SS2 Chemistry Lesson Note on Oxidation – Reduction (Redox) Reaction

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TOPIC:       OXIDATION – REDUCTION (REDOX) REACTION

CONTENT: 1. OXIDATION AND REDUCTION

1. REDOX REACTIONS
2. OXIDATION NUMBERS OF CENTRAL ELEMENTS IN SOME COMPOUNDS
3. CONNECTION OF OXIDATION NUMBER WITH IUPAC NAME
4. OXIDISING AND REDUCING AGENTS
5. REDOX EQUATION

PERIOD 1: OXIDATION AND REDUCTION

Redox is a short form for reduction and oxidation reactions. The two reactions are opposing and complementary and they occur simultaneously. Redox has the following definitions

1. Definition of Oxidation reactions with examples

• In terms of addition of oxygen: Oxidation is the addition of oxygen or removal of hydrogen from a substance. Any reaction where there is addition of oxygen to a reactant is regarded as Oxidation.

Some examples are:

(i) 2Mg_(s) + O_2(g)  →    2MgO_(s)           oxidation of magnesium

(ii) C_(s) + 2ZnO_(s)        →     CO_{2 (g)} + 2Zn_(s)

In this reaction, oxygen (O) was added to carbon (C) to form Carbon (iv) oxide (CO₂). That is carbon was oxidized to Carbon (iv) oxide.

• In terms of hydrogen removal: Oxidation is the removal of hydrogen from a compound e.g. (i) combustion of hydrogen sulphide

2H₂S_(g) + O_2(g)  →   2H₂O_(l) + 2S_(s)

H₂S_(g) + Cl_2(g)  →     2HCl_2(g) + S_(s)

In both reactions, hydrogen sulphide was oxidized to Sulphur. The second reaction shows that oxidation can occur without the involvement of oxygen

• In terms of loss of electron:  Oxidation is the loss of electron.

4Na_(s) + O_2(g)    →     2Na₂O_(s)

Na  →       Na⁺ + e^– the loss of electron by sodium atom (Na) to form sodium ion (Na+) in sodium oxide

2. Definition of reduction with examples

Definitions of reduction are the opposites of the definitions of oxidation.

• Removal of oxygen

E.g.  Cu_(s) + ZnO_(s)    →  CuO_(s) + Zn_(s)

In this reaction zinc oxide was reduced to zinc

• Addition of hydrogen

e.g.   H₂S_(g) +Cl_2(g)    →    2HCl _(g) + S_(s)

chlorine is reduced to hydrogen chloride.

• Gain of electrons

e.g.  4Na_(s) + O_2(g) →   2Na₂O_(s)    i.e oxygen gained electron

0₂ + 4e^–  →     20^2-

• In terms Of Decrease In Oxidation Number

Zn_(s) + CuSO_4(aq)   →    ZnSO_4(aq) + Cu_(s)

The oxidation number of copper decreased from +2 in copper (ii) salt to zero in copper.

• Reduction occurs when electro negative element is added

3. Redox Reaction And Electron Transfer: considering all the reactions used as examples so far, it would be seen that both oxidation and reactions occur together see illustration below:

Because 2Na  →   2Na⁺ + 2e^– (loss of electron is oxidation)

O₂ + 4e^{–        →}      2O^2- (gain of electron is reduction)

The above electron transfer equations can be written for most redox reactions.

More examples are

• 2KI + Fe₂(SO₄)_3(aq) → I_2(aq) + K₂SO_4(aq) + 2FeSO_4(aq)

Ionically

2I^–_(aq)  →     I_{2 (aq)} + 2e^– (loss of electrons in oxidation)

The above equation is the redox reaction. Each half reaction cannot occur alone because an atom cannot receive or donate electron without another atom around that must donate or receive the electron.

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