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Hydrocarbons: Where They Come From, What They Do, and Rings

Hydrocarbons are compounds of only carbon and hydrogen. We get them from crude oil and natural gas by heating and separating (fractional distillation), and long chains are broken into useful short ones by cracking. They give us fuels and plastics, but burning them makes CO2. If a chain closes into a ring we get a cycloalkane, CnH2n, which has 2 H fewer than the chain.

🎬 Step-by-step story

  1. Crude oil is a thick black mix of many hydrocarbons. We heat it at the bottom of a tall tower. Nothing is separated yet.
  2. Hot gas rises in the tower and cools. Small molecules rise to the top, big ones stay low. Each level gives one fraction: gas, petrol, diesel, bitumen.
  3. Long chains are thick and not so useful. In cracking, a long chain is broken into a shorter alkane and a short alkene.
  4. Hydrocarbons burn with oxygen. Look at propane: 3 carbons become 3 CO2, and 4 water molecules form. Heat comes out.
  5. A chain has two loose ends. Join the ends and you get a ring. Count the hydrogens: the ring has 2 fewer. C6H14 becomes C6H12.
  6. Your turn. Move the slider to pick 3 to 8 carbons. Press the button to change chain to ring. Watch the formula.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why do small molecules rise to the top?

They are light and weakly attracted to each other, so they stay a gas until it is cool near the top.

Is crude oil one substance?

No. It is a mix of hundreds of hydrocarbons. The tower separates it into groups.

Why break big molecules at all?

Short chains make better fuel, and the alkene piece makes plastics. Cracking gives us more of what we need.

Where does the CO2 come from when fuel burns?

Every carbon atom in the fuel joins two oxygen atoms. 3 carbons give 3 CO2.

Why does the ring have two fewer H?

The two end carbons each had one extra H. When they join, those two H go away and the ends bond to each other.

Does the rule CnH2n work for any ring size?

Yes for one ring with single bonds, from 3 carbons upward. Try 3 to 8 on the slider.

What is a hydrocarbon?

A hydrocarbon is a compound made of only two elements: carbon and hydrogen. Carbon can join to other carbons, so the molecules can be short or very long.

Where do hydrocarbons come from?

Most hydrocarbons come from crude oil (petroleum) and natural gas. These formed over millions of years from tiny sea plants and animals. Crude oil is a mixture, so we must separate it.

Fractional distillation. Crude oil is heated until it turns to vapour and is fed into a tall tower that is hot at the bottom and cooler at the top. Each hydrocarbon rises until it is cool enough to turn back into liquid. Small molecules have low boiling points, so they reach the top. Big molecules have high boiling points, so they stop low. Each liquid group is called a fraction.

FractionCarbonsMain use
Refinery gasC1–C4cooking gas (LPG)
PetrolC5–C10fuel for cars, scooters
DieselC11–C20fuel for buses, trucks, trains
BitumenC20 and moreroad surface, roofing

Cracking. There is more of the long chains than we need and less of the short ones. So we heat long chains very hot (with a catalyst) and they break. Example: C12H26 → C8H18 + C4H8. One piece is a useful alkane (petrol), the other is an alkene used to make plastics.

Other ways: alkanes can also be made by adding hydrogen to alkenes (C2H4 + H2 → C2H6), and alkenes by removing water from alcohols.

Uses and effect on the environment

Uses. Fuels (cooking gas, petrol, diesel, aircraft fuel), raw material for plastics, fibres, paints and medicines (alkenes such as ethene and propene), solvents, and bitumen for roads.

Burning (combustion). With enough air a hydrocarbon gives carbon dioxide and water and a lot of heat: C3H8 + 5 O2 → 3 CO2 + 4 H2O. With too little air we get carbon monoxide (a poisonous gas) and black soot.

Problems. CO2 traps heat in the air and adds to global warming. Burning vehicle fuel makes smog. Oil spills harm sea life. Crude oil is not renewable. So we save fuel, use public transport, and recycle plastics.

Cycloalkanes: hydrocarbons in a ring

When the two ends of a carbon chain join, we get a cycloalkane. The general formula is CnH2n. Every carbon has two H, so the ring has 2 fewer hydrogens than the alkane with the same carbons.

NameCarbonsFormula
cyclopropane3C3H6
cyclobutane4C4H8
cyclopentane5C5H10
cyclohexane6C6H12

The name is "cyclo" + the alkane name. A cycloalkane has the same formula as an alkene (CnH2n), so the two are isomers. Cyclohexane is a common solvent. Small rings (3 and 4) are strained and react more easily; the 5 and 6 rings are steady. Like alkanes, cycloalkanes burn and react by substitution; cyclopropane can also open to a chain.

Find the formula from data

1. From relative density. The molar mass is M = D × M(reference gas). With hydrogen, M = 2 × D(H2). With air, M ≈ 29 × D(air).

2. From percentages. Divide each percentage by the atomic mass (C = 12, H = 1) and get the simplest ratio. That gives the empirical formula. Then divide the real molar mass by the empirical mass to get the multiplier.

3. From burning. The moles of CO2 equal the moles of carbon; the moles of H2O × 2 equal the moles of hydrogen.

Try it: in the 3D, set 4 carbons. Count the H in the chain (10) and in the ring (8). Predict 5 carbons first, then check.

Key formulas and definitions

Worked examples

1. A hydrocarbon is 85.7% carbon and 14.3% hydrogen by mass. Find its empirical formula.

C: 85.7 ÷ 12 = 7.14. H: 14.3 ÷ 1 = 14.3. Ratio C : H = 7.14 : 14.3 = 1 : 2. Empirical formula = CH2.

2. The same gas is 21 times as dense as hydrogen. Find its molecular formula.

M = 2 × 21 = 42. Empirical mass of CH2 = 14. 42 ÷ 14 = 3, so the formula is C3H6 (propene or cyclopropane).

3. What is the formula of cyclopentane, and what is its molar mass?

n = 5, so C5H10. M = 5 × 12 + 10 × 1 = 70 g/mol.

4. 2 L of propane burns completely. What volume of CO2 forms (same conditions)?

C3H8 + 5 O2 → 3 CO2 + 4 H2O. 1 volume propane gives 3 volumes CO2, so 2 L gives 6 L.

5. 0.1 mol of a hydrocarbon burns to give 0.4 mol CO2 and 0.4 mol H2O. Find its formula.

Per mole: 4 mol CO2 so 4 C. 4 mol H2O so 8 H. Formula C4H8.

6. What mass of CO2 is made when 16 g of methane burns completely?

CH4 + 2 O2 → CO2 + 2 H2O. 16 g CH4 = 1 mol, so 1 mol CO2 = 44 g.

7. 2.24 L of a gaseous hydrocarbon at STP has mass 2.8 g. Find its formula.

n = 2.24 ÷ 22.4 = 0.1 mol. M = 2.8 ÷ 0.1 = 28 g/mol. CH2 has mass 14, so 28 ÷ 14 = 2: C2H4 (ethene).

Common mistakes

Practice quiz

1. A hydrocarbon contains:
2. Which fraction comes out at the top of the tower?
3. Cracking is done to:
4. The formula of cyclohexane is:
5. Burning propane in plenty of air gives:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

Why is crude oil separated by boiling points?

Each hydrocarbon has its own boiling point. Longer chains have stronger attraction between molecules, so they need more heat to boil. This lets us separate them level by level.

Are cycloalkanes and alkenes the same?

They have the same formula, CnH2n, but different structure: a ring with single bonds versus a chain with one C=C. So they are isomers, and only the alkene decolourises bromine water quickly.

Why is burning hydrocarbons a problem?

It puts CO2 in the air, which traps heat, and with poor air supply it makes carbon monoxide and soot. Crude oil is also used up and cannot be made again quickly.

Where this is taught

Ukraine10 класHydrocarbons
Ukraine10 класHydrocarbons
Japan高校（専門学科）1〜3年Industrial Chemistry

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