Page 6 - 1202 Question Bank Chemistry Form 5 KSSM

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Chapter Redox Equilibrium

NOTES

1.1 Oxidation and Reduction MnO (aq) + 8H (aq) + 5e → Mn (aq) +
−
−
2+
+
4
4H O(l)
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2
1. Definition of oxidation and reduction. (b) Acidified potassium dichromate(VI) solution:
2−
+
3+
−
Oxidation Reduction Cr O (aq) + 14H (aq) + 6e → 2Cr (aq) +
2
7
7H O(l)
• Gain of oxygen • Loss of oxygen 2 − −
(c) Chlorine water: Cl (aq) + 2e → 2Cl (aq)
2
• Loss of hydrogen • Gain of hydrogen (d) Bromine water: Br (aq) + 2e → 2Br (aq)
−
−
2
• Loss of electron • Gain of electron
6. A reducing agent is a substance that reduces another
• Increase in oxidation • Decrease in substance and oxidises itself.
number oxidation number
Example:
2+
(a) Magnesium: Mg(s) → Mg (aq) + 2e -
2. Oxidation number or oxidation state is the charge of (b) Sulphur dioxide:
the element in a compound if the transfer of electrons SO (aq) + 2H O(l) → SO (aq) + 4H (aq) + 2e −
2−
+
2
2
4
occurs in an atom to form chemical bonds with other (c) Iron(II) ion: Fe (aq) → Fe (aq) + e −
2+
3+
atoms.
7. Examples of redox reactions:
3. Rules for assigning oxidation number: (a) Conversion of Fe ion to Fe ion
2+
3+
(a) The oxidation number of each atom of a free Ionic equation:
element is 0. Br (aq) + 2Fe (aq) → 2Br (aq) + 2Fe (aq)
3+
−
2+
2
(b) The oxidation number of a monoatomic ion is Reduction half-equation:
equal to the charge of the ion. Br (aq) + 2e → 2Br (aq)
−
−
2
(c) Fluorine in its compounds has a fixed oxidation Oxidation half-equation:
number of −1. Fe (aq) → Fe (aq) + e −
2+
3+
(d) Alkali metals (Group 1 elements) in their (b) Displacement of copper by zinc from
compounds have a fixed oxidation number of +1. copper(II) sulphate solution
(e) Alkaline earth metals (Group 2 elements) in Ionic equation:
their compounds have a fixed oxidation number Zn(s) + Cu (aq) → Zn (aq) + Cu(s)
2+
2+
of +2. Reduction half-equation:
(f) Hydrogen in a compound normally has an Cu (aq) + 2e → Cu(s)
−
2+
oxidation number of +1 when it combines with Oxidation half-equation:
non-metals, but hydrogen has an oxidation Zn(s) → Zn (aq) + 2e −
2+
number of −1 when it combines with metals (c) Displacement of bromine by chlorine from
hydrides. potassium bromide solution
(g) Oxygen in a compound normally has an oxidation Ionic equation:
number of −2 except peroxide compounds Cl (aq) + 2Br (aq) → 2Cl (aq) + Br (aq)
−
−
2
(oxidation number is −1) and compounds with Reduction half-equation: 2
fluorine (oxidation number is +2). Cl (aq) + 2e → 2Cl (aq)
−
−
2
(h) Halogens (Cl, Br, I) in their compounds have Oxidation half-equation:
oxidation number of −1 except when combined 2Br (aq) → Br (aq) + 2e −
−
with fluorine and oxygen. 2
(i) The sum of oxidation numbers of all atoms in a 8. Displacement of metal from its salt solution
neutral compound is always 0. (a) A more electropositive metal displaces a less
electropositive metal from its salt solution.
4. A redox reaction is a chemical reaction in which (b) The electropositivity of a metal is determined
oxidation and reduction occur simultaneously. from its position in the electrochemical series.
(c) A metal can displace any metal below it from its
5. An oxidising agent is a substance that oxidises salt solution.
another substance and reduces itself. Example:
(a) Acidified potassium manganate(VII) solution:
1