Unit 3 Topic 2: Oxidation and reduction

Write both half-equations first: the more positive E° is reduced at the cathode, and E°cell is E°(reduction) minus E°(oxidation).

A lesson for this topic

Quick checks: multiple choice

Question 1

What is the oxidation state of manganese in KMnO₄?

Question 2

What is the oxidation state of chromium in the dichromate ion, Cr₂O₇²⁻?

Question 3

A bag of lawn fertiliser at a Toowoomba garden centre lists Fe₂(SO₄)₃ as an ingredient. What is the systematic name of this compound?

Question 4

A Year 12 class in Toowoomba places a zinc strip in copper(II) sulfate solution and a red-brown coating forms: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Which species is the oxidising agent?

Question 5

Which equation represents a redox reaction?

Question 6

Natural gas burns in a power station near Gladstone: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Which statement describes what happens to carbon?

Question 7

Steel fencing near the beach at Hervey Bay begins to rust. The first step is 2Fe(s) + O₂(g) + 2H₂O(l) → 2Fe(OH)₂(s). Which statement is correct?

Question 8

In acidic solution, nitrate ions are reduced to nitrogen monoxide, NO. Which half-equation is balanced?

Question 9

Aluminium foil reacts with copper(II) chloride solution: 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s). How many electrons are transferred in the equation as written?

Question 10

Hydrogen peroxide, H₂O₂, is used to bleach hair. What is the oxidation state of oxygen in H₂O₂?

Question 11

A winery near Stanthorpe tests wine with acidified potassium dichromate, which oxidises ethanol to ethanoic acid. The half-equations are Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ → 2Cr³⁺(aq) + 7H₂O(l) and C₂H₅OH(aq) + H₂O(l) → CH₃COOH(aq) + 4H⁺(aq) + 4e⁻. Which is the balanced overall equation?

Question 12

Sulfur dioxide from a smelter stack in Mount Isa is bubbled through acidified dichromate solution, which turns green. The half-equations, as listed in the data book, are Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ → 2Cr³⁺(aq) + 7H₂O(l) and SO₄²⁻(aq) + 4H⁺(aq) + 2e⁻ → SO₂(aq) + 2H₂O(l). When the overall equation is balanced with the smallest whole numbers, how many H⁺ ions does it contain, and on which side?

Question 13

A 5.00 g sample of iron(II) sulfate fertiliser is dissolved in dilute sulfuric acid and made up to 250.0 mL. A 25.00 mL aliquot needs 21.40 mL of 0.02000 mol L⁻¹ KMnO₄ to reach the end point: MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq). Using Fe 55.85, what is the percentage by mass of iron in the fertiliser?

Question 14

Results of placing metals W, X and Y in solutions of each other's 2+ ions
Metal	W²⁺(aq)	X²⁺(aq)	Y²⁺(aq)
W	not tested	no change	metal coated
X	metal coated	not tested	metal coated
Y	no change	no change	not tested

Students at a Cairns school place strips of three metals, W, X and Y, in solutions of each other's 2+ ions and record whether a coating of another metal forms. Which lists the metals from the strongest reducing agent to the weakest?

Question 15

In a galvanic cell, which statement describes the anode?

Question 16

A galvanic cell is built from a zinc electrode in 1.0 mol L⁻¹ Zn²⁺(aq) and a copper electrode in 1.0 mol L⁻¹ Cu²⁺(aq), joined by a wire and a salt bridge. E°(Zn²⁺/Zn) = −0.76 V and E°(Cu²⁺/Cu) = +0.34 V. How do electrons move?

Question 17

A cell is made from magnesium in Mg²⁺(aq) (E° = −2.36 V) and iron in Fe²⁺(aq) (E° = −0.44 V), joined by a salt bridge soaked in potassium nitrate solution. Which movement occurs in the salt bridge while the cell runs?

Question 18

A cell is written as Ni(s) | Ni²⁺(aq) || Ag⁺(aq) | Ag(s). E°(Ni²⁺/Ni) = −0.24 V and E°(Ag⁺/Ag) = +0.80 V. Which half-equation occurs at the cathode?

Question 19

A student in Rockhampton builds a cell from magnesium in Mg²⁺(aq) (E° = −2.36 V) and copper in Cu²⁺(aq) (E° = +0.34 V). Which cell diagram represents this cell?

Question 20

Which statement correctly distinguishes a galvanic cell from an electrolytic cell?

Question 21

A half-cell contains a solution of iron(III) and iron(II) ions, Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V. What should be used as its electrode?

Question 22

In the cell Ni(s) | Ni²⁺(aq) || Ag⁺(aq) | Ag(s), the silver electrode gains 0.540 g while the cell runs. Using Ag 107.87 and Ni 58.69, what mass of nickel is lost from the anode?

Question 23

Which description fits the standard hydrogen electrode?

Question 24

A nickel half-cell (E°(Ni²⁺/Ni) = −0.24 V) is joined to a silver half-cell (E°(Ag⁺/Ag) = +0.80 V) under standard conditions. What is E°cell?

Question 25

For the cell 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s), E°(Al³⁺/Al) = −1.68 V and E°(Cu²⁺/Cu) = +0.34 V. What is the standard cell potential?

Question 26

Using E°(Mg²⁺/Mg) = −2.36 V, E°(Zn²⁺/Zn) = −0.76 V, E°(Cu²⁺/Cu) = +0.34 V and E°(Ag⁺/Ag) = +0.80 V, which of these species is the strongest reducing agent: Mg²⁺(aq), Zn(s), Cu(s) or Ag⁺(aq)?

Question 27

E°(Fe²⁺/Fe) = −0.44 V, E°(Ni²⁺/Ni) = −0.24 V, E°(Cu²⁺/Cu) = +0.34 V and E°(Ag⁺/Ag) = +0.80 V. Which species is the strongest oxidising agent?

Question 28

Use E°(Zn²⁺/Zn) = −0.76 V, E°(Fe²⁺/Fe) = −0.44 V, E°(Cu²⁺/Cu) = +0.34 V, E°(Fe³⁺/Fe²⁺) = +0.77 V and E°(Ag⁺/Ag) = +0.80 V. Which mixture reacts spontaneously under standard conditions?

Question 29

E°(I₂/I⁻) = +0.54 V, E°(Br₂/Br⁻) = +1.08 V and E°(Cl₂/Cl⁻) = +1.36 V. A student adds a few drops of a halogen solution to a solution of a potassium halide. Which mixture reacts?

Question 30

E°(Pb²⁺/Pb) = −0.13 V and E°(H⁺/H₂) = 0.00 V, so E° values predict that lead reacts with 1 mol L⁻¹ hydrochloric acid. A student sees almost no bubbles after ten minutes. Which explanation is best?

Question 31

Cell potentials of metals A, B and C, each joined to a standard copper half-cell
Metal half-cell	Measured cell potential (V)	Positive electrode
A in 1 mol L⁻¹ A²⁺(aq)	1.10	copper
B in 1 mol L⁻¹ B²⁺(aq)	0.20	metal B
C in 1 mol L⁻¹ C²⁺(aq)	0.47	copper

Students in a Townsville laboratory join half-cells of three unknown metals, A, B and C, each to a standard copper half-cell, E°(Cu²⁺/Cu) = +0.34 V, and record the results in the table. Which two of the metal half-cells would make the cell with the largest E°cell, and what is that value?

Question 32

E°(Zn²⁺/Zn) = −0.76 V, E°(Cu²⁺/Cu) = +0.34 V and E°(Ag⁺/Ag) = +0.80 V, measured against the standard hydrogen electrode. Suppose chemists had chosen the copper half-cell as the reference and given it 0.00 V instead. What would E°cell be for a cell of zinc and silver half-cells?

Question 33

E° values: Mg²⁺/Mg −2.36 V, Al³⁺/Al −1.68 V, Zn²⁺/Zn −0.76 V, Ni²⁺/Ni −0.24 V, Cu²⁺/Cu +0.34 V, Ag⁺/Ag +0.80 V. Which pair of standard half-cells gives the galvanic cell with the largest E°cell?

Question 34

A steel hull in a Mackay marina needs a block of metal attached that is oxidised in place of iron. E° values: Zn²⁺/Zn −0.76 V, Fe²⁺/Fe −0.44 V, Sn²⁺/Sn −0.14 V, Pb²⁺/Pb −0.13 V, Cu²⁺/Cu +0.34 V. Which metal is suitable?

Question 35

Copper(I) ions are unstable in water and change by the reaction 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s). E°(Cu²⁺/Cu⁺) = +0.16 V and E°(Cu⁺/Cu) = +0.52 V. Which statement is correct?

Question 36

Molten sodium chloride is electrolysed between inert electrodes. What forms at each electrode?

Question 37

Dilute sodium chloride solution is electrolysed with graphite electrodes. E° values: Na⁺/Na −2.71 V, 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) −0.83 V, O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) +1.23 V, Cl₂/Cl⁻ +1.36 V. What are the main products?

Question 38

When concentrated sodium chloride solution is electrolysed with inert electrodes, chlorine forms at the anode, even though E°(O₂/H₂O) = +1.23 V is lower than E°(Cl₂/Cl⁻) = +1.36 V. Which explanation is best?

Question 39

Copper(II) sulfate solution is electrolysed with graphite electrodes. E° values: Cu²⁺/Cu +0.34 V, 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) −0.83 V, O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) +1.23 V. Sulfate ions are not oxidised. Which describes the products?

Question 40

At a copper refinery in Townsville, impure copper anodes containing some silver are electrolysed in acidified copper(II) sulfate. E°(Cu²⁺/Cu) = +0.34 V and E°(Ag⁺/Ag) = +0.80 V. Why does the silver collect as a sludge below the anode?

Question 41

A jeweller in Brisbane silver plates a brass ring using a silver anode and silver nitrate solution. How is the ring connected?

Question 42

Which of these does not affect which products form when an aqueous solution is electrolysed?

Question 43

A current of 2.50 A passes through copper(II) sulfate solution for 30.0 minutes. Using F = 96 485 C mol⁻¹ and Cu 63.55, what mass of copper is deposited on the cathode?

Question 44

A spoon is silver plated from silver nitrate solution with a current of 0.750 A. Using F = 96 485 C mol⁻¹ and Ag 107.87, how long does it take to deposit 0.500 g of silver?

Question 45

One cell at an aluminium smelter near Gladstone runs at 2.00 × 10⁵ A for 24.0 hours. The cathode reaction is Al³⁺ + 3e⁻ → Al. Using F = 96 485 C mol⁻¹ and Al 26.98, what mass of aluminium does the cell produce?

Question 46

A student electrolyses a solution of a metal sulfate, MSO₄, which contains M²⁺ ions, and plots the mass of metal deposited against the charge passed. Using F = 96 485 C mol⁻¹, which metal is M?

Question 47

Concentrated brine is electrolysed at 3.00 A for 40.0 minutes; the anode reaction is 2Cl⁻(aq) → Cl₂(g) + 2e⁻. Using F = 96 485 C mol⁻¹ and a molar gas volume of 22.7 L mol⁻¹ at STP (273 K and 100 kPa), what volume of chlorine forms at STP?

Question 48

A 10.00 mL sample of hair bleach is diluted to 250.0 mL in a volumetric flask. 20.00 mL aliquots of the diluted solution need a mean titre of 18.60 mL of 0.02000 mol L⁻¹ KMnO₄: 2MnO₄⁻(aq) + 6H⁺(aq) + 5H₂O₂(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5O₂(g). What is the concentration of hydrogen peroxide in the original bleach?

Question 49

A zinc and copper cell has 100.0 mL of 1.00 mol L⁻¹ Cu²⁺(aq) in its copper half-cell. After it has run for some time, the zinc electrode has lost 0.654 g. Using Zn 65.38, what is the concentration of Cu²⁺ ions now?

Question 50

A tap is chromium plated from a solution of Cr³⁺ ions: Cr³⁺(aq) + 3e⁻ → Cr(s). In 40.0 minutes, 0.520 g of chromium is deposited. Using F = 96 485 C mol⁻¹ and Cr 52.00, what was the average current?

Question 51

Copper(II) sulfate solution is electrolysed between platinum electrodes at 0.800 A for 1.50 hours. E° values: Cu²⁺/Cu +0.34 V; 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) −0.83 V; O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) +1.23 V. Sulfate ions are not oxidised. Using F = 96 485 C mol⁻¹ and 22.7 L mol⁻¹ at STP, what volume of gas forms at the anode, measured at STP?

Question 52

Sodium oxalate, Na₂C₂O₄, is used to standardise permanganate solutions. The half-equations are C₂O₄²⁻(aq) → 2CO₂(g) + 2e⁻ and MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l). What mass of sodium oxalate reacts exactly with 22.40 mL of 0.0200 mol L⁻¹ KMnO₄? Use Na 22.99, C 12.01 and O 16.00.

Question 53

Acidified potassium dichromate, Cr₂O₇²⁻/Cr³⁺ E° = +1.36 V, is added to a solution containing 1 mol L⁻¹ each of Cl⁻, Br⁻ and I⁻ ions. E° values: I₂/I⁻ +0.54 V, Br₂/Br⁻ +1.08 V, Cl₂/Cl⁻ +1.36 V. Under standard conditions, which ions are oxidised, and what is E°cell for the reaction with bromide ions?

Question 54

Potassium iodide solution is electrolysed with graphite electrodes at 0.500 A for 20.0 minutes. E° values: K⁺/K −2.94 V; 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) −0.83 V; I₂/I⁻ +0.54 V; O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) +1.23 V. Using F = 96 485 C mol⁻¹ and I 126.90, what forms at the cathode, and what mass of iodine forms at the anode?

Question 55

Copper dissolves in dilute nitric acid. The half-equations are Cu(s) → Cu²⁺(aq) + 2e⁻ and NO₃⁻(aq) + 4H⁺(aq) + 3e⁻ → NO(g) + 2H₂O(l). What volume of nitrogen monoxide, measured at STP (22.7 L mol⁻¹), forms when 1.00 g of copper dissolves completely? Use Cu 63.55.

Question 56

A galvanic cell is made from a tin half-cell (E°(Sn²⁺/Sn) = −0.14 V) and a silver half-cell (E°(Ag⁺/Ag) = +0.80 V). Which electrode is the cathode, and what is E°cell?

Question 57

Copper(II) sulfate solution is electrolysed using two copper electrodes. E°(Cu²⁺/Cu) = +0.34 V and E°(O₂/H₂O) = +1.23 V. Which describes the cell after an hour?

Short response questions

Iron in a supplement tabletShort response · 7 marks

A student in Ipswich crushes one iron supplement tablet (0.850 g), which contains iron(II) sulfate, dissolves it in dilute sulfuric acid and makes the solution up to 100.0 mL. She titrates 20.00 mL aliquots with 0.00500 mol L⁻¹ potassium permanganate solution; the mean titre is 13.65 mL. The reaction is MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq). Use Fe 55.85.

(a) Identify the oxidising agent and the reducing agent in the titration, and state the change in the oxidation state of manganese. [3]

0 words · about 50 to 80 words

(b) Calculate the amount of iron(II) ions in the 100.0 mL solution. [2]

mol

A nickel and silver cellShort response · 6 marks

A student in Townsville places a nickel strip in 1.0 mol L⁻¹ nickel(II) nitrate solution and a silver wire in 1.0 mol L⁻¹ silver nitrate solution, then joins them with a wire, a voltmeter and a salt bridge soaked in potassium nitrate solution. E°(Ni²⁺/Ni) = −0.24 V and E°(Ag⁺/Ag) = +0.80 V.

(a) Calculate the standard cell potential. [1]

(b) Identify the anode, the direction in which electrons flow in the wire, and the direction in which nitrate ions move in the salt bridge. [3]

0 words · about 50 to 80 words

0 words · a sentence or two

Ranking unknown metalsShort response · 8 marks

In a Rockhampton school laboratory, students join half-cells of three metals, Q, R and T, each in 1.0 mol L⁻¹ of its 2+ ion, to a standard copper half-cell, E°(Cu²⁺/Cu) = +0.34 V, at 25 °C. They record the cell potential and which electrode is positive.

Cell potentials of metals Q, R and T, each joined to a standard copper half-cell
Metal half-cell	Measured cell potential (V)	Positive electrode
Q in 1.0 mol L⁻¹ Q²⁺(aq)	0.73	copper
R in 1.0 mol L⁻¹ R²⁺(aq)	0.24	metal R
T in 1.0 mol L⁻¹ T²⁺(aq)	1.52	copper

(a) Determine the standard electrode potential of the R half-cell. [2]

(b) Calculate E°cell for a galvanic cell made from the R and T half-cells. [2]

0 words · a sentence or two

(d) State two limitations of using E° values to make predictions like this one. [2]

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Electrolysing salt waterShort response · 8 marks

A Year 12 class electrolyses two sodium chloride solutions with graphite electrodes: a dilute 0.05 mol L⁻¹ solution and a concentrated 5 mol L⁻¹ solution. E° values: Na⁺(aq) + e⁻ → Na(s) −2.71 V; 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) −0.83 V; O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) +1.23 V; Cl₂(g) + 2e⁻ → 2Cl⁻(aq) +1.36 V.

(a) Explain, using the E° values, the products formed at the cathode and at the anode when the dilute solution is electrolysed. [4]

0 words · about 80 to 120 words

(b) The dilute cell runs at 2.00 A for 25.0 minutes. Calculate the volume of hydrogen gas formed at STP (molar volume 22.7 L mol⁻¹; F = 96 485 C mol⁻¹). [2]

0 words · a sentence or two

A model copper refineryShort response · 8 marks

A class models the copper refinery at Townsville. An impure copper anode, containing small amounts of zinc and silver, and a pure copper cathode stand in acidified copper(II) sulfate solution. A current of 1.50 A flows for 2.00 hours; the anode loses 3.712 g. E° values: Zn²⁺/Zn −0.76 V, Cu²⁺/Cu +0.34 V, Ag⁺/Ag +0.80 V. Use F = 96 485 C mol⁻¹ and Cu 63.55.

(a) Calculate the mass of copper deposited on the cathode. [3]

(b) Explain why the anode loses more mass than the cathode gains. [3]

0 words · about 50 to 80 words

(c) Identify the electrode joined to the positive terminal of the power supply, and write the half-equation for the reaction at the cathode. [2]

0 words · a sentence or two

Rusting iron nailsShort response · 6 marks

Students in Cairns place three clean iron nails in salty water for one week. Nail A is alone, nail B is wrapped in magnesium ribbon and nail C is wrapped in copper wire. After a week nail A shows some rust, nail B shows no rust but the magnesium is partly corroded, and nail C is heavily rusted. E° values: Mg²⁺/Mg −2.36 V, Fe²⁺/Fe −0.44 V, Cu²⁺/Cu +0.34 V, O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq) +0.40 V.

(a) Identify the species oxidised and the oxidising agent when nail A rusts. [2]

0 words · a sentence or two

(b) Explain the results for nails B and C, using the E° values. [4]

0 words · about 80 to 120 words