3.1.7 Oxidation, Reduction and Redox Equations

Question 1

What is oxidation?

Answer 1

The loss of electrons.
OR
Gain of oxygen (or any electronegative element)
OR
Loss of hydrogen (or any electropositive element)

Question 2

What is reduction?

Answer 2

The gain of electrons
OR
Loss of oxygen (or any electronegative element)
OR Gain of hydrogen (or any electropositive element)

Question 3

What is an oxidising agent?

Answer 3

A species that gains electrons, reduing itself.

Question 4

What is a reducing agent?

Answer 4

A species that loses electrons, oxidising itself.

Question 5

What are the half equations and the ionic equation for?
SnO + Zn -> ZnO + Sn

Answer 5

Half equations:
- Sn²⁺ + 2e⁻ -> Sn
- Zn -> Zn²⁺ + 2e⁻
Ionic equation:
- Sn²⁺ + Zn -> Sn + Zn²⁺

Question 6

Define oxidation state.

Answer 6

A number which represents the number of electrons lost or gained by an atom of that element in the compound.
(If electrons are lost number is positive, if electrons are gained number is negative.)

Question 7

What is the oxidation state of oxygen in OF₂?

Answer 7

[O] = +2
(Otherwise [O] = -2 in most other compounds)

Question 8

What is the oxidation state of hydrogen in KH?

Answer 8

[H] = -1
([H] = -1 is for metal hydrides. Otherwise [H] = +1 in most other compounds)

Question 9

What is the oxidation state of chlorine in NaClO?

Answer 9

[Cl] = +1
(Otherwise [Cl] = -1 in most other compounds. NaClO is formed in a disproportionation reaction.)

Question 10

Define the term disproportionation.

Answer 10

Where in a redox reaction, the oxidation states of atoms of the same element, increase for some atoms, whereas decrease for some atoms.

Question 11

What is the oxidation state of phosphorus in PCl₅?

Answer 11

[P] = +5

Question 12

What is the oxidation state of nitrogen in ammonia?

Answer 12

[N] = -3

Question 13

What is the oxidation state of arsenic in AsO₄³⁻?

Answer 13

[As] = +5

Question 14

What is the oxidation state of iron in K₄Fe(CN)₆?

Answer 14

[Fe] = +2

Question 15

Why is 1s²2s²2p⁵ a weaker reducing agent that 1s²2s²2p⁶3s²3p⁶4s²?

Answer 15

The 2p electron is closer to the nucleus (smaller atom) than the 4s electron. Hence the nuclear attraction is stronger so the 2p electron is lost less easily than the 4s electron.

Question 16

What happens in a redox reaction?

Answer 16

• Electrons are transferred from one species (element) to another.
• One element is oxidised whilst another is reduced.

Question 17

Why is,
2CrO₄²⁻ + 2H⁺ -> Cr₂O₇²⁻ + H₂O
not a redox reaction?

Answer 17

Chromium and hydrogen have the same oxidation state (no element is reduced and oxidised).