Topic 3 - redox

Question 1

What is oxidation?

Answer 1

When atoms lose electrons, it is called oxidation

Question 2

What is reduction?

Answer 2

When atoms gain electrons, it is called reduction​

Question 3

Redox in ionic compounds​

Answer 3

An atom becomes an ion when it loses or gains one or more electrons​

The electrons are transferred from one atom to another so BOTH processes must occur simultaneously​
Reduction + oxidation is known as REDOX

Question 4

Half Equations​

Answer 4

Oxidation:​
Mg → Mg2+ + 2e-​
Electrons are lost – the atom is oxidised​
​
Reduction:​
O2 + 4e- → 2O2-​
Electrons are gained – the atom is reduced​
​
The two half equations combined show the reaction between magnesium and oxygen:​
2Mg + O2 → 2MgO

Question 5

Redox in covalent compounds​

Answer 5

It is easy to see where electrons are gained and lost in ionic compounds​

We can’t write half equations for reactions where covalent compounds are formed​

OXIDATION STATES are used instead

Question 6

Oxidation states

Answer 6

In a molecule of an element, the oxidation states of each atom are zero, there is no difference in electronegativity so no transfer of electrons​
So in O2, both oxygen atoms have an oxidation state of 0​

If there is a difference in electronegativity then the electrons will be more attracted to one atom than the other​

The more electronegative atom can be given a number as if it has gained those electrons in the covalent bond​
In H2O, the oxygen has an oxidation state of -2. Each hydrogen has an oxidation state of +1​

Compounds and molecules always have an overall oxidation state of zero so the oxidation numbers will all add up to 0

Question 7

Redox and Oxidation states

Answer 7

Oxidation occurs when the oxidation state increases​

Reduction occurs when the oxidation state decreases​

In the formation of water:​

the oxidation state of hydrogen changes from 0 to +1​

The hydrogen is oxidised​

The oxidation state of oxygen changes from 0 to -2​
The oxygen is reduced

Question 8

Other reactions

Answer 8

Not all reactions are redox reactions​

If the oxidation state for each atom do not change, it is not redox​

E.g. ​
HCl + NaOH → NaCl + H2O​
+1 -1 +1 -2 +1 +1 -1 (2x+1) -2​
​
Each compound’s oxidation states add up to 0​

Each atom has the same oxidation state throughout so this is not a redox reaction

Question 9

Variable oxidation states​

Answer 9

The transition metals have variable oxidation states because their highest energy electrons are in the 3d sub-shell​

When a transition metal loses electrons to form a positive ion, the 4s electrons are lost first, followed by the 3d electrons​

The maximum oxidation state increases as you go along the period until manganese, which has a maximum oxidation state of +7 (2 from 4s and 5 from 3d)​

eg. Scandium and zinc only have one oxidation state when in a compound​
Other metals in P4 have two or more oxidation states​
Iron has possible oxidation states of +2 or +3, written as Fe(II) and Fe(III)

Question 10

Uses of transition metals​

Answer 10

Usual metallic properties make them useful for various purposes​

The variable oxidation states make transition metals and their compounds useful as catalysts:​

Iron used for Haber process​

Platinum used in catalytic converters​

Vanadium (V) oxide used to make sulfur dioxide​

Manganese (IV) oxide used for decomposition of hydrogen peroxide

Question 11

Transition metal compounds as catalysts

Answer 11

Their use as catalysts is due to their ability to change between oxidation states​

E.g. Manganese (IV) oxide + hydrogen peroxide → manganese (VII) oxide + water + oxygen​
2MnO2 + 5H2O2 → Mn2O7 + 5H2O +O2​

Then manganese (VII) oxide → Manganese (IV) oxide + oxygen​
2Mn2O7 → 4MnO2 + 3O2​

This leaves the Manganese (IV) oxide ready to be used again