Overview
This lecture covers all key concepts required for AQA GCSE Chemistry Paper 1, including atoms, bonding, quantitative chemistry, chemical and energy changes, with relevant notes for both higher and foundation tiers.
Atomic Structure & The Periodic Table
- Atoms are the smallest units of elements, shown by symbols in the periodic table.
- Compounds consist of two or more different atoms chemically bonded, e.g. H₂O.
- Chemical reactions change atom arrangements; equations must be balanced.
- Mixtures contain elements and compounds not chemically bonded, e.g. air.
- Separation techniques: filtration (large particles), crystallization (solute recovery), distillation (solvent recovery), fractional distillation (separating liquids by boiling point).
- States of matter: solid, liquid, gas; changes are physical, not chemical.
- Atoms have a nucleus (protons, neutrons) and orbiting electrons in shells; isotopes have the same protons but different neutrons.
- Atomic number = protons; mass number = protons + neutrons; use RAM (relative atomic mass) for calculations.
- Periodic table groups elements by properties and electron configurations.
Bonding & Structure
- Metals: lose electrons to form positive ions; good conductors; metallic bonding involves delocalized electrons.
- Non-metals: gain electrons or share electrons (covalent bonding) to achieve full outer shells.
- Ionic bonding: metal + non-metal; compounds must be charge balanced; form crystals, high melting/boiling points, conduct electricity when molten or dissolved.
- Covalent bonding: non-metals share electrons; can be simple molecules (low melting points, do not conduct) or giant covalent structures (e.g. diamond, graphite, graphene).
- Transition metals form multiple ions (e.g., Fe²⁺, Fe³⁺), form colored compounds, and are harder/less reactive than alkali metals.
- Alloys are stronger than pure metals due to disrupted lattice structure.
Quantitative Chemistry
- Mass is conserved in chemical reactions; equations must balance.
- RAM used to calculate formula mass (Mr) for compounds.
- A mole is the relative mass in grams; moles = mass (g) / RAM or Mr.
- Stoichiometry: use balanced equations to find ratios of reactants/products.
- Limiting reactant: the reactant that runs out first.
- Solution concentration: g/dm³ or mol/dm³ (1 dm³ = 1000 cm³).
- For triple only: percentage yield = (actual mass / theoretical mass) x 100; atom economy = (Mr of desired product / total Mr of all products) x 100.
- One mole of any gas occupies 24 dm³ at room temperature and pressure.
Chemical Changes
- Reactivity series: more reactive metals displace less reactive ones from compounds.
- Extraction: metals below carbon can be reduced by carbon; those above require electrolysis.
- Oxidation is loss, reduction is gain of electrons (OIL RIG).
- Acids react with metals to produce salts and hydrogen; acids + alkalis (bases) neutralize to form salt + water.
- pH scale (0–14): acids <7, neutral = 7, alkalis >7; logarithmic scale.
- Strong acids fully ionize, weak acids partially ionize; pH depends on strength and concentration.
Electrolysis
- Electrolysis splits ionic compounds using electricity; only works when ions are free to move (molten or dissolved).
- At cathode (−): cations reduced (gain electrons); at anode (+): anions oxidized (lose electrons).
- In solution, less reactive element is discharged at electrodes.
- Used to extract reactive metals and purify substances (e.g. copper, aluminium).
Energy Changes
- Breaking bonds requires energy (endothermic); forming bonds releases energy (exothermic).
- Exothermic reactions release more energy than they use; temperature rises (e.g. combustion).
- Endothermic reactions absorb energy; temperature falls.
- Activation energy: minimum energy needed to start a reaction.
- Bond energy calculations: total energy in minus total energy out gives net energy change.
- (Triple only) Cells and batteries produce voltage from chemical reactions; fuel cells generate electricity using hydrogen and oxygen.
Key Terms & Definitions
- Atom — Smallest unit of an element, with protons, neutrons, electrons.
- Compound — Substance formed from two or more elements chemically bonded.
- Ion — Atom or molecule with a net electric charge due to electron loss/gain.
- Isotope — Atoms of the same element with different numbers of neutrons.
- Mole — Amount of substance with mass equal to its formula mass in grams.
- Stoichiometry — Ratio of reactants/products in a chemical reaction.
- Electrolysis — Chemical decomposition via electricity.
- Exothermic — Reaction that releases energy (temperature rises).
- Endothermic — Reaction that absorbs energy (temperature falls).
Action Items / Next Steps
- Review example calculations for moles, concentration, percentage yield, and atom economy.
- Practice balancing chemical equations and drawing dot-and-cross diagrams.
- Memorize the reactivity series and key separation techniques.
- Complete any assigned exercises on chemical changes and energy changes.