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Fabric Stain Removal Guide

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.

The oxidation number is a number identical with the valency but with a sign, expressing the nature of the charge of the species in question when formed from the neutral atom. Thus, the oxidation number of chlorine in hydrochloric acid is -1, while it is +1 in hypochlorous acid. Similarly we can say that the oxidation number of chlorine in chloric acid (HClO3) is +5, and in perchloric acid (HClO4) +7.

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+ + SO42- + 2Na+ + 2OH- -> (Cu2+ + 2OH-) Solid + 2Na+ + SO42-

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).

Now, a redox reaction is the release and uptake of electrons.

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).

Redox reactions then, are the transfer of electrons from one reactant to another...

When there is oxidation, there is also reduction.
The substance which loses electrons is oxidised.
The substance which gains electrons is reduced.

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).

Simply put:

The chemical which gains electrons is reduced (reduces its valency) and is called the oxidising agent.

The chemical which loses electrons, is oxidised (increases its valency) and is called the reducing agent.

Please do not get these terms "oxidised" and "oxidising agent" confused.

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

Fe + Cu2+ -> Fe2+ + Cu
Oxidised
Reducing
Agent
Reduced
Oxidising
Agent

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+ + 3S2- + 8H+ + 8NO3- -> 3Cu2+ + 3SO42- + 8NO(g) + 4H2O (equation written in ionic nomenclature)

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

Sulphur S2- goes to S6+O4 ((S6+ (O42-))2-
So has changed from 2- to 6+ (Sulphur). Sulphur has lost electrons, therefore has been oxidised.

Nitrogen has gone from N5+ (N5+ O32-) to N2+ (N2+ O2-)
So has gained electrons, therefore has been reduced.

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  • 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+ + 2e- -> 2Fe2+ (reduction)

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

Sn2+ -> Sn4+ + 2e- (oxidation)

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

Add the two half equations: 2Fe3+ + 2e- + Sn2+ -> 2Fe2+ + Sn4+ + 2e-
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.

The atom which gains electrons reduces its valency, therefore is reduced  and is called the oxidising agent.

The atom which loses electrons, increases its oxidation number, therefore is oxidised, and is called the reducing agent.

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