Redox

Question 1

Define a redox reaction.

Answer 1

A redox reaction is a chemical reaction involving both oxidation and reduction simultaneously.

Question 2

Define oxidation, in terms of oxygen / hydrogen gain / loss.

Answer 2

A process whereby a substance gains oxygen or loses hydrogen in a chemical reaction.

Question 3

Define reduction, in terms of oxygen / hydrogen gain / loss.

Answer 3

A process whereby a substance loses oxygen or gains hydrogen in a chemical reaction.

Question 4

Define the oxidation number of an atom.

Answer 4

The charge that the atom carries as a monoatomic ion.

Question 5

In a compound, state the nature of the oxidation number of the more electronegative element.

Answer 5

The oxidation number of the more electronegative element is negative.

Question 6

State the oxidation number (O.S.) for the following elements as well as the exceptions, if any.
1: hydrogen, H
2: oxygen, O
3: Group 1 metals
4: Group 2 metals
5: aluminium, Al
6: fluorine, F
7: chlorine, Cl

Answer 6

1: +1
Exception: O.S. of H in metal hydride is -1
2: -2
Exception: O.S. of O in peroxide is -1
3: +1
4: +2
5: +3
6: -1
7: -1

Question 7

Why does fluorine have an oxidation number of -1 in all compounds?

Answer 7

Fluorine is the most electronegative element and it forms one covalent bond. The shared electron pair in the single covalent bond will be assigned to fluorine and hence its oxidation number is -1.

Question 8

State the oxidation number (O.S.) of each element in the following compounds.
1: MgBr2
2: CO2
3: Cl2
4: H2O2
5: SF4
6: SO3

Answer 8

1:
O.S. of Mg= +2
O.S. of Br= -1
2:
O.S. of C= +4
O.S. of O= -2
3:
O.S. of Cl= 0, since there is no difference in electronegativity between the 2 Cl atoms
4:
O.S. of H= +1
O.S. of O= -1
5:
O.S. of S= +4
O.S. of F= -1
6:
O.S. of S= +6
O.S. of O= -2

Question 9

Define oxidation, in terms of electron transfer.

Answer 9

Oxidation is a process whereby a substance loses electrons, causing an increase in oxidation number.

Question 10

Define reduction, in terms of electron transfer.

Answer 10

Reduction is a process whereby a substance gains electrons, causing a decrease in oxidation number.

Question 11

In terms of changes in oxidation number, state which substance is oxidised and reduced.
2NH3 (g) + 3CuO (s) -> N2 (g) + 3Cu (s) + 3H2O (l)

Answer 11

NH3 is oxidised as there is an increase in oxidation number of N from -3 in NH3 to 0 in N2.
CuO is reduced as there is a decrease in oxidation number of Cu from +2 in CuO to 0 in Cu.

Question 12

Identify the oxidising agents and reducing agents in the following substances:
1. oxygen, O2
2. carbon, C
3. hydrogen, H2
4. chlorine, Cl2
5. carbon monoxide, CO
6. aluminium, Al
7. zinc, Zn
8. potassium manganate (VII), KMnO4
9. hydrogen peroxide, H2O2
10. potassium iodide, KI
11. potassium dichromate (VI), K2Cr2O7

Answer 12

Oxidising agents:
1, 4, 8, 9, 11

Reducing agents:
2, 3, 5, 6, 7, 10

Question 13

For the redox reactions below, identify the reducing and oxidising agents and explain why.
Reaction 1:
H2O2 (aq) + 2Fe^2+ (aq) + 2H^+ (aq) → 2Fe^3+ (aq) + 2H2O (l)

Reaction 2:
NO3^− (aq) + 4Zn (s) + 7OH^– (aq) + 6H2O (l) → 4Zn(OH)4^2– (aq) + NH3 (aq)

Answer 13

Reaction 1:
Fe^2+ is the reducing agent as it reduces H2O2 since there is a decrease in oxidation number of O from -1 in H2O2 to -2 in H2O.
H2O2 is the oxidising agent as it oxidises Fe^2+ since there is an increase in oxidation number of Fe from +2 in Fe^2+ to +3 in Fe^3+.

Reaction 2:
Zn is the reducing agent as it reduces NO3^− since there is a decrease in oxidation
number of N from +5 in NO3^− to −3 in NH3.
NO3^− is the oxidising agent as it oxidises Zn since there is an increase in oxidation number of Zn from 0 in Zn to +2 in Zn(OH)4^2−.

Question 14

Which substance can act as both oxidising agent and reducing agent?

Answer 14

hydrogen peroxide, H2O2

Question 15

State the reducing agent used to test for oxidising agent.

Answer 15

aqueous potassium iodide, KI
(the K+ ion is the spectator ion as I- ion is the species that causes reduction of another substance)

Question 16

Describe the procedure of testing for oxidising agent using aqueous potassium iodide, KI, and state the positive observation.

Answer 16

Procedure: Add a few drops of aqueous potassium iodide to the solution to be tested.

Positive observation: Colourless solution (containing KI) turns brown. This is because I- ion is colourless and during the reaction, it is oxidised to I2 which is brown.

Question 17

List the oxidising agents that can be used to test for reducing agent.

Answer 17

1. acidified potassium manganate(VII), KMnO4

(the K+ ion is the spectator ion as MnO4- ion is the species that causes oxidation of another substance)

2. acidified potassium dichromate(VI), K2Cr2O7

(the K+ ion is the spectator ion as Cr2O7 ^2- ion is the species that causes oxidation of another substance)

Question 18

Describe the procedure of testing for reducing agent using aqueous potassium manganate(VII), KMnO4, and state the positive observation.

Answer 18

Procedure: Add dilute sulfuric acid, followed by a few drops of aqueous potassium manganate.

Positive observation: Purple solution (containing KMnO4) turns colourless. This is because MnO4- ion is purple in colour and during the reaction, it is reduced to Mn2+ which is colourless.

Question 19

Describe the procedure of testing for reducing agent using aqueous potassium dichromate(VI), KCr2O7, and state the positive observation.

Answer 19

Add dilute sulfuric acid followed by aqueous potassium dichromate(VI).

Positive observation: Orange solution turns green. This is because Cr2O7 ^2- ion is orange in colour and during the reaction, it is reduced to Cr ^3+ which is green.

Question 20

Describe the colour change that will be observed when potassium iodide is gradually added to a solution containing acidified potassium manganate(VI).

Answer 20

Purple solution turns brown.

Question 21

Given that Fe is above Cu in the reactivity series, write the end products (with state symbols) of the reaction below.
Fe(s) + CuSO4(aq)

Answer 21

FeSO4(aq) and Cu(s)

Question 22

A more reactive metal loses electrons and become oxidised while a less reactive metal gains electron and become reduced.
True / False?

Answer 22

True

Question 23

Reactions of three metals, W, X and Y, or compounds of these metals, are described.
1. The oxide of W is reduced both by heating with carbon and by heating with hydrogen.
2. The oxide of X is not reduced by heating with carbon or by heating with hydrogen.
3. W displaced Y from an aqueous solution of the sulfate of Y.
List the three metals from least reactive to most reactive.

Answer 23

Y, W, X

Question 24

Define disproportionation.

Answer 24

Disproportionation is a redox reaction in which a single substance is simultaneously oxidised and reduced to form two different products.

Question 25

Identify the disproportionation reactions from the following reactions.
Reaction 1:
H2SO4 + 2HBr → SO2 + Br2 + 2H2O
Reaction 2:
3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O
Reaction 3:
IO3 ^- + 5I ^- + 6H ^+ -> 3I2 + 3H2O
Reaction 4:
2NO2 + H2O → HNO3 + HNO2
Reaction 5:
2H2O2 → 2H2O + O2

Answer 25

Reactions 2, 4, 5

Question 26

Why is an external indicator not required to be added in potassium manganate(VII) titration?

Answer 26

Since KMnO4(aq) is purple in colour while its reduced product, Mn ^2+(aq), is faint pink which is essentially colourless, an external indicator is not required to be added in the course of the titration. The endpoint is taken when the first permanent pink coloration is formed because of an excess drop of potassium
manganate(VII) added.

Question 27

Why is an external indicator not required to be added in potassium dichromate(VI) titration?

Answer 27

Upon reduction in an acidic medium, the orange dichromate(VI) ion changes to green chromium(III) ions.

Question 28

State the 2 main substances involved in an iodometric titration and briefly state its function.

Answer 28

Thiosulfate ions and iodine

Function: To determine the amount of iodine present in a solution.

Question 29

Describe an iodometric titration as well as the endpoint of the titration.

Answer 29

As thiosulfate ions are added, it would reduce the iodine, resulting in a decrease in the concentration of iodine. The resultant solution will approach a pale yellow coloration.
The endpoint of the titration occurs when a distinct colour change from deep blue to colourless is observed.

Question 30

Suggest what can be done to obtain a more accurate detection of the endpoint during an iodometric titration.

Answer 30

Add a few drops of starch solution when the solution turns pale yellow.