5.2.3 Redox and Electrode Potentials: Module 5: Physical Chemistry & Transition Elements: Chemistry OCR A A Level

Define oxidising agent.

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

Define reducing agent.

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

Define oxidation.

- Loss of electrons.

- An increase in the oxidation number.

Define reduction.

- Gain of electrons.

- Decrease in the oxidation number.

What happens in a redox reaction?

Electrons are transferred from one species to another. One element is oxidised whilst the other is reduced.

Define standard electrode potential.

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 100 kPa.

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

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

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

Zn (s) ⇌ Zn^2+ (aq) + 2e^-

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

Cu^2+ (aq) ⇌ Cu^3+ (aq) + e^-

What is a standard hydrogen half cell made of?

● Hydrochloric acid 1 mol•dm^-3

● Hydrogen gas at 100 kPa

● Inert platinum electrode

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

Easy to control its purity and reproducibility.

How to make a simple salt bridge?

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

Why are salt bridges necessary?

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.

Why might you use other standard electrodes occasionally?

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

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

Better reducing agent (easier to oxidise).

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

Better oxidising agent (easier to reduce).

When would you use a Platinum electrode?

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

Why is Platinum chosen?

Inert and good conductor to complete circuit.

How would you predict if a reaction would occur?

1. Take the 2 half equations.

2. Find the species that is being reduced.

3. Calculate its E° value minus the E° value of the species that is being oxidised.

4. If E° overall > 0.4V , reaction will occur.

What are the 3 main types of electrochemical cells?

● Non rechargeable cells

● Rechargeable cells

● Fuel cells

Describe how non rechargeable cells work.

They provide electrical energy until all the chemicals have reacted.

Describe how rechargeable cells work.

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

Give some examples of rechargeable cells.

● Nickel and cadmium batteries

● Lithium ion batteries

● Lithium polymer batteries

Explain why lithium is used in laptop batteries.

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

What are the drawbacks of using lithium batteries?

● They are toxic if ingested.

● Rapid discharge of current can cause fire or even explosions.

Describe how fuel cells work.

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

Modern fuel cells are based on what type of fuels?

● Hydrogen

● Hydrogen rich fuels e.g methanol

What are the disadvantages of fuel cells?

1. Hydrogen is a flammable gas with a low b.p. → hard and

dangerous to store and transport → expensive to buy

2. Fuel cells have a limited lifetime and use toxic chemicals in their manufacture.

What is the reason that some cells cannot be recharged?

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

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

Concentrations of the ions change - the reagents are used up.

How can the E.M.F. of a cell be kept constant?

Reagents are supplied constantly, so the concentrations of the ions are constant; E° remains constant.