Crude Oil, Hydrocarbons, Alkanes & Fractional Distillation
Crude oil is a finite fossil fuel formed over millions of years from the buried remains of ancient marine plankton, consisting of a complex mixture of hydrocarbons (compounds containing hydrogen and carbon atoms only). Most hydrocarbons in crude oil belong to the homologous series of alkanes, which are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. Examiners frequently test the step-by-step physical separation of crude oil in a fractionating column, how chain length governs boiling point, viscosity, and flammability, and balanced equations for complete and incomplete combustion.
- Crude Oil & Hydrocarbons
- Crude oil is a finite (non-renewable) resource found in porous rocks, formed from ancient biomass (mainly plankton) buried in mud at high temperature and pressure without oxygen. It is an unbonded mixture of a very large number of compounds, most of which are hydrocarbons (molecules made of hydrogen and carbon atoms only).
- Alkanes Homologous Series (CₙH₂ₙ₊₂)
- Alkanes are saturated hydrocarbons because every carbon atom forms 4 single covalent bonds (C-C and C-H) with no C=C double bonds. General formula: CₙH₂ₙ₊₂. First four members: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀).
- Fractional Distillation Process
- Separates crude oil into fractions (groups of hydrocarbons with similar chain lengths and boiling points): 1. Heat crude oil so most hydrocarbons evaporate into vapour. 2. Pass vapour into a fractionating column that is hot at the bottom (~350°C) and cool at the top (~40°C). 3. Vapours rise, cool, and condense at different levels according to their boiling points. Fractions from top to bottom: LPG (refinery gases), Petrol (gasoline), Kerosene, Diesel oil, Heavy fuel oil, and Bitumen (residue).
Curly-Arrow Organic Mechanisms (Nucleophilic Substitution & Addition)
3 Golden Rules for Curly Arrows:
1. Start at electrons: Every curly arrow MUST start at a lone pair of electrons or the middle of a covalent bond (never start an arrow on a δ+ or positive charge).
2. Show dipoles: Always label δ+ and δ− on polar bonds like C–Br or H–Br.
3. Balance charges: If a neutral molecule loses a leaving ion (:Br⁻), the intermediate carbocation must carry a formal + charge.
Key Concepts & Exam Definitions (6)
- Crude Oil & Hydrocarbons
- Crude oil is a finite (non-renewable) resource found in porous rocks, formed from ancient biomass (mainly plankton) buried in mud at high temperature and pressure without oxygen. It is an unbonded mixture of a very large number of compounds, most of which are hydrocarbons (molecules made of hydrogen and carbon atoms only).
- Alkanes Homologous Series (CₙH₂ₙ₊₂)
- Alkanes are saturated hydrocarbons because every carbon atom forms 4 single covalent bonds (C-C and C-H) with no C=C double bonds. General formula: CₙH₂ₙ₊₂. First four members: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀).
- Fractional Distillation Process
- Separates crude oil into fractions (groups of hydrocarbons with similar chain lengths and boiling points): 1. Heat crude oil so most hydrocarbons evaporate into vapour. 2. Pass vapour into a fractionating column that is hot at the bottom (~350°C) and cool at the top (~40°C). 3. Vapours rise, cool, and condense at different levels according to their boiling points. Fractions from top to bottom: LPG (refinery gases), Petrol (gasoline), Kerosene, Diesel oil, Heavy fuel oil, and Bitumen (residue).
- Properties vs Hydrocarbon Chain Length
- Shorter-chain alkanes (at the top of the column) have weak intermolecular forces: they have low boiling points, low viscosity (very runny), and high flammability (ignite easily and burn cleanly, making ideal fuels). Longer-chain alkanes (at the bottom) have stronger intermolecular forces: high boiling points, high viscosity (thick/treacly), and low flammability.
- Complete Combustion of Hydrocarbons
- Occurs when a hydrocarbon burns in plenty of oxygen/air. Carbon and hydrogen in the fuel are completely oxidised: Hydrocarbon + Oxygen -> Carbon Dioxide + Water (+ thermal energy). When balancing symbol equations for alkane combustion, always balance in alphabetical order: C first, H second, and O last.
- Incomplete Combustion & Carbon Monoxide
- Occurs in a restricted supply of oxygen, producing carbon monoxide (CO) and/or solid carbon particulates (soot) instead of CO₂. Carbon monoxide is a colourless, odourless, toxic gas that binds irreversibly to haemoglobin in red blood cells, preventing them from transporting oxygen around the body.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Define the terms 'hydrocarbon' and 'saturated'.
What is the molecular formula of an alkane containing 12 carbon atoms?
List the 6 main fractions obtained from the fractional distillation of crude oil from lowest boiling point (top) to highest boiling point (bottom).
Explain why decane (C₁₀H₂₂) has a higher boiling point than propane (C₃H₈).
Write the balanced symbol equation for the complete combustion of propane (C₃H₈).
Name two products obtained from crude oil other than fuels (petrochemical feedstocks).
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Explain how crude oil is separated into useful fractions by fractional distillation. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Crude oil is heated in a furnace until most of the hydrocarbons evaporate (turn into gas/vapour) [1 mark]. 2. The hot vapour enters the bottom of a fractionating column, which has a temperature gradient: it is very hot at the bottom and cooler at the top [1 mark]. 3. As the hydrocarbon vapours rise up the column, they cool down and condense back into liquids when they reach a tray at a temperature below their individual boiling points [1 mark]. 4. Hydrocarbons with long chains and high boiling points condense near the hot bottom, while hydrocarbons with short chains and low boiling points rise to the cool top before condensing [1 mark].
- Q2
Compare the boiling point, viscosity, and flammability of petrol (average formula C₈H₁₈) with heavy fuel oil (average formula C₂₅H₅₂), and explain why their boiling points differ. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Boiling point: Petrol has a much lower boiling point than heavy fuel oil [1 mark]. 2. Explanation: C₈H₁₈ molecules are much smaller (shorter carbon chains) than C₂₅H₅₂ molecules, so the intermolecular forces between petrol molecules are weaker and require less energy to overcome [1 mark]. 3. Viscosity: Petrol has a much lower viscosity (it is much runnier / flows more easily) than heavy fuel oil [1 mark]. 4. Flammability: Petrol has a much higher flammability (ignites much more easily and burns with a cleaner, less smoky flame) than heavy fuel oil [1 mark]. 5. Note that covalent bonds inside the hydrocarbon molecules do not break during boiling [1 mark].
- Q3
Butane (C₄H₁₀) is burned in a camping stove. Write the balanced symbol equation for the complete combustion of butane, and explain the health hazard if the camping stove is used inside a closed tent with poor ventilation. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Correct formulas for complete combustion: C₄H₁₀ + O₂ -> CO₂ + H₂O [1 mark]. 2. Correctly balanced equation: 2C₄H₁₀ + 13O₂ -> 8CO₂ + 10H₂O (or C₄H₁₀ + 6½O₂ -> 4CO₂ + 5H₂O) [1 mark]. 3. Inside a closed tent, the oxygen supply becomes limited, so butane undergoes incomplete combustion, producing carbon monoxide (CO) gas [1 mark]. 4. Carbon monoxide is colourless and odourless, so campers cannot detect it [1 mark]. 5. Carbon monoxide is toxic because it binds irreversibly to haemoglobin in red blood cells, reducing the blood's capacity to carry oxygen to the brain and heart, causing fainting, coma, or death [1 mark].
- Q4
An alkane has a relative formula mass (Mr) of 114. Given that the relative atomic masses are C = 12 and H = 1, determine the molecular formula of this alkane. [3 marks]
Click “Check Mark Scheme” to compare your answer, 1. The general formula of an alkane is CₙH₂ₙ₊₂ [1 mark]. 2. Using relative atomic masses: 12n + 1(2n + 2) = 114 -> 14n + 2 = 114 [1 mark]. 3. 14n = 112 -> n = 112 / 14 = 8; therefore the molecular formula is C₈H₁₈ (octane) [1 mark].
Common Exam Questions (4)
What are the first four alkanes and their molecular formulas?
Remember the mnemonic 'Monkeys Eat Peeled Bananas' for n = 1, 2, 3, 4: 1. Methane (CH₄), 2. Ethane (C₂H₆), 3. Propane (C₃H₈), 4. Butane (C₄H₁₀). All alkanes follow the general formula CₙH₂ₙ₊₂.
How do boiling point, viscosity, and flammability change as alkane chain length increases?
As hydrocarbon chain length increases (larger molecules): 1. Boiling point increases (because intermolecular forces between larger molecules are stronger and need more energy to overcome). 2. Viscosity increases (they become thicker and flow less easily). 3. Flammability decreases (they are harder to ignite and burn with a smokier flame).
How does fractional distillation separate crude oil into fractions?
Crude oil is heated until it evaporates (vaporises) and enters the bottom of a fractionating column, which has a temperature gradient (hot at the bottom, cool at the top). Vapours rise up the column and condense back into liquids at different heights when they reach a tray at or below their boiling point: long-chain hydrocarbons with high boiling points condense near the hot bottom, while short-chain hydrocarbons with low boiling points rise and condense near the cool top.
What is the difference between complete combustion and incomplete combustion?
Complete combustion occurs in a plentiful supply of oxygen, oxidising all carbon to carbon dioxide (CO₂) and all hydrogen to water vapour (H₂O), releasing maximum energy. Incomplete combustion occurs when oxygen is limited, producing water plus toxic carbon monoxide gas (CO) and/or black soot particulates (unburned carbon, C).
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