Oxidation And Reduction

Question 1

What is oxidation

Answer 1

Oxidation is the process of electron loss:

Zn —> Zn2+ + 2e-

It involves an increase in oxidation number

Question 2

What is reduction

Answer 2

Reduction is the process of electron gain: Cl2 + 2e- —> 2Cl-
It involves a decrease in oxidation number

Question 3

Rules for assigning oxidation numbers

Answer 3

1. All uncombined elements have an oxidation number of zero
2. The oxidation numbers of the elements in a compound add up to zero
3. The oxidation number of a monoatomic ion is equal to the ionic charge
4. In a polyatomic ion (CO32-) the sum of the individual oxidation numbers of the elements adds up to the charge on the ion
5. Several elements have invariable oxidation numbers in their common compounds.

Question 4

Group 1 metals oxidation number

Answer 4

1

Question 5

Group 2 metals oxidation number

Answer 5

+2

Question 6

Al oxidation number

Answer 6

+3

Question 7

H oxidation number

Answer 7

+1 (except in metal hydrides where it is –1 eg NaH)

Question 8

F oxidation number

Answer 8

-1

Question 9

Cl, Br and I oxidation states

Answer 9

-1 except in compounds with oxygen and fluorine

Question 10

O oxidation state

Answer 10

-2, except in peroxides (H2O2 ) where it is –1 and in compounds with fluorine.

Question 11

Balancing redox equasions

Answer 11

1. Work out oxidation numbers for element being oxidised/ reduced
2. Add electrons equal to the change in oxidation number For reduction add e’s to reactants
   For oxidation add e’s to products
3. check to see that the sum of the charges on the reactant side equals the sum of the charges on the product side

Question 12

More complex half equasions

Answer 12

1. Write half equasion ( balance change in oxidation number with electrons)
2. Add H20 to products to balance the oxygen
3. Add H+ to reactants to balance the h20
4. Check to see that the sum of the charges on the reactant side equals the sum of the charges on the product side

Question 13

Oxidising agent

Answer 13

An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
An oxidising agent itself gets reduced – it gains electrons
Therefore, the oxidation state of the oxidising agent decreases

Question 14

Oxidising agent

Answer 14

An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
An oxidising agent itself gets reduced – it gains electrons
Therefore, the oxidation state of the oxidising agent decreases

Question 15

Reducing agent

Answer 15

A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons
A reducing agent itself gets oxidised – it loses/donates electrons
Therefore, the oxidation state of the reducing agent increases