23 Redox and Electrode Potentials

Question 1

Oxidising agent

Answer 1

Takes electrons from species being oxidised, contains species which is reduced.

Question 2

Reducing agent

Answer 2

Adds electrons to species being reduced, contain species being oxidised.

Question 3

How do you construct redox equations using half equations?

Answer 3

1. Balance the electrons.
2. Add the two equations together and cancel out electrons.
3. Cancel any species on both sides of the equations.

Question 4

Predicting half equations using oxidation numbers

Answer 4

1. Assign oxidation numbers.
2. Balance the electrons.
3. Balance remaining atoms and predict any further species e.g. to balance O, add H2O and balance H.

Question 5

Manganate (VII) redox titration

Answer 5

• Titrate solution with standard solution of potassium manganate.
• Read burette from top of meniscus due to dark colour of KMnO4.
• End point is judged by first permanent pink colour.
• Repeat until concordant titres are obtained.

Question 6

Standard electrode potential

Answer 6

The e.m.f. of a half-cell connected to a standard hydrogen half-cell under standard conditions.

Electrode with more negative standard electrode potential is the more reactive metal and is oxidised.

Vice versa for electrode with more positive value.

Question 7

Equation for calculating standard cell potential

Answer 7

Standard electrode potential of positive electrode - standard electrode potential of negative electrode.

Question 8

What are the limitations of making predictions using standard electrode potential values?

Answer 8

• Cannot indicate rate of a reaction.
• If concentration is not 1 mol dm-3, then standard electrode potential will differ from standard value.
• Actual conditions may not be standard.

Question 9

Characteristics of a primary cell

Answer 9

Non-rechargeable, one-time use, chemicals get used up.

Question 10

Characteristics of a secondary cell

Answer 10

Rechargeable, reversible reactions, chemicals can be regenerated.

Question 11

What are the benefits of lithium ion cells?

Answer 11

Light metal, flexible so it can be formed into different shapes.

Question 12

What are the risks associated with lithium ion cells?

Answer 12

They are unstable at high temperatures, must be recycled carefully since Li is very reactive.

Question 13

Hydrogen fuel cells

Answer 13

• Different reactions at electrodes in acidic and alkaline conditions.
• Water is the only combustion product.
• Energy from reaction is used to create a voltage.