3.2.2Organic Chemistry
Alkane Fuels — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 5Fractional Distillation of Crude Oil
Fractional distillation separates compounds according to their boiling points.
1
Crude oil is a mixture of hydrocarbons, mostly alkanes, and is heated until most of it is vapour.
2
The vapour enters a fractionating column that is hot at the bottom and cooler at the top.
3
As the vapour rises, each hydrocarbon condenses at the level where the temperature falls below its boiling point.
4
Short-chain molecules with low boiling points reach the top, while long-chain molecules with high boiling points condense near the bottom.
5
Each fraction is a mixture of hydrocarbons with similar boiling points, not a pure compound.
Section 2 of 5Fractions and Uses
Fractions and Uses
Refinery gases
- Refinery gases The smallest molecules are collected at the top and used as bottled fuel gas.
Petrol
- Petrol Petrol contains molecules of around 5 to 10 carbon atoms and fuels cars.
Kerosene
- Kerosene Kerosene has somewhat longer chains and is used as aircraft fuel.
Diesel
- Diesel Diesel oil has longer chains again and fuels lorries, trains and buses.
Bitumen
- Bitumen The largest molecules stay at the bottom and are used to surface roads.
Section 3 of 5Cracking and Combustion
Why crack
- Why crack Demand for shorter-chain fuels is greater than supply, so long chains are broken into smaller, more useful molecules.
Thermal cracking
- Thermal cracking High temperature and high pressure break the chain, mostly giving alkenes and alkanes.
Catalytic cracking
- Catalytic cracking A zeolite catalyst at about 450 °C and slight pressure gives branched alkanes and aromatic compounds for petrol.
Example
- Example C₁₀H₂₂ → C₈H₁₈ + C₂H₄ shows an alkane being cracked into a shorter alkane and an alkene.
Complete combustion
- Complete combustion Plenty of oxygen gives carbon dioxide and water: C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O.
Incomplete combustion
- Incomplete combustion Limited oxygen gives carbon monoxide or carbon (soot) with water: CH₄ + 1½O₂ → CO + 2H₂O.
Section 4 of 5Worked Example
Question
- Calculate the mass of carbon dioxide produced by the complete combustion of 11.4 g of octane, C₈H₁₈.
- (Ar: C 12.0, H 1.0, O 16.0.)
- Mr of C₈H₁₈ = 8 × 12.0 + 18 × 1.0 = 114.0, so moles = 11.4 ÷ 114.0 = 0.100 mol.
- EquationC₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O, so 1 mol octane gives 8 mol CO₂.
- Moles of CO₂ = 0.100 × 8 = 0.800 mol.
- Mr of CO₂ = 44.0, so mass = 0.800 × 44.0 = 35.2 g.
Answer 35.2 g of CO₂
Section 5 of 5Exam Checklist & Tips
3.2.2 – Key Points for the Exam
- Fraction A mixture of hydrocarbons with similar boiling points separated from crude oil.
- Cracking Breaking long-chain hydrocarbons into shorter, more useful molecules.
- Complete combustion Burning in excess oxygen to give carbon dioxide and water only.
- Incomplete combustion Burning in limited oxygen to give carbon monoxide or carbon as well as water.
- Catalyst A substance that speeds up a reaction without being used up.
Common Mistakes
- Saying fractional distillation breaks bonds, when it only separates molecules by boiling point.
- Stating that cracking produces only alkanes, when it also gives alkenes.
- Forgetting to balance oxygen when it needs half-integer coefficients, for example 12½O₂ for octane.
- Referring to a fraction as a pure compound.
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