Topic 3 - Redox 1

Question 1

What is an oxidation number?

Answer 1

The number of electrons which would be added or removed to make the species neutral

Question 2

What are the three benchmark elements when determining oxidation numbers, and what are their oxidation numbers?

Answer 2

F, O and H
F always -1
H = +1 (except in H2 = 0 and in a metal hydride eg NaH = -1)
O = -2 (except in O2 = 0 and H2O2 = -1)

Question 3

Define oxidation

Answer 3

The loss of electrons (and an increase in oxidation number)

Question 4

Define reduction

Answer 4

The gain of electrons (and a decrease in oxidation number)

Question 5

Define disproportionation

Answer 5

When the same element in a compound is simultaneously oxidised and reduced

Question 6

What steps should you follow to balance a half equation (in acidic conditions)

Answer 6

1. Balance atoms undergoing oxidation or reduction
2. Balance O by adding H2O
3. Balance H by adding H+
4. Balance charge by adding e-

Question 7

What steps should you follow to balance a half equation in alkaline conditions

Answer 7

1. Balance as if in acidic conditions
2. Look how many H+ there are and add that same amount of OH- to both sides
3. On the side with the H+, turn the H+ and OH- into waters
4. Cancel any water appearing on both sides

Question 8

What are the steps to balance using oxidation numbers?

Answer 8

1. Identify which species are being oxidised and reduced and work out their initial and final oxidation numbers
2. Calculate how many electrons have been lost and gained by each species
3. Cross multiply to generate the stoichiometric coefficients for the reaction
4. Place these big numbers in front of the reactants and balance the rest by inspection.