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