Ion - Combination Reactions
(Not Redox)

A great number of reactions are ion-combination reactions. These reactions involve NO change to the valency (oxidation number) of the reacting chemicals.

For example: CuSO₄ + 2NaOH -> Cu(OH)₂ + Na₂SO₄

In this ion-combination reaction Copper (Cu) is in the ionic form Cu²⁺ on the left hand side of the equation, and as Cu²⁺ on the right hand side of the equation.

Cu²⁺ + SO₄^2- + 2Na⁺ + 2OH^- -> (Cu²⁺ + 2OH^-) _Solid + 2Na⁺ + SO₄^2-

What we are showing here, is that the Copper (Cu) has stayed as Cu²⁺. It has not gained any electrons (nor lost any electrons). This is typical of ion-combination reactions.

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• Redox Reactions
  (Oxidation - Reduction Reactions)

In these reactions, the valency (oxidation number) of the reactants change.

For example: 2Fe³⁺ + Sn²⁺ -> 2Fe²⁺ + Sn⁴⁺ (8⁺ each side of the equation)

The iron (iii) + tin (ii) have reacted to give iron (ii) + tin (iv) of course, this reaction is carried out in the presence of HCl (Hydrochloric Acid), but the oxidation reduction reaction is only between the iron (iii) and tin (ii).

So, the Fe³⁺ is reduced to Fe²⁺, and the Sn²⁺ is oxidised to Sn⁴⁺.

What happened in this reaction?

Sn²⁺ donated electrons to the Fe³⁺ (an electron transfer took place).

For example: Fe (metal) + Cu²⁺ -> Fe²⁺ + Cu (metal)

Fe donates two electrons to the Cu²⁺ to form Cu (metal). The Fe lost 2 electrons, so is oxidised.

The Cu²⁺ gained 2 electrons, so is reduced (in its valency).

Confusing isn’t it! - Lets try some more...

The chemical which is oxidised is the reducing agent.
The chemical which is reduced is the oxidising agent.

For Example: Zn + 2HCl -> Zn²⁺ + H₂ +2Cl^-

In this reaction Zn + 2H⁺ -> Zn²⁺ + H2 (the chlorine is not changed in its ion state, so is not oxidised or reduced).

Zn is oxidised to Zn²⁺ (loses 2 electrons)

H+ is reduced to H₂ (gains 2 electrons)

H+ has an oxidation number (valency) of +1, and is reduced to an oxidation number (valency) of 0.

So, reduction decreases the oxidation number (valency).

Zn has an oxidation number (valency) of 0, and is oxidised to Zn²⁺, an oxidation number of 2+,

So oxidation increases the oxidation number (valency).

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• Redox Reactions
  Involving Acidic and Basic Solutions

Not only are there an exchange of electrons in these reactions, but also an exchange of protons (hydronium ions), as in any base system.

CuS + HNO₃ -> Cu SO4 + NO (g) + H2O (equation not balanced).

3CuS + 8HNO₃ -> 3 CuSO₄ + 8NO(g) + 4H₂O (equation balanced)

3CuS²⁺ + 3S^2- + 8H⁺ + 8NO₃^- -> 3Cu²⁺ + 3SO₄^2- + 8NO(g) + 4H₂O (equation written in ionic nomenclature)

Copper Cu²⁺ does not change.
8H⁺ goes to 4H₂O (exchange of hydronium ions)

Sulphur S^2- goes to S⁶⁺O₄ ((S⁶⁺ (O₄^2-))^2-
So has changed from 2^- to 6⁺ (Sulphur). Sulphur has lost electrons, therefore has been oxidised.

Nitrogen has gone from N⁵⁺ (N⁵⁺ O₃^2-) to N²⁺ (N²⁺ O^2-)
So has gained electrons, therefore has been reduced.

• Redox
  (Half Cells)

Sometimes it is easier to see the transfer of electrons in the system if it is split into definite steps. This will be oxidation of one substance and reduction of the other substance.

2Fe³⁺ + Sn²⁺ -> 2Fe²⁺ + Sn⁴⁺

Split into 2 separate steps.

2Fe³⁺ + 2^e- -> 2Fe²⁺ (reduction)

(6+) + (2-) -> (4+) (balanced for charges)

Sn²⁺ -> Sn⁴⁺ + 2^e- (oxidation)

(2+) -> (4+) + (2-)

Add the two half equations: 2Fe³⁺ + 2^e- + Sn²⁺ -> 2Fe²⁺ + Sn⁴⁺ + 2^e-
The electrons cancel each other out, so equation is: 2Fe³⁺ + Sn²⁺ -> 2F²⁺ + Sn⁴⁺
By breaking down the equation into half cells, the oxidation or reduction of each chemical can be determined.