Organic chemistry & analysis flashcards

Key points

• A hydrocarbon is a compound made of hydrogen and carbon atoms only.
• Crude oil is a mixture of a very large number of hydrocarbons, most of them alkanes.
• It formed over millions of years from the remains of ancient sea plankton, buried in mud.
• It forms far more slowly than we use it, so it is a finite resource.

Key points

• The crude oil is heated until most of it evaporates.
• The vapours rise up a column that is hot at the bottom and cooler towards the top.
• Each hydrocarbon condenses at the height that matches its boiling point. A fraction is a group with similar boiling points.
• Short molecules with low boiling points come off near the top; long ones, such as bitumen, at the bottom.

Fractions, top to bottom

Key points

• Longer chains have stronger forces between the molecules.
• So as the chain gets longer, the boiling point rises.
• Viscosity rises too: long-chain fractions are thick and flow slowly.
• Flammability falls: long chains are harder to set alight. That is why short-chain fractions make better fuels.

Key points

• Alkanes are saturated hydrocarbons: only single bonds between their carbon atoms.
• Each carbon atom forms four bonds.
• The first four: methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀.

Formula

An alkane with 5 carbon atoms

Key points

• An alkene has a carbon–carbon double bond, so it is unsaturated and more reactive than an alkane.
• The first alkenes are ethene C₂H₄ and propene C₃H₆. Their names end in -ene.
• Shake with bromine water: an alkene turns it from orange to colourless; an alkane leaves it orange.

Formula

Key points

• Complete combustion needs plenty of oxygen.
• The carbon becomes carbon dioxide and the hydrogen becomes water.
• Energy is given out: it is exothermic, which is why hydrocarbons make good fuels.
• In too little oxygen, combustion is incomplete and makes carbon monoxide or soot.

Methane burning

Key points

• Cracking breaks long hydrocarbon molecules into shorter, more useful ones. It is thermal decomposition.
• Catalytic cracking: heat the vapour over a hot catalyst. Steam cracking: mix the vapour with steam at a very high temperature.
• It makes shorter alkanes for fuels and alkenes for making polymers.
• Crude oil holds more long chains than people want and fewer short ones, so cracking matches supply to demand.

Cracking decane

Key points

• Hydrogen: hold a lit splint at the mouth of the tube. It burns with a squeaky pop.
• Oxygen: put a glowing splint into the gas. It relights.
• Carbon dioxide: bubble it through limewater. The limewater turns milky (cloudy).
• Chlorine: hold damp litmus paper in the gas. It is bleached white.

Key points

• A pure substance is a single element or compound, not mixed with anything else.
• It melts and boils at a sharp, fixed temperature.
• A mixture melts over a range of temperatures. Impurities usually lower the melting point.
• So compare a measured melting point with the known value: pure water melts at 0 °C and boils at 100 °C.

Key points

• A formulation is a mixture made to a precise recipe, so that it does a particular job.
• Each ingredient is measured in an exact amount, because each has its own job.
• Examples: paints, medicines, fuels, cleaning products, alloys, fertilisers and foods.

A paint

Key points

• Spot the mixture on a start line drawn in pencil, then stand the paper in a little solvent, kept below the line.
• Substances with different solubilities and attractions to the paper move up at different speeds.
• A pure substance gives one spot; a mixture gives two or more.
• Rf has no units and is always less than 1. Match Rf values to known substances to identify them.

Formula

Spot moves 3 cm, solvent 12 cm