Topic 3 - Redox I

Question 1

What does an atom’s oxidation number tell you?

Answer 1

How many electrons an atom has donated or accepted to form an ion or form part of a compound.

Question 2

What is an atom’s oxidation number the same as?

Answer 2

Its charge.

Question 3

How does an atom’s oxidation number relate to a molecular ion’s overall charge?

Answer 3

The charge is equal to the sum of the oxidation numbers.

Question 4

Describe the oxidation number of hydrogen.

Answer 4

• Usually -> +1
• In metal hydrides -> -1
• In molecular hydrogen -> 0

Question 5

Describe the oxidation number of oxygen.

Answer 5

• Usually -> -2
• In peroxides (O₂²⁻) -> -1
• In molecular oxygen -> 0

Question 6

How can the oxidation number of an atom or group be indicated in a formula?

Answer 6

Using Roman numerals.

Question 7

How does the position of a Roman numeral in a formula relate to the oxidation numbers of the atoms or groups?

Answer 7

The Roman numeral is always after the thing it describes.

Question 8

What is oxidation?

Answer 8

A loss of electrons.

Question 9

What is oxidation?

Answer 9

Loss of electrons

Question 10

What is reduction?

Answer 10

Gain of electrons

Question 11

What is a redox reaction?

Answer 11

Where oxidation and reduction happen simultaneously.

Question 12

What happens to the oxidation number when an electron is lost?

Answer 12

Increases by 1

Question 13

What happens to the oxidation number when an electron is gained?

Answer 13

Decreases by 1

Question 14

How can you work out if something has been oxidised or reduced?

Answer 14

• Assign oxidation numbers to the element before and after the reaction
• If the oxidation number increases -> This is oxidation
• If the oxidation number decreases -> This is reduction

Question 15

What is a disproportionation reaction?

Answer 15

When a single species is simultaneously oxidised and reduced.

Question 16

What types of equation show oxidation or reduction?

Answer 16

Half-equations

Question 17

Describe the process of writing a full equation using half-equations of reduction and oxidation.

Answer 17

1) Write down the half-equations
Look at each equation and:
2) Balance the oxygens using water on the right
3) Balance the hydrogens by adding H⁺’s on the left
4) Balance the charges using electrons
5) Make sure the electrons in each half-equation are the same
6) Now, combine the two half-equations

(See example pg 41 of revision guide)

Question 18

When balancing redox reaction, what 3 things can be used?

Answer 18

• H⁺
• H₂O
• e⁻

Question 19

Acidified manganate(VII) ions (MnO₄⁻) can be reduced to Mn²⁺ by Fe²⁺ ions. Write the full equation for this reaction.

Answer 19

For manganate:
• MnO₄⁻ -> Mn²⁺
• MnO₄⁻ -> Mn²⁺ + 4H₂O
• MnO₄⁻ + 8H⁺ -> Mn²⁺ + 4H₂O
• MnO₄⁻ + 8H⁺ + 5e⁻ -> Mn²⁺ + 4H₂O
For iron:
• Fe²⁺ -> Fe³⁺ + e⁻
• 5Fe²⁺ -> 5Fe³⁺ + 5e⁻
Overall:
• MnO₄⁻ + 8H⁺ + 5Fe²⁺ -> Mn²⁺ + 5Fe³⁺ + 4H₂O