1.7 Oxidation, reduction and redox

Question 1

Define term oxidation and reduction

Answer 1

Oxidation : Loss of electrons
Reduction : gain of electrons

Question 2

Two reactions are shown. State which one is oxidation and which one is reduction
(i) F2 + 2e^- -> 2F^-
(ii) Cu^+ -> Cu^2+ + e^-

Answer 2

(i) Reduction
(ii) Oxidation

Question 3

Define oxidising agent, reducing agent and redox reaction

Answer 3

Oxidising agent : A species gains electrons and it causes the other species to lose electrons (get oxidised) and so it is an electron acceptor
Reducing agent : A species loses electrons and causes the other species to gain electrons (get reduced) and so it is an electron donor
Redox reaction : Reaction involving oxidation of one species and reduction of the other

Question 4

What order should you assign oxidation states and what is the acronym for it

Answer 4

Under My Face Heres Our Chin
Uncombined elements (0)
Metals
Fluorine (-1)
Hydrogen (+1)
Oxygen (-2)
Chlorine (-1)

Question 5

What are the oxidation states of the following
(i) MgF2
(ii) SO3

Answer 5

(i) Mg : +2 F : -1
(ii) S : +6 O : -2

Question 6

Use oxidation numbers to identify what has been oxidised and reduced for reaction
2Sr + O2 -> 2SrO

Answer 6

2Sr + O2 -> 2SrO
Ox states : 0 0 Sr :+2 O : -2
Sr is oxidised and Oxygen is reduced

Question 7

Calculate the oxidation states for the metal in :
copper (i) oxide : Cu2O
copper (ii) oxide : CuO
Manganese (iv) oxide : MnO2

Answer 7

• The roman numerals gives a hint as to what the oxidation state will be
  Copper (i) oxide : +1
  Copper (ii) oxide : +2
  Manganese (iv) oxide : +4

Question 8

Give formula of chlorate (iii) ion and overall charge

Answer 8

Overall charge : -1
ClO2^-

Question 9

State what gets oxidies and reduced in the equation : KMnO4 + HCl -> MnCl2 + Cl2 + H2O

Answer 9

Oxidised : Cl^-
* Its an ion because the chlorine in HCl is a negative ion
Reduced : KMnO4
* K and O dont get oxidised nore reduced but they are just spectator ions

Question 10

Deduce half eqaution of chloride ions to chlorine 16H^+ + 2MnO4^- + 10Cl^- > 2Mn^2+ + 8H2O + 5Cl2

Answer 10

2CL^- -> Cl2 + 2e^-

Question 11

The ionic equation for this redox reaction is 16H^+ + 2MnO4^− + 10Cl− -> 2Mn2^+ + 8H2O + 5Cl2
(i) Give the oxidation state of manganese in the MnO4^− ion
(ii) Deduce the half-equation for the reduction of the MnO4^−
ions in acidified solution to manganese(II) ions and water.

Answer 11

(i) +7
(ii) MnO4^ − + 8H^+ + 5e^−
-> Mn^2+ + 4H2O

Question 12

Which of these is the best reducing agent
1. Cl2
2. I2
3. Cl^-
4. I^-

Answer 12

3 and 4 are better than 1 and 2 as they have a lower oxidation state
4 is better than 3 because it is a bigger atom as it has stronger forces etc.
4 has a larger atomic radius so it has a weaker attraction from nucleus to outer electrons and can be lost more easily

Question 13

Deduce an equation for hydrogen iodide from its elements

Answer 13

0.5 H2 + 0.5 I2 -> HI

Question 14

(a) Write a half-equation for the conversion of NO2^- in an acidic solution into NO
(b) Write a half-equation for the conversion of I^− into I2

Answer 14

(a) NO2^− + e^− + 2H^+
⟶ NO + H2O
(b) 2I− ⟶ I2 + 2e−

Question 15

When chlorine is added to water, there is a redox reaction, as shown by the equation
Cl2 + H2O ⇌ HClO + HCl
Give two half-equations to show the oxidation and reduction processes that occur in this redox reaction.

Answer 15

Oxidation half-equation : Cl2 + 2H2O → 2HClO + 2H^+ + 2e^–
Reduction half-equation : Cl2 + 2H^+ + 2e^– → 2HCl

Question 16

Chlorine is reacted with cold, aqueous sodium hydroxide in the manufacture of bleach. Give an equation for this reaction between chlorine and sodium hydroxide.

Answer 16

2NaOH + Cl2 → NaCl + NaClO + H2O

Question 17

Give the equation for the reaction between iron and sulfuric acid

Answer 17

Fe + H2SO4 → FeSO4 + H2

Question 18

Give the overall ionic equation for the oxidation of Fe2+ by manganate(VII) ions, in
acidic conditions.

Answer 18

5Fe2+ + MnO4^– + 8 H^+ → 5Fe3^+ + Mn2^+ + 4 H2O

Question 19

Name the pieces of equipment
(i) to take 25.0cm^3 portions
(ii) To add potassium manganese solution

Answer 19

(i) pipette
(ii) burette
Graduated pipette and graduated burette are also allowed

Question 20

The balance used to weigh the 680 mg of iron wire has an uncertainty of ±0.005 g. A container was weighed and its mass was subtracted from the total mass of the container and wire. Calculate the percentage uncertainty in using the balance.

Answer 20

(0.005 divided by 680m) x 100 = 0.735
0.735 x 2 = 1.47%

Question 21

State formula of Chlorate(i) ions

Answer 21

ClO^-

Question 22

A solution of sodium chlorate(l) was added to a colourless solution of potassium iodide.
Suggest what is observed.
Explain the reaction that leads to this observation.

Answer 22

Goes brown
Due to iodine as iodine is oxidised

Question 23

Define in terms of electron transfer meaning of terms
(i) oxidising agent
(ii) reducing agent

Answer 23

(I) Electron acceptor
(ii) electron donor

Question 24

Suggest one reason why the reaction of sodium fluoride with concentrated
sulfuric acid is different from the reaction with sodium iodide.

Answer 24

Fluoride less powerful reducing agent (than iodide)
or fluorine less easily oxidised than iodide

Question 25

Household bleach contains sodium chlorate(I) and sodium chloride.
State and explai, why it is
dangerous to acidify an aqueous mixture of sodium chlorate(I) and sodium chloride.

Answer 25

Producing chlorine is toxic

Question 26

Chlorine behaves as an oxidising agent in the extraction of bromine from seawater. In this process, chlorine gas is bubbled through a solution containing bromide ions.
Write the simplest ionic equation for the reaction of chlorine with bromide ions.

Answer 26

Cl2 + 2Br^− ->
2Cl^− + Br2

Question 27

In sunlight, chlorine can also oxidise water slowly to form oxygen.
Write an equation for this reaction.

Answer 27

2Cl2 + 2H2O -> 4HCl + O2

Question 28

The pool manager maintains the water at a pH slightly greater than 7.0
Explain how this affects the equilibrium established when chlorine is added to water

Answer 28

Hydroxide / alkali ions react with the acids
Equilibrium moves to the right

Question 29

Explain why chlorine is used to kill bacteria in swimming pools, even though chlorine is toxic.

Answer 29

Only used in small amounts
The health benefits outweigh the risks

Question 30

Antimony can be extracted by reacting scrap iron with low-grade ores that contain antimony sulfide (Sb2S3).
Write an equation for the reaction of iron with antimony sulfide to form
antimony and iron(II) sulfide.
(ii) Write a half-equation to show what happens to the iron atoms in this reaction.

Answer 30

3Fe + Sb2S3 -> 3FeS + 2Sb
(ii) ) Fe -> Fe2^+ + 2e^−

Question 31

In the first stage of the extraction of antimony from a high-grade ore, antimony sulfide is roasted in air to convert it into antimony(III) oxide (Sb2O3) and sulfur dioxide. Write an equation for this reaction.
(ii) Identify one substance that is manufactured directly from the sulfur dioxide formed in this reaction.

Answer 31

Sb2S3 + 4.5O2 -> Sb2O3 + 3SO2
(ii) SO3 or sulfur trioxide

Question 32

The student found that the calculated mass of FeSO4.7H2O was greater than the actual mass of the sample that had been weighed out. The student realised that this could be due to the nature of the impurity.
Suggest one property of an impurity that would cause the calculated mass of FeSO4.7H2O in X to be greater than the actual mass of X.
Explain your answer.

Answer 32

(Impurity is a) reducing agent

Such that for a given mass, the impurity would react with more dichromate
than a similar mass of FeSO4.7H2O
OR
for equal masses of the impurity and FeSO4.7H2O , the impurity would
react with more dichromate.

Question 33

Use your knowledge of metal reactivity to state and explain a low-cost method for the extraction of copper from a dilute aqueous solution of copper(II) sulfate.
Write the simplest ionic equation for the reaction that occurs during this extraction process.

Answer 33

M1 add scrap / recycled / waste iron (or steel) to the aqueous solution
M2 the iron is a more reactive metal OR Fe is a better reducing agent
M3 Cu2+ / copper ions are reduced / gain electrons
OR Cu2+ + 2e− Cu
OR copper / Cu is displaced by Fe
M4 Fe + Cu 2+ Fe 2+ + Cu ONLY

Question 34

Give one reason why titanium cannot be extracted directly from titanium(IV) oxide using carbon.

Answer 34

Ti is not produced
TiC / carbide is produced OR titanium reacts with carbon
OR
Product is brittle
OR
Product is a poor engineering material

Question 35

Scrap iron is used to extract copper from dilute aqueous solutions containing copper(II) ions.
Explain, in terms of redox, what happens to the copper(II) ions in this extraction

Answer 35

M1 The Cu2+ / copper(II) ions / they have gained (two) electrons
OR Cu2+ + 2e– Cu
M2 The Cu2+ / copper(II) ions / they have been reduced

Question 36

Aluminium is an expensive metal because it is extracted from molten aluminium oxide using electrolysis.
Write the half-equation for the reaction that occurs at the positive electrode during this extraction

Answer 36

2O2^– -> O2 + 4e–

Question 37

State why iodide ions are better reducing agents than chloride ions

Answer 37

Iodide ions have a larger atomic radius than chloride ions
Electron lost from an iodide ion is less strongly held by the nucleus compared with that lost from a chloride ion