Crude Oil and Hydrocarbons

Almost everything made from oil — the petrol in cars, the diesel in lorries, the plastics all around you — begins as crude oil, a thick, dark liquid pumped from deep underground. Crude oil is a finite resource and a mixture of many different compounds, most of which are hydrocarbons — molecules made of hydrogen and carbon only. The most important hydrocarbons are the alkanes. This lesson, part of Topic C6 of OCR Gateway Combined Science A, explains what crude oil is and how it formed, defines a hydrocarbon and an alkane, shows the first members of the alkane family, describes how their properties change with chain length, and covers complete combustion.

By the end of this lesson you should be able to describe crude oil as a finite mixture of hydrocarbons, define a hydrocarbon and an alkane, give the general formula and the first four alkanes, predict how the properties of alkanes change with chain length, and write balanced equations for complete combustion.

This lesson runs from AO1 (recalling what crude oil, hydrocarbons and alkanes are) through AO2 (writing balanced symbol equations for complete combustion) to AO3 (predicting how the properties of alkanes change as the chain lengthens).

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What Crude Oil Is

Crude oil is a finite resource formed over millions of years from the remains of ancient biomass — mainly tiny sea creatures and plants such as plankton that died and were buried under layers of sediment. Over a very long time, high pressure and temperature and the absence of oxygen turned this buried biomass into crude oil (and natural gas). Because it forms so slowly, crude oil is being used up far faster than it is replaced, so it is a finite (non-renewable) resource.

Crude oil is a mixture of many different compounds, the great majority of which are hydrocarbons. It is the raw material — the feedstock — for an enormous range of useful products: fuels such as petrol, diesel and kerosene, and the starting materials (petrochemicals) for plastics, solvents, detergents and much more.

Exam Tip: Two facts examiners want about crude oil: it is finite (formed over millions of years from ancient biomass/plankton) and it is a mixture of hydrocarbons. A common misconception is to call it "a single compound" — it is a mixture.

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Hydrocarbons and Alkanes

A hydrocarbon is a compound made of hydrogen and carbon only — no other elements. Most of the hydrocarbons in crude oil are alkanes.

The alkanes are a family of hydrocarbons with three key features:

• they are a homologous series — a family of compounds with the same general formula, similar chemical properties, and a gradual change in physical properties along the series;
• they are saturated, meaning they contain only single carbon–carbon bonds (C–C) — there are no double bonds, and each carbon is bonded to as many hydrogen atoms as possible;
• they have the general formula $\text{C}_n\text{H}_{2n+2}$Cn​H2n+2​, where $n$n is the number of carbon atoms.

The first four alkanes are:

Name	Formula	Number of carbons ($n$n)	Check: $2n+2$2n+2
Methane	$\text{CH}_4$CH4​	1	$2(1)+2 = 4$2(1)+2=4 ✓
Ethane	$\text{C}_2\text{H}_6$C2​H6​	2	$2(2)+2 = 6$2(2)+2=6 ✓
Propane	$\text{C}_3\text{H}_8$C3​H8​	3	$2(3)+2 = 8$2(3)+2=8 ✓
Butane	$\text{C}_4\text{H}_{10}$C4​H10​	4	$2(4)+2 = 10$2(4)+2=10 ✓

The displayed formula shows every atom and every bond. Here are the first three alkanes drawn out:

Exam Tip: Learn the first four alkanes (meth-, eth-, prop-, but- = 1, 2, 3, 4 carbons) and the general formula $\text{C}_n\text{H}_{2n+2}$Cn​H2n+2​. Given any value of $n$n you can write the formula, and given a formula you can check it is an alkane.

Worked Example 1 — Using the general formula

What is the formula of the alkane with 5 carbon atoms (pentane)?

Step 1 — use $\text{C}_n\text{H}_{2n+2}$Cn​H2n+2​ with $n = 5$n=5.

Step 2 — number of hydrogens $= 2n+2 = 2(5)+2 = 12$=2n+2=2(5)+2=12.

Step 3 — write the formula: $\text{C}_5\text{H}_{12}$C5​H12​.

Answer: pentane is $\text{C}_5\text{H}_{12}$C5​H12​.

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How the Properties of Alkanes Change with Chain Length

As the hydrocarbon chains get longer (more carbon atoms), the physical properties change in a predictable way. The reason lies in the intermolecular forces between molecules: longer molecules have more, stronger intermolecular forces, so more energy is needed to separate them.

Property	Short chains (e.g. methane)	Long chains (e.g. bitumen)
Boiling point	Low	High
Viscosity (how thick/runny)	Runny (low viscosity)	Thick (high viscosity)
Volatility (how easily it evaporates)	High (evaporates easily)	Low
Flammability (how easily it burns)	High (burns easily)	Low

So as the chains get longer: boiling point rises, the liquid becomes more viscous, it is less volatile, and it is less flammable. Short-chain hydrocarbons are gases or runny, volatile, flammable liquids (useful as fuels); long-chain hydrocarbons are thick, less flammable liquids or solids (useful as lubricants, fuel oil and bitumen).

Exam Tip: Connect the trend to intermolecular forces: longer chains have more/stronger intermolecular forces, so a higher boiling point. A classic misconception is to say longer chains are more flammable — they are less flammable.

Worked Example 2 — Predicting properties

Two hydrocarbons are compared: hexane ($\text{C}_6\text{H}_{14}$C6​H14​) and an oil with chains of about 30 carbons. Which has the higher boiling point and which is more flammable?

Step 1 — compare the chain lengths: the oil has much longer chains (≈30 carbons) than hexane (6 carbons).

Step 2 — apply the trend: longer chains → higher boiling point and less flammable.

Step 3 — conclude: the oil has the higher boiling point; hexane (shorter chains) is more flammable.

Answer: the long-chain oil boils at a higher temperature; hexane is the more flammable of the two.

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Complete Combustion

Hydrocarbons are widely used as fuels because they release energy when they burn. When a hydrocarbon burns in plenty of oxygen, complete combustion occurs: the carbon is oxidised to carbon dioxide and the hydrogen is oxidised to water.

$\text{hydrocarbon} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$hydrocarbon+oxygen→carbon dioxide+water

For methane (the main gas in natural gas):

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$CH4​+2O2​→CO2​+2H2​O

Complete combustion releases the most energy and is the cleanest form of burning, producing only carbon dioxide and water. (In a limited supply of oxygen, incomplete combustion occurs instead, producing carbon monoxide and soot — covered in the later lesson on atmospheric pollutants.)

Worked Example 3 — Balancing a combustion equation

Write a balanced equation for the complete combustion of propane, $\text{C}_3\text{H}_8$C3​H8​.

Step 1 — write the unbalanced equation: $\text{C}_3\text{H}_8 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$C3​H8​+O2​→CO2​+H2​O.

Step 2 — balance carbon: 3 carbons on the left, so put a 3 in front of $\text{CO}_2$CO2​ → $3\text{CO}_2$3CO2​.

Step 3 — balance hydrogen: 8 hydrogens on the left, so put a 4 in front of $\text{H}_2\text{O}$H2​O (giving $4 \times 2 = 8$4×2=8 H) → $4\text{H}_2\text{O}$4H2​O.

Step 4 — balance oxygen last: the right now has $(3 \times 2) + (4 \times 1) = 6 + 4 = 10$(3×2)+(4×1)=6+4=10 oxygen atoms, so put a 5 in front of $\text{O}_2$O2​.