4bCrude oil

Crude oil is a mixture of hydrocarbons.

Because it is a mixture and not a compound, it can be separated by a physical process — no bonds are broken, and the molecules that come out are the ones that went in.

How the column separates

1. The crude oil is heated until most of it evaporates.
2. The vapours enter a fractionating column that is hot at the bottom and cooler at the top.
3. As the vapours rise they cool.
4. Each fraction condenses at the height where the temperature matches its boiling range.

A fraction is a mixture of hydrocarbons of similar chain length, so it condenses over a range of temperatures rather than at a single boiling point.

The temperature gradient is what does the separating. A column at one temperature would separate nothing at all.

The six fractions

In order up the column — increasing height, decreasing boiling point:

Bitumen is the only one not used as a fuel.

The trends

Going down the column, the molecules get longer, and three properties rise together:

• Boiling point increases
• Viscosity increases — the fractions get thicker
• Colour darkens

Longer molecules have stronger intermolecular forces, which is why they boil higher and condense first, low in the column.

As always with molecular substances, it is the forces between molecules that change. The covalent bonds inside them are not broken by boiling.

Why gasoline burns and bitumen does not

Gasoline has short molecules, so it is volatile — it evaporates easily, mixes with air and ignites in an engine.

Bitumen has very long molecules, so it is thick and barely evaporates. It is difficult to ignite and useless as a fuel, which is why it surfaces roads instead.

Fractions are still mixtures

Crude oil contains far too many different hydrocarbons for distillation to isolate single compounds. Molecules of similar length have similar boiling points and condense together, so each fraction is itself a mixture and boils over a range.

A fuel is a substance that releases heat energy when burned.

Note what that definition does not require: it says nothing about carbon. Hydrogen is a fuel and contains none.

Complete combustion

With plenty of oxygen, a hydrocarbon burns to give carbon dioxide and water.

CH₄ + 2O₂ → CO₂ + 2H₂O

Only those two products are possible, because a hydrocarbon contains only carbon and hydrogen.

Incomplete combustion

With a limited supply of oxygen, the carbon is not fully oxidised. The possible products are:

• Carbon monoxide, CO
• Carbon, as soot
• Water — produced either way

Water appears in both complete and incomplete combustion. Only the carbon-containing product changes.

Reading a flame

A Bunsen burner shows both cases:

The yellow colour comes from glowing soot particles — unburned carbon. A yellow, sooty flame on a gas appliance is a visible warning that combustion is incomplete.

Complete combustion also releases more energy per molecule of fuel, which is why the blue flame is hotter.

Why carbon monoxide is dangerous

Two reasons together, and a full answer gives both.

It is poisonous. Carbon monoxide is absorbed by the blood and reduces the blood's capacity to carry oxygen, so less oxygen reaches the body's cells. (The specification does not require haemoglobin to be named.)

It cannot be detected. It is colourless and has no smell, so a person can be affected without any warning.

The combination is what makes a poorly ventilated gas fire so hazardous: the same lack of oxygen produces both the visible yellow flame and the invisible poison.

Two pollutants come from burning fuels that have nothing to do with the hydrocarbon itself.

Sulfur dioxide

Hydrocarbons contain only carbon and hydrogen, so they cannot produce sulfur dioxide. The sulfur is an impurity in the fuel, left over from the crude oil.

When the fuel burns, that sulfur burns too:

sulfur + oxygen → sulfur dioxide

Say impurity. A pure hydrocarbon produces no sulfur dioxide at all, which is why removing sulfur at the refinery prevents the problem at source.

Oxides of nitrogen

Nitrogen makes up 78% of air and is normally very unreactive — its triple bond is hard to break.

Inside a car engine the temperature is high enough to supply that energy, so nitrogen and oxygen from the air react to form oxides of nitrogen.

The nitrogen comes from the air, not the fuel. Hydrocarbon fuels contain no nitrogen.

Acid rain

Both gases dissolve in water in the atmosphere to form acidic solutions, which fall as acid rain.

Its effects:

• Damages trees and plants
• Makes lakes and rivers acidic, killing fish
• Corrodes limestone buildings and statues, and attacks metal structures

When asked for problems, give effects on both living things and buildings — two effects of the same kind may count as one point.

Two different problems, two different gases

This distinction is examined regularly:

Carbon dioxide is a greenhouse gas, not a cause of acid rain. Mixing them up is a straightforward error.

Catalytic converters

Fitted to car exhausts to remove oxides of nitrogen and carbon monoxide — one an environmental problem through acid rain, the other directly toxic to people.

A question asking why both are removed wants a separate reason for each gas.

The problem cracking solves

Crude oil comes out of the ground with the wrong proportions.

It contains far more long-chain hydrocarbons than anyone wants, and not nearly enough of the short-chain fractions — especially gasoline — that are in heavy demand.

Supply exceeds demand at the heavy end; demand exceeds supply at the light end.

Cracking converts the surplus into the shortage.

What cracking does

It breaks long-chain alkanes into:

• a shorter-chain alkane, and
• an alkene

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Atoms are conserved, so you can find a missing product by subtraction: C₁₀H₂₂ minus C₈H₁₈ leaves C₂H₄, which is ethene.

Conditions

Catalytic cracking uses:

• a catalyst of silica or alumina
• a high temperature, around 600–700 °C

Cracking is a chemical change

Worth keeping separate from fractional distillation:

Confusing the two is a common error, and the test is simple — are new substances made?

Why the alkenes matter

Cracking is not only about making more petrol. The alkenes it produces are arguably the more valuable product.

Alkenes are unsaturated, with a carbon-carbon double bond, which makes them far more reactive than alkanes. That reactivity lets them be used to make polymers and other chemicals such as alcohols.

Alkanes are too unreactive to polymerise, so without cracking there would be far less alkene feedstock from crude oil — which is why cracking, and not just distillation, is what supports the plastics industry.