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C6.1 Introduction to Electrolysis

GCSE Combined Science · AQA · Chapter C6: Electrolysis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: electrolytes, electrodes, the electrolysis of molten ionic compounds, and half equations. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain why an ionic compound conducts electricity only when it is molten or dissolved in water
  • Describe electrolysis using the words electrolyte, electrode, cathode and anode
  • Predict the products of electrolysing a molten binary ionic compound and write the overall equation
  • Describe oxidation and reduction at the electrodes in terms of electrons
  • Write balanced half equations for the reactions at each electrode (Higher tier)

Key words

  • Electrolysis: Breaking down an ionic compound into simpler substances using electricity.
  • Electrolyte: A molten or dissolved ionic compound. Its ions are free to move, so it conducts and is broken down.
  • Electrode: A conducting rod or plate that carries the current into or out of the electrolyte.
  • Cathode: The negative electrode. Positive ions (cations) move towards it.
  • Anode: The positive electrode. Negative ions (anions) move towards it.
  • Discharged: An ion is discharged when it gains or loses electrons at an electrode and becomes a neutral atom or molecule.
  • Oxidation / reduction: Oxidation is loss of electrons. Reduction is gain of electrons.
  • Half equation: (Higher tier) An equation for the change at one electrode, with electrons written as e−.

Why must the ions be free to move?

An ionic compound, such as potassium chloride, is a giant lattice of positive and negative ions held together by strong electrostatic forces. In the solid the ions can only vibrate about fixed positions, so the solid does not conduct electricity. When the compound is melted or dissolved in water, the lattice breaks up and the ions are free to move. Moving ions carry charge through the liquid, so it conducts. A liquid like this is called an electrolyte.

Key fact: In the metal wires the current is a flow of electrons. In the electrolyte the current is carried by moving ions. Electrons never travel through the electrolyte.

Not every liquid is an electrolyte. Most covalent substances, such as sugar solution or ethanol, contain no ions, so they cannot be electrolysed. Metals conduct as solids and liquids, but the current is carried by delocalised electrons and the metal is not broken down.

The electrolysis cell

Two electrodes dip into the electrolyte and are connected to a direct current (d.c.) supply. Because opposite charges attract, positive ions move to the negative electrode (cathode) and negative ions move to the positive electrode (anode). At the electrodes the ions are discharged: they gain or lose electrons and become atoms or molecules. These are the products.

  • Cathode; Charge negative; Ions attracted positive ions, e.g. metal ions such as Zn²⁺; What the ions do gain electrons; Type of change reduction
  • Anode; Charge positive; Ions attracted negative ions, e.g. Cl−, I−, O²−; What the ions do lose electrons; Type of change oxidation

Memory aid: OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). PANiC: Positive is Anode, Negative is Cathode.

The electrodes are usually inert, for example graphite (carbon) or platinum. Inert electrodes conduct but do not react with the electrolyte or with the products.

Electrolysing molten ionic compounds

A binary ionic compound is made from just two elements: one metal and one non-metal. When it is molten there is only one kind of positive ion and one kind of negative ion, so it always breaks down into its two elements. The metal forms at the cathode and the non-metal forms at the anode.

  • lead(II) bromide, PbBr₂; At the cathode (−) lead; At the anode (+) bromine; What you would see molten lead collects under the cathode; brown vapour at the anode
  • zinc iodide, ZnI₂; At the cathode (−) zinc; At the anode (+) iodine; What you would see grey metal on the cathode; purple vapour at the anode
  • sodium chloride, NaCl; At the cathode (−) sodium; At the anode (+) chlorine; What you would see molten sodium; pale green, choking gas
  • calcium oxide, CaO; At the cathode (−) calcium; At the anode (+) oxygen; What you would see grey metal; bubbles of colourless gas

Melting an ionic compound needs a lot of energy because the electrostatic forces between the ions are strong. Lead(II) bromide is often used in demonstrations because it melts at a fairly low temperature for an ionic compound. Its products are toxic, so it is done in a fume cupboard.

Worked example: overall equation with state symbols

Molten sodium iodide, NaI, is electrolysed. Write the word equation and the balanced symbol equation.
Products: sodium at the cathode and iodine at the anode. sodium iodide → sodium + iodine
Iodine is made of I₂ molecules, so two NaI are needed: 2NaI → 2Na + I₂.
The cell is above the melting point of sodium, so sodium forms as a liquid. Iodine forms as a vapour (gas).

Answer: 2NaI(l) → 2Na(l) + I₂(g)

Exam tip: Write (l) for a molten electrolyte and (aq) for a dissolved one. Halogens and oxygen are made of diatomic molecules: Cl₂, Br₂, I₂, O₂.

Half equations (Higher tier)

A half equation shows what happens at one electrode. Electrons are written as e−. The atoms must balance and the total charge on each side must be the same.

  • Cathode: positive ions gain electrons, so the electrons go on the left, e.g. Zn²⁺ + 2e− → Zn
  • Anode: negative ions lose electrons, so the electrons go on the right, e.g. 2I− → I₂ + 2e−
  • The anode equation can also be written with the electrons taken away on the left: 2I− − 2e− → I₂. Both forms are accepted.
  • Oxide ions: 2O²− → O₂ + 4e− (each O²− loses two electrons).

Worked example: molten calcium bromide, CaBr₂

Ions present: Ca²⁺ and Br−.
Cathode: each Ca²⁺ needs two electrons: Ca²⁺ + 2e− → Ca. Charge: +2 − 2 = 0 on the left, 0 on the right.
Anode: bromine is Br₂, so start with two bromide ions: 2Br− → Br₂ + 2e−. Charge: −2 on each side.
Calcium ions gain electrons, so they are reduced. Bromide ions lose electrons, so they are oxidised.

Answer: Ca²⁺ + 2e− → Ca and 2Br− → Br₂ + 2e−

Common mistakes

  • Saying that electrons flow through the electrolyte. Ions move through the electrolyte; electrons move through the wires.
  • Mixing up the electrodes. The cathode is negative and attracts positive ions.
  • Saying a solid ionic compound has “no ions”. It does have ions, but they cannot move.
  • Writing Br or Cl as a product. Halogens are diatomic: Br₂, Cl₂.
  • Putting the electrons on the wrong side of a half equation. Check that the charges balance.

Revision questions

Try these, then check your answers below each one.

Question 1. Why does solid potassium bromide not conduct electricity? (Choose from: Its ions are held in fixed positions and cannot move, It contains no ions, Its electrons are delocalised, Its molecules are too large to move.)

Answer: Its ions are held in fixed positions and cannot move

Working: Potassium bromide is ionic. In the solid the ions are fixed in the lattice, so no charge can flow. Melting or dissolving lets the ions move.

Question 2. Molten zinc iodide is electrolysed using inert electrodes. Name the product formed at the anode.

Answer: iodine

Working: Iodide ions (I⁻) move to the anode, lose electrons and form iodine, I₂. Zinc forms at the cathode.

Question 3. True or false? In the electrolyte, the electric current is carried by moving ions, not by electrons.

Answer: True

Working: True. Electrons flow in the wires; in the electrolyte the charge is carried by ions moving towards the electrodes.

Related lessons

Continue in the full course

This lesson is section C6.1 (AGCSC2301) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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