Electron Transfer Reactions

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Oxidation and Reduction

Oxidation is the loss of electrons and an increase in oxidation number.
Reduction is the gain of electrons and a decrease in oxidation number.
An oxidising agent oxidises another species by accepting electrons; it is itself reduced and its oxidation number decreases.
A reducing agent reduces another species by donating electrons; it is itself oxidised and its oxidation number increases.
In a redox reaction, both an oxidising agent and a reducing agent are present.
Some substances can act as either an oxidising or reducing agent depending on what they react with and the reaction conditions.
Oxidation numbers are used in naming compounds, e.g. iron(II) sulfate contains Fe²⁺ and iron(III) sulfate contains Fe³⁺.

Oxidation and reduction by electron transfer

Half Equations and Redox Titrations

To balance a redox equation: write the unbalanced equation, deduce oxidation number changes, balance the changes, balance charges, then balance atoms.
Redox titrations involve titrating an oxidising agent against a reducing agent; electrons are transferred between species.
In manganate(VII) titrations, acidified MnO₄⁻ reacts with Fe²⁺: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O.
Manganate(VII) titrations are self-indicating because the purple MnO₄⁻ colour disappears at the endpoint.
In iodine-thiosulfate titrations, iodine reacts with thiosulfate: 2S₂O₃²⁻ + I₂ → 2I⁻ + S₄O₆²⁻.
Starch is added near the endpoint in iodine-thiosulfate titrations; the solution turns blue/black until all iodine reacts.
These titrations can be used for analysis, e.g. iron in tablets or chlorine in bleach.

Titration apparatus

Relative Ease of Oxidation and Reduction

Down Group 1 and Group 2, the relative ease of oxidation increases; reactions with water become more vigorous.
Lithium reacts with water: 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g); Li is oxidised from 0 to +1.
Magnesium reacts very slowly with water, while calcium reacts much more quickly.
The oxidising power of halogens decreases down Group 17: F₂ is the strongest oxidising agent and I₂ the weakest.
A more reactive halogen can displace a less reactive halogen from its halide solution.
Chlorine displaces bromine from potassium bromide: 2KBr(aq) + Cl₂(aq) → 2KCl(aq) + Br₂(aq).
Bromine displaces iodine from sodium iodide: Br₂(l) + 2NaI(aq) → 2NaBr(aq) + I₂(aq).
Metals can be ranked in a reactivity series; more reactive metals displace less reactive metal ions from solution.

The reactivity series of metals

Acids with Reactive Metals

The general reaction is: acid + metal → salt + hydrogen.
Example: 2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g).
Example: H₂SO₄(aq) + Fe(s) → FeSO₄(aq) + H₂(g).
A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal or another positive ion.
The extent of reaction depends on the reactivity of the metal and the strength of the acid.
Very reactive metals react dangerously with acids; metals low in reactivity, such as copper, do not react with dilute acids.
Stronger acids react more vigorously, showing faster effervescence and quicker metal dissolution.
Ionic equation for zinc and hydrochloric acid: 2H⁺(aq) + Zn(s) → Zn²⁺(aq) + H₂(g); Zn is oxidised and H⁺ is reduced.

Primary Cells

Voltaic (galvanic) cells generate electricity from spontaneous redox reactions.
A half-cell consists of a metal electrode dipped in a solution of its own ions, setting up an equilibrium and an electrode potential.
Example equilibrium: Zn(s) ⇌ Zn²⁺(aq) + 2e⁻.
Electrons flow through the external circuit from the anode (oxidation) to the cathode (reduction).
A salt bridge completes the circuit; anions migrate toward the anode and cations toward the cathode.
Cell notation uses a single vertical line for a phase boundary and a double vertical line for a salt bridge.
Example: Zn(s) | Zn²⁺(aq) ∥ Cu²⁺(aq) | Cu(s) E_cell = +1.10 V.
Fuel cells continuously generate electricity from a fuel and oxygen; the hydrogen-oxygen fuel cell produces only water: 2H₂(g) + O₂(g) → 2H₂O(l) E = +1.23 V.

Secondary Cells

Secondary (rechargeable) cells employ reversible chemical reactions; applying a voltage greater than the cell voltage reverses the reaction.
Lead-acid batteries use lead as the negative electrode, lead(IV) oxide as the positive electrode, and sulfuric acid as the electrolyte.
A lead-acid cell generates about 2 V; six cells in series give about 12 V.
Lead-acid batteries are heavy, bulky, contain toxic lead and corrosive sulfuric acid, and pose disposal challenges.
NiCad cells use cadmium as the negative electrode and a nickel(II) hydroxide-oxide system as the positive electrode; overall E = +1.2 V.
Cadmium is toxic and NiCad cells can suffer from the memory effect, losing capacity if repeatedly recharged without full discharge.
Lithium-ion cells use lithium cobalt oxide as the positive electrode, carbon as the negative electrode, and a solid polymer electrolyte.
Lithium-ion cells are lightweight, deliver high voltage (3.5–4.0 V), contain no toxic heavy metals, but degrade over time and pose fire risks.

Electrolytic Cells

Electrolysis is the splitting of an ionic compound by electricity when molten or in solution.
The electrolyte contains positive and negative ions; cations move to the cathode and anions move to the anode.
At the cathode, positive ions gain electrons (reduction); at the anode, negative ions lose electrons (oxidation).
Electrolysis of molten lead bromide: Pb²⁺(aq) + 2e⁻ ⇌ Pb(s) at the cathode; 2Br⁻(l) ⇌ Br₂(l) + 2e⁻ at the anode.
For molten salts, metals are always formed at the cathode and non-metals at the anode.
In voltaic cells, the anode is negative and the cathode is positive; in electrolytic cells, the anode is positive and the cathode is negative.
Uses of electrolysis include purifying copper, plating metals, extracting reactive metals like aluminium, and making chlorine, hydrogen and sodium hydroxide.

The basic set-up of an electrolytic cell

Oxidation of Alcohols

Alcohols contain at least one hydroxy (-OH) group and have the general formula CₙH₂ₙ₊₁OH.
Primary alcohols have the -OH carbon attached to one other carbon; secondary alcohols to two; tertiary alcohols to three.
The oxidising agent for alcohols is acidified potassium dichromate(VI), K₂Cr₂O₇; the colour change is orange to green.
Primary alcohols can be oxidised to aldehydes and further to carboxylic acids.
Secondary alcohols are oxidised to ketones only; ketones cannot be further oxidised.
Tertiary alcohols do not undergo oxidation because there is no hydrogen on the functional group carbon.
Formation of a carboxylic acid uses reflux; formation of an aldehyde uses distillation to remove the lower-boiling aldehyde as it forms.
Example: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (reflux); CH₃CH₂OH + [O] → CH₃CHO + H₂O (distillation).

Reduction of Carbonyl Compounds and Unsaturated Compounds

Sodium borohydride, NaBH₄, is a reducing agent used to reduce aldehydes and ketones to alcohols.
Propanone reacts with NaBH₄ to form propan-2-ol.
Butanone reacts with NaBH₄ to form butan-2-ol.
Reduction of a carbonyl compound (C=O) converts it to an alcohol (-OH).
Unsaturated compounds containing C=C bonds can be reduced by addition of hydrogen.
Example: C₂H₄ + H₂ → C₂H₆ (ethene to ethane).

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Practice questions

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1.Which row correctly describes oxidation and reduction in terms of electron transfer and change in oxidation state?
Easy
- AOxidation: gain of electrons; oxidation number decreases
- BOxidation: loss of electrons; oxidation number increases
- COxidation: loss of electrons; oxidation number decreases
- DOxidation: gain of electrons; oxidation number increases
2.Which row describes the most common oxidation numbers of fluorine, oxygen and hydrogen in compounds?
Easy
- AF = -1, O = -2, H = +1
- BF = +7, O = -1, H = -1
- CF = +1, O = +2, H = +1
- DF = -7, O = -1, H = -1
3.A student added a solution of iodine to a solution of sodium bromide. Which statement correctly describes what happens?
Easy
- ANo reaction occurs
- BThe bromide ions are oxidised
- CThe iodine atoms are oxidised
- DBoth the bromide and iodide ions undergo changes in their oxidation state
4.What substance will be obtained at the positive electrode (anode) when molten KBr is electrolysed?
Medium
- AK(l)
- BBr(l)
- CBr2(l)
- DK(s)
5.What is the product of the reaction between propanone and sodium borohydride, NaBH4?
Medium
- Apropan-1-ol
- Bpropan-2-ol
- Cpropanoic acid
- Dpentanal
6.4-methylpentanal can be formed by the oxidation of 4-methylpentan-1-ol. Which reagent is used as an oxidising agent for this conversion?
Medium
- ANaBH4
- BNaOH
- CK2Cr2O7
- DLiAlH4
7.The chemistry of the Group VII elements often involves redox processes. Which of the following statements is correct?
Medium
- ABromine can oxidise chloride ions
- BIodide ions are the weakest reducing agent of the first four Group VII ions
- CIn reactions with water, chlorine is oxidised and reduced
- DFluorine is a weaker oxidising agent than chlorine
8.In which of the following reactions is the species shown in bold acting as an oxidising agent?
Medium
- ACr2O7²- + 8H+ + 3SO3²- → 2Cr³+ + 4H2O + 3SO4²-
- BMg + Fe²+ → Mg²+ + Fe
- CCl2 + 2Br^- → 2Cl^- + Br2
- DFe2O3 + 3CO → 2Fe + 3CO2

More questions

9.Below are four statements about voltaic and electrolytic cells. Identify the statement that is correct for voltaic cells but not for electrolytic cells.
Medium
- AAn electrolyte is needed in the cell
- BIons are moving in the electrolyte
- COxidation occurs at the anode
- DElectrons flow from the negative to the positive electrode
10.Which of the following statements about oxidising agents and reducing agents are correct? (select all that apply)
Medium
- AAn oxidising agent gains electrons and is itself reduced
- BA reducing agent donates electrons and is itself oxidised
- CAn oxidising agent causes another species to gain electrons
- DThe oxidation number of an oxidising agent increases during the reaction
- EA reducing agent causes another species to lose electrons
11.In a voltaic cell, oxidation takes place at the negative electrode and reduction takes place at the positive electrode.
Easy
True or false?
12.Tertiary alcohols can be oxidised to ketones by acidified potassium dichromate(VI).
Easy
True or false?
13.Match each alcohol classification to the product it forms on oxidation with acidified potassium dichromate(VI).
Medium
- Primary alcohol
- Secondary alcohol
- Tertiary alcohol
- Aldehyde then carboxylic acid
- Ketone only
- No reaction
14.Place the following metals in order of decreasing reactivity (most reactive first).
Medium
- Zn
- Sn
- Cu
- Ag
15.Which of the following is the correct definition of a salt?
Easy
- AA compound formed when a metal reacts with an acid
- BAn ionic compound formed when the hydrogen of an acid is replaced by a metal or another positive ion
- CA covalent compound formed when an acid is neutralised by a base
- DA compound formed when an acid reacts with a carbonate
16.Which of the following statements about the relative ease of oxidation of Group 1 metals is correct?
Medium
- AThe ease of oxidation decreases down the group
- BThe ease of oxidation increases down the group
- CAll Group 1 metals react with water at the same rate
- DGroup 1 metals do not undergo oxidation
17.In a redox reaction, the oxidising agent is itself reduced and the reducing agent is itself oxidised.
Easy
True or false?
18.Which of the following statements about fuel cells are correct? (select all that apply)
Medium
- AThe hydrogen-oxygen fuel cell produces water as the only product
- BFuel cells convert all bond energy into electrical energy without producing heat or light
- CFuel cells can operate continuously as long as fuel and oxygen are supplied
- DHydrogen storage is easy and inexpensive
- EFuel cells produce harmful oxides of nitrogen
19.Match each term to its correct definition.
Medium
- Oxidising agent
- Reducing agent
- Oxidation
- Reduction
- Accepts electrons and is itself reduced
- Donates electrons and is itself oxidised
- Loss of electrons
- Gain of electrons
20.Place the steps for balancing a redox equation using oxidation numbers in the correct order.
Medium
- Write the unbalanced equation and identify the atoms that change oxidation number
- Deduce the oxidation number changes
- Balance the oxidation number changes
- Balance the charges
- Balance the atoms
21.Which of the following statements about the oxidising power of the Group 17 elements are correct? (select all that apply)
Medium
- AFluorine is the strongest oxidising agent
- BThe oxidising power of the halogens decreases going down the group
- CIodine is the weakest oxidising agent of the three halogens chlorine, bromine and iodine
- DHalogens become oxidised when they react with metals
- EBromine is a stronger oxidising agent than chlorine
22.Which of the following are correct about the hydrogen-oxygen fuel cell? (select all that apply)
Medium
- AThe overall reaction is 2H2 (g) + O2 (g) → 2H2O (l)
- BWater is the only reaction product
- CThe cell operates continuously as long as hydrogen and oxygen are supplied
- DThe energy is stored within the cell itself
- EHydrogen is oxidised at one electrode while oxygen gains electrons at the other
23.Place the following metals in order of decreasing strength as reducing agents (strongest reducing agent first).
Hard
- Magnesium
- Zinc
- Iron
- Copper
24.Which of the following statements about secondary (rechargeable) cells are correct? (select all that apply)
Medium
- AThey can be recharged by applying a voltage greater than the cell voltage
- BA lead-acid battery uses lead as the negative electrode and lead(IV) oxide as the positive electrode
- CCadmium in NiCad cells is toxic, making disposal hazardous
- DLithium-ion cells contain toxic heavy metals such as lead and cadmium
- ESecondary cells employ chemical reactions that can be reversed
25.Match each cell or battery to a correct feature.
Medium
- Lead-acid battery
- NiCad cell
- Lithium-ion cell
- Hydrogen-oxygen fuel cell
- Can suffer from the memory effect
- Lightweight with a high voltage output of about 3.5–4.0 V
- Delivers a high current for a short period, ideal for starting engines
- Produces water as its only reaction product
26.Which of the following statements about electrolytic cells are correct? (select all that apply)
Medium
- ACations move towards the cathode and are reduced
- BAnions move towards the anode and are oxidised
- CIn electrolytic cells the anode is positive
- DIn electrolytic cells the anode is negative
- EElectrolysis of a molten binary ionic compound produces its component elements
27.Place the stages of the iodine-thiosulfate titration procedure in the correct order.
Medium
- Titrate iodine solution with sodium thiosulfate solution
- The light brown/yellow colour of the iodine turns paler as it becomes colourless iodide
- When the solution is a straw colour, add starch
- The solution turns blue/black until all the iodine reacts, then the colour disappears
28.A student adds chlorine solution to colourless potassium bromide solution. Which statements about what is observed and why are correct? (select all that apply)
Medium
- AThe solution becomes orange as bromine is formed
- BChlorine displaces bromine because it is more reactive
- CThe equation for the reaction is 2KBr (aq) + Cl2 (aq) → 2KCl (aq) + Br2 (aq)
- DThe bromide ions are reduced to bromine
- EChlorine is below bromine in Group 17, so it is less reactive
29.Which of the following statements about the salt bridge in a voltaic cell are correct? (select all that apply)
Medium
- AIt allows ions to flow from one solution to the other, completing the circuit
- BNegative ions (anions) migrate toward the anode
- CPositive ions (cations) migrate toward the cathode
- DIt is often a piece of filter paper saturated with KNO3 (aq)
- EIt allows electrons to flow between the two half-cells
30.In a voltaic cell, the anode is the negative electrode where oxidation occurs.
Easy
True or false?
31.In a lead-acid battery, the electrolyte is sulfuric acid.
Easy
True or false?
32.The oxidising power of the Group 17 elements increases going down the group.
Medium
True or false?
33.A student sets up a voltaic cell using an aluminium electrode in aluminium ions solution connected by a salt bridge to a zinc electrode in zinc ions solution. The voltmeter reads 0.94 V and the aluminium electrode is negative. Which conventional cell diagram correctly represents this cell?
Medium
- AAl (s) | Al3+ (aq) || Zn2+ (aq) | Zn (s) Ecell = +0.94 V
- BZn (s) | Zn2+ (aq) || Al3+ (aq) | Al (s) Ecell = +0.94 V
- CAl3+ (aq) | Al (s) || Zn (s) | Zn2+ (aq) Ecell = +0.94 V
- DAl (s) | Al3+ (aq) || Zn2+ (aq) | Zn (s) Ecell = -0.94 V
34.A student adds bromine solution to a colourless solution of potassium iodide. What colour change would be observed and what is the reason?
Easy
- AThe solution turns brown because iodine is formed; bromine is reduced.
- BThe solution turns orange because bromine is formed; iodide is oxidised.
- CThe solution stays colourless because no reaction occurs.
- DThe solution turns pale green because chlorine is formed; iodide is reduced.
35.A student investigates the reaction between zinc and copper(II) sulfate solution. Which statements about this reaction are correct? (select all that apply)
Medium
- AZinc is oxidised because it loses electrons.
- BCopper(II) ions are reduced because they gain electrons.
- CZinc acts as the reducing agent.
- DCopper(II) ions act as the reducing agent.
- EThe oxidation number of copper changes from +2 to 0.
36.In a voltaic cell, the anode is the negative electrode where oxidation takes place.
Easy
True or false?
37.In the reaction 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O, which species is acting as the oxidising agent and why?
Medium
- AH₂O₂ because it gains electrons and is reduced.
- BFe²⁺ because it loses electrons and is oxidised.
- CH⁺ because it gains electrons to form water.
- DH₂O₂ because it loses electrons and is oxidised.
38.A student sets up a voltaic cell with zinc and copper half-cells connected by a salt bridge containing KNO₃(aq). Which statements about the salt bridge are correct? (select all that apply)
Medium
- ANegative ions migrate toward the anode.
- BPositive ions migrate toward the cathode.
- CThe salt bridge completes the circuit.
- DElectrons flow through the salt bridge.
- EThe salt bridge prevents the solutions from mixing.
39.When solid potassium bromide is added to concentrated sulfuric acid, the following reaction occurs: 2KBr + 2H₂SO₄ → K₂SO₄ + SO₂ + Br₂ + 2H₂O. What is the change in oxidation number of sulfur in this reaction?
Hard
- AFrom +6 to +4
- BFrom +6 to +2
- CFrom +4 to +6
- DFrom +6 to 0
40.In the electrolysis of molten lead(II) bromide, what are the products at the anode and cathode respectively?
Medium
- ABromine at the anode, lead at the cathode
- BLead at the anode, bromine at the cathode
- COxygen at the anode, hydrogen at the cathode
- DBromine at the cathode, lead at the anode
41.Name the reagent used as the oxidising agent in the oxidation of alcohols.
Easy
42.Complete the sentence about the colour change in the oxidation of alcohols.
Easy
When acidified potassium dichromate(VI) oxidises an alcohol, the colour changes from ____ to ____.
43.A health supplement tablet containing iron(II) sulfate weighing 2.25 g was dissolved and titrated against 0.100 mol dm^-3 KMnO4. The titration required 26.50 cm³ for complete reaction. Calculate the percentage by mass of iron in the tablet. (Molar mass of Fe = 55.85 g mol^-1)
Medium
44.2.30 g of ethanol was oxidised using excess acidified potassium dichromate(VI) and the product was collected by distillation. If the percentage yield of the product was 50.0%, what mass of product was collected? (Molar masses: ethanol = 46.0 g mol^-1, ethanal = 44.0 g mol^-1)
Medium
45.What is the name of the process in which a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide?
Easy
46.Complete the sentences about the reactivity of halogens.
Easy
The most reactive halogen is ____ and the least reactive halogen is ____.
47.Complete the sentence about the colour change seen when an alcohol is oxidised by acidified potassium dichromate(VI).
Easy
The solution changes from ____ to ____ as the alcohol is oxidised.
48.Complete the sentence about the products of electrolysis of a molten binary ionic compound.
Medium
Metals are always formed at the ____ and non-metals at the ____.
49.A redox titration uses the reaction MnO4- (aq) + 8H+ (aq) + 5Fe2+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l). If 0.00265 mol of MnO4- reacts completely, how many moles of Fe2+ react?
Hard
50.Using the same reaction, a 2.25 g iron(II) sulfate tablet required 26.50 cm3 of 0.100 mol dm-3 KMnO4. Given that 0.01325 mol of Fe2+ reacted and the molar mass of iron is 55.85 g mol-1, calculate the mass of iron in the tablet in grams.
Hard
51.For the same tablet, calculate the percentage by mass of iron, given that the mass of iron is 0.740 g and the tablet weighs 2.25 g. Give your answer to three significant figures.
Hard
52.Name the process in which a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide.
Medium
53.Explain why tertiary alcohols do not undergo oxidation with acidified potassium dichromate(VI).
Hard
54.Describe the structure and function of the salt bridge in a voltaic cell.
Medium
55.Explain why the oxidising power of the halogens decreases going down Group 17.
Hard
56.A health supplement tablet containing iron(II) sulfate weighing 2.25 g was dissolved and titrated against 0.100 mol dm^-3 KMnO4. The titration required 26.50 cm³ for complete reaction. Calculate the percentage by mass of iron in the tablet. (Molar mass of Fe = 55.85 g mol^-1)
Medium
- A29.6 %
- B36.2 %
- C39.5 %
- D32.9 %
57.2.30 g of ethanol was oxidised using excess acidified potassium dichromate(VI) and the product was collected by distillation. If the percentage yield of the product was 50.0%, what mass of product was collected? (Molar masses: ethanol = 46.0 g mol^-1, ethanal = 44.0 g mol^-1)
Medium
- A0.1 g
- B1.1 g
- C2.1 g
- D3.1 g
58.A redox titration uses the reaction MnO4- (aq) + 8H+ (aq) + 5Fe2+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l). If 0.00265 mol of MnO4- reacts completely, how many moles of Fe2+ react?
Hard
- A1.0132 mol
- B-0.9868 mol
- C2.0133 mol
- D0.01325 mol
59.Using the same reaction, a 2.25 g iron(II) sulfate tablet required 26.50 cm3 of 0.100 mol dm-3 KMnO4. Given that 0.01325 mol of Fe2+ reacted and the molar mass of iron is 55.85 g mol-1, calculate the mass of iron in the tablet in grams.
Hard
- A0.74 g
- B1.74 g
- C-0.26 g
- D2.74 g
60.For the same tablet, calculate the percentage by mass of iron, given that the mass of iron is 0.740 g and the tablet weighs 2.25 g. Give your answer to three significant figures.
Hard
- A36.2 %
- B29.6 %
- C39.5 %
- D32.9 %

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