Chapter 23 - Redox and electrode potentials

Question 1

What is an oxidising agent?

Answer 1

A species which oxidises other species and is itself reduced.

Question 2

What is a reducing agent?

Answer 2

A species which reduces other species and is itself oxidised.

Question 3

What is a redox reaction?

Answer 3

A reaction containing species which are oxidised and species that are reduced.

Question 4

What is a redox titration?

Answer 4

A titration in which a solution of reducing agent is titrated with a solution of an oxidising agent.

Question 5

What are manganate (VII) redox titrations used for?

Answer 5

Acidified manganate(VII) is an oxidising agent and manganate(VII) (MnO^4-) titrations are used for analysing reducing agents.

Question 6

What are iodine/thiosulfate titrations used for?

Describe the initial process.

Answer 6

Analysing oxidising agents.
The oxidising agent is reacted with an excess of iodide ions (e.g. 2I- found in KI). The iodide ions are oxidised to iodine.

The iodine generated is the titrated with thiosulfate ions, S203^2- (aq), until the solution turns colourless..

Question 7

What is an assumption made in iodine/thiosulfate calculations?

Answer 7

n(Cu^2+) = n(S2O3^2-)

Cu^2+ is an example of an oxidising agent that can be analysed using iodine/thiosulfate titrations.

Question 8

What is a half-cell?

Answer 8

A cell which contains the chemical species present in a redox half-equation.

Question 9

What is a voltaic cell?

Answer 9

A type of electrochemical cell which converts chemical energy into electrical energy.

A voltaic cell can be made by connecting two different half-cells, creating an electrode potential, which then allows electrons to flow.

Question 10

What is a metal/metal ion half-cell?

Answer 10

A metal rod dipped into a solution of its metal ion.
An equilibrium is set up between the metal ion and the metal.

e.g. Zn/Zn^2+ half-cell:

Zn^2+ + 2e- ⇌ Zn (s)
→ reduction
← oxidation

Question 11

What is a ion/ion half-cell?

Answer 11

A solution containing ions of the same element in different oxidation states (e.g. Fe^2+ and Fe^3+).

An inert platinum electrode is used to transport electrons into or out of the half-cell.

e.g. Fe^2+/Fe^3+ half-cell:

Fe^3+(aq) + e- ⇌ Fe^2+(aq)
→ reduction
← oxidation

Question 12

What is electrode potential, E?

What is the unit?

Answer 12

The tendency for electrons to be gained and for reduction to take place in a half-cell.
Unit: V

Question 13

What is the standard electrode potential?

Answer 13

The e.m.f. of a half-cell connected to a standard hydrogen half-cell under standard conditions (298K, 100kPa, 1 moldm^-3).

The standard electrode potential of a standard hydrogen half-cell under is 0V.

Question 14

What is the equation for the standard electrode potential of a cell?

Answer 14

E(positive electrode) - E(negative electrode)

Question 15

How are the standard electrode potentials of half-cells measured?

Answer 15

By connecting to a standard hydrogen electrode.

Question 16

What are the limitations of feasibility predictions using standard electrode potentials?

Answer 16

Standard electrode potentials only apply to aqueous equilbria.
Standard electrode potentials do not take account of activation energy and rate of reactions.

Question 17

What is a fuel cell?

Answer 17

A cell which uses the energy from the reaction of a fuel with oxygen to create a voltage.

• Oxygen is at the positive electrode where reduction takes place.
• The fuel is at the negative electrode where oxidation takes place.

Question 18

What is a hydrogen fuel cell?

Answer 18

A fuel cell which can operate with an acid or alkali electrolyte.

2H+(aq) + 2e- ⇌ H2(g)
O2(g) + 4H+(aq) + 4e- ⇌ 2H2O(l)
Overall cell reaction: 2H2(g) + O2(g) → 2H2O(l)