C7 Organic chemistry

Crude oil is a finite (non-renewable) resource found in rocks. It was formed over millions of years from the remains of ancient biomass, mainly tiny sea organisms called plankton, that were buried in mud and compressed.

It is a mixture of a very large number of compounds that are not chemically joined together. Most of these compounds are hydrocarbons: molecules that contain ONLY carbon and hydrogen atoms.

Most of the hydrocarbons in crude oil are alkanes. Alkanes are a homologous series (a family of compounds) with the general formula C_nH_2n+2.

To find the formula of an alkane, use C_nH_2n+2. This means you multiply the number of carbon atoms by 2 and then add 2 to find the number of hydrogen atoms.

The first four alkanes are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).

The displayed formulae of methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).

An alkane contains 7 carbon atoms. How many hydrogen atoms does it have in its molecular formula?

Use the general formula C_nH_2n+2 where n = 7.

Hydrogen atoms = (2 × 7) + 2 = 14 + 2 = 16.

The molecular formula is C₇H₁₆.

A hydrocarbon must contain carbon and hydrogen only. If it has oxygen in it, it is not a hydrocarbon.

Crude oil was not formed from dinosaurs, and it is not a single compound.

When applying the general formula C_nH_2n+2, always remember the '+ 2' at the end. Don't confuse it with other general formulae.

In a displayed formula, every carbon atom must have exactly 4 single bonds, and every hydrogen atom must have 1.

Crude oil is a mixture of a huge number of compounds, mostly hydrocarbons. Fractional distillation separates it into fractions.

Each fraction is still a mixture, but its hydrocarbons have a similar number of carbon atoms, so they have similar boiling points.

Fractions are processed into fuels (such as petrol, kerosene and diesel oil) and into feedstock for the petrochemical industry, which makes solvents, lubricants, polymers and detergents.

Crude oil is heated until most of it evaporates. The vapour enters the bottom of a tall fractionating column.

There is a temperature gradient in the column: it is hottest at the bottom and coolest at the top.

Vapours rise up the column. Each hydrocarbon condenses (turns back to liquid) when it reaches a level where the temperature is below its boiling point. The liquid is collected on a tray and piped off.

Small molecules have low boiling points, so they rise furthest and leave near the top. Large molecules have high boiling points, so they condense low down or leave the bottom as a liquid residue.

Fractions are piped off at different heights: liquefied petroleum gases at the cool top, residue at the hot bottom.

Kerosene is collected from the column above diesel oil but below petrol. What does this tell you about kerosene molecules?

Fractions higher up the column condense at lower temperatures, so they have lower boiling points and smaller molecules.

Kerosene molecules are smaller than diesel oil molecules but larger than petrol molecules, and its boiling points lie between theirs.

Substances condense where the temperature drops to or below their boiling point, not above it.

Fractional distillation separates a mixture by evaporation and condensation. It does not break any chemical bonds. Breaking large molecules into smaller ones is a different process called cracking.

As hydrocarbon molecules get larger (longer chains with more carbon atoms), their properties change. Three key trends occur as size increases: their boiling point increases, their viscosity increases (they become thicker and harder to pour), and their flammability decreases (they are harder to ignite).

These trends happen because larger molecules have stronger intermolecular forces between them, which require more energy to overcome.

Hydrocarbons are used as fuels because they release energy when they burn. This is an exothermic reaction. During combustion, the carbon and hydrogen in the fuel are oxidised.

If there is plenty of oxygen, complete combustion happens. The only products are carbon dioxide and water.

The boiling point of alkanes steadily increases as the number of carbon atoms in the chain gets larger.

Write the balanced equation for the complete combustion of heptane, C₇H₁₆.

First, balance the carbon: 7 carbon atoms need 7CO₂.

Next, balance the hydrogen: 16 hydrogen atoms need 8H₂O.

Finally, count the oxygen atoms on the right: (7 × 2) from CO₂ plus 8 from H₂O = 22. This needs 11O₂.

The equation is: C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O.

When balancing, you may get an odd number of oxygen atoms on the right, needing a half number of O₂ molecules (e.g. 6.5). If you want whole numbers, double everything in the entire equation.

Remember that water (H₂O) has only one oxygen atom. A common mistake is counting it as two.

Good fuels need to flow easily and ignite easily. Small hydrocarbons make better fuels than very large ones.

Cracking breaks down large hydrocarbon molecules into smaller, more useful ones. It is a thermal decomposition reaction because it uses heat to break chemical bonds. It does not separate a mixture and does not need oxygen.

Cracking produces smaller alkanes and alkenes. Alkenes are more reactive than alkanes.

There is a higher demand for small-molecule fuels (like petrol) than the crude oil supply provides. Cracking converts surplus large molecules to meet demand.

Catalytic cracking: The hydrocarbon is heated to vaporise it. The vapour passes over a hot catalyst at about 550 °C.

Steam cracking: The hydrocarbon vapour is mixed with steam and heated to a very high temperature (about 850 °C) without a catalyst.

Testing for alkenes: Shake the hydrocarbon with orange bromine water. An alkene turns it colourless. An alkane does not react, staying orange.

In the lab, liquid paraffin on mineral wool is heated. Vapour passes over hot broken porcelain (catalyst), and gas is collected over water.

A hydrocarbon C₁₄H₃₀ is cracked into C₈H₁₈ and an alkene, C₂H₄. Balance the equation:

C₁₄H₃₀ → C₈H₁₈ + ? C₂H₄

Carbon and hydrogen atoms are conserved. Carbon: 14 on the left. C₈H₁₈ has 8, leaving 6 carbon atoms.

Each C₂H₄ has 2 carbon atoms, so 6 ÷ 2 = 3 molecules. Check hydrogen: 18 + (3 × 4) = 30. The blank is 3.

Remove the delivery tube from the water before stopping heating in the lab, otherwise cold water sucks back and shatters the hot glass.

Bromine water turns colourless with an alkene, not "clear".

Cracking does not use oxygen. Some products are used as fuels, and alkenes are used to make polymers.