Organic Chemistry and Fuels
Core ideas
Organic chemistry is the chemistry of carbon compounds. Hydrocarbons contain only carbon and hydrogen. Alkanes are saturated (single bonds) with general formula CₙH₂ₙ₊₂; alkenes are unsaturated (one double bond) with formula CₙH₂ₙ. Functional groups include alcohols (–OH), carboxylic acids (–COOH) and halogenoalkanes. Isomers have the same molecular formula but different structures.
Crude oil is separated by fractional distillation based on boiling points. Cracking breaks large molecules into smaller, more useful ones, producing alkenes. Combustion of hydrocarbons releases carbon dioxide and water; incomplete combustion produces carbon monoxide and soot. Polymers form by addition (alkenes) or condensation polymerisation. Fuels include fossil fuels and alternatives such as hydrogen and biofuels.
Traps to watch
- Alkenes decolourise bromine water; alkanes do not - a key test.
- Cracking produces alkenes and hydrogen, not just smaller alkanes.
- Carbon monoxide is toxic and forms in limited oxygen.
- Addition polymerisation has one product; condensation has two (often water).
Rule of thumb
Count bonds: alkane = all single; alkene = one double. For polymers, the monomer must contain a C=C for addition polymerisation.
Worked example
Ethene (C₂H₄) is bubbled into bromine water. The orange colour disappears because the double bond reacts with bromine, showing unsaturation.
Air, Water, Environmental and Industrial Chemistry
Core ideas
Air is about 78% nitrogen, 21% oxygen, with small amounts of argon, carbon dioxide and water vapour. Pollutants include carbon monoxide, sulfur dioxide (acid rain), nitrogen oxides (photochemical smog) and particulates. Greenhouse gases such as carbon dioxide and methane trap heat. Water is treated by filtration and chlorination; hard water contains dissolved calcium and magnesium ions and forms scum with soap.
Industrial processes include the Haber process (N₂ + 3H₂ ⇌ 2NH₃) and the contact process (SO₂ to SO₃ for sulfuric acid). Conditions are chosen for economic yield and rate. Sustainability involves recycling, renewable energy and reducing waste.
Traps to watch
- Carbon dioxide is not the only greenhouse gas; methane is more potent per molecule.
- Hard water is not unsafe; it is a problem mainly with soap and limescale.
- Catalysts do not increase yield in the Haber process; they increase rate.
- Acid rain is caused mainly by sulfur dioxide and nitrogen oxides, not carbon dioxide.
Rule of thumb
Link each pollutant to its source and effect: SO₂ from fossil fuels → acid rain; CO from incomplete combustion → toxic; CO₂ → greenhouse effect.
Worked example
State two conditions used in the Haber process and their purposes. Answer: about 450 °C (compromise temperature for rate and yield) and high pressure (favours ammonia formation); an iron catalyst increases rate.
Experimental Techniques, Measurement and Data Handling
Core ideas
Good practical chemistry depends on choosing the right apparatus and reading it correctly. Measuring cylinders give approximate volumes; burettes and pipettes give precise volumes; thermometers measure temperature; balances measure mass. Read scales at eye level to avoid parallax error. Filtration separates an insoluble solid from a liquid; evaporation recovers a soluble solid; distillation separates liquids with different boiling points; chromatography separates dissolved substances.
Data handling includes tables, graphs and calculations. Identify variables: independent (changed), dependent (measured) and control (kept constant). Plot the independent variable on the x-axis. Calculate means and identify anomalies. Accuracy is closeness to true value; precision is consistency of repeats; reliability improves with repeats. Errors may be random or systematic.
Traps to watch
- Do not read a meniscus from above; read the bottom of the curve at eye level.
- Anomalous results should be identified and excluded from means, not silently averaged.
- A large titre is not necessarily more accurate; precision depends on apparatus.
- Correlation is not causation; control variables matter.
Rule of thumb
For any experiment: what is changed, what is measured, what is kept the same. Repeat and average to improve reliability.
Worked example
A student measures the volume of gas produced every 10 s and plots a graph. The curve rises steeply then levels off. The gradient shows the rate; the plateau shows the reaction has finished.