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 oxidised

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

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

Answer 6

Half Equations:
Sn²⁺ + 2e⁻ -> Sn
Zn -> Zn²⁺ + 2e⁻

Question 7

Define standard electrode potential

Answer 7

The emf of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentration of 1 mol dm^-3 and a gas pressure of 100kPa

Question 8

What happens when a rod of a metal is dipped into a solution of its own ions?

Answer 8

An equilibrium is set up between the solid metal and the aqueous metal ions

Question 9

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

Answer 9

Zn (s) ⇌ Zn²⁺ (aq) + 2e⁻

Question 10

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

Answer 10

Cu²⁺ ⇌ Cu³⁺ + e⁻

Question 11

What is a standard hydrogen half cell made of?

Answer 11

• Hydrochloric acid 1 mol dm^-3
  -Hydrogen gas 100kPa
• Inert platinum electrode

Question 12

Why is hydrogen half cell used as a standard half cell?

Answer 12

• Easy to control its purity and reproducibility

Question 13

How to make a simple salt bridge?

Answer 13

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

Question 14

Why are salts bridges necessary?

Answer 14

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 15

Why might you use other other standard electrodes occasionally?

Answer 15

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

Platinum is expensive

Question 16

If an E⁰ value is more negative, what does it mean in terms of oxidising/reducing power?

Answer 16

better reducing agent
(easier to oxidise)

Question 17

If an E⁰ value is more positive, what does it mean in terms of oxidising/reducing power?

Answer 17

Better oxidising agent (easier to reduce)

Question 18

How do you calculate the emf of a cell from E⁰ values?

Answer 18

E⁰ (cell) = E⁰ (positive) - E⁰ (negative)

Question 19

When would you use a platinum electrode?

Answer 19

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

Question 20

Why is platinum chosen?

Answer 20

Inert and good conductor to complete circuit

Question 21

How would you predict if a reaction would occur?

Answer 21

Take the 2 half equations
Find the species that is being reduced
Calculate the E⁰ value minus the E⁰ value of the species that is being oxidised
If E⁰ overall > 0.4V reaction will occur

Question 22

What are the 3 main types of electrochemical cells?

Answer 22

• Non rechargeable cells
• Rechargeable cells
• Fuel cells

Question 23

Describe how non rechargeable cells work

Answer 23

They provide electrical energy until all the chemicals have reacted

Question 24

Describe how rechargeable cells work

Answer 24

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

Question 25

Give some examples of rechargeable cells

Answer 25

Nickel and cadmium batteries
Lithium ion batteries
Lithium polymer batteries

Question 26

Explain why lithium is used in laptop batteries

Answer 26

Lithium has low density so the electrode is light and very reactive

Question 27

What are the drawbacks of using lithium batteries?

Answer 27

• They are toxic if indigested
• Rapid discharge of current can cause fire or even explosions

Question 28

Describe how fuel cells work

Answer 28

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

Question 29

Modern fuel cells are based on which type of fuels?

Answer 29

• Hydrogen
• Hydrogen rich fuels

Question 30

What are the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?

Answer 30

2H₂ + 4OH⁻ -> 4H₂O + 4e⁻

O₂ + 2H₂O + 4e⁻ -> 4OH⁻

Question 31

What are the disadvantages of fuel cells?

Answer 31

Hydrogen is a flammable gas with a low bp
Hard and dangerous to store and transport
expensive to buy
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture

Question 32

What is the reason that some cells cannot be recharged?

Answer 32

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

Question 33

Why might the emf of a cell change over a period of time?

Answer 33

Concentrations of the ion change
the reagents are used up

Question 34

How can the emf of a cell be kept constant?

Answer 34

Reagent are supplied constantly so the concentrations of the ions are constant.
Eo remains constant.