CARA'S TIPSRedox and electrolysis
JAMBWAECNECO
What the syllabus asks
JAMB: redox reactions, oxidation numbers, electrochemical series, electrolysis, Faraday's laws, electrochemical cells, corrosion
WAEC: oxidation and reduction (oxygen/hydrogen transfer, electron transfer, change in oxidation number), oxidation numbers and IUPAC naming, oxidising and reducing agents, balancing redox equations, electrochemical series, electrochemical (galvanic) cells, electrolysis, electrolytes and non-electrolytes, preferential discharge, electrolysis of named electrolytes, Faraday's laws, applications…
NECO: oxidation and reduction (oxygen/hydrogen transfer, electron transfer, change in oxidation number), oxidation numbers and IUPAC naming, oxidising and reducing agents, balancing redox equations, electrochemical series, electrochemical (galvanic) cells, electrolysis, electrolytes and non-electrolytes, preferential discharge, electrolysis of named electrolytes, Faraday's laws, applications…
What you will learn
- Define oxidation and reduction in terms of oxygen, hydrogen, electrons and oxidation number.
- Calculate oxidation numbers and use them in IUPAC names.
- Identify oxidising and reducing agents and balance redox equations by half-equations.
- Use the electrochemical series to predict displacement reactions and cell e.m.f.
- Explain electrolysis and predict products for molten and aqueous electrolytes.
- State and apply Faraday's first and second laws.
- Describe electroplating, purification of copper, extraction of metals and the prevention of corrosion.
Key points
- Oxidation: gain of O, loss of H, loss of electrons, increase in oxidation number; reduction is the reverse (OIL RIG).
- An oxidising agent is reduced; a reducing agent is oxidised.
- Oxidation numbers: element 0, O −2, H +1, sum = charge; used in IUPAC names (e.g. tetraoxosulphate(VI)).
- Balance redox by half-equations: atoms, O with H₂O, H with H⁺, charge with e⁻, equalise electrons.
- Higher metals in the electrochemical series displace lower ones and are stronger reducing agents.
- Galvanic cell: anode (−) oxidation, cathode (+) reduction; E°cell = E°cathode − E°anode (Daniell 1.10 V).
- Electrolysis: cations to cathode (reduction), anions to anode (oxidation); molten binary: metal at cathode, non-metal at anode.
- Preferential discharge depends on series position, concentration and electrode type.
- Faraday: Q = It; 1 F = 96,500 C; m = MIt/(nF); in series, moles of electrons are the same in every cell.
- Applications: electroplating (object = cathode), copper refining, Al and Na extraction, brine electrolysis.
- Rusting needs water and oxygen; prevent by barriers, galvanising, sacrificial metals and alloying.
Start here: Oxidation and reduction: the older definitions
Leave a new cutlass outside in the rainy season and it turns red-brown with rust. Burn firewood and it turns to ash and smoke. Your body "burns" glucose to release energy. All of these involve a chemical process chemists first called oxidation, because oxygen was involved.
- 2Mg + O₂ → 2MgO: magnesium gains oxygen, so it is oxidised.
- CuO + H₂ → Cu + H₂O: copper(II) oxide loses oxygen, so it is reduced; hydrogen gains oxygen, so it is oxidised.
- H₂S + Cl₂ → S + 2HCl: H₂S loses hydrogen (oxidised); Cl₂ gains hydrogen (reduced).
Notice that in every example, one substance is oxidised while another is reduced. You cannot have one without the other. Reactions in which both happen are called redox reactions (reduction + oxidation).
The substance that supplies the oxygen (or removes hydrogen) is the oxidising agent; the substance that removes oxygen (or supplies hydrogen) is the reducing agent. In CuO + H₂ → Cu + H₂O, CuO is the oxidising agent and H₂ the reducing agent.
In the blast furnace, Fe₂O₃ + 3CO → 2Fe + 3CO₂. Identify the substances oxidised and reduced, and the reducing agent.
- Fe₂O₃ loses oxygen to become Fe: reduced.
- CO gains oxygen to become CO₂: oxidised. Because CO removes oxygen from the iron ore, it is the reducing agent.
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Practice questions
- loss of electrons
- gain of electrons
- gain of hydrogen
- loss of oxygen
Show answer
A: loss of electrons
OIL RIG.
- +4
- +2
- +6
- +7
Show answer
D: +7
+1 + Mn + 4(−2) = 0 → Mn = +7.
- salt bridge
- electrolyte surface
- cathode
- anode
Show answer
C: cathode
Cations gain electrons at the cathode.
- 4
- 2
- 1
- 0.5
Show answer
B: 2
Cu²⁺ + 2e⁻ → Cu; 2 mol electrons = 2 F.
- oxygen
- hydrogen
- chlorine
- sulphur(IV) oxide
Show answer
A: oxygen
OH⁻ is discharged at the anode giving O₂.
Key words
- Oxidation
- Loss of electrons, or increase in oxidation number.
- Reduction
- Gain of electrons, or decrease in oxidation number.
- Redox reaction
- A reaction in which oxidation and reduction occur together.
- Oxidation number
- The charge an atom would have if all its bonds were ionic.
- Oxidising agent
- A substance that accepts electrons and is itself reduced.
- Reducing agent
- A substance that donates electrons and is itself oxidised.
- Electrochemical series
- Arrangement of elements in order of their standard electrode potentials.
- Electrolyte
- A compound which, molten or in solution, conducts electricity and is decomposed by it.
- Electrolysis
- Decomposition of an electrolyte by passing an electric current through it.
- Anode
- Electrode connected to the positive terminal; where oxidation occurs.
- Cathode
- Electrode connected to the negative terminal; where reduction occurs.
- Preferential discharge
- Selective discharge of one ion rather than another at an electrode.
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