AQA GCSE Chemistry Paper 2 Summary

Overview

• Overview of everything that could come up on Paper 2
• Useful for last-minute cramming before the exam
• Combined science students should watch for green headings (triple science content)
• Use timestamps in the description for navigation

Rate and Extent of Chemical Change

Key Concepts

• Rate of chemical reaction: Speed of the reaction
• Measure via reactants used or products made per unit time
• Rate calculation formula: amount of substance / time
• Units: Mass (grams) or volume (cm³)
• Higher tier: Rate in moles per second

Examples

• E.g., 20 cm³ of gas in 2 seconds ⇒ 10 cm³/s
• Units guide the calculation method: units like cm³/s mean divide cm³ by seconds

Graphs

• Plot raw data; interpret rate graphs
• Gradient represents the rate: Steeper gradient = faster rate
• Constant rate: Straight line, Curved graph: Changing rate
• Gradient calculation: change in y / change in x
• Draw tangents for specific time stamps to determine rates

Factors Affecting Rate

• Collision Theory: Reactions occur when particles collide with sufficient energy (activation energy)
• 5 ways to increase rate:
  • Increase pressure
  • Increase concentration
  • Increase surface area
  • Increase temperature
  • Add a catalyst
• Importance of terms like “collide more frequently”

Specific Methods

• Pressure: More particles or smaller volume ⇒ higher collision frequency
• Concentration: More particles in same volume ⇒ higher collision frequency
• Surface Area: Smaller pieces ⇒ higher surface area to volume ratio ⇒ higher collision frequency
• Temperature: Higher kinetic energy ⇒ more frequent and energetic collisions
• Catalysts: Provide an alternative pathway with lower activation energy (e.g., enzymes)

Practical Applications

• Energy profile diagrams for reactions: Exothermic (high reactants, low products); lowering activation energy with catalysts
• Required Practical: Investigate rate changes with concentration changes (gas collection method and turbidity method)

Reversible Reactions and Equilibrium

Key Concepts

• Reversible reactions: Products can react to form reactants
• Represented by double-headed arrow (⇌)
• Equilibrium: Rate of forward and backward reactions are equal in a closed system

Named Example

• Hydrated copper sulfate (blue) ⇌ Anhydrous copper sulfate (white) + Water

Le Chatelier’s Principle (Higher Tier)

• Le Chatelier's Principle: System at equilibrium will counteract changes
• Adding reactants, changing pressure, and temperature shifts equilibrium to favor forward or backward reactions to counteract changes
• E.g., Adding hydrochloric acid shifts equilibrium to produce more green copper compound from blue copper compound

Organic Chemistry

Crude Oil

• Finite resource, mixtures of hydrocarbons mainly of alkanes
• Formation: ancient biomass (plankton) under pressure over millions of years

Alkanes

• First four: Methane, Ethane, Propane, Butane (Mnemonic: Most Elephants Prefer Bacon)
• General formula: CnH₂n+₂

Fractional Distillation

• Separates crude oil into fractions based on boiling points
• Provides fuels and raw materials for petrochemical industry

Combustion

• Complete combustion in sufficient O₂ forms CO₂ and H₂O
• E.g., Propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• Cracking: Breaking long alkanes into shorter alkanes and alkenes

Alkenes

• Unsaturated hydrocarbons with a double bond (General formula: CnH₂n)
• Test with bromine water (will turn colorless in presence of double bond)

Additional Organic Compounds (Triple Science)

• Alcohols: Methanol, Ethanol, Propanol, Butanol; uses and reactions
• Carboxylic Acids: Functional group COOH, weak acids, reactions with carbonates and alcohols
• Esters: Formed from alcohols and carboxylic acids, used in perfumes and flavorings

Chemical Analysis

Pure Substances vs. Mixtures

• Pure substances: Single element/compound
• Melting/boiling points indicate purity

Chromatography

• Method of separation and analysis
• Stationary phase: Chromatography paper, Mobile phase: Solvent
• Calculate Rf value: distance moved by substance / distance moved by solvent
• Required Practical: Paper chromatography to separate and analyze mixtures

Gas Tests

• Oxygen: Relights a glowing splint
• Hydrogen: Burns with a squeaky pop
• Chlorine: Bleaches damp litmus paper
• Carbon Dioxide: Turns limewater milky

Flame Tests (Triple Science)

• Nichrome wire loop: Clean with HCl, then test sample
• Colors: Lithium (crimson), Sodium (yellow), Potassium (lilac), Calcium (orange-red), Copper (blue-green)
• Flame emission spectroscopy: Analyzes light spectra to identify elements

Tests for Anions

• Carbonates: React with acids to release CO₂
• Halides: Form precipitates with silver nitrate
• Sulfates: React with barium chloride to form precipitate

Earth’s Atmosphere

Modern Atmosphere

• ~80% Nitrogen, 20% Oxygen, small amounts of other gases

Early Atmosphere

• Volcanic activity emitted CO₂, water vapor, and other gases
• Formation of oceans absorbed CO₂
• Algae and plants increased O₂ levels through photosynthesis

Greenhouse Gases and Global Warming

• Named gases: CO₂, Methane, Water Vapor
• Greenhouse effect traps infrared radiation, warming Earth
• Consequences: Rising sea levels, extreme weather events, loss of habitats

Human Impact on Greenhouse Gases

• Burning fossil fuels, deforestation, agriculture, waste decomposition

Carbon Footprint

• Total greenhouse gas emissions of a product/service
• Reduction methods: Local sourcing, energy-efficient manufacturing

Pollutants from Fuels

• CO₂: Global warming
• Carbon Monoxide: Toxic
• Particulates: Global dimming
• Sulfur Dioxide and Nitrogen Oxides: Acid rain and respiratory issues

Earth’s Resources

Natural vs. Synthetic Resources

• Natural: Cotton, wool
• Synthetic: Nylon, polyester

Potable Water

• Safe to drink, low levels of salts and microorganisms
• Methods: Filtration, sterilization, desalination

Waste Water Treatment

• Screening, sedimentation, biological treatments for potable water

Metal Extraction

• Phytomining: Using plants to absorb metals
• Bioleaching: Using bacteria to extract metals

Life Cycle Assessments

• Evaluate environmental impact from raw material extraction to disposal
• Challenges: Subjectivity and potential bias

Recycling

• Glass: Color sorting, crushing, melting
• Metals: Melting, recasting

Materials

Corrosion

• Breakdown of materials due to chemical reactions (e.g., rusting of iron)

Protecting Metals

• Methods: Painting, electroplating, galvanizing

Alloys

• Mixtures of metals with useful properties
• Examples: Bronze, Brass, Steel types, Aluminium alloys

Ceramics

• Clay ceramics and glass, properties and uses

Polymers

• Made from monomers, properties depend on monomers and reaction conditions
• Thermo-setting vs. Thermo-softening polymers

Composites

• Made from two/more materials, Examples: Wood (natural), Steel-reinforced concrete (synthetic)

Harbor Process

• Industrial process for making ammonia for fertilizers
• Conditions: Iron catalyst, 450°C, 200 atmospheres pressure

Fertilizers

• NPK fertilizers contain salts of nitrogen, phosphorus, potassium

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