Crude oil as a complex hydrocarbon mixture

Chemistry · Fuels

Mixture or one compound?

Put each name in the right group. Tip: "one compound" means every molecule is the same kind.

10 of 10 still to sort.

Ten names you have probably heard. Decide where each one goes, then read why. Commit first — the reason shows after you place it.

The molecules inside the mixture

Most of the hydrocarbon molecules in crude oil are alkanes. Spot the pattern in the hydrogen count as the carbon count goes up.

How crude oil formed

Step through it in order. The last step is the one that matters for the future.

1. 1Plankton dieDead plankton settle on the ocean floor.+
2. 2BuriedLayers of mud and other sediment cover the dead plankton.+
3. 3Squeezed and heatedHigh pressure from the sediments and rocks above, and high temperatures, act on what is buried.+
4. 4Crude oilOver millions of years, the buried plankton are turned into crude oil — a fossil fuel, like natural gas and coal.+
5. 5FiniteCrude oil exists in limited quantities. Once it is used, it cannot be regenerated or replaced in our lifetime, so it is a finite (non-renewable) resource.+

What you need to know

• Crude oil is a mixture of different hydrocarbons plus some impurities, so it is not a pure substance.
• A hydrocarbon is a covalent compound whose molecules contain hydrogen and carbon atoms only.
• Have a goDani says: "A hydrocarbon is any molecule with carbon in it." Which atoms does a hydrocarbon molecule contain?

  Hydrogen and carbon atoms only.

  Carbon alone is not enough: a hydrocarbon's molecules contain hydrogen and carbon atoms only, nothing else.

• Most hydrocarbons in crude oil are alkanes, a homologous series with only single bonds between carbon atoms.
• Alkanes have the general formula CₙH₂ₙ₊₂, and every carbon atom in them forms four covalent bonds.
• Have a goAn alkane molecule has 7 carbon atoms. Use CₙH₂ₙ₊₂ to predict how many hydrogen atoms it has.

  16 hydrogen atoms, so C₇H₁₆.

  Put n = 7 into 2n + 2: double it to get 14, then add 2. Forgetting the +2 is the usual slip.

• The first four alkanes are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀.
• Crude oil is a fossil fuel, like natural gas and coal, formed over millions of years from dead plankton.
• It is finite (non-renewable): once used, it cannot be replaced in our lifetime.
• Have a goA friend says: "No problem, we'll just wait for the oil we use to be replaced." What does "finite" say about that?

  Once used, crude oil cannot be replaced in our lifetime.

  It formed over millions of years, so replacing it is not something that happens on a human timescale.

• Because it is a mixture, crude oil can be separated by fractional distillation, according to boiling point, into fractions.
• A fraction is a group of hydrocarbons of similar size and similar boiling points, and it is still a mixture.

The big picture

Crude oil is a finite mixture of many different hydrocarbons, mostly alkanes, formed from dead plankton over millions of years. Fractional distillation separates the hydrocarbons already there into fractions, and each fraction is still a mixture. Those fractions give us fuels and other useful products.

⚠ Watch out

Thinking that petrol, diesel oil or any other fraction is a single pure compound. A fraction is a group of hydrocarbons with similar boiling points, so it is still a mixture. A named alkane such as propane is the one that is a single compound.

Memory hook

Crude oil is a crowd of hydrocarbons, and distillation seats the crowd in groups. Nobody new walks in, and each table is still a crowd.

Check yourself

Cover the page and explain, in two sentences: why is crude oil not a pure substance, and why does fractional distillation not give you pure compounds?