The process of electrolysis — AQA GCSE Chemistry
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The process of electrolysis explained
The process of electrolysis covers 7 specification points, including When an ionic compound is melted or dissolved… and These liquids and solutions are able to….
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When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution.
In a solid ionic compound, the ions are held in a giant lattice and cannot move, so it does not conduct. Melting or dissolving in water breaks down this lattice and releases the ions. Once released, the ions are free to move about within the liquid or solution. For example, solid sodium chloride does not conduct, but molten sodium chloride and sodium chloride solution both contain mobile Na⁺ and Cl⁻ ions. The key idea is that mobile ions, not electrons, carry charge in these liquids and solutions. This explains why an ionic compound must be molten or dissolved before it can be electrolysed.
These liquids and solutions are able to conduct electricity and are called electrolytes.
A liquid or solution that contains mobile ions can conduct electricity and is called an electrolyte. The mobile ions carry charge through the electrolyte, so a current flows. For example, molten lead bromide and copper sulfate solution are electrolytes because they contain mobile ions. In contrast, pure water and solid ionic compounds are not electrolytes in this sense because they do not contain mobile ions. The term electrolyte therefore describes the substance being electrolysed, not the electrodes or the wires. Recognising an electrolyte means checking that ions are free to move within the liquid or solution.
Passing an electric current through electrolytes causes the ions to move to the electrodes.
When a direct current is passed through an electrolyte, the mobile ions move towards the electrodes. Positively charged ions are attracted to the negative electrode, and negatively charged ions are attracted to the positive electrode. This movement of ions through the electrolyte completes the circuit and allows charge to flow. For example, in molten sodium chloride, Na⁺ ions move towards the negative electrode and Cl⁻ ions move towards the positive electrode. The ions then gain or lose electrons at the electrodes, producing new substances. This process is electrolysis, and it requires a direct current supply and mobile ions.
Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode).
During electrolysis, oppositely charged ions are attracted to the electrodes. Positively charged ions, called cations, move to the negative electrode, which is the cathode. Negatively charged ions, called anions, move to the positive electrode, which is the anode. For example, in molten lead bromide, Pb²⁺ ions move to the cathode and Br⁻ ions move to the anode. At the electrodes, the ions gain or lose electrons and form neutral products. Remembering that cations go to the cathode and anions go to the anode helps avoid confusion. The names cathode and anode are fixed by the polarity of the electrodes, not by the direction of ion charge.
Ions are discharged at the electrodes producing elements.
During electrolysis, ions in an electrolyte move to the electrode of opposite charge. Positive ions (cations) are attracted to the negative cathode, where they gain electrons and are discharged as neutral atoms. Negative ions (anions) are attracted to the positive anode, where they lose electrons and are discharged as neutral atoms. For example, in molten lead bromide, Pb²⁺ ions gain electrons at the cathode to form lead metal, while Br⁻ ions lose electrons at the anode to form bromine molecules. The discharge of ions produces elements because the neutral atoms formed are the elements themselves. This process requires a direct current supply. Inert electrodes are often used so they do not react, but reactive electrodes are sometimes used, such as carbon anodes in aluminium extraction.
This process is called electrolysis.
Electrolysis is the process of breaking down an ionic compound into its elements using an electric current. It occurs when a direct current is passed through an electrolyte, which is a molten or aqueous ionic compound containing mobile ions. The ions move to the electrodes: positive ions to the cathode and negative ions to the anode. At the electrodes, ions gain or lose electrons and are discharged as neutral atoms, producing elements. For example, electrolysis of molten lead bromide produces lead at the cathode and bromine at the anode. The term 'electrolysis' literally means 'breaking down with electricity'. It is used industrially to extract reactive metals and to produce useful chemicals.
(HT only) Throughout Section 4.4.3 Higher Tier students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations.
Higher Tier students must be able to write half equations for electrode reactions during electrolysis. A half equation shows the gain or loss of electrons by ions at an electrode. At the cathode (negative electrode), positive ions gain electrons (reduction). For example, Pb²⁺ + 2e⁻ → Pb. At the anode (positive electrode), negative ions lose electrons (oxidation). For example, 2Br⁻ → Br₂ + 2e⁻. Students may also need to complete and balance supplied half equations by adding electrons, ions, or adjusting coefficients to ensure conservation of charge and mass. Balancing requires that the number of atoms of each element and the total charge are the same on both sides. State symbols may be required.
Your focus
- Describe why solid ionic compounds do not conduct electricity.
- Explain how melting or dissolving releases ions so that they are free to move.
- Identify mobile ions as the charge carriers in molten and dissolved ionic compounds.
Show all 21 objectives
- Define the term electrolyte.
- Explain why an electrolyte conducts electricity.
- Distinguish between electrolytes and non-electrolytes using examples.
- Describe how passing a direct current through an electrolyte causes ions to move.
- Explain how ion movement allows charge to flow through the electrolyte.
- Relate ion movement to discharge and product formation at the electrodes.
- Identify the cathode as the negative electrode and the anode as the positive electrode.
- Describe the movement of positive and negative ions to the correct electrodes.
- Explain how ions are discharged at the electrodes to form products.
- Identify the electrode at which each type of ion is discharged.
- Explain how the gain or loss of electrons leads to the formation of elements.
- Apply the concept of discharge to predict the products of electrolysis of a given binary ionic compound.
- Define electrolysis and identify the necessary conditions for it to occur.
- Describe the movement of ions during electrolysis.
- Explain why electrolysis requires a molten or aqueous electrolyte.
- Write balanced half equations for reactions at the cathode and anode.
- Complete and balance supplied half equations by adding electrons or adjusting coefficients.
- Apply the principles of conservation of mass and charge to electrode reactions.
The process of electrolysis exam tips
Marking Points
- Solid ionic compounds contain ions fixed in a giant lattice, so the ions cannot move and the solid does not conduct.
- Melting provides energy to break down the lattice, releasing ions that are free to move in the liquid.
- Dissolving in water separates the ions from the lattice and allows them to move about within the solution.
- Mobile ions are the charge carriers in molten and dissolved ionic compounds, not free electrons.
- A named example such as molten sodium chloride or sodium chloride solution can be used to illustrate mobile Na⁺ and Cl⁻ ions.
- An electrolyte is a liquid or solution that contains mobile ions and can conduct electricity.
- Conduction occurs because mobile ions carry charge through the electrolyte.
- Molten ionic compounds and aqueous solutions of ionic compounds are examples of electrolytes.
- Pure water and solid ionic compounds are not electrolytes because they lack mobile ions.
- The term electrolyte refers to the substance being electrolysed, not to the electrodes or external circuit.
- Passing a direct current through an electrolyte causes ions to move towards the electrodes.
- The movement of ions through the electrolyte allows charge to flow and completes the circuit.
- Ions are discharged at the electrodes, where they gain or lose electrons.
- Electrolysis requires mobile ions and a direct current supply.
- A named example such as molten sodium chloride can be used to show Na⁺ and Cl⁻ ions moving to opposite electrodes.
- Positively charged ions are attracted to the negative electrode, called the cathode.
- Negatively charged ions are attracted to the positive electrode, called the anode.
- The terms cathode and anode refer to the negative and positive electrodes respectively.
- At the electrodes, ions gain or lose electrons to form neutral products.
- A named example such as molten lead bromide can show Pb²⁺ ions moving to the cathode and Br⁻ ions moving to the anode.
- Cations (positive ions) move to the cathode (negative electrode) and gain electrons to form neutral atoms.
- Anions (negative ions) move to the anode (positive electrode) and lose electrons to form neutral atoms.
- Discharge produces elements because the neutral atoms formed are the elements themselves.
- The electrolyte must contain mobile ions, either molten or in aqueous solution.
- Electrodes can be inert (like platinum) to prevent reaction, or reactive (like carbon in aluminium extraction) depending on the process.
- Electrolysis is defined as the breakdown of an ionic compound using electricity.
- The electrolyte must be molten or in aqueous solution to allow ion movement.
- A direct current (d.c.) supply is required.
- Ions are discharged at the electrodes, producing elements.
- The process involves oxidation at the anode and reduction at the cathode.
- Write half equations for reduction at the cathode, showing electrons gained by positive ions.
- Write half equations for oxidation at the anode, showing electrons lost by negative ions.
- Balance half equations for both atoms and charge.
- Include state symbols where appropriate (e.g., (l), (g), (aq)).
- Complete supplied half equations by adding missing electrons, ions, or coefficients.
Examiner Tips
- 💡Link the state of the compound directly to ion mobility: solid means fixed ions, molten or dissolved means mobile ions.
- 💡Use a named ionic compound and identify the ions present, such as Na⁺ and Cl⁻, to make the explanation concrete.
- 💡When asked why a solid does not conduct, refer to the lattice and the inability of ions to move rather than saying there are no ions.
- 💡Define an electrolyte in terms of mobile ions and electrical conduction.
- 💡Give a named example and state the mobile ions present to support the definition.
- 💡Contrast an electrolyte with a non-electrolyte such as pure water to show understanding.
- 💡State that a direct current is needed and explain that it causes directed ion movement.
- 💡Describe the movement of both positive and negative ions to show a complete picture.
- 💡Link ion movement to discharge at the electrodes and the formation of new substances.
- 💡Use the spelling link that cations are positive and go to the cathode.
- 💡State the polarity of each electrode whenever you name the cathode or anode.
- 💡Support your answer with a named electrolyte and the formulas of the ions moving to each electrode.
- 💡When describing discharge, always state the electrode, the ion, and whether electrons are gained or lost.
- 💡Use the terms cation and anion correctly to show understanding of charge.
- 💡Link the discharge to the formation of a neutral element, naming the product.
- 💡Define electrolysis clearly, mentioning the use of electricity to break down an ionic compound.
- 💡State that the electrolyte must be molten or aqueous to allow ions to move.
- 💡Use the term 'discharge' when describing the formation of products at electrodes.
- 💡Always check that both atoms and charge are balanced in half equations.
- 💡Remember that reduction occurs at the cathode (electrons on the left) and oxidation at the anode (electrons on the right).
- 💡Practice completing half equations by identifying the missing species, often electrons or water molecules in aqueous solutions.
Common Mistakes
- Thinking that solid ionic compounds conduct because they contain charged ions; the correction is that the ions are fixed in the lattice and cannot move.
- Believing that electrons carry charge through molten or dissolved ionic compounds; the correction is that mobile ions carry the charge.
- Assuming that dissolving always produces mobile ions for every compound; the correction is that this applies to ionic compounds that dissolve and release their ions.
- Calling any liquid an electrolyte; the correction is that an electrolyte must contain mobile ions.
- Confusing the electrolyte with the electrodes; the correction is that the electrolyte is the liquid or solution being electrolysed.
- Thinking that all solutions conduct equally; the correction is that conduction depends on the presence and mobility of ions.
- Thinking that ions move only when the current is switched on but not before; the correction is that ions are already mobile in the electrolyte and the current causes their directed movement.
- Believing that electrons flow through the electrolyte; the correction is that ions carry charge through the electrolyte while electrons flow in the external circuit.
- Assuming that alternating current is used; the correction is that electrolysis uses a direct current so that the electrodes keep fixed polarities.
- Mixing up the electrodes and sending positive ions to the anode; the correction is that positive ions move to the cathode.
- Thinking that cathode means positive electrode; the correction is that the cathode is the negative electrode.
- Believing that ions change into electrons at the electrodes; the correction is that ions gain or lose electrons to form neutral atoms or molecules.
- Confusing the polarity of electrodes: the cathode is negative and attracts positive ions; the anode is positive and attracts negative ions. Correction: remember 'cations go to cathode' and 'anions go to anode'.
- Thinking that ions gain or lose protons rather than electrons. Correction: discharge involves electron transfer, not proton transfer.
- Assuming that solid ionic compounds conduct electricity. Correction: the ions must be mobile, so the compound must be molten or dissolved in water.
- Thinking that electrolysis can occur with solid ionic compounds. Correction: the ions must be free to move, so the compound must be molten or dissolved.
- Confusing electrolysis with simple conduction. Correction: electrolysis involves chemical change at the electrodes, not just the flow of charge.
- Believing that alternating current (a.c.) is used. Correction: electrolysis requires direct current (d.c.) to ensure consistent electrode reactions.
- Forgetting to balance charge in half equations. Correction: ensure the total charge on each side is equal by adding the appropriate number of electrons.
- Placing electrons on the wrong side of the equation. Correction: electrons are gained (reactants) at the cathode and lost (products) at the anode.
- Omitting state symbols when they are required. Correction: include (l), (g), (aq), or (s) as appropriate for the species involved.