9: Redox reactions

Question 1

What happens in a redox reaction

Answer 1

One reactant loses electrons (oxidation) and another gains electrons (reduction)

Question 2

What is a oxidising agent

Answer 2

The substance that is reduced

Question 3

What is a reducing agent

Answer 3

The substance that is oxidised

Question 4

If the oxidation number becomes more positive has the element been oxidised or reduced

Answer 4

Oxidised

Question 5

If the oxidation number becomes less positive has the element been oxidised or reduced

Answer 5

Reduced

Question 6

What is it called when a substance is oxidised and reduced in a single reaction

Answer 6

Disproportionation

Question 7

Method for constructing Half Equations

Answer 7

1. Balance elements other than H and O
2. Balance oxygen using H₂O
3. Balance hydrogen using H†
4. Balance charges using e¯

Question 8

Method for constructing half equations in alkaline solutions

Answer 8

1. Balance elements other than H and O
2. Balance oxygen using H₂O
3. Balance hydrogen using H†
4. Balance charges using e¯
5. Add an OH¯ to each side for each H† in the equation
6. Cancel any waters that are on both sides

Question 9

How to combine half equations

Answer 9

1. Check that one half equation is going in the reduction direction and the other in the oxidation direction.
2. If necessary multiply one or other half equation by whatever will give you equal numbers of electrons in both half equations.
3. Add the two half equations together. The electrons should cancel out.
4. Cancel out anything else that appears on both sides of the equation.

Question 10

Is KMnO₄(aq) a oxidising or reducing agent

Answer 10

It is a powerful oxidising agent

Question 11

What condition is needed for Potassium Manganate(VII) titrations

Answer 11

Acidic solution is needed because the reduction half equation involves a lot of H† ions

Question 12

What colour change is seen in Potassium Manganate (VII) titrations

Answer 12

Colourless → pale pink

Question 13

What is the half equation for potassium manganate

Answer 13

MnO₄¯(aq) + 8H†(aq) + 5e¯ → Mn²†(aq) + 4H₂0(l)

Question 14

What is the most commonly used reducing agent for reducing I₂(aq) to I¯(aq)

(for titrations)

Answer 14

Thiosulfate (S₂O₃²¯)

Question 15

What is the colour change seen in iodine titration

Answer 15

Brown to colourless

Question 16

What is the colour change seen in iodine titration with a starch indicator

Answer 16

Blue/black to colourless

Question 17

What piece of information can you use to work out the oxidation state of an element

Answer 17

The moles of electrons lost or gained

eg. When Sn²† is oxidised to Sn⁴† , the oxidation state increases by 2 which means that 2 moles of electrons are released for each mole of Sn⁴† that reacts.