Redox chemistry
Oxidation and reduction, oxidation states, tests for oxidising and reducing agents, electrolysis, and simple and fuel cells for G3 Pure and O-Level Chemistry.
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Before you begin
Whenever one substance is oxidised, another is reduced, because the electrons or oxygen lost by one must be gained by the other. The lessons first show four ways of recognising redox (oxygen, hydrogen, electrons and oxidation states) and the colour tests for oxidising and reducing agents. They then use the same electron transfer to explain electrolysis, where electricity drives a redox reaction, and simple cells and fuel cells, where a redox reaction produces electricity.
Be comfortable with: ions and ionic bonding, writing and balancing chemical equations and the reactivity series.
Learning goals
- Define oxidation and reduction as gain or loss of oxygen or hydrogen
- Define redox as electron transfer and changes in oxidation state
- Identify redox reactions by oxygen or hydrogen gain or loss, electron gain or loss and changes in oxidation state
- Describe how aqueous potassium iodide and acidified potassium manganate(VII) test for oxidising and reducing agents by their colour changes
- Describe electrolysis as the passage of electricity through a molten or dissolved ionic compound, causing chemical changes at the electrodes
- Describe how electrolysis shows that ions exist, fixed in a solid lattice but free to move when molten or dissolved
- Describe the electrolysis of molten sodium chloride with inert electrodes in terms of ion movement and electrode products
- Predict the products of electrolysing a molten binary ionic compound with inert electrodes
- Predict which cation is discharged using the reactivity series
- Predict which anion is discharged from halides, hydroxides and sulfates, as in aqueous copper(II) sulfate and dilute sodium chloride
- Explain how concentration affects which ion is discharged, as in concentrated and dilute aqueous sodium chloride
- Predict the products of electrolysing an aqueous electrolyte from given information
- Write ionic equations for the reactions at the electrodes during electrolysis
- Describe how copper is purified by electrolysing aqueous copper(II) sulfate with copper electrodes
- Describe electroplating, such as copper plating, and state one use of it
- Describe how a simple cell of two electrodes in an electrolyte produces electricity, linked to the reactivity series and electron transfer
- Describe hydrogen from water or hydrocarbons as a potential fuel that reacts with oxygen in a fuel cell to generate electricity
Syllabus statements covered
- define oxidation and reduction (redox) in terms of oxygen/hydrogen gain/loss
- define redox in terms of electron transfer and changes in oxidation state
- identify redox reactions in terms of oxygen/hydrogen gain/loss, electron gain/loss and changes in oxidation state
- describe the use of aqueous potassium iodide and acidified potassium manganate(VII) in testing for oxidising and reducing agents from the resulting colour changes.
- describe electrolysis as the conduction of electricity through an ionic compound (an electrolyte), when molten or dissolved in water, leading to chemical changes (including decomposition) at the electrodes
- describe electrolysis as evidence for the existence of ions which are held in a lattice when solid but which are free to move when molten or in solution
- describe, in terms of the mobility of ions present and the electrode products, the electrolysis of molten sodium chloride, using inert electrodes
- predict the likely products of the electrolysis of a molten binary ionic compound using inert electrodes
- apply the idea of selective discharge based on — cations: linked to the reactivity series (see also 8.4)
- apply the idea of selective discharge based on — anions: halides, hydroxides and sulfates (e.g. aqueous copper(II) sulfate and dilute sodium chloride solution (as essentially the electrolysis of water))
- apply the idea of selective discharge based on — concentration effects (as in the electrolysis of concentrated and dilute aqueous sodium chloride) (in all cases above, inert electrodes are used)
- predict the likely products of the electrolysis of an aqueous electrolyte, given relevant information
- construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given relevant information
- describe the electrolysis of aqueous copper(II) sulfate with copper electrodes as a means of purifying copper (no technical details are required)
- describe the electroplating of metals, e.g. copper plating, and state one use of electroplating
- describe the production of electrical energy from simple cells (i.e. two electrodes in an electrolyte) linked to the reactivity series (see also 8.4) and redox reactions (in terms of electron transfer)
- describe hydrogen, derived from water or hydrocarbons, as a potential fuel, reacting with oxygen to generate electricity directly in a hydrogen fuel cell (details of the construction and operation of a fuel cell are not required).
Lessons
Work through them in order.
Redox foundations
- Redox reactionsSpot oxidation and reduction from oxygen, hydrogen, electrons and oxidation states, and name the oxidising and reducing agents.
- Assigning and calculating oxidation statesWork out oxidation states in elements, compounds and ions, then use their changes to identify redox.
- Tests for oxidising and reducing agentsTest for oxidising agents with potassium iodide and reducing agents with acidified manganate(VII), and explain the colour changes.
Electrochemistry
- What is electrolysis?Why an electrolyte needs mobile ions, which way ions and electrons move, and what happens when ions are discharged.
- Electrolysis of molten compoundsPredict the products of electrolysing a molten binary compound and write the half-equations and overall equation.
- Electrolysis of aqueous compoundsUse selective discharge to predict the products when water competes with the dissolved ions.
- Types of electrodes in electrolysisCompare inert and copper anodes in copper(II) sulfate: electrode masses, products and colour.
- Writing and checking electrode half-equationsBuild electrode half-equations by balancing atoms and charge, and combine them into an overall equation.
- Electroplating: growing a metal coatingSet up electrolysis to coat an object with metal, and why objects are electroplated.
- Purifying copper by electrolysisHow copper moves from an impure anode to a pure cathode, and where the impurities go.
- Simple electric cellsHow a simple cell turns a redox reaction into electricity, and how the reactivity series decides its electrodes and voltage.
- Hydrogen as a fuel and fuel cellsHydrogen as a fuel: where it comes from, how a fuel cell generates electricity, and how clean it really is.
Practise and check
Or choose
Topic reference
Quick reference
| Item | Quick rule / reminder |
|---|---|
| Oxidation | gain of oxygen / loss of hydrogen / loss of electrons / increase in oxidation state |
| Reduction | loss of oxygen / gain of hydrogen / gain of electrons / decrease in oxidation state |
| Agents | oxidising agent accepts electrons and is reduced; reducing agent gives electrons and is oxidised |
| Oxidation-state sum | 0 for a neutral compound; the ion’s charge for an ion |
| Test for oxidising agent | aqueous potassium iodide: colourless → brown |
| Test for reducing agent | acidified potassium manganate(VII): purple → colourless |
| Electrolysis signs | anode positive (oxidation), cathode negative (reduction): “PANIC”, Positive Anode, Negative Is Cathode |
| Molten binary compound | metal at the cathode, non-metal at the anode |
| Aqueous: cathode | metal below hydrogen in the reactivity series → metal; above hydrogen → H₂ |
| Aqueous: inert anode | sulfate → O₂; dilute chloride → O₂; concentrated chloride → Cl₂ |
| Copper anode | dissolves, Cu → Cu²⁺ + 2e⁻, instead of giving O₂ |
| Electroplating | object = cathode; plating metal = anode; electrolyte contains ions of the plating metal |
| Simple cell signs | more reactive metal = negative anode; less reactive metal = positive cathode; electrons flow anode → cathode |
Core knowledge to remember
- Redox reaction: a reaction in which oxidation and reduction happen together.
- Oxidation state: a number given to an atom to keep track of electrons; it is 0 in an element and equal to the charge in a one-atom ion.
- Electrolysis: the conduction of electricity through an ionic compound, molten or in solution, causing chemical changes at the electrodes.
- Electrolyte: a molten ionic compound or solution that conducts because it contains mobile ions.
- Selective discharge: in aqueous electrolysis, only one of the competing species reacts at each electrode.
- Inert electrode: an electrode, such as graphite or platinum, that conducts but does not react.
- Purifying copper: impure copper anode, pure copper cathode, copper(II) sulfate solution; impurities fall as anode sludge.
- Simple cell: two different metals in an electrolyte, changing chemical energy into electrical energy; the further apart the metals in the reactivity series, the larger the voltage.
- Hydrogen fuel cell: hydrogen, made from water or hydrocarbons, reacts with oxygen to generate electricity directly, forming water.
Common mistakes
- Agents: the oxidising agent is reduced, and the reducing agent is oxidised.
- Half-equations: electrons go on the right for oxidation and on the left for reduction; check both atoms and charge.
- Oxidation-state sums: a polyatomic ion adds up to its charge, not 0.
- Test answers: name the reagent in full and give both colours, then the conclusion.
- Electrode signs: PANIC applies to electrolysis. In a simple cell the anode is negative.
- Aqueous electrolysis: water can be reduced or oxidised; sodium is never deposited from an aqueous solution.
- Concentration: dilute sodium chloride gives oxygen at the anode; concentrated brine gives chlorine.
- Reactive anode: a copper anode dissolves instead of producing oxygen.
- Charge carriers: electrons move in the wires; ions move in the electrolyte.