1.7 Redox

Question 1

What is Oxidation?

Answer 1

The Process of electron loss.

Question 2

What is Oxidation?

Answer 2

The Process of electron gain.

Question 3

What are the 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.

eg . Zn, Cl2, O2, Ar all have oxidation numbers of zero
In NaCl Na= +1 Cl= -1 Sum = +1 -1 = 0
e.g. Zn2+ = +2 Cl- = -1
e.g. in CO32- C = +4 and O = -2 sum = +4 + (3 x -2) = -2

Question 4

What is the oxidation number of Group 1 metals?

Answer 4

+1

Question 5

What is the Oxidation number of Group 2 metals?

Answer 5

+2

Question 6

What is the Oxidation number for Al?

Answer 6

+3

Question 7

What is the Oxidation number for H?

Answer 7

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

Question 8

What is the Oxidation number for F?

Answer 8

-1

Question 9

What is the Oxidation number of Cl, Br, and I?

Answer 9

–1 except in compounds with oxygen and fluorine

Question 10

What is the Oxidation number of 0?

Answer 10

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

Question 11

What is the Oxidation number of Fe in FeCL3?

Answer 11

Using rule 5, Cl has an O.N. of –1
Using rule 2, the O.N. of the elements must add up to 0
Fe must have an O.N. of +3
in order to cancel out 3 x –1 = -3 of the Cl’s

Question 12

What does a reduction half equation show?

Answer 12

A reduction half equation only shows the parts of a chemical equation involved in reduction. The electrons are on the left

Question 13

What does an Oxidation half equation show?

Answer 13

An oxidation half equation only shows the parts of a chemical equation involved in oxidation.
The electrons are on the right

Question 14

What is a reducing agent?

Answer 14

Electron donors

Question 15

What is an oxidising agent?

Answer 15

Electron acceptors

Question 16

What does an oxidising agent do?

Answer 16

An oxidising agent (or oxidant) is the species that causes another element to oxidise. It is itself reduced in the reaction.

Question 17

What does a reducing agent do?

Answer 17

A reducing agent (or reductant) is the species that causes another element reduce. It is itself oxidised in the reaction.

Question 18

How do you write half equations?

Answer 18

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 19

If the substance that is being oxidised or reduced contains a varying amount of O then the half equations are balanced by adding…

Answer 19

H+, OH- ions and H2O.

Question 20

What must there be to combine half equations?

Answer 20

To combine two half equations there must be equal numbers of electrons in the two half equations so that the electrons cancel out