Crude Oil

Crude oil (also called 'petroleum') is a complex MIXTURE of mainly hydrocarbons — primarily alkanes, plus smaller amounts of cycloalkanes and aromatic compounds. It is a dark, oily, viscous liquid found in underground deposits.

Origin. Crude oil formed over MILLIONS OF YEARS from the remains of MARINE ORGANISMS (mostly tiny plankton and algae) that died and sank to the sea bed. Their remains were buried under layers of mud and rock; under high pressure and temperature over geological timescales, they were chemically transformed into a mixture of hydrocarbons.

Natural gas (mainly methane) was formed similarly but from smaller / more volatile fragments of decay.

Why finite and non-renewable. Crude oil takes millions of years to form. We're extracting it MILLIONS OF TIMES FASTER than it can naturally replenish. The known reserves will be depleted within a few generations at current usage rates. 'Non-renewable' means: once burnt, the resource is gone for practical timescales. This contrasts with renewable resources (solar, wind, biomass) that replenish on timescales of seconds to years.

Composition. A typical crude oil contains:

• 85% hydrocarbons (mostly C1 to C60 alkanes; some cycloalkanes and aromatics).
• Sulfur compounds (small amounts; the source of SO₂ pollution).
• Nitrogen and oxygen compounds (trace).
• Metals (very small amounts).

The exact composition varies between oil fields — 'light' oils have more short-chain alkanes (good for petrol); 'heavy' oils have more long-chain alkanes (more cracking needed).

Why crude oil is so important. Crude oil + natural gas provide ~ 50% of all human energy use (heating, transport, electricity). They're also the FEEDSTOCK for nearly all plastics (~ 400 million tonnes/year globally), most fertilisers (via the Haber process — needs natural gas for H₂), many medicines, paints, dyes, detergents. Modern industrial life is built on hydrocarbons.

Environmental impact. Burning fossil hydrocarbons releases CO₂ → enhanced greenhouse effect → climate change. Plus various local pollutants (CO, NOₓ, SO₂, particulates — see later sections). Transitioning away from fossil fuels is a major 21st-century challenge.

The path from oil well to product.

1. Extraction: drill into oil-bearing rock (often kilometres underground); oil flows out under pressure (or is pumped). Sometimes water or gas is injected to push more oil out.
2. Transport: pipelines or oil tankers move crude oil to refineries.
3. Refining: at the oil refinery, crude oil undergoes (a) FRACTIONAL DISTILLATION to separate it; (b) CRACKING of heavy fractions; (c) further conversion / purification.
4. Distribution: refined products (petrol, kerosene, plastics feedstock) go to fuel stations, airports, factories.

Fractional distillation and cracking are the two key chemistry processes in step 3.

Fractional distillation is the industrial process for separating crude oil into useful FRACTIONS — each fraction is a group of hydrocarbons with similar boiling points (and similar chain lengths).

The fractionating column.

A tall steel tower (typically 30-50 m high) inside the oil refinery. Key features:

• Vertical with a continuous TEMPERATURE GRADIENT: HOT at the bottom (~ 400 °C), gradually COOLER moving up, room temperature (~ 25 °C) at the top.
• Internal trays / shelves at multiple heights — small platforms with collection pipes.
• Operated CONTINUOUSLY (crude oil flows in 24/7).

Step-by-step process.

1. Heating: crude oil is heated in a furnace to ~ 400 °C. Most components vaporise; only the heaviest (bitumen) remains liquid.
2. Vapour enters the column near the bottom.
3. Vapours rise: as they go up, the column gets cooler.
4. Condensation: each fraction CONDENSES (returns to liquid) when the column temperature drops to its boiling-point range. Smaller molecules (low bp) need to rise further (where it's cooler) before condensing; larger molecules condense lower down.
5. Collection: condensed liquid pools on the tray at the appropriate height and is piped out to storage tanks.
6. Residue: bitumen does not vaporise at 400 °C; it exits at the very bottom as a viscous dark liquid.

The column thus separates crude oil into fractions by boiling point in a SINGLE continuous operation. Each fraction is a MIXTURE of similar-sized alkanes, ready for further processing.

The fractions — names, ranges, and uses.

Position in column	Fraction	Carbon range	Bp range	Main uses
TOP	Refinery gases	C1-C4	< 25 °C	LPG: bottled gas (cooking, heating)
2	Petrol / gasoline	C5-C10	40-180 °C	Petrol cars
3	Naphtha	C5-C11	60-180 °C	Petrochemicals feedstock (plastics, dyes)
4	Kerosene / paraffin	C10-C16	175-300 °C	Jet fuel, paraffin lamps, heating
5	Diesel	C14-C20	250-340 °C	Diesel cars/trucks/ships, generators
6	Lubricating oil	C20-C50	350-400 °C	Machine oil, candle wax
BOTTOM	Bitumen (residue)	C70+	> 400 °C	Roads (asphalt), roofing

Memorise the order top-to-bottom (RGPNKDLB — Refinery Gas, Petrol, Naphtha, Kerosene, Diesel, Lubricating oil, Bitumen) and at least 3-4 uses.

Trends in physical properties going DOWN the column (i.e. as chain length increases):

1. Boiling point INCREASES — stronger intermolecular forces in longer molecules.
2. Melting point INCREASES — same reason.
3. Viscosity INCREASES — longer chains tangle more → thicker liquid. Petrol is runny; bitumen is almost solid.
4. Flammability DECREASES — combustion needs vapour; longer molecules are less volatile.
5. Volatility DECREASES — fewer molecules evaporate at room T.
6. Density INCREASES slightly.

Why bp increases — intermolecular forces.

Hydrocarbons are simple molecular substances. On boiling, the WEAK INTERMOLECULAR FORCES (van der Waals / London dispersion forces) between molecules are overcome. The covalent C–C and C–H bonds WITHIN molecules are NOT broken.

Two reasons longer alkanes have stronger intermolecular forces:

1. More electrons — bigger molecule → larger temporary dipole → stronger force.
2. Larger surface area in contact between adjacent molecules → more sites where forces can act.

So bp ↑ as chain length ↑. The relationship is roughly linear for the first few members of a series and gets less steep as molecules get very large.

Real-world implications.

• Petrol (C5-C10, bp ~ 100 °C) — VOLATILE; ignites easily; perfect for car engines that need quick ignition. Stored carefully because the vapour is flammable.
• Diesel (C14-C20, bp ~ 300 °C) — LESS volatile; needs higher compression / heat to ignite. More energy per litre than petrol.
• Lubricating oil — HIGH bp; viscous; coats machine parts to reduce friction.
• Bitumen — NON-VOLATILE at room T; sticky; used in road surfaces and roofing.

The product mix of a refinery depends on regional needs: more petrol where many cars; more diesel where many trucks; more kerosene where many flights.

Why crude oil needs separating.

The crude oil straight from the well is unusable as a fuel — it has too much variation (some short volatile components + some heavy waxy ones). Burning it directly would burn the volatile fraction first (with bad flame) and leave the heavy fraction unburnt. Fractional distillation gives us PRODUCTS WITH KNOWN PROPERTIES — petrol behaves consistently because it's a defined mix of similar-sized molecules.

Cracking is the industrial process of breaking LONG-CHAIN alkanes (from heavier fractions like naphtha, kerosene, diesel) into shorter, more useful molecules.

The products are always: a SHORTER ALKANE + an ALKENE. The alkene provides a 'home' for the 2 H atoms that would otherwise be 'missing' (because the C=C double bond uses 2 H atoms' worth of bonding capacity).

Example reaction.

Decane (C₁₀H₂₂) can crack into octane + ethene:

$\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$

Balance check: 10 C = 8 + 2 ✓; 22 H = 18 + 4 ✓.

Other splits are also possible: C₁₀H₂₂ → C₆H₁₄ + 2 C₂H₄ (hexane + 2 ethene), or C₁₀H₂₂ → C₇H₁₆ + C₃H₆ (heptane + propene), etc. Cracking gives a MIXTURE of products.

WHY cracking is carried out — two economic reasons.

Reason 1: Supply doesn't match demand.

The natural composition of crude oil doesn't match what we WANT:

• Crude oil supply is rich in MEDIUM-to-LONG fractions (naphtha, kerosene, diesel, lubricating oil).
• Market demand is dominated by SHORT fractions (petrol, gasoline — C5-C10).

This mismatch creates a SURPLUS of heavy fractions and a SHORTAGE of light ones. Cracking converts the surplus heavy fractions into light ones → matches supply to demand → increases the value of the crude oil.

Reason 2: Alkenes for the plastics industry.

Crude oil contains NO alkenes naturally (alkenes are too reactive to survive geological time). But alkenes are extremely valuable:

• Ethene (C₂H₄): monomer for poly(ethene) — the most-used plastic. Also makes ethanol (hydration), ethylene glycol (antifreeze), PVC.
• Propene (C₃H₆): monomer for poly(propene) — used in containers, fibres, car parts.
• Butenes: synthetic rubber, butan-1-ol.

Cracking is THE main industrial source of alkenes. Without cracking, there would be no plastics industry.

Conditions for cracking — two methods.

Thermal cracking.

• Temperature: ~ 800-900 °C (very hot).
• Pressure: ~ 70 atm (high).
• No catalyst.
• Products: tends to give a high yield of alkenes (especially ethene). Used when ethene is the desired product.

Catalytic cracking.

• Temperature: ~ 500 °C (much cooler than thermal).
• Pressure: atmospheric / slightly elevated.
• CATALYST: aluminium oxide (Al₂O₃, 'alumina') — often as a zeolite (silica/alumina) catalyst.
• Products: more controlled mix; tends to give branched (high-octane) petrol-range alkanes. Used when high-quality petrol is the goal.

Why use a catalyst? Provides an alternative pathway with LOWER ACTIVATION ENERGY → reaction occurs at LOWER temperature → less energy needed → cheaper and more efficient. Also gives BETTER CONTROL over which products form (more branching, fewer side reactions).

Most modern refineries use CATALYTIC CRACKING for the bulk of their cracking — it's more energy-efficient.

Industrial scale.

A typical large oil refinery cracks ~ 30-50% of its crude oil throughput. The cracker is a major piece of equipment (~ 20 m tall, processes thousands of tonnes per day). The fluidised-bed cracker (FCC) is the most common design.

Both products from cracking are valuable.

There's NO WASTE in cracking — both the shorter alkane and the alkene are useful:

• Shorter alkane → petrol pool (cars), naphtha (chemicals feedstock), or further cracking.
• Alkene → sold to the plastics industry as feedstock for polymerisation.

This is why cracking is so profitable: a low-value heavy fraction becomes TWO high-value products.

Comparison: distillation vs cracking.

• Distillation SEPARATES the components of crude oil — physical change, no bonds broken.
• Cracking CHEMICALLY CHANGES one component into different molecules — bonds are broken and new ones formed.

Both are essential to the refining process. Distillation comes first (to sort by chain length); cracking is applied to the surplus heavy fractions.

Worked example — different splits.

Cracking reaction	Yield per mole of decane
C₁₀H₂₂ → C₈H₁₈ + C₂H₄	1 octane + 1 ethene
C₁₀H₂₂ → C₇H₁₆ + C₃H₆	1 heptane + 1 propene
C₁₀H₂₂ → C₆H₁₄ + 2 C₂H₄	1 hexane + 2 ethene
C₁₀H₂₂ → C₅H₁₂ + C₃H₆ + C₂H₄	1 pentane + 1 propene + 1 ethene

All are valid! The mix depends on the conditions (T, catalyst, residence time).

Burning hydrocarbon fuels (petrol, diesel, kerosene, natural gas, coal) releases a mixture of gases. The composition depends on whether combustion is COMPLETE or INCOMPLETE.

Complete combustion.

With PLENTY of oxygen (excess O₂), every carbon atom is oxidised to CO₂ and every hydrogen to H₂O:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$ $2\text{C}_8\text{H}_{18} + 25\text{O}_2 \rightarrow 16\text{CO}_2 + 18\text{H}_2\text{O}$ (octane in petrol)

• Highly EXOTHERMIC — maximum heat released.
• BLUE NON-LUMINOUS flame (Bunsen with air hole open) — hot, no soot.
• ~ 1500 °C in a well-mixed flame.

Incomplete combustion.

With LIMITED oxygen, carbon is only partially oxidised:

• Carbon monoxide (CO) — toxic, formed from partial oxidation.
• Carbon (C) — soot / particulates, from no oxidation at all.
• H atoms still become H₂O.

Example equations:

$2\text{C}_3\text{H}_8 + 7\text{O}_2 \rightarrow 6\text{CO} + 8\text{H}_2\text{O}$ $\text{C}_3\text{H}_8 + 2\text{O}_2 \rightarrow 3\text{C} + 4\text{H}_2\text{O}$ (soot only)

In practice, a MIXTURE of CO + C + CO₂ + H₂O forms depending on how oxygen-starved the combustion is.

• Less heat released (partial oxidation, less energy out).
• YELLOW LUMINOUS flame (glowing soot particles) — Bunsen with air hole closed.
• Deposits BLACK SOOT on cool surfaces.

Where incomplete combustion happens:

• Faulty gas boilers / heaters in poorly-ventilated rooms.
• Old / poorly-tuned engines.
• A Bunsen burner with the air hole closed (deliberately for some experiments).

Toxicity of carbon monoxide.

CO is the MAIN HEALTH HAZARD from incomplete combustion. It binds to HAEMOGLOBIN in red blood cells ~ 200× more strongly than O₂, forming carboxyhaemoglobin. This:

• BLOCKS the haemoglobin from carrying O₂.
• Causes oxygen starvation in tissues.
• Symptoms: headache, dizziness, drowsiness, unconsciousness, death.
• CO is COLOURLESS AND ODOURLESS — undetectable without an alarm. 'Silent killer'.

The five main air pollutants from fossil-fuel combustion.

(1) Carbon dioxide (CO₂).

• Source: complete combustion of any hydrocarbon.
• Impact: GREENHOUSE GAS. CO₂ absorbs IR radiation re-emitted from Earth's surface and re-emits it back, warming the atmosphere (the greenhouse effect). Excess CO₂ from fossil fuels enhances this effect → CLIMATE CHANGE: rising T, melting ice, sea level rise, extreme weather. Major 21st-century issue.
• Reduction: switch to renewable energy; electric vehicles; carbon capture & storage; reforestation.

(2) Carbon monoxide (CO).

• Source: incomplete combustion of hydrocarbons.
• Impact: TOXIC. Binds to haemoglobin → oxygen starvation. Causes death at high doses. Colourless, odourless.
• Reduction: catalytic converters in cars; proper ventilation in gas-heated rooms; CO alarms.

(3) Nitrogen oxides (NOₓ, mainly NO and NO₂).

• Source: formed at HIGH TEMPERATURES inside engines and power-station boilers: N₂ + O₂ → 2NO (high-T endothermic reaction). NO further oxidises in air: 2NO + O₂ → 2NO₂.
• Impact 1: ACID RAIN. NOₓ dissolves in water to form nitric acid (HNO₃) — damages stonework (especially limestone CaCO₃ — buildings, statues), kills lakes (acidifies water, fish die), kills trees (leaches nutrients from soil).
• Impact 2: respiratory irritant. NO₂ aggravates asthma and bronchitis.
• Impact 3: photochemical smog (NOₓ + hydrocarbons + sunlight → ground-level ozone, toxic).
• Reduction: CATALYTIC CONVERTER — converts NO to N₂ (see below). Lower combustion temperatures.

(4) Sulfur dioxide (SO₂).

• Source: from SULFUR IMPURITIES in fuel: S + O₂ → SO₂. Modern petrol has < 10 ppm S (very low). Diesel and coal still have more.
• Impact: ACID RAIN. SO₂ dissolves in water → sulfurous acid → further oxidation to sulfuric acid. Same damage as NOₓ. Historical driver of acid rain in Europe and North America.
• Reduction: LOW-SULFUR FUELS (mandatory in many countries); FLUE-GAS DESULFURISATION in power stations (gases passed through Ca(OH)₂ slurry → CaSO₃ → can become gypsum CaSO₄ for plasterboard).

(5) Particulates (soot / unburned carbon).

• Source: incomplete combustion of hydrocarbons. Fine carbon particles (PM2.5, PM10).
• Impact: lung damage — fine particles penetrate deep into lungs → bronchitis, emphysema, lung cancer. Linked to ~ 4 million premature deaths/year globally. Also blackens buildings; reduces visibility (smog).
• Reduction: PARTICULATE FILTERS in diesel exhausts; cleaner combustion; switch to electric vehicles.

Catalytic converter — how it works.

A device in the exhaust pipe of every modern petrol car (and diesel cars too, with modifications). It contains a HONEYCOMB ceramic block coated with PLATINUM and RHODIUM catalysts.

The hot exhaust gases pass over the catalyst surface. Three reactions occur (in three different 'zones' of the converter or simultaneously):

$2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2$ (CO and NO together → harmless products)

$2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2$ (remaining CO oxidised)

$2\text{NO} \rightarrow \text{N}_2 + \text{O}_2$ (remaining NO decomposed)

Unburned hydrocarbons also oxidised to CO₂ + H₂O.

The result: the harmful gases (CO, NO, unburned HCs) are CONVERTED to harmless gases (CO₂, N₂, H₂O). The catalyst is NOT used up; it lasts for the life of the car (~ 100,000+ miles).

Limits of catalytic converters. They DON'T remove CO₂ (it's just produced more efficiently). They CAN be poisoned by lead (which is why leaded petrol was phased out in the 1990s). They need to be warm to work properly — cold-start emissions remain a problem.

Summary table.

Pollutant	From	Damage	Reduction
CO₂	Complete combustion	Climate change	Renewable energy
CO	Incomplete combustion	Toxic (haemoglobin)	Catalytic converter
NOₓ	High-T in engines	Acid rain + respiratory	Catalytic converter
SO₂	S impurities	Acid rain	Low-S fuels
Particulates	Incomplete combustion	Lung damage	Particulate filter