Topic R3.2 · HL only

Electron transfer reactions: notes and practice questions

9 min read Jump to 11 practice questions

Summary

This topic extends electron transfer reactions to electrochemical cells, standard electrode potentials, and electrolysis.
The standard hydrogen electrode, $\text{H}^+(\text{aq}) + \text{e}^- \rightleftharpoons \frac{1}{2} \text{H}_2(\text{g})$ , is assigned $E^\circ = 0 \text{ V}$ .
$E^\circ_{\text{cell}}$ is calculated from standard electrode potentials; a positive value indicates a spontaneous reaction.
Standard Gibbs energy change relates to cell potential: $\Delta G^\circ = -nFE^\circ_{\text{cell}}$ .
Electrolysis of aqueous solutions involves competing electrode reactions, deduced from standard electrode potentials.
Electroplating uses electrolytic cells for metallic coating, requiring deduction of electrode reactions.

How it is examined

Oxidation state deduction is a 1-mark part in both papers. Balancing half-equations is 1 to 2 marks and the balance must be right in both atoms and charge. Voltaic cell diagrams attract "identify the anode", "state the direction of electron flow" and "explain the role of the salt bridge" at 1 mark each, and the salt bridge answer must mention ion movement to maintain electrical neutrality, not just "completes the circuit". At HL, E⦵cell calculations are [2] and the sign error from reversing the wrong half-cell is the classic loss. The organic oxidation sequence (primary alcohol to aldehyde with distillation, to carboxylic acid with reflux) is examined by asking for conditions as well as products.

Given in the booklet

HL: standard reduction potentials, the Faraday constant F, and ΔG⦵ = −nFE⦵cell. At SL nothing is given, but the guidance says displacement data will be supplied in the question. What is recall: oxidation state rules, anode is oxidation and cathode is reduction, the sign convention (anode negative in a voltaic cell, positive in an electrolytic cell), and the oxidation products of primary versus secondary versus tertiary alcohols.

Key ideas

3.2.1 Oxidation and reduction can be described in terms of electron transfer, change in oxidation state, oxygen gain or loss, or hydrogen loss or gain. Students deduce oxidation states of an atom in a compound or an ion, and identify the oxidized and reduced species and the oxidizing and reducing agents in a chemical reaction.
3.2.2 Half-equations separate the processes of oxidation and reduction, showing the loss or gain of electrons. Students deduce redox half-equations and equations in acidic or neutral solutions.
3.2.3 The relative ease of oxidation and reduction of an element in a group can be predicted from its position in the periodic table. The reactions between metals and aqueous metal ions demonstrate the relative ease of oxidation of different metals. Students predict the relative ease of oxidation of metals and of reduction of halogens, and interpret data regarding metal and metal ion reactions.
3.2.4 Acids react with reactive metals to release hydrogen. Students deduce equations for reactions of reactive metals with dilute HCl and H₂SO₄.

Not assessed

Half-equations and equations are required in acidic or neutral solutions only. Balancing in alkaline solution is not on the syllabus.
Names and formulas of specific oxidizing agents, and the mechanisms of oxidation, will not be assessed. The same for reducing agents and the mechanisms of reduction. Include the role of hydride ions in the reduction reaction.

Guiding questions

What happens when electrons are transferred?

Linking questions

Structure 3.1 What are the advantages and limitations of using oxidation states to track redox changes? Why does metal reactivity increase, and non-metal reactivity decrease, down the main groups? How can oxidation states be used to show that CH₄, CH₃OH, HCHO, HCOOH, CO₂ are given in increasing order of oxidation?
Structure 2.3 The surface oxidation of metals is often known as corrosion. What are some of the consequences of this process?
Structure 2.1 Under what conditions can ionic compounds act as electrolytes?
Structure 3.2 How does the nature of the functional group in a molecule affect its physical properties, such as boiling point?
Reactivity 1.3 Electrical energy can be derived from the combustion of fossil fuels or from electrochemical reactions. What are the similarities and differences? What is the difference between combustion and oxidation of an alcohol?
Reactivity 2.3 Secondary cells rely on electrode reactions that are reversible. What are the common features of these reactions?
Reactivity 3.4 Why are some reactions of alkenes classified as reduction reactions while others are classified as electrophilic addition reactions?
Reactivity 1.4 (HL) How can thermodynamic data also be used to predict the spontaneity of a reaction?
Structure 3.1 (HL) Why is there a colour change when an alcohol is oxidized by a transition element compound?
Tool 1, Inquiry 2 Why are some redox titrations described as "self-indicating"? How is an electrolytic cell used for electroplating?

Practice questions

11 questions · 1 easy · 8 medium · 2 hard

Showing 11 of 11

Question 1

EasyPaper 1A · calculator1 mark

The reactivity of several metals is given below, from most reactive to least reactive.

Which reaction will occur spontaneously under standard conditions?

A. $Mg^{2+}(aq) + Fe(s) \rightarrow Fe^{2+}(aq) + Mg(s)$

B. $Cr^{3+}(aq) + Al(s) \rightarrow Al^{3+}(aq) + Cr(s)$

C. $Pb^{2+}(aq) + 2Ag(s) \rightarrow 2Ag^{+}(aq) + Pb(s)$

D. $Fe^{2+}(aq) + Pb(s) \rightarrow Pb^{2+}(aq) + Fe(s)$

Question 2

MediumPaper 2 · calculator1 mark

Use the following table of standard reduction potentials to answer the question.

$E^\ominus / V$

$Na^+(aq) + e^- \rightleftharpoons Na(s)$ -2.71

$Zn^{2+}(aq) + 2e^- \rightleftharpoons Zn(s)$ -0.76

$Cr^{3+}(aq) + 3e^- \rightleftharpoons Cr(s)$ -0.74

$Fe^{2+}(aq) + 2e^- \rightleftharpoons Fe(s)$ -0.45

$2H^+(aq) + 2e^- \rightleftharpoons H_2(g)$ 0.00

$Cu^{2+}(aq) + 2e^- \rightleftharpoons Cu(s)$ +0.34

$Cu^+(aq) + e^- \rightleftharpoons Cu(s)$ +0.52

$I_2(aq) + 2e^- \rightleftharpoons 2I^-(aq)$ +0.54

$Fe^{3+}(aq) + e^- \rightleftharpoons Fe^{2+}(aq)$ +0.77

$Ag^+(aq) + e^- \rightleftharpoons Ag(s)$ +0.80

$Cl_2(aq) + 2e^- \rightleftharpoons 2Cl^-(aq)$ +1.36

A student sets up a standard voltaic cell using an iron electrode in a solution of $Fe^{2+}(aq)$ and a silver electrode in a solution of $Ag^+(aq)$ .

For the spontaneous reaction that occurs in this cell, what is its standard Gibbs free energy change, $\Delta G^\ominus$ ?

The value of the Faraday constant is $96\ 500\ C\ mol^{-1}$ and $\Delta G^\ominus = -nFE^\ominus$ .

A -241 kJ mol $^{-1}$

B -185 kJ mol $^{-1}$

C -120 kJ mol $^{-1}$

D +241 kJ mol $^{-1}$

Question 3

HardPaper 2 · calculator24 marks

(a)(i)

Antimony (Sb) and Bismuth (Bi) are elements in group 15 of the periodic table.

(a) Antimony has two stable isotopes. 57.21% of antimony atoms contain 70 neutrons and the remainder contain 72 neutrons.

(i) Deduce the nuclear symbol of the isotope of antimony containing 72 neutrons. Use section 6 of the data booklet.

[1]

(a)(ii)

(ii) Calculate, to two decimal places, the relative atomic mass of antimony.

[2]

(b)(i)

Bismuth(III) nitrate, $Bi(NO_3)_3$ , is a common salt of bismuth.

(b) (i) The compound contains both ionic and covalent bonds. State which particles are joined by covalent bonds and which are joined by ionic bonds.

[2]

(b)(ii)

(ii) Distinguish between covalent and ionic bonding in terms of electron distribution.

[2]

(b)(iii)

(iii) State the enthalpy term that characterizes the strength of the bonding between the ions in an ionic solid.

[1]

(b)(iv)

(iv) Write an equation for the formation of aqueous bismuth(III) nitrate from solid bismuth(III) oxide and nitric acid.

[2]

(b)(v)

(v) Calculate the volume, in $cm^3$ , of $1.50 \text{ mol dm}^{-3}$ nitric acid required to react completely with $5.00 \text{ g}$ of solid bismuth(III) oxide.

[3]

(b)(vi)

(vi) Predict, with a reason, whether bismuth(III) oxide is expected to be primarily acidic, basic or amphoteric.

[1]

(b)(vii)

(vii) Discuss how the relative reactivity of zinc and bismuth could be established using the metals and aqueous solutions of their nitrates.

[2]

(b)(viii)

(viii) Discuss the products formed at the electrodes during the electrolysis of aqueous bismuth(III) nitrate. Use the standard electrode potential $E^\ominus(Bi^{3+}/Bi) = +0.32 \text{ V}$ and section 24 of the data booklet.

[2]

(c)(i)

Bismuth compounds are sometimes used in fireworks to produce special effects.

[1]

(c)(ii)

(ii) Calculate the wavelength, in nm, that corresponds to the first ionization energy of bismuth. Use sections 1, 2 and 8 of the data booklet.

[3]

(c)(iii)

(iii) Explain why the first ionization energy of bismuth is lower than that of polonium (Po), in terms of nuclear charge and electron shielding.

[2]

Question 4

MediumPaper 2 · calculator1 mark

An excess of magnesium turnings is added to an acidified solution of potassium dichromate(VI). Determine the final chromium-containing product of the reaction. The following standard electrode potentials are provided:

$Cr_2O_7^{2-}(aq) + 14H^+(aq) + 6e^- \rightleftharpoons 2Cr^{3+}(aq) + 7H_2O(l) \quad E^{\ominus} = +1.33 \text{ V}$

$Cr^{3+}(aq) + e^- \rightleftharpoons Cr^{2+}(aq) \quad E^{\ominus} = -0.41 \text{ V}$

$Cr^{2+}(aq) + 2e^- \rightleftharpoons Cr(s) \quad E^{\ominus} = -0.91 \text{ V}$

$Mg^{2+}(aq) + 2e^- \rightleftharpoons Mg(s) \quad E^{\ominus} = -2.37 \text{ V}$

A. $Cr(s)$

B. $Cr^{2+}(aq)$

C. $Cr^{3+}(aq)$

D. $Cr_2O_7^{2-}(aq)$

Question 5

HardPaper 2 · calculator23 marks

(a)

A sample of chlorine consists of two isotopes, $^{35}Cl$ and $^{37}Cl$ .

(a) Contrast the sub-atomic structure of these two isotopes.

[1]

(b)(i)

(b) (i) The sample of chlorine is analysed in a mass spectrometer, producing a spectrum for the $Cl_2^+$ ion. The spectrum shows three peaks at m/z values of 70, 72 and 74. Explain the origin and relative heights of these three peaks, given that the abundance of $^{35}Cl$ is approximately three times that of $^{37}Cl$ .

[3]

(b)(ii)

(ii) A more precise measurement finds the composition by mass to be: $^{35}Cl$ : 75.76%, $^{37}Cl$ : 24.24%. Calculate the relative atomic mass of chlorine from this sample, giving your answer to two decimal places. (Use isotopic masses of 35.0 and 37.0 for this calculation).

[2]

(c)(i)

Magnesium chloride, $MgCl_2$ , and manganese(II) chloride, $MnCl_2$ , are two ionic compounds.

(c) (i) Deduce the type of bonding in magnesium chloride, $MgCl_2$ , using electronegativity values from section 9 of the data booklet.

[2]

(c)(ii)

(ii) Determine the lattice enthalpy of magnesium chloride, assuming the bonding is purely ionic. Use sections 9, 10 and 12 of the data booklet and the following data:

Enthalpy of formation of magnesium chloride = $-641 \text{ kJ mol}^{-1}$

[3]

(c)(iii)

(iii) Explain, with reference to electron configurations, why the ionic radii of $Mg^{2+}$ , $Mn^{2+}$ and $Cl^-$ are different. Use section 10 of the data booklet.

[3]

(c)(iv)

(iv) Predict, with a reason, which has the stronger ionic bonding, manganese(II) chloride, $MnCl_2$ , or magnesium chloride.

[2]

(d)(i)

Magnesium chloride is white, but manganese(II) chloride is pale pink.

(d) (i) State the condensed electron configuration of a manganese atom.

[1]

(d)(ii)

(ii) State the reason, in terms of electron configuration, why manganese(II) chloride is coloured.

[1]

(d)(iii)

(iii) Manganese(II) chloride absorbs light with a wavelength of approximately 530 nm. Describe why this is consistent with the observed colour of the compound. Use sections 2 and 15 of the data booklet.

[2]

(e)(i)

A copper key is to be electroplated with manganese using an aqueous solution of manganese(II) chloride as the electrolyte.

(e) (i) Deduce the half-equations for the reactions occurring at the anode (made of pure manganese) and the cathode (the copper key).

[2]

(e)(ii)

(ii) Deduce a balanced chemical equation for the reaction of fluorine gas with the aqueous chloride ions in the electrolyte.

[1]

Question 6

MediumPaper 1A · calculator1 mark

What are the products at the anode and cathode respectively during the electrolysis of aqueous zinc bromide, $ZnBr_2(aq)$ , using inert electrodes?

A. bromine, zinc

B. oxygen, zinc

C. bromine, hydrogen

D. oxygen, hydrogen

Question 7

MediumPaper 1A · calculator1 mark

Consider the following standard electrode potentials:

Half-reaction	$E^\ominus$ / V
$Pb^{2+}(aq) + 2e^- \rightleftharpoons Pb(s)$	-0.13
$Cu^{2+}(aq) + 2e^- \rightleftharpoons Cu(s)$	+0.34
$I_2(s) + 2e^- \rightleftharpoons 2I^-(aq)$	+0.54
$Ag^+(aq) + e^- \rightleftharpoons Ag(s)$	+0.80

Which combination of reactants will result in a spontaneous reaction under standard conditions?

A. $Ag(s)$ and $Cu^{2+}(aq)$

B. $I_2(s)$ and $Ag(s)$

C. $Cu(s)$ and $Pb^{2+}(aq)$

D. $Pb(s)$ and $Ag^+(aq)$

Question 8

MediumPaper 1A · calculator1 mark

A student constructs a voltaic cell using two half-cells: a zinc electrode in a $Zn^{2+}(aq)$ solution and a copper electrode in a $Cu^{2+}(aq)$ solution.

Given the standard electrode potentials:

$Zn^{2+}(aq) + 2e^- \rightleftharpoons Zn(s)$ $E^\ominus = -0.76$ V

$Cu^{2+}(aq) + 2e^- \rightleftharpoons Cu(s)$ $E^\ominus = +0.34$ V

What will be the standard cell potential, in $V$ , when these two half-cells are connected?

A. $-1.10$

B. $-0.42$

C. $+0.42$

D. $+1.10$

Question 9

MediumPaper 1A · calculator1 mark

What are the main products when a dilute aqueous solution of copper(II) sulfate, $CuSO_4(aq)$ , is electrolysed using inert graphite electrodes?

	Anode	Cathode
A.	Oxygen	Copper
B.	Copper	Oxygen
C.	Sulfur dioxide	Hydrogen
D.	Oxygen	Hydrogen

Question 10

MediumPaper 1A · calculator1 mark

A student performs a redox titration to determine the concentration of iron(II) ions in a sample. The iron(II) ions are oxidized to iron(III) ions by dichromate(VI) ions, which are themselves reduced to chromium(III) ions.

$Cr_2O_7^{2-} (aq)$ reduced to $Cr^{3+}(aq)$

$Fe^{2+} (aq)$ oxidized to $Fe^{3+} (aq)$

What volume, in $cm^3$ , of $0.020\ mol\ dm^{-3}\ Cr_2O_7^{2-} (aq)$ is required to reach the equivalence point in the titration of $25.00\ cm^3$ of $0.120\ mol\ dm^{-3}\ Fe^{2+} (aq)$ ?

A. 12.5

B. 25.0

C. 50.0

D. 150.0

Question 11

MediumPaper 1A · calculator1 mark

The overall reaction occurring during the discharge of a lead-acid battery is:

$Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)$

Which statement about this reaction is correct?

A. The element lead is both oxidized and reduced.

B. The oxidation state of sulfur changes.

C. Lead(IV) oxide, $PbO_2$ , is the reducing agent.

D. The reaction is an example of disproportionation.

Every Electron transfer reactions question, marked for you

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
Joining the dots instead of drawing a curve.
Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.

FAQ

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What does Electron transfer reactions cover in IB Chemistry?

This topic extends electron transfer reactions to electrochemical cells, standard electrode potentials, and electrolysis. The standard hydrogen electrode, H^+(aq) + e^- rightleftharpoons (1)/(2) H_2(g), is assigned E^° = 0 V. E^°_cell is calculated from standard electrode potentials; a positive value indicates a spontaneous reaction.

Is Electron transfer reactions SL or HL?

Electron transfer reactions is HL only. SL students are not examined on it.

How do I revise Electron transfer reactions for IB Chemistry?

Start from the core idea: this topic extends electron transfer reactions to electrochemical cells, standard electrode potentials, and electrolysis. In the exam: oxidation state deduction is a 1-mark part in both papers. Balancing half-equations is 1 to 2 marks and the balance must be right in both atoms and charge. Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

How does FourtyFive help me practise Electron transfer reactions?

FourtyFive has 11 Electron transfer reactions questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for Electron transfer reactions practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite Electron transfer reactions answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

Electron transfer reactions: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

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Where marks are lost

Practise this topic as a session

Questions,
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How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Every Electron transfer reactions question, marked for you

Where marks are lost

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answered.

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