REDOX REACTION

REDOX REACTION

Chemical reactions involving oxidation and reduction occurring simultaneously

REDOX REACTIONS CAN BE EXPLAINED IN TERM OF :

Loss or gain of oxygenLoss or gain of hydrogenTransfer of electronsChanges in oxidation number

REDOX REACTIONS BASED ON LOSS OR GAIN OF OXYGEN

Oxidation : refers to the combination (gain) of a

substance with oxygen

Reduction : refers to the removal (loss) of oxygen

from a substance

Oxidizing agent (oxidant ) : substance that causes oxidation

Reducing agent (reductant) : substance that causes reduction

EXAMPLE : Heating of copper(II) oxide, CuO with carbon

gains oxygen(oxidation) 2CuO (s) + C (s) → 2Cu (s) + CO2 (g) Oxidizing Reducing agent agent

loses oxygen (reduction)

REDOX REACTIONS BASED ON LOSS OR GAIN OF HYDROGEN

Oxidation : loss of hydrogenReduction : gain of hydrogen

EXAMPLE : reaction between hydrogen sulphide, H2S and

chlorine

gains hydrogen(reduction)

H2S (g) + Cl2 (g) → S (s) + 2HCl (g) Reducing Oxidizing agent agent

loses hydrogen (oxidation)

REDOX REACTIONS BASED ON TRANSFER OF ELECTRONS

Oxidation : loss of electrons

Reduction : gain of electrons

EXAMPLE : Zn (s) + Cu 2+ → Zn 2+ (aq) + Cu (s)

Oxidation half equation : Zn(s) → Zn 2+ (aq) +2e (loss of electrons )

Reduction half equation : Cu 2+ (aq) + 2e → Cu(s) (gain of electrons)

REDOX REACTIONS BASED ON CHANGES IN OXIDATION NUMBER

Oxidation number :

Is the charge that the atom of the element would have if complete transfer of electrons occurs

RULES OF ASSIGNING OXIDATION NUMBERS

1) The oxidation number of an atom in its elemental state is zero.

For example, the oxidation number of each atom in Mg, Cu, Na, H2, O2.

2)The oxidation number of a monoatomic ion is equal to its charge

Ion Na+ Mg 2+ Cl - N3-

Oxidation number

+1 +2 -1 -3

3)The oxidation number of hydrogen in a compound is always +1 except in metal hydrides, where it is -1.

For example, the oxidation number of H in H2O and NH3 is +1,

However, the oxidation number of H in sodium hydride, NaH is -1

4) The oxidation number of oxygen in a compound is always -2 except in peroxides.

For example, the oxidation number of O in H2O and MgO is -2.

However, the oxidation number of O in hydrogen peroxide, H2O2 is -1

5) The oxidation number of fluorine in all its compounds is -1.

The oxidation number of other halogens (chlorine, bromine and iodine) in their compounds is -1 except when they combine with more electronegative elements such as oxygen or nitrogen

6) The sum of the oxidation numbers of all the elements in the formula of a compound must be zero.

The sum of the oxidation numbers of all the elements in the formula of a polyatomic ion must be equal to the charge of the ion

THE ROMAN NUMERALS REPRESENT THE OXIDATION NUMBERS OF ELEMENTS IN COMPOUNDS

Formula Name Formula Name

FeSO4 Iron(II) sulphate NaClO Sodium Chlorate(I)

Fe2(SO4)3 Iron(III) sulphate

NaClO3 Sodium Chlorate(V)

PbO Lead (II) Oxide MnO2 Manganese(IV) oxide

PbO 2 Lead(IV) Oxide KMnO4 Potassium manganate(VII)

OXIDATION NUMBERS IN OXIDATION AND REDUCTION

Oxidation : increase in oxidation number

Reduction : decrease in oxidation number

EXAMPLE :

increase in oxidation number (oxidation) 0 +2 2Mg (s) + O2 (g) → 2MgO (s) Reducing Oxidizing -2 agent agent 0

decrease in oxidation number (reduction)

TYPES OF REDOX REACTIONS

Oxidation Reduction

Gain of oxygen Loss of oxygen

Loss of hydrogen Gain of hydrogen

Loss of electrons Gain of electrons

Increase in oxidation number

Decrease in oxidation number