GCSE Chemistry Key Topics

Overview

This lecture provides a comprehensive review of key topics for the AQA GCSE Chemistry Paper 2, covering rates of reaction, organic chemistry, chemical analysis, environmental chemistry, and industrial processes.

Rates of Reaction

• The rate of reaction describes how quickly reactants are used or products formed.
• Mean rate = amount of reactant used or product formed ÷ time taken.
• Rate units depend on measurement: g/s, cm³/s, or mol/s.
• Steep reaction curves on graphs indicate a faster rate.
• Methods include measuring change in mass or volume of gas with a balance/syringe and stopwatch.
• Tangents on graphs show the rate at a specific point (gradient = rate).
• Factors affecting rate: temperature, pressure, concentration, surface area, catalysts.
• Collision theory: particles must collide with enough energy (activation energy) to react.
• Catalysts lower activation energy, increasing rate.

Reversible Reactions and Equilibrium

• Reversible reactions can go in both directions; denoted by ⇌.
• At equilibrium, forward and backward reactions occur at the same rate in a closed system.
• Le Chatelier’s Principle: system shifts to oppose changes in concentration, temperature, or pressure.
• Changing concentration shifts equilibrium to the opposite side; increasing temperature favors endothermic direction; increasing pressure favors fewer gas molecules.

Organic Chemistry: Hydrocarbons and Polymers

• Crude oil is a mixture of hydrocarbons (compounds with only C and H).
• Alkanes: saturated hydrocarbons, general formula CnH2n+2 (e.g., methane, ethane, propane, butane).
• Fractional distillation separates hydrocarbons by boiling points.
• Cracking breaks long alkanes into shorter alkanes and alkenes.
• Alkenes: unsaturated hydrocarbons, CnH2n, with a C=C double bond; react with bromine water (turns colorless).
• Polymers: Addition polymers (from alkenes) and condensation polymers (from diols and dicarboxylic acids); condensation forms water as a byproduct.
• Biopolymers: proteins from amino acids, DNA from nucleotides.

Chemical Analysis

• Pure substance: single element/compound with a sharp melting point.
• Formulation: carefully measured mixture for a specific use (e.g., paint, medicine).
• Chromatography separates mixtures; Rf = distance traveled by substance ÷ distance by solvent.
• Gas tests: hydrogen (squeaky pop), oxygen (relights glowing splint), CO₂ (limewater turns cloudy), chlorine (bleaches litmus paper).
• Flame tests for metal ions: Li+ (crimson), Na+ (yellow), K+ (lilac), Ca²+ (orange-red), Cu²+ (green).
• Sodium hydroxide tests form colored precipitates with specific metal ions.
• Halide ions tested with silver nitrate: chloride (white ppt), bromide (cream), iodide (yellow).
• Sulfate ions: barium chloride and HCl form white ppt.
• Instrumental methods (e.g., flame emission spectroscopy) are accurate, sensitive, rapid.

Environmental Chemistry: Atmosphere and Pollution

• Modern atmosphere: ~80% nitrogen, ~20% oxygen, traces of CO₂, water vapor, noble gases.
• Early atmosphere: CO₂-rich due to volcanism; O₂ increased via photosynthesis.
• Greenhouse gases (CO₂, methane, H₂O) trap heat, causing global warming.
• Human activities increase greenhouse gases via fossil fuel burning, deforestation, agriculture.
• Climate change: rising temperatures, melting ice, sea level rise.
• Complete combustion: hydrocarbon + O₂ → CO₂ + H₂O.
• Incomplete combustion: less O₂, forms CO, carbon particulates; both are pollutants.
• Acid rain: caused by SO₂ from burning sulfur-containing fuels.

Using Earth’s Resources and Sustainable Development

• Finite resources like fossil fuels will run out; sustainable development meets current needs without harming future generations.
• Potable water requires low dissolved salts/microbes; obtained via filtration and sterilization.
• Desalination (distillation/reverse osmosis) is energy-intensive.
• Wastewater treatment: screening, sedimentation, anaerobic/aerobic digestion.
• Metal extraction: bioleaching (bacteria) and phytoextraction (plants).
• Life cycle assessment evaluates a product’s environmental impact from production to disposal.
• Recycling and reusing glass/metals conserves raw materials and energy.
• Metals corrode (oxidize) over time; rusting is iron/steel oxidizing with air and water.

Materials: Ceramics, Polymers, Alloys

• Alloys: mixtures of metals with enhanced properties (e.g., brass, bronze, steel).
• Ceramics: brittle, hard, heat-resistant (e.g., clay, glass).
• Low/high-density polyethene produced under different conditions with varying flexibility/strength.
• Thermosoftening polymers melt when heated; thermosetting polymers do not.

Industrial Processes and Fertilisers

• Haber process: N₂ + 3H₂ ⇌ 2NH₃; uses 450°C, 200 atm, iron catalyst.
• Fertilisers: NPK manufactured from raw materials, e.g., potassium chloride/sulfate (soluble), phosphate rock (processed).
• Laboratory vs. industrial production: batch vs. continuous, scale, automation, cost.

Key Terms & Definitions

• Collision Theory — reactants must collide with enough energy for a reaction.
• Activation Energy — minimum energy needed for a reaction.
• Homologous Series — compounds with same general formula and similar properties.
• Saturated/Unsaturated — saturated: single C-C bonds; unsaturated: C=C double bonds.
• Rf Value — ratio of distance travelled by substance to solvent in chromatography.
• Pure Substance — only one type of element or compound.
• Finite Resource — resource that will run out.
• Sustainable Development — meets current needs without harming future generations.
• Catalyst — substance that speeds up reactions without being used up.

Action Items / Next Steps

• Review and memorize key formulas and definitions.
• Practice drawing and interpreting reaction graphs.
• Complete homework or practice papers referenced in lesson.
• Revise flame test and chemical test colors and results.
• Study life cycle assessment steps and sustainability concepts.