Organic chemistry, Year 11: crude oil, alkanes and fractional distillation

A hydrocarbon

HintThe name is built from the names of the two elements.

The whyThe word 'only' matters: a hydrocarbon contains no other element at all. Most of the compounds in crude oil are hydrocarbons.

C3H8

HintLook for the formula with exactly two elements in it, and the right two.

The whyC3H8 contains carbon and hydrogen and nothing else. CO2 has no hydrogen, H2O has no carbon, and C2H6O has both but also contains oxygen.

Mixture

HintIts parts can be separated by a physical method, distillation.

The whyBecause the compounds in crude oil are only mixed, not bonded to each other, each keeps its own boiling point. That is what allows them to be separated by fractional distillation.

Mud (layers of sediment)

HintIt is fine, wet and soft, and over time it hardens into rock.

The whyBuried and compressed over millions of years, the remains of this ancient biomass turned into crude oil, which is now found in rocks. Because it forms so slowly, it is a finite resource.

Alkanes

HintIts members include methane and propane; the family name shares their ending.

The whyThe alkanes are a family of hydrocarbons that all fit one general formula. The smallest is methane, CH4, the main gas in natural gas.

14

HintPut 6 in place of n, double it, then add two.

The whyThe number of hydrogen atoms is 2n + 2. With n = 6 that is 2 × 6 + 2 = 14, so the formula is C6H14.

ethane; propane

HintTheir molecules have two and three carbon atoms.

The whyMethane has one carbon atom, ethane two, propane three and butane four. These four are the only alkane names that have to be known.

Butane

HintIt is the fourth member of the family, and the fuel in many camping stoves.

The whyThe formula fits CnH2n+2 with n = 4: 2 × 4 + 2 = 10 hydrogen atoms. Written on one line to show the chain, butane is CH3CH2CH2CH3.

Yes: 2 × 7 + 2 = 16, so it fits CnH2n+2

HintTest the numbers of the two kinds of atom against the general formula.

The whyAn alkane with n carbon atoms has 2n + 2 hydrogen atoms. With n = 7 that predicts 16, which matches, so C7H16 is an alkane (its name, heptane, is not needed).

7

HintCount the bonds to hydrogen, then add the bond that joins the carbon atoms.

The whyEach of the 6 hydrogen atoms is joined to a carbon atom by one bond, and the two carbon atoms are joined to each other by one more: 6 + 1 = 7. Written on one line, ethane is CH3CH3.

A fraction (of crude oil)

HintThe word also means a part of a whole in mathematics.

The whyCrude oil contains too many compounds to separate one by one, so it is split into groups. The molecules in one fraction are of similar size, so they have similar boiling points and condense together.

It cools and condenses, at a height that depends on its boiling point

HintEach vapour turns back into a liquid somewhere on the way up.

The whyA vapour condenses when it reaches a part of the column that is cooler than its boiling point. Different hydrocarbons therefore turn to liquid at different heights, where they are collected as separate fractions.

Near the top: their boiling points are the lowest

HintThese stay as gases the longest while the vapour cools.

The whyThe column is hottest at the bottom and coolest at the top. Small molecules have low boiling points, so they do not condense until they reach the cool upper part; the largest molecules condense low down, or never evaporate at all.

fuels; feedstock

HintOne is burned for energy; the other is the raw material that chemical plants are 'fed' with.

The whyThe fuels include petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases. Feedstock is turned into other useful materials: solvents, lubricants, polymers and detergents.

Carbon atoms can bond to each other in chains and rings, forming families of similar compounds

HintConsider how this element behaves towards more of itself.

The whyBecause carbon atoms link to one another, a molecule can have a carbon skeleton of almost any length or shape. Each pattern gives a family of similar compounds, such as the alkanes.