REDOX REACTIONS Lesson Note
Sub-Topics: Definition, IUPAC/Stock System, Types of Redox, Balancing Redox Equations
This SS2 Chemistry lesson note covers REDOX REACTIONS for First Term, Week 5.
Lesson Note
Sub-Topics: Definition, IUPAC/Stock System, Types of Redox, Balancing Redox Equations
LEARNING OBJECTIVES
At the end, students should be able:
1. to give definition of redox reaction
2. balance redox reaction
DEFINITION OF REDOX REACTION
Redox reaction is a reaction that involves both reduction and oxidation occurring simultaneously.
Old definition:
Oxidation: Addition of oxygen, removal of hydrogen, addition of electronegative element
Reduction: Removal of oxygen, addition of hydrogen, removal of electronegative element
Modern definition (Electronic concept):
Oxidation: Loss of electrons, increase in oxidation number. OIL (Oxidation Is Loss)
Reduction: Gain of electrons, decrease in oxidation number. RIG (Reduction Is Gain)
In redox, one substance is oxidized (reducing agent) while another is reduced (oxidizing agent).
OXIDATION NUMBER / IUPAC (STOCK SYSTEM)
Oxidation number is the apparent charge an atom has in a compound if all bonds were ionic.
Rules for Oxidation Number:
Free element = 0 e.g. Na, O2, Fe = 0
Group I = +1, Group II = +2, Al = +3
Oxygen = -2 except in peroxide (-1) and OF2 (+2)
Hydrogen = +1 except in metal hydrides (-1)
Sum in neutral compound = 0, in ion = charge of ion
Fluorine always -1
IUPAC Nomenclature (Stock System) for Redox Compounds:
We use Roman numerals to show oxidation state of transition metals/variable valency.
Examples:
1. FeO = Iron(II) oxide (Fe is +2)
2. Fe2O3 = Iron(III) oxide (Fe is +3)
3. Cu2O = Copper(I) oxide (Cu is +1)
4. CuO = Copper(II) oxide (Cu is +2)
MnO2 = Manganese(IV) oxide (Mn is +4)
H2SO4 = Tetraoxosulphate(VI) acid (S is +6)
How to calculate: Example Fe2O3: Let x be Fe. 2x + 3(-2) = 0 ⇒ 2x = 6 ⇒ x = +3
TYPES OF REDOX REACTIONS
1. Combination Reaction:
Two elements combine. A + B → AB
e.g. 2Mg + O2 → 2MgO (Mg: 0 to +2 oxidized, O: 0 to -2 reduced)
2. Decomposition Reaction:
Compound breaks down. AB → A + B
e.g. 2HgO → 2Hg + O2
3. Displacement Reaction:
More reactive element displaces less reactive.
e.g. Zn(s) + CuSO4(aq) → ZnSO4 + Cu
Zn: 0 to +2 (oxidized), Cu: +2 to 0 (reduced)
4. Disproportionation Reaction:
Same element is both oxidized and reduced.
e.g. Cl2 + 2NaOH → NaCl + NaOCl + H2O
Cl: 0 to -1 (in NaCl) and 0 to +1 (in NaOCl)
5. Combustion Reaction: All combustion are redox e.g. C + O2 → CO2
OXIDIZING AND REDUCING AGENTS
Oxidizing agent: Substance that causes oxidation in others and itself gets reduced. It gains electrons. e.g. O2, KMnO4, K2Cr2O7, Cl2, conc. H2SO4, HNO3
Reducing agent: Substance that causes reduction in others and itself gets oxidized. It loses electrons. e.g. H2, C, CO, H2S, SO2, metals
BALANCING REDOX EQUATIONS
Two methods: Oxidation number method and Half-reaction (Ion-electron) method. We will use Oxidation number method for SS2.
Steps (Oxidation number method):
Write unbalanced equation and assign oxidation numbers
Identify atoms that change in oxidation number
Find increase and decrease in oxidation number
Equate increase = decrease with coefficients
Balance other atoms (H, O) by inspection, add H2O, H+ if in acidic medium
EXAMPLE 1: (Simple)
Cu + HNO3 → Cu(NO3)2 + NO2 + H2O
Cu: 0 → +2 (increase by 2)
N in HNO3: +5 → +4 in NO2 (decrease by 1)
To equal: need 2 N to balance 1 Cu
Balanced: Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O
EXAMPLE 2: (Acidic medium - for higher)
MnO4- + Fe2+ + H+ → Mn2+ + Fe3+ + H2O
Mn: +7 → +2 (gain 5e)
Fe: +2 → +3 (lose 1e)
Multiply Fe by 5
Result: MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O
Evaluation
OBJECTIVE QUESTIONS
1. Redox reaction involves:
A. Only oxidation
B. Only reduction
C. Both oxidation and reduction
D. Neither
2. In electronic concept, oxidation is:
A. Gain of electrons
B. Loss of electrons
C. Gain of hydrogen
D. Loss of oxygen
3. The oxidation number of free element like O2 is:
A. -2
B. +1
C. 0
D. +2
4. Oxidation number of oxygen in H2O2 is:
A. -2
B. -1
C. +2
D. +1
5. In Fe2O3, the oxidation number of Fe is:
A. +2
B. +3
C. -2
D. -3
6. Using IUPAC system, Cu2O is named:
A. Copper (II) oxide
B. Copper (I) oxide
C. Copper oxide
D. Dicopper oxide
7. Which of these is an oxidizing agent?
A. H2
B. CO
C. KMnO4
D. C
8. A reaction where same element is both oxidized and reduced is called:
A. Displacement
B. Combination
C. Disproportionation
D. Decomposition
9. In Zn + CuSO4 → ZnSO4 + Cu, Zn is:
A. Oxidizing agent
B. Reducing agent
C. Both
D. Neither
10. OIL RIG means:
A. Oxidation Is Loss, Reduction Is Gain of electrons
B. Oxidation Is Gain
C. Reduction Is Loss
D. None
THEORY QUESTIONS
1. a) Define redox reaction in terms of: i) Oxygen/Hydrogen ii) Electron transfer iii) Oxidation number.
b) Calculate the oxidation number of: i) Mn in KMnO4 ii) Cr in K2Cr2O7 iii) S in H2SO4
c) Name the following compounds using IUPAC (Stock) system: i) FeO ii) Fe2O3 iii) SO2 iv) Cu2O
2. a) Distinguish between oxidizing agent and reducing agent with two examples each.
b) Explain with examples: i) Displacement reaction as redox ii) Disproportionation reaction
c) Balance the following redox equations using oxidation number method:
i) Cu + HNO3 → Cu(NO3)2 + NO2 + H2O
ii) Zn + HCl → ZnCl2 + H2