Leaving Certificate ChemistryLeaving Certificate Chemistry

Oxidation and ReductionOxidation and Reduction

Leaving Certificate ChemistryLeaving Certificate Chemistry

Oxidation and ReductionOxidation and Reduction

Michael O’Leary

Patrician Academy, Mallow

Earlier TheoriesAn early theory on oxidation was that when a substance combined with oxygen it is oxidised.

e.g. C + O2 CO2

Reduction is the opposite of oxidation.When oxygen is removed from a substance, it is said to be reduced.

e.g.

The carbon is said to be oxidised

The haematite, Fe2O3 is reduced

Fe2O3 + 2Al 2Fe + Al2O3

Place a piece of zinc in a solution of copper sulfate and observe

‘OIL RIG TheoryA later theory stated that oxidation was loss of electrons

e.g. Zn + Cu2+ Zn2+ + Cu

Copper has left the solution to become plated onto the zinc plate

Here, also zinc has lost two electrons

Zinc is oxidised

loses 2 electrons

‘OIL RIG TheoryThis theory also stated that reduction was gain of electrons

e.g. Zn + Cu2+ Zn2+ + Cu

Cu2+ has gained 2 electrons

Copper is reduced

There is a complete transfer of electrons from zinc to copper

gain of 2 electrons

‘OIL RIG TheoryThus we have the theory

OXIDATION IS LOSS OF ELECTRONS

REDUCTION IS GAIN OF ELECTRONS

Oxidising & Reducing Agents

A substance that allows oxidation to take place by gaining electrons itself is called the oxidising agent.

The oxidising agent gains electrons

The oxidising agent is reduced

Similarly,

The reducing agent is oxidised

Consider the reaction between carbon and oxygen

C + O2 CO2

According to the earlier theory, here the carbon is oxidised.

But as CO2 is a covalent molecule and NO IONS here,there is NO complete transfer of electrons from Carbon to form Carbon Dioxide.

i.e. CO2 is a covalent molecule where the electrons are shared.

A new theory of oxidation and reduction is needed…

Oxidation NumberIn order to overcome this problem, the concept of OXIDATION NUMBER was introduced.

OXIDATION NUMBER is defined as -

the charge that an atom has or appears to have when electrons are distributed according to certain rules.

Rules for Assigning Oxidation Numbers

The oxidation number (O.N.) for atoms of elements in the uncombined state is zero (0)e.g. O.N. of sodium in Na is 0 (zero), the O.N. of hydrogen in H2 is 0 (zero)

In neutral molecules such as H20, the sum of the O.N.s is 0 (zero)Here, each Hydrogen is +1 and Oxygen is -2

In complex ions such as CO32-, the sum of the O.N.s of

the elements is equal to the charge on the ion

Rules for Assigning Oxidation Numbers - continued

In ions, the O.N. of a monatomic ion equals the charge of the ione.g. the oxidation number of Na+ is +1; the oxidation number of N3- is -3 The O.N. of F- is -1

The O.N. of oxygen is -2, except in peroxides (H2O2) when it is -1, and when in OF2 when it is +2

The O.N. of hydrogen is +1 except in metal hydridese.g. NaH, MgH2 when it is -1

The O.N. of chlorine is -1 except when combined with oxygen or fluorine

Calculating Oxidation NumbersWhat are the oxidation numbers of the elements in CO2?

The O.N. of oxygen is -2

Since there are 2 oxygen atoms, the total contribution of oxygen is -4, therefore the O.N. of the carbon atom is +4.

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QuestionWhat are the oxidation numbers of the elements in H2SO4?

Solution:

Start with elements whose O.N. can be deduced directly from the rules

H2SO4

The O.N. of hydrogen is +1

+1

The O.N. of oxygen is -2

-2

The sum of the O.N.s = 0

X = O.N. of sulfur

2(+1) + X + 4(-2) = 0

X = 8 - 2

X = 6

+6

Quiz

What is the oxidation number of

Hydrogen in H2O +1

+1Hydrogen in H2O2

Oxygen in Cu2O -2

+2Calcium in CaCl2

-3Nitrogen in NH3

-1Oxygen in H2O2

And Finally

What is the oxidation number of

Carbon in CO32- +4

+1Hydrogen in OH-

Hydrogen in MgH2 -1

0Carbon in C6H12O6

+7Manganese in MnO4-

+6Sulfur in SO42-