nota kimia t5 no pengoksidaan

Azlee Chemistry 2009

Oxidation number

(NOMBOR PENGOKSIDAAN)

Definition:

The oxidation number of an element is the

charge that the atom of the element would

have if complete transfer of electron occurs

Nombor pengoksidaan unsur adalah cas atom unsur itu

sendiri jika pemindahan elektron berlaku dengan

lengkap

Tips from the rules:

the oxidation number for atom and molecule is

zero =0

Atom or molecule Zn C Mg Ag H2 O2 Cl2

Oxidation number 0 0 0 0 0 0 0

the oxidation number for monoatomic ion is

equal to its charge

Monoatomic ion H+ Fe2+ Mn2+ Fe3+ Cr3- O2- N3-

Oxidation number +1 +2 +2 +3 -1 -2 -3

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the sum of oxidation numbers of all elements in

the compound is zero

Compound H2SO4 KMnO4 SO2 Na2SO4 K2Cr2O7

Oxidation number 0 0 0 0 0

the sum of oxidation number of all elements in

polyatomic ions is equal to the charge of the

ions

Polyatomic

ion NH4

+ SO4

2- MnO4

- Cr2O7

2- S2O3

2- Fe(CN)6

3-

Oxidation

number +1 -2 -1 -2 -2 -3

Azlee Chemistry 2009

Calculate the oxidation numbers for the underlined

elements.

(i) SO2 [compound] [refer to rules above]

Element S O Sum of oxidation

numbers

Number of element 1 2 0

Oxidation number x -2 [let the unknown oxidation number is equal to x]

1(x) + 2(-2) = 0

x - 4 = 0

x = +4

Thus;

Oxidation number of S in SO2 = +4

Latihan

Dapatkan nombor pengoksidaan bagi

H2SO4

Na2S2O3

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NH3

S2O32-

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(ii) H2SO4

Element H S O Sum of oxidation

numbers

Number of element 2 1 4 0

Oxidation number +1 x -2

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

2 + x - 8 = 0

x = +6

Thus;

Oxidation number of S in H2SO4 = +6

Azlee Chemistry 2009

(iii) Na2S2O3

Element Na S O Sum of oxidation

numbers

Number of element 2 2 3 0

Oxidation number +1 x -2

2(+1) + 2(x) + 3(-2) = 0

2 + 2x - 6 = 0

x = 4

2

= +2

Thus;

Oxidation number of S in Na2S2O3 = +2

Azlee Chemistry 2009

(iv) NH3

Element N H Sum of oxidation

numbers

Number of element 1 3 0

Oxidation number x +1

1(x) + 3(+1) = 0

x = -3

Thus;

Oxidation number of N in NH3 = -3

Azlee Chemistry 2009

(v) S2O32- [polyatomic ion]

[refer to rules above]

Element S O Sum of oxidation

numbers

Number of element 2 3 -2

Oxidation number x -2

2(x) + 3(-2) = -2

2x - 6 = -2

x = (6 – 2) /2

x = +2

Thus;

Oxidation number of S in S2O32- = +2

(vi) NH4+ : ammonium ion [polyatomic ion]

Element N H Sum of oxidation

numbers

Number of element 1 4 +1

Oxidation number x +1

x(1) + 4(+1) = +1

x + 4 = +1

x = 1 - 4

x = -3

Thus;

Oxidation number of N in NH4+ = -3

Azlee Chemistry 2009

Exercises;

Calculate the oxidation numbers for the underlined

elements.

i. KMnO4 potassium manganate(VII)

ii. MnO4-

iii. K2Cr2O7 Potassium dichromate(VI)

iv. Cr2O72-

Azlee Chemistry 2009

Answer:

i. KMnO4

Element K Mn O Sum of oxidation

numbers

Number of element 1 1 4 0

Oxidation number +1 x -2

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

1 + x – 8 = 0

x = +7

Thus;

Oxidation number of Mn in KMnO4 = +7

Azlee Chemistry 2009

ii. MnO4-

Element Mn O Sum of oxidation

numbers

Number of element 1 4 -1

Oxidation number x -2

1(x) + 4(-2) = -1

x – 8 = -1

x = +7

Thus;

Oxidation number of Mn in KMnO4 = +7

Azlee Chemistry 2009

iii. K2Cr2O7

Element K Cr O Sum of oxidation

numbers

Number of element 2 2 7 0

Oxidation number +1 x -2

2(+1) + 2(x) + 7(-2) = 0

2 + 2x – 14 = 0

2x = 12

2 x = 12 / 2

x = +6

Thus;

Oxidation number of Cr in Cr2O72- = +6

Azlee Chemistry 2009

iv. Cr2O72-

Element Cr O Sum of oxidation

numbers

Number of element 2 7 -2

Oxidation number x -2

2(+1) + 2(x) + 7(-2) = 0

2 + 2x – 14 = 0

2x = 12

2 x = 12 / 2

x = +6

Thus;

Oxidation number of Cr in Cr2O72- = +6