Chapter 7 Redox reactions

Question 1

Oxidation Numbers

Answer 1

• The oxidation numbers (also known as oxidation state) is a number given to each atom or ion in a compound to keep track of how many electrons they have
• In a single ion or molecular ion, the oxidation number tells us how many electrons have been lost or gained
• Positive oxidation number = loss of electrons
• Negative oxidation number = gain of electrons

Question 2

Oxidation number rules

Answer 2

1. The ox.no of any uncombined element is zero
2. Many atoms or ions have fixed ox.no in compounds
3. The ox.no of an element in a mono-atomic ion i always the same as the charge
4. The sum of the ox.no in a compound is zero
5. The sum of ox.no in an ion is equal to the charge on the ion
6. In either a compound or an ion, the more electronegative element is given by the ox.no

Question 3

Balancing Redox Reactions

Answer 3

• Oxidation numbers can be used to balance chemical equations
• Roman numerals between brackets are used to show the ox. no. of an atom that can have multiple oxidation states, eg:

Fe(III) = iron with ox. no. +3

Question 4

Oxidation

Answer 4

• Oxidation is the gain of oxygen
• Oxidation is also the loss of a hydrogen
• Oxidation is also the loss of electrons

Question 5

Reduction

Answer 5

• Reduction is the loss of oxygen
• Reduction is also the gain of a hydrogen
• Reduction is also the gain of electrons

Question 6

Redox reactions

Answer 6

• Redox reactions are reactions in which oxidation and reduction take place together
• While one species is oxidising, another is reducing in the same reaction, eg:

Cu2++ Mg → Mg2+ + Cu

(Cu has been reduced and Mg has been oxidised)

Question 7

Disproportionation reactions

Answer 7

is a reaction in which the same species is both oxidised and reduced

Question 8

Oxidising agent

Answer 8

is a substance that oxidises another atom or ion by causing it to lose electrons

• An oxidising agent itself gets reduced – gains electrons
• Therefore, the ox. no. of the oxidising agent decreases

Question 9

Reducing agent

Answer 9

is a substance that reduces another atom or ion by causing it to gain electrons

• A reducing agent itself gets oxidised – loses/donates electrons
• Therefore, the ox. no. of the reducing agent increases

Question 10

For a reaction to be recognised as a redox reaction

Answer 10

there must be both an oxidising and reducing agent

• Some substances can act both as oxidising and reducing agents
• Their nature is dependent upon what they are reacting with and the reaction conditions