SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 2)

by BerryBerryTeacher

in Berry Reference (Notes)

Berry Berry Easy would like to present all with Part 2 of SPM Form 5 Chemistry “Oxidation and Reduction”. In the previous part, the concept of redox reaction was introduced to all students. By now, all of you should be comfortable in identifying the “two reactions” (reduction and oxidation) that make up a redox reaction. In this part, the intention is to convey the concept of “oxidation number“. Ample of examples on it and sample calculations are also given. However, do take note also that some metals might break convention and show different oxidation numbers (as shown in the tables towards the end).

SPM Chemistry Form 5 – Terminology and Concepts: Oxidation and Reduction(Part 2)

Rusting of Iron Nails

Rusting of Iron Nails

Oxidation Number – is the charge that the atom of the element would have if complete transfer of electron takes place.

Oxidation number

(i) Free elements have an oxidation number of zero.

Na = 0
Mg = 0
C = 0
H2 = 0
Br2 = 0

(ii) Oxidation number of a simple monoatomic ions is the same as its charge.

Na+ ion = +1
Mg2+ ion = +2
O2- ion = -2
Cl- ion = -1
H+ ion = +1

(iii) Sum of the oxidation number for a neutral compound is zero.

CaH2
(+2) + 2(-1)
= 0
Sum of oxidation number is 0

Al2O3
2(+3) + 3(-2)
= 0
Sum of oxidation number is 0

Iodine, Bromine, Chlorine, Nitrogen, Oxygen, Fluorine

—> Electronegativity increase

Cl2O
2(+1) + (-2)
= 0
Sum of oxidation number is 0.

(Chlorine, bromine and iodine usually have the oxidation number of -1 except when combine with a more electronegative element.)

HClO
(+1) + (+1) + (-2)
= 0
Sum of oxidation number is 0.

(Chlorine, bromine and iodine usually have the oxidation number of -1 except when combine with a more electronegative element.)

(iv) Polyatomic ion, the sum of the oxidation numbers of all the atoms equals the charge on the ion.

SO4 2-
(+6) + 4 (-2)
= +6 + (-8)
= -2
Sum of oxidation number is -2

Cr2O72-
2(+6) + 7(-2)
= -2
Sum of oxidation number is -2

(v) Calculating the oxidation numbers of elements in compounds or ions.

K2Cr2O7
2 (+1) + 2x + 7 (-2) = 0
x = +6
Oxidation number of chromium in K2Cr2O7 is +6

NO3-
x + 3(-2) = -1
x = +5
Oxidation number of nitrogen in NO3- is +5

Hydrogen peroxide, H2O2
2(+1) + 2x = 0
x = -1
Oxidation number of oxygen in H2O2 is -1 (and not -2)
(Usually oxidation number for combined oxygen usually is -2 except in peroxides)

F2O
2(-1) + x = 0
x = +2
Oxidation number of oxygen in F2O is +2 (and not -2)
(Usually oxidation number for combined oxygen usually is -2 except in fluorine compounds)

NaH
(+1) + x = 0
x = -1
Oxidation number of hydrogen in NaH is -1 (and not +1)
(Usually oxidation number for combined hydrogen usually is +1 except in metal hydrides)

AlH3
(+3) + 3x = 0
x = -1
Oxidation number of hydrogen in AlH3 is -1 (and not +1)
(Usually oxidation number for combined hydrogen usually is +1 except in metal hydrides)

MgH2
(+2) + 2x = 0
x = -1
Oxidation number of hydrogen in MgH2 is -1 (and not +1)
(Usually oxidation number for combined hydrogen usually is +1 except in metal hydrides)

(vi) Some metals show different oxidation numbers.

Compound Oxidation number of manganese
MnSO4 +2
MnO2 +4
K2MnO4 +6
KMnO4 +7
Compound Oxidation number of chromium
K2CrO4 +6
K2Cr2O7 +6

(vii) Usually non-metals have negative oxidation numbers but Cl, Br & I can have positive or negative oxidation number.

Compound Oxidation number of chlorine
HCl -1
ClO2 +4
HClO4 +7

Stay tune for the next installment in the “Oxidation and Reduction” series with focus on the difference between redox reaction and non-redox reactions.

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August 31, 2011
March 11, 2012

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