5.2.3: Redox and electrode potentials

Question 1

Define oxidising agent

Answer 1

A species that is reduced in a reaction
and causes another species to be
reduced

Question 2

Define reducing agent

Answer 2

A species that is oxidised in a reaction
and causes another species to be
reduced

Question 3

Define oxidation

Answer 3

Loss of electrons

An increase in the oxidation number

Question 4

Define reduction

Answer 4

Gain of electrons

Decrease in the oxidation number

Question 5

What happens in a redox reaction?

Answer 5

Electrons are transferred from one species to another.

One element is oxidised whilst the other is reduced

Question 6

Why is 2CrO4 2- + 2H+ → Cr2O7 2- + H2O not a redox reaction?

Answer 6

Chromium is oxidised whereas hydrogen remains the same oxidation state (no element is reduced).

Question 7

What are the half equations and the ionic equation

for: SnO + Zn → ZnO + Sn

Answer 7

Half Equations:
● Sn2+ + 2e- → Sn
● Zn → Zn2+ + 2e
Ionic Equation:
● Sn2+ + Zn → Sn + Zn2+

Question 8

Define standard electrode potential

Answer 8

The e.m.f. Of a half cell compared with a
standard hydrogen half cell measured at
298 K with solution concentration of 1
mol dm-3 and a gas pressure of 100kPa

Question 9

What happens when a rod of a metal is dipped into a

solution of its own ions?

Answer 9

An equilibrium is set up between the

solid metal and the aqueous metal ions

Question 10

Write a half-equation for zinc (s) to zinc (II).

Answer 10

Zn (s) ⇌ Zn2+(aq) + 2e

Question 11

Write a half-equation for copper (II) to copper (III).

Answer 11

Cu2+(aq) ⇌ Cu3+(aq) + e-

Question 12

What is a standard hydrogen half cell made of? (3 marks)

Answer 12

● Hydrochloric acid 1 mol dm-3
● Hydrogen gas at 100 kPa
● Inert platinum electrode

Question 13

Why is hydrogen half cell used as a standard half

cell?

Answer 13

Easy to control its purity and reproducibility

Question 14

How to make a simple salt bridge?

Answer 14

Soak a piece of filter paper in an aqueous solution of KNO3 or NH4NO3

Question 15

Why are salt bridges necessary?

Answer 15

To complete the circuit by connecting the
two solutions. This enables charge to be
transferred between the half cells. They
do not react with the electrodes

Question 16

Why might you use other standard electrodes

occasionally?

Answer 16

They are cheaper/easier/quicker to use
and can provide just as good a
reference.
Platinum is expensive

Question 17

If an Eo value is more negative, what does it mean in

terms of oxidising/reducing power?

Answer 17

Better reducing agent (easier to oxidise)

Question 18

If an Eo value is more positive, what does it mean in

terms of oxidising/reducing power?

Answer 18

Better oxidising agent (easier to reduce)

Question 19

How do you calculate the emf of a cell from Eo

values?

Answer 19

Eo cell = Eo positive - Eo negative

Question 20

When would you use a Platinum electrode?

Answer 20

When both the oxidised and reduced
forms of the metal are in aqueous
solution

Question 21

Why is Platinum chosen?

Answer 21

Inert and good conductor to complete circuit

Question 22

How would you predict if a reaction would occur?

Answer 22

Take the 2 half equations.
Find the species that is being reduced
Calculate its Eo value minus the Eo value of the species that is being oxidised
If Eo overall > 0.4V , reaction will occur.

Question 23

What are the 3 main types of electrochemical cells?

Answer 23

● Non rechargeable cells
● Rechargeable cells
● Fuel cells

Question 24

Describe how non rechargeable cells work

Answer 24

They provide electrical energy until all the chemicals have reacted

Question 25

Describe how rechargeable cells work

Answer 25

Chemicals in the cell provide electrical energy. When recharging the reactions of the cells can be reversed

Question 26

Give some examples of rechargeable cells

Answer 26

● Nickel and cadmium batteries
● Lithium ion batteries
● Lithium polymer batteries

Question 27

Explain why lithium is used in laptop batteries

Answer 27

Lithium has low density so the electrode

is light and it is very reactive.

Question 28

What are the drawbacks of using lithium batteries?

Answer 28

● They are toxic if ingested
● Rapid discharge of current can cause
fire or even explosions

Question 29

Describe how fuel cells work

Answer 29

The cell uses external supplies of fuel and an oxidant. These external supplies need to be continuously supplied.

Question 30

Modern fuel cells are based on what type of fuels?

Answer 30

● Hydrogen

● Hydrogen rich fuels e.g methanol

Question 31

What are the reactions that take place at the two

electrons in an alkaline hydrogen fuel cell?

Answer 31

2H2 + 4OH- → 4H2O + 4e-

O2 + 2H2O + 4e- → 4OH

Question 32

What are the disadvantages of fuel cells?

Answer 32

Hydrogen is a flammable gas with a low b.p. → hard and
dangerous to store and transport → expensive to buy
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture

Question 33

What is the reason that some cells cannot be

recharged?

Answer 33

Reaction of the cell is not reversible - a product is produced that either dissipates or cannot be converted back into the reactants

Question 34

Why might the e.m.f. Of a cell change after a period

of time?

Answer 34

Concentrations of the ions change - the reagents are used up

Question 35

How can the e.m.f. Of a cell be kept constant?

Answer 35

Reagents are supplied constantly, so the concentrations of the ions are constant;
Eo remains constant