are some worked examples of electrolysis of solutions?

The products depend on the ions present, their concentration and the electrodes used.

Concentrated sodium chloride (brine), inert electrodes

• Cathode: H⁺ is discharged, giving hydrogen gas: 2H⁺ + 2e⁻ → H₂.
• Anode: Cl⁻ is discharged, giving chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻.
• Left in solution: Na⁺ and OH⁻ ions, which form sodium hydroxide solution.

Dilute sulphuric acid, platinum electrodes

• Cathode: hydrogen gas: 2H⁺ + 2e⁻ → H₂.
• Anode: oxygen gas: 4OH⁻ → 2H₂O + O₂ + 4e⁻.
• Gas volumes: hydrogen and oxygen are collected in the ratio 2 : 1.
• Net effect: water is decomposed and the acid becomes more concentrated.

Copper(II) sulphate solution

What are the uses of electrolysis?

Electrolysis is used to coat, purify and extract metals and to make chemicals.

• Electroplating: coating an object with a thin layer of metal, such as chromium or silver. The object is the cathode.
• Purifying copper: impure copper is the anode and pure copper is the cathode.
• Extracting metals: sodium from molten NaCl and aluminium from molten aluminium oxide in cryolite.
• Making chemicals: chlorine, hydrogen and sodium hydroxide from brine.

Evaluation

1. Define electrolysis and electrolyte.
2. State the difference between the anode and the cathode.
3. Which process, oxidation or reduction, occurs at the anode?
4. Why is there no competition for discharge when molten NaCl is electrolysed?
5. List the four ions present in a solution of NaCl in water.

Assignment

1. Compare the electrolysis of molten NaCl with that of concentrated NaCl solution using inert electrodes. State the ions present, the product at each electrode and the equation at each electrode. Explain why the solution is more complicated.