Electron Transfer Reactions

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Oxidation & Reduction

Oxidation is the loss of electrons, while reduction is the gain of electrons.
An oxidising agent oxidises another species by causing it to lose electrons; the oxidising agent itself is reduced and its oxidation number decreases.
A reducing agent reduces another species by causing it to gain electrons; the reducing agent itself is oxidised and its oxidation number increases.
In a redox reaction, both oxidation and reduction occur simultaneously, so both an oxidising agent and a reducing agent are present.
Some substances can act as either oxidising or reducing agents depending on what they react with and the reaction conditions.
Oxidation numbers are used to identify what is oxidised or reduced; they are also 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

Half equations show the separate oxidation and reduction processes, including electrons.
To balance a redox equation: write the unbalanced equation, deduce oxidation number changes, balance the changes, balance charges, then balance atoms.
In acidic conditions, balance oxygen by adding H₂O and hydrogen by adding H⁺.
Example half equations: Fe²⁺(aq) → Fe³⁺(aq) + e⁻ (oxidation) and MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l) (reduction).
The overall equation is obtained by combining half equations so that electrons cancel: MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l).
Redox titrations involve an oxidising agent titrated against a reducing agent; the endpoint can be shown by a colour change or by an indicator.
In manganate(VII) titrations, the purple MnO₄⁻ ion acts as its own indicator, turning colourless at the endpoint.
In iodine–thiosulfate titrations, iodine (brown/yellow) is reduced to colourless iodide; starch is added near the endpoint to sharpen the colour change.

Relative Ease of Oxidation & Reduction

Down Group 1 and Group 2, the relative ease of oxidation of the metal increases; reactivity with water becomes more vigorous.
For example, lithium reacts vigorously with water, and down Group 1 the reaction becomes even more violent.
Magnesium reacts very slowly with water, while calcium reacts much more quickly.
Halogens are oxidising agents: they gain an electron from a metal, oxidising the metal and becoming reduced themselves.
The oxidising power of halogens decreases down Group 17: F₂ > Cl₂ > Br₂ > I₂.
A more reactive halogen displaces a less reactive halogen from its halide solution, e.g. Cl₂ + 2KBr → 2KCl + Br₂.
Metals can be ranked in a reactivity series; more reactive metals displace less reactive metals from their compounds.
In metal displacement, the more reactive metal acts as the reducing agent, e.g. Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s).

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).
The ionic equation is 2H⁺(aq) + Zn(s) → Zn²⁺(aq) + H₂(g); chloride ions are spectator ions.
The half equations are 2H⁺(aq) + 2e⁻ → H₂(g) (reduction) and Zn(s) → Zn²⁺(aq) + 2e⁻ (oxidation).
The metal is the reducing agent (it is oxidised), and H⁺ from the acid is the oxidising agent (it is reduced).
The extent of reaction depends on the reactivity of the metal and the strength of the acid; stronger acids react more vigorously.
Very reactive metals react dangerously with acids, while metals low in reactivity (e.g. copper) do not react with dilute acids.
Relative reducing power of metals (strongest to weakest): Mg > Al > Zn > Fe > Pb > H > Cu > Ag.

Primary Cells

A voltaic (galvanic) cell generates electricity from a spontaneous redox reaction.
The potential difference is called the electromotive force (EMF) or cell potential, symbol E.
A half-cell consists of a metal electrode dipped in a solution of its own ions, setting up an equilibrium such as 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, allowing ions to migrate and balance charge; it often contains KNO₃(aq) or KCl(aq).
Cell notation uses a single vertical line for a phase boundary and a double vertical line for the salt bridge, e.g. Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s) E = +1.10 V.
In a hydrogen–oxygen fuel cell, the overall reaction is 2H₂(g) + O₂(g) → 2H₂O(l) with E = +1.23 V.
Fuel cells produce only water, are more efficient than combustion engines, and avoid nitrogen oxides, but hydrogen storage and production pose safety and economic challenges.

Secondary Cells

Secondary (rechargeable) cells can be reversed by applying a voltage greater than the cell voltage, pushing electrons in the opposite direction.
A lead-acid battery uses lead as the negative electrode, lead(IV) oxide as the positive electrode, and sulfuric acid as the electrolyte.
Lead-acid half equations: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻ and PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l); overall E = +2.06 V.
Lead-acid batteries deliver high current for short periods (ideal for car engines) but are heavy, bulky, and contain toxic lead and corrosive acid.
Nickel-cadmium (NiCad) cells use cadmium as the negative electrode and a nickel(II) hydroxide-oxide system as the positive electrode; overall E = +1.2 V.
NiCad cells can be recharged hundreds of times but cadmium is toxic and they can suffer from the memory effect.
Lithium-ion cells use a lithium cobalt oxide positive electrode, a carbon negative electrode, and a solid polymer electrolyte.
Lithium-ion cells are lightweight, deliver high voltage (3.5–4.0 V), and contain no toxic heavy metals, but degrade over time, are expensive, and pose a fire risk.

Electrolytic Cells

Electrolysis is the splitting of an ionic compound by electricity when molten or in aqueous solution.
In an electrolytic cell, 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).
For molten lead bromide: Pb²⁺(l) + 2e⁻ → Pb(l) at the cathode and 2Br⁻(l) → Br₂(l) + 2e⁻ at the anode.
In electrolysis of 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.
RED CAT reminds you that reduction takes place at the cathode in both cell types.

The basic set-up of an electrolytic cell

Oxidation of Alcohols

Alcohols contain the –OH group and have the general formula CₙH₂ₙ₊₁OH; they are classified as primary, secondary, or tertiary.
A primary alcohol has the –OH carbon bonded to one other carbon; a secondary alcohol to two; a tertiary alcohol 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 then further to carboxylic acids.
Secondary alcohols are oxidised to ketones only; tertiary alcohols do not undergo oxidation.
To form an aldehyde, use distillation apparatus so the lower-boiling aldehyde distils off as it forms.
To form a carboxylic acid, use reflux apparatus with excess oxidising agent.
Tertiary alcohols do not oxidise because there is no hydrogen on the functional group carbon to break off and form water.

Reduction of Carboxylic Acids, Aldehydes & Ketones

Carboxylic acids can be reduced to primary alcohols.
Aldehydes can be reduced to primary alcohols.
Ketones can be reduced to secondary alcohols.
These reductions typically use reducing agents such as lithium aluminium hydride (LiAlH₄) in dry ether, or sodium borohydride (NaBH₄).
The reduction of an aldehyde or ketone involves the addition of hydrogen across the C=O bond.

Reduction of Unsaturated Compounds

Unsaturated compounds contain carbon–carbon double or triple bonds and can undergo addition reactions.
Alkenes can be reduced to alkanes by addition of hydrogen across the C=C bond.
This hydrogenation reaction requires a metal catalyst, such as nickel, platinum, or palladium.
For example, ethene reacts with hydrogen to form ethane: CH₂=CH₂ + H₂ → CH₃CH₃.
The process is used industrially to harden vegetable oils into margarine.

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练习题

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1.Which statement correctly defines an oxidising agent?
Easy
- AA substance that oxidises another species by causing it to lose electrons
- BA substance that oxidises another species by causing it to gain electrons
- CA substance that reduces another species by causing it to lose electrons
- DA substance that reduces another species by causing it to gain electrons
2.A reducing agent is itself oxidised and its oxidation number increases.
Easy
True or false?
3.Which of the following statements about oxidising and reducing agents are correct? (select all that apply)
Medium
- AOxidising agents accept electrons.
- BReducing agents donate electrons.
- COxidising agents are themselves oxidised.
- DReducing agents are themselves reduced.
- EOxidising agents cause other species to lose electrons.
4.Match each term with its correct description.
Medium
- Oxidising agent
- Reducing agent
- Redox reaction
- Spectator ion
- A species that gains electrons and is itself reduced
- A species that loses electrons and is itself oxidised
- A reaction in which both oxidation and reduction occur
- An ion that does not change in the reaction
5.Place the steps for balancing a redox equation into the correct order.
Medium
- Write the unbalanced equation and identify the atoms which change in oxidation number.
- Deduce the oxidation number changes.
- Balance the oxidation number changes.
- Balance the charges.
- Balance the atoms.
6.In the reaction 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O, which species is acting as the oxidising agent?
Medium
- AFe²⁺
- BH₂O₂
- CH⁺
- DH₂O
7.Which of the following is a redox reaction?
Medium
- AHCl + NaOH → NaCl + H₂O
- B2HCl + Zn → ZnCl₂ + H₂
- CCaCO₃ → CaO + CO₂
- DAgNO₃ + NaCl → AgCl + NaNO₃
8.In a voltaic cell, oxidation takes place at the negative electrode.
Easy
True or false?

更多题目

9.Which statements about the hydrogen–oxygen fuel cell are correct? (select all that apply)
Medium
- AThe overall reaction is 2H₂(g) + O₂(g) → 2H₂O(l).
- BThe electrolyte is aqueous sodium hydroxide.
- CWater is the only reaction product.
- DThe cell stores energy for later use.
- EThe cell produces harmful oxides of nitrogen during operation.
10.Which of the following statements about the relative ease of oxidation down Group 1 are correct? (select all that apply)
Medium
- AThe reaction of the metal with water becomes more vigorous down the group
- BLithium is oxidised from Li to Li⁺ when it reacts with water
- CThe relative ease of oxidation decreases down the group
- DPotassium reacts more violently with water than lithium
- EThe oxidation number of the Group 1 metal decreases when it reacts with water
11.Which of the following statements about halogen displacement reactions are correct? (select all that apply)
Medium
- AChlorine can displace bromine from potassium bromide solution
- BBromine can displace iodine from sodium iodide solution
- CIodine can displace chlorine from sodium chloride solution
- DFluorine is a weaker oxidising agent than chlorine
- EThe oxidising power of the halogens decreases going down Group 17
12.Which of the following are differences between a voltaic cell and an electrolytic cell? (select all that apply)
Medium
- AIn a voltaic cell, oxidation occurs at the negative electrode, while in an electrolytic cell oxidation occurs at the positive electrode
- BA voltaic cell generates electricity from a spontaneous redox reaction, while an electrolytic cell uses electrical energy to drive a non-spontaneous reaction
- CIn a voltaic cell, reduction occurs at the cathode, while in an electrolytic cell reduction occurs at the anode
- DA voltaic cell converts chemical energy to electrical energy, while an electrolytic cell converts electrical energy to chemical energy
- EIn a voltaic cell, the anode is positive, while in an electrolytic cell the anode is negative
13.Which of the following statements about the hydrogen–oxygen fuel cell are correct? (select all that apply)
Medium
- AThe overall reaction is 2H₂(g) + O₂(g) → 2H₂O(l)
- BThe electrolyte is aqueous sodium hydroxide
- CThe cell produces carbon dioxide and water as products
- DThe cell can operate continuously as long as hydrogen and oxygen are supplied
- EThe cell stores energy for later use
14.Which of the following statements about the oxidation of alcohols are correct? (select all that apply)
Medium
- APrimary alcohols can be oxidised to aldehydes and then to carboxylic acids
- BSecondary alcohols can be oxidised to ketones
- CTertiary alcohols can be oxidised to ketones
- DAcidified potassium dichromate(VI) changes from orange to green when it oxidises an alcohol
- ETertiary alcohols do not undergo oxidation because there is no hydrogen on the functional group carbon
15.Match each alcohol type with the product formed when it is oxidised.
Medium
- Primary alcohol
- Secondary alcohol
- Tertiary alcohol
- Aldehyde, then carboxylic acid
- Ketone
- No reaction
16.Match each cell type with its correct description.
Medium
- Primary cell
- Secondary cell
- Fuel cell
- Cannot be recharged and is disposed of after one use
- Can be recharged by applying a voltage greater than the cell voltage
- Operates continuously as long as fuel and oxygen are supplied
17.Place the following metals in order of decreasing reactivity (most reactive first).
Medium
- Magnesium
- Zinc
- Iron
- Copper
- Silver
18.Place the following halogens in order of decreasing oxidising power (strongest oxidising agent first).
Medium
- Fluorine
- Chlorine
- Bromine
- Iodine
19.In an electrolytic cell, reduction takes place at the anode.
Easy
True or false?
20.A reducing agent itself gets oxidised and its oxidation number increases.
Easy
True or false?
21.In a hydrogen–oxygen fuel cell, the only reaction product is water.
Easy
True or false?
22.In the reaction 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O, H₂O₂ acts as a reducing agent.
Easy
True or false?
23.A student writes the half-equation for a half-cell containing a silver electrode in silver nitrate solution. Which is correct?
Medium
- AAg⁺(aq) + e⁻ ⇌ Ag(s)
- BAg(s) ⇌ Ag⁺(aq) + e⁻
- CAg⁺(aq) ⇌ Ag(s) + e⁻
- DAg(s) + e⁻ ⇌ Ag⁺(aq)
24.A voltaic cell is constructed using the Ag⁺/Ag half-cell and the Pb²⁺/Pb half-cell. Using standard electrode potentials, which overall equation is correct?
Medium
- APb(s) + 2Ag⁺(aq) → Pb²⁺(aq) + 2Ag(s)
- BPb²⁺(aq) + 2Ag(s) → Pb(s) + 2Ag⁺(aq)
- CPb(s) + Ag⁺(aq) → Pb²⁺(aq) + Ag(s)
- DPb²⁺(aq) + Ag(s) → Pb(s) + Ag⁺(aq)
25.A student investigates the electrolysis of dilute sodium chloride solution using inert electrodes. Which statements about the products are correct? (select all that apply)
Medium
- AHydrogen gas is produced at the cathode.
- BChlorine gas is produced at the anode.
- CSodium metal is produced at the cathode.
- DOxygen gas is produced at the anode.
- EThe solution becomes alkaline due to hydroxide ions.
26.A student adds chlorine solution to colourless potassium bromide solution. What colour change is observed and why?
Medium
- AColourless to orange, because chlorine displaces bromine.
- BColourless to brown, because bromine displaces chlorine.
- CColourless to pale green, because chlorine is reduced.
- DColourless to colourless, because no reaction occurs.
27.In a voltaic cell, the anode is the positive electrode.
Easy
True or false?
28.A voltaic cell is set up as Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s). What is the standard cell potential, E°cell?
Medium
- A+1.10 V
- B-1.10 V
- C+0.76 V
- D+0.34 V
29.In a redox titration using acidified potassium manganate(VII), why is no indicator added?
Medium
- AThe manganate(VII) ion is a strong purple colour which disappears at the endpoint, so the titration is self-indicating
- BStarch is added at the start to sharpen the endpoint
- CThe iron(II) ions change colour from green to orange at the endpoint
- DA pH indicator such as phenolphthalein is used because the reaction is acidic
30.In an iodine–thiosulfate titration, when should starch indicator be added?
Medium
- AAt the very start, before any thiosulfate is added
- BWhen the iodine colour has faded to a straw yellow
- COnly after the solution has turned completely colourless
- DStarch is never used in this titration
31.A student sets up the standard hydrogen electrode. Why is platinum used as the electrode?
Medium
- APlatinum is a reactive metal that readily donates electrons
- BPlatinum is inert and provides a surface for the H⁺/H₂ equilibrium without taking part in the reaction
- CPlatinum is a good reducing agent that oxidises hydrogen gas
- DPlatinum dissolves to give Pt²⁺ ions, which conduct the current
32.Which apparatus is used to prepare an aldehyde from a primary alcohol?
Medium
- AReflux apparatus
- BDistillation apparatus
- CA separating funnel
- DA burette and conical flask
33.A student adds bromine water to an unknown colourless solution. The solution turns brown. What can be concluded?
Hard
- AThe solution contains chloride ions
- BThe solution contains iodide ions
- CThe solution contains bromide ions
- DThe solution contains fluoride ions
34.During the electrolysis of aqueous copper(II) sulfate with inert electrodes, what is produced at the anode?
Medium
- ACopper metal
- BOxygen gas
- CHydrogen gas
- DSulfur dioxide gas
35.During the electrolysis of aqueous copper(II) sulfate, the inert electrodes are replaced with copper electrodes. What happens at the anode?
Hard
- ACopper metal is deposited
- BOxygen gas is evolved
- CThe copper anode dissolves to form Cu²⁺ ions
- DHydrogen gas is evolved
36.Which statements about the electrolysis of molten lead bromide are correct? (select all that apply)
Medium
- ALead is produced at the cathode
- BBromine is produced at the anode
- CThe cathode half-equation is Pb²⁺ + 2e⁻ → Pb
- DHydrogen gas is produced at the cathode
- EThe anode half-equation is 2Br⁻ → Br₂ + 2e⁻
37.A lead-acid battery is used to start a car engine. Which statements about it are correct? (select all that apply)
Hard
- AIt consists of six cells connected in series
- BThe negative electrode is lead and the positive electrode is lead(IV) oxide
- CThe electrolyte is sulfuric acid
- DThe overall cell reaction is not reversible
- EEach cell generates an EMF of about 12 V
38.In a redox titration using acidified potassium dichromate(VI), the colour change at the endpoint is from orange to green.
Easy
True or false?
39.In an iodine–thiosulfate titration, the starch indicator is added at the beginning of the titration before any thiosulfate is added.
Medium
True or false?
40.Which product is formed at the cathode?
Medium
- AHydrogen gas
- BOxygen gas
- CChlorine gas
- DSodium metal
41.Complete the definitions using the words 'electrons' and 'oxygen'.
Easy
Oxidation is the gain of ____ or the loss of ____.
42.What is the name of the process in which an ionic compound is broken down into its elements by passing an electric current through it when molten or in aqueous solution?
Medium
43.A 2.25 g iron tablet was dissolved and titrated against 0.100 mol dm⁻³ KMnO₄, requiring 26.50 cm³. Calculate the percentage by mass of iron in the tablet. (Mᵣ Fe = 55.85)
Medium
44.A 1.35 g iron tablet was titrated against 0.0200 mol dm⁻³ K₂Cr₂O₇, requiring 31.00 cm³. Calculate the number of moles of K₂Cr₂O₇ used.
Medium
45.Using the same titration (0.0200 mol dm⁻³ K₂Cr₂O₇, 31.00 cm³), determine the number of moles of Fe²⁺ in the sample. The reaction is Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺.
Medium
46.Using the same titration (0.0200 mol dm⁻³ K₂Cr₂O₇, 31.00 cm³), calculate the mass of iron in the original sample. (Mᵣ Fe = 55.85)
Medium
47.Using the same titration (0.0200 mol dm⁻³ K₂Cr₂O₇, 31.00 cm³), calculate the percentage of iron in the original 1.35 g iron tablet. (Mᵣ Fe = 55.85)
Medium
48.Complete the sentence about the lead-acid battery.
Medium
In a lead-acid battery, the negative electrode is made of ____ and the positive electrode is made of ____.
49.Complete the sentence about the lithium-ion cell.
Medium
In a lithium-ion cell, the positive electrode is ____ and the negative electrode is ____.
50.What is the name of the reagent used to oxidise alcohols, which changes from orange to green?
Easy
51.What is the name of the electrode at which reduction occurs in any electrochemical cell?
Medium
52.A 2.25 g iron tablet was dissolved and titrated against 0.100 mol dm⁻³ KMnO₄, requiring 26.50 cm³. Calculate the mass of iron in the tablet, in grams. (Mᵣ Fe = 55.85)
Hard
53.Explain why tertiary alcohols do not undergo oxidation.
Hard
54.Explain why 2-methylpropan-2-ol will not form a carboxylic acid when heated with acidified potassium dichromate(VI).
Hard
55.State whether the sulfite ions, SO₃²⁻(aq), are acting as an oxidising or reducing agent in the reaction Cr₂O₇²⁻(aq) + 8H⁺(aq) + 3SO₃²⁻(aq) → 2Cr³⁺(aq) + 4H₂O(l) + 3SO₄²⁻(aq). Justify your answer.
Medium
56.Deduce if the reaction between hydrochloric acid and sodium hydroxide is a redox reaction. Justify your answer. HCl + NaOH → NaCl + H₂O
Medium
57.Explain why the products of electrolysis of concentrated sodium chloride solution differ from those of the dilute solution.
Hard
58.A 2.25 g iron tablet was dissolved and titrated against 0.100 mol dm⁻³ KMnO₄, requiring 26.50 cm³. Calculate the percentage by mass of iron in the tablet. (Mᵣ Fe = 55.85)
Medium
- A29.6 %
- B39.5 %
- C36.2 %
- D32.9 %
59.A 1.35 g iron tablet was titrated against 0.0200 mol dm⁻³ K₂Cr₂O₇, requiring 31.00 cm³. Calculate the number of moles of K₂Cr₂O₇ used.
Medium
- A2.0006 mol
- B1.0006 mol
- C0.00062 mol
- D-0.9994 mol
60.Using the same titration (0.0200 mol dm⁻³ K₂Cr₂O₇, 31.00 cm³), determine the number of moles of Fe²⁺ in the sample. The reaction is Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺.
Medium
- A1.0037 mol
- B-0.9963 mol
- C2.0037 mol
- D0.00372 mol
61.Using the same titration (0.0200 mol dm⁻³ K₂Cr₂O₇, 31.00 cm³), calculate the mass of iron in the original sample. (Mᵣ Fe = 55.85)
Medium
- A-0.792 g
- B1.208 g
- C0.208 g
- D2.208 g
62.Using the same titration (0.0200 mol dm⁻³ K₂Cr₂O₇, 31.00 cm³), calculate the percentage of iron in the original 1.35 g iron tablet. (Mᵣ Fe = 55.85)
Medium
- A14.4 %
- B16.4 %
- C15.4 %
- D17.4 %
63.A 2.25 g iron tablet was dissolved and titrated against 0.100 mol dm⁻³ KMnO₄, requiring 26.50 cm³. Calculate the mass of iron in the tablet, in grams. (Mᵣ Fe = 55.85)
Hard
- A2.74 g
- B0.74 g
- C1.74 g
- D-0.26 g

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