2.7.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Types of Reaction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 44 of 53 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Addition, Elimination and Substitution
These three types are distinguished by what happens to the number of reactants and products.
Addition
- Addition Two reactants combine to give one product, as when ethene reacts with bromine: C₂H₄ + Br₂ → C₂H₄Br₂.
Elimination
- Elimination A small molecule such as H₂O or HBr is removed from one reactant, leaving a double bond, as in CH₃CH₂Br → C₂H₄ + HBr.
Substitution
- Substitution One atom or group is replaced by another, as in CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻.
Addition example
- Addition example Ethene and hydrogen bromide give bromoethane: C₂H₄ + HBr → C₂H₅Br.
Elimination example
- Elimination example Ethanol heated with concentrated acid loses water to form ethene.
Counting products
- Counting products A quick check is to count reactants and products: two reactants forming one product suggests an addition reaction.
Section 2 of 4Oxidation, Reduction and Hydrolysis
Oxidation, Reduction and Hydrolysis
Oxidation
- Oxidation In organic chemistry oxidation is gain of oxygen or loss of hydrogen, for example CH₃CHO + [O] → CH₃COOH.
Reduction
- Reduction Reduction is loss of oxygen or gain of hydrogen, for example CH₃CHO + 2[H] → CH₃CH₂OH.
Hydrolysis
- Hydrolysis A compound is split by reaction with water, as in CH₃Cl + H₂O → CH₃OH + HCl.
[O] and [H]
- [O] and [H] Square brackets show an oxidising or reducing agent whose exact identity is not given.
Polymer example
- Polymer example Poly(ethene) forms when many ethene molecules join, as in n CH₂=CH₂ → (-CH₂-CH₂-)ₙ.
Section 3 of 4Addition Polymerisation
1
Many identical alkene molecules, called monomers, are involved, each with a C=C double bond.
2
The double bond in each monomer opens up and the monomers join end to end.
3
No atoms are lost, so the polymer has the same empirical formula as the monomer.
4
The product is a long saturated chain with repeating units, such as poly(ethene) from ethene.
5
Addition polymerisation is an addition reaction repeated many times.
6
Because no by-product forms, addition polymerisation has an atom economy of 100%.
Section 4 of 4Exam Checklist & Tips
2.7.1 – Key Points for the Exam
- Addition A reaction in which two molecules combine to form a single product.
- Elimination A reaction in which a small molecule is removed from a larger one, leaving a double bond.
- Substitution A reaction in which an atom or group is replaced by another.
- Hydrolysis A reaction with water that breaks a bond in a compound.
- Monomer A small molecule that can join with others to form a polymer.
Common Mistakes
- Calling the reaction of ethene with bromine substitution because bromine is added; it is addition.
- Mixing up elimination, which loses a small molecule, with substitution, which swaps groups.
- Forgetting that hydrolysis is a reaction with water, not any reaction in solution.
- Describing polymerisation as losing small molecules, which is true for condensation but not for addition polymerisation.
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