Overview
This lecture rapidly reviews core content for AQA GCSE Chemistry Paper 1, covering atoms, bonding, quantitative chemistry, chemical/energy changes, and key exam formulas and concepts.
Atoms, Elements & Compounds
- Atoms are basic units; elements contain one type, compounds have two or more types chemically bonded.
- Chemical formulas show atom types and ratios (e.g., H₂O is two hydrogens, one oxygen).
- Chemical equations must be balanced; atoms aren't created or destroyed in reactions.
- Mixtures are combinations of elements/compounds not chemically bonded (e.g., air, salt water).
- Mixtures can be separated by filtration, crystallization, distillation, or fractional distillation (physical changes, not chemical).
The Structure of the Atom & Periodic Table
- Atoms have a nucleus (protons, neutrons) with electrons in shells.
- Protons: +1 charge; Neutrons: 0; Electrons: -1, very low mass.
- Atomic number = protons; mass number = protons + neutrons.
- Isotopes: same element, different neutrons.
- Relative atomic mass may be an average due to isotopic abundance.
- Electron shells fill 2, 8, 8, 2 for up to 20 electrons.
- Metals lose electrons (form positive ions); non-metals gain them (form negative ions).
- Group number = number of outer shell electrons; reactivity varies by group and position.
- Group 1: alkali metals (increasing reactivity downward); Group 7: halogens (decreasing reactivity downward); Group 0: noble gases (unreactive).
Chemical Bonding
- Metals: metallic bonding (lattice of positive ions, delocalized electrons).
- Metal + non-metal: ionic bonding (transfer of electrons, oppositely charged ions).
- Ionic compounds: repeating lattice, high melting/boiling points, conduct electricity when molten/dissolved.
- Non-metals: covalent bonding (shared electron pairs).
- Simple molecular structures: low boiling points, non-conductors.
- Giant covalent structures (e.g., diamond, graphite): hard, high boiling/melting points.
- Alloys: mixture of metals, stronger than pure metals.
- Nanoparticles: very high surface area to volume ratio (triple only).
Quantitative Chemistry
- Total mass conserved in reactions; equations must balance.
- Relative formula mass (sum of atomic masses in a compound).
- Mole: standard unit for amount of substance; 1 mole = relative mass in grams.
- Moles = mass / relative atomic/formula mass.
- Stoichiometry: mole ratios in equations.
- Limiting reactant: reactant that runs out first, limiting product formed.
- Solution concentration: grams or moles per decimetre cubed.
- Percentage yield = (actual mass / theoretical mass) × 100.
- Atom economy = (mass of desired product / total mass of reactants) × 100.
- 1 mole of any gas occupies 24 dm³ at room temperature/pressure.
Reactivity & Chemical Changes
- Reactivity series: ranks metals and includes carbon and hydrogen.
- Displacement reactions: more reactive metal displaces less reactive from compound.
- Extraction: metals less reactive than carbon can be reduced by carbon (smelting).
- Oxidation is loss, reduction is gain of electrons (OIL RIG).
- Acids + metals: produce salt + hydrogen.
- Acid + base/alkali: neutralization (forms salt + water).
- pH scale: logarithmic (acids pH < 7, alkalis pH > 7); every decrease of 1 in pH = 10x increase in H⁺ concentration.
- Strong acids fully dissociate; weak acids partially dissociate.
Electrolysis
- Electrolysis: uses electricity to decompose ionic compounds (must be molten/dissolved for ion movement).
- At the cathode (negative): cations gain electrons (reduction).
- At the anode (positive): anions lose electrons (oxidation).
- Metal formed at cathode if less reactive than hydrogen; otherwise, hydrogen forms.
- Halide ions produce halogen gas at anode; otherwise, oxygen forms.
Energy Changes
- Chemical reactions transfer energy: breaking bonds absorbs energy, making bonds releases it.
- Exothermic: releases net energy (temperature increases); endothermic: absorbs net energy (temperature decreases).
- Activation energy: minimum energy needed to start a reaction.
- Energy profiles show energy of reactants/products, activation energy, and whether reaction is exo/endothermic.
- Net bond energy = energy in (breaking bonds) - energy out (forming bonds).
Triple Only: Quantitative/Cells & Batteries
- Titrations find concentration via neutralization and volume measurement.
- Cells/batteries: use chemicals to produce voltage; rechargeables reverse reaction; fuel cells use hydrogen/oxygen to generate electricity.
Key Terms & Definitions
- Atom — basic unit of matter.
- Mole — amount of substance containing Avogadro's number of particles.
- Isotope — atoms of same element, different neutrons.
- Ionic bond — electron transfer, forms positive and negative ions.
- Covalent bond — sharing of electron pairs between atoms.
- Exothermic reaction — releases energy.
- Endothermic reaction — absorbs energy.
- Electrolysis — decomposition by electricity.
Action Items / Next Steps
- Practice balancing chemical equations and calculating moles.
- Review group trends and periodic table layout.
- Memorize reactivity series and definitions.
- Triple only: review titration steps and energy change calculations.