Overview
This lecture covers all key content for the AQA GCSE Chemistry Paper 2, including reaction rates, organic chemistry, analysis techniques, Earth's atmosphere, resources, and industrial processes, with concise explanations and essential definitions.
Rates of Reaction and Collision Theory
- The rate of reaction is how quickly reactants are used or products formed.
- Mean rate = quantity of reactant used or product made ÷ time taken.
- Units can be g/s, cm³/s, or mol/s, based on what is measured.
- Steep graph curves indicate fast reactions; shallow curves mean slower reactions.
- Measure rate by mass loss (using a balance) or gas volume produced (using a gas syringe or measuring cylinder with a stopwatch).
- To find rate on a graph, calculate the gradient: change in amount ÷ change in time.
- Tangent lines at a point on a curve give the instantaneous rate there.
- Five main factors affect rate: temperature, pressure, concentration, surface area, catalysts.
- Collision Theory: reactions need particles to collide with enough (activation) energy.
- Increasing temp, pressure, concentration, or surface area increases collision frequency and/or energy, increasing rate.
- Catalysts lower activation energy, increasing the proportion of successful collisions.
Reversible Reactions and Equilibrium
- Reversible reactions: products can react to remake reactants; shown by ⇌ symbol.
- Direction of reversible reactions can be changed by altering conditions.
- If endothermic one way, it's exothermic the other, and energy change is equal in both.
- Equilibrium: in a closed system, forward and backward reactions occur at the same rate.
- The position of equilibrium is affected by concentration, temperature, and pressure, not by catalysts.
- Le Chatelier’s Principle: if conditions change, equilibrium shifts to counteract the change.
- Increasing concentration shifts equilibrium away from that substance; decreasing shifts toward it.
- Increasing temp favors the endothermic direction; increasing pressure favors the side with fewer gas molecules.
Organic Chemistry: Hydrocarbons, Alkanes, Alkenes
- Crude oil is a finite resource of hydrocarbon mixtures from ancient biomass.
- Hydrocarbon: molecule of hydrogen and carbon only.
- Alkanes: saturated hydrocarbons, general formula CnH2n+2; first four: methane, ethane, propane, butane.
- Fractional distillation separates crude oil by boiling points into fractions for fuels/materials.
- Larger hydrocarbon molecules: higher boiling points, higher viscosity, lower flammability.
- Combustion: hydrocarbon + O₂ → CO₂ + H₂O (complete); may produce CO or C (incomplete).
- Cracking: breaks long-chain hydrocarbons (low demand) into short alkanes (fuel) and alkenes (feedstocks).
- Alkenes: unsaturated hydrocarbons, general formula CnH2n, contain C=C double bond, more reactive than alkanes.
- Test for alkenes: decolorize orange bromine water (turns colorless).
Organic Chemistry: Alcohols, Carboxylic Acids, Esters, Polymers
- Alcohols: functional group -OH; methanol, ethanol, propanol, butanol.
- Alcohols combust to produce CO₂ and H₂O; react with sodium to make hydrogen.
- Oxidizing alcohols forms carboxylic acids, e.g., ethanol → ethanoic acid.
- Short-chain alcohols are soluble in water; solubility decreases with chain length.
- Ethanol can be produced by fermentation of glucose with yeast in anaerobic, 25-35°C conditions.
- Carboxylic acids: functional group -COOH; weak acids; react with metals, bases, carbonates to form salts.
- Esters form by reacting carboxylic acids with alcohols; used as solvents, perfumes, flavorings.
- Addition polymerization of alkenes forms polymers (e.g., polyethene); monomers join without other products.
- Condensation polymers form when monomers with two functional groups react, releasing water (e.g., amino acids form proteins).
Chemical Analysis
- Pure substance: single element/compound, melts at specific temperature; impure melts over a range.
- Formulation: mixture designed as a useful product (e.g. fuels, medicines).
- Chromatography separates substances; RF value = distance substance moved ÷ distance solvent moved.
- Flame tests identify metals: lithium (crimson), sodium (yellow), potassium (lilac), calcium (orange-red), copper (green).
- Sodium hydroxide test: different metals give colored precipitates; Al³⁺ dissolves in excess NaOH.
- Carbonate test: add acid, bubble gas through limewater; turns cloudy with CO₂.
- Halide test: add nitric acid then silver nitrate; chloride (white), bromide (cream), iodide (yellow) precipitates.
- Sulfate test: add barium chloride and HCl; white precipitate forms.
- Instrumental methods (e.g., flame emission spectroscopy): more accurate, sensitive, and rapid than chemical tests.
The Atmosphere and Pollution
- Current atmosphere: ≈80% nitrogen, ≈20% oxygen, small amounts of other gases.
- Early atmosphere: formed by volcanic activity; high CO₂, little/no O₂.
- Oxygen increased due to photosynthesis by algae/plants; CO₂ decreased by dissolution, photosynthesis, rock/fossil fuel formation.
- Greenhouse effect: greenhouse gases (CO₂, CH₄, H₂O vapor) trap infrared radiation, warming atmosphere.
- Human activity (fossil fuel burning, deforestation, farming) increases greenhouse gases, causing global warming.
- Global warming effects: glacier melt, drought, sea level rise, habitat loss.
- Reducing carbon footprint: use renewables, less travel, plant trees.
- Combustion releases pollutants: CO₂ (greenhouse), CO (poisonous), SO₂ (acid rain), NOx (smog), particulates (health, dimming).
- Complete combustion: CO₂ and water; incomplete: CO and/or particulates.
- Acid rain damages wildlife and buildings; NOx cause breathing issues, smog.
Using Resources and Sustainability
- Finite resources (e.g. fossil fuels) run out; sustainable development meets present needs without harming future needs.
- Potable water: low dissolved salts/microbes; made by filtration and sterilization.
- Desalination (distillation/reverse osmosis) makes water from seawater; energy intensive.
- Wastewater treated by screening, sedimentation, anaerobic digestion (sludge), aerobic treatment (effluent).
- Metal extraction: bioleaching uses bacteria; phytomining uses plants to accumulate then extract metals.
- LCA (Life Cycle Assessment) assesses environmental impact at all stages of a product.
- Recycling/reusing glass and metals saves energy/resources.
Corrosion, Alloys, Materials
- Metal corrosion: formation of oxide layer; rusting needs iron/steel, oxygen, and water.
- Prevent rust: barrier methods (paint, oil), removing air/water, sacrificial protection (e.g., zinc for iron).
- Alloys: brass (copper+zinc), bronze (copper+tin), jewelry gold (gold+other metals), steel (iron+elements).
- Ceramics: clay (bricks/pottery), glass (soda-lime for everyday use, borosilicate for high temp).
- Polymers: LDPE (flexible, low density), HDPE (strong, high density), thermosoftening (melt), thermosetting (do not melt).
- Composites: matrix + reinforcement (e.g., concrete, plywood).
The Haber Process and Fertilisers
- Haber process: N₂ + 3H₂ ⇌ 2NH₃; uses 450°C, 200 atmospheres, iron catalyst.
- Industrial conditions balance yield and rate while considering costs and safety.
- Fertilisers: mined or processed from potassium/sulfur/nitrogen compounds; phosphate rock can be treated with different acids to make soluble fertilisers.
- Industrial vs. lab production: industrial is continuous, large-scale, automated, fast; lab is batch, small-scale, manual.
- Ammonium sulfate made by titration (lab) or continuous process (industry).
Key Terms & Definitions
- Activation energy — minimum energy required for a reaction.
- Homologous series — compounds with same general formula and similar properties.
- Saturated/Unsaturated — alkanes have single bonds (saturated), alkenes have double bonds (unsaturated).
- Functional group — atom/group giving characteristic reactions (e.g., -OH for alcohols).
- Reversible reaction — products can reform reactants.
- Equilibrium — forward and backward reactions occur at the same rate in a closed system.
- Formulation — a mixture designed for a specific purpose.
- Chromatography — technique for separating mixtures.
- Greenhouse effect — warming due to gases trapping infrared radiation.
- Finite resource — will eventually run out.
- Sustainable development — meets current needs without harming future generations.
- Alloy — mixture of metals for improved properties.
Action Items / Next Steps
- Review and memorize key formulas and definitions listed above.
- Practice calculations for rates, equilibrium, and balanced equations.
- Learn flame/precipitate test results for metal ions and common qualitative tests.
- Study the flow charts/steps for processes like the Haber process, water treatment, and polymerization.
- Complete any additional homework or practice questions assigned on these topics.