Overview
This lecture gives a rapid, comprehensive summary of everything required for AQA GCSE Chemistry Paper 1, covering atoms, bonding, quantitative chemistry, chemical and energy changes, and key specifics for both double and triple science.
Atoms, Elements & Compounds
- An atom is the smallest unit of an element, represented by a symbol on the periodic table.
- Compounds are substances of two or more different atoms chemically bonded together; chemical formulas show ratios.
- In chemical reactions, atoms are rearranged but not created or destroyed, so equations must be balanced.
- Mixtures are physical combinations of elements/compounds that can be separated (e.g., filtration, crystallization, distillation).
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Atomic Structure & The Periodic Table
- Atoms have a nucleus (protons+neutrons) and electrons in shells around it.
- Protons = atomic number; mass number = protons + neutrons.
- Isotopes: same element, different numbers of neutrons.
- Early periodic tables used atomic weights; Mendeleev arranged by properties/predicted undiscovered elements.
- Electron shells fill 2, 8, 8, 2 up to calcium (20 electrons).
Groups in the Periodic Table
- Metals (left of staircase) lose electrons, forming positive ions (cations); nonmetals (right) gain electrons (anions).
- Group 1 (alkali metals) get more reactive down the group; Group 7 (halogens) get less reactive.
- Group 0 (noble gases) are very unreactive.
Bonding
- Metals: Metallic bonding creates a lattice of positive ions and delocalized electrons; conducts electricity/heat.
- Ionic bonding: Metal and nonmetal exchange electrons; ionic compounds have high melting points, conduct electricity when molten/dissolved.
- Covalent bonding: Nonmetals share electrons to form molecules; simple molecules have low boiling points, do not conduct electricity.
- Giant covalent structures (e.g., diamond, graphite) have high melting points; graphite can conduct electricity.
Quantitative Chemistry
- Relative formula mass: Sum of atomic masses in a compound.
- Moles = mass (g) / relative atomic or formula mass.
- Stoichiometry: Use balanced equations to relate amounts in moles.
- Limiting reactant: The reactant that runs out first in a reaction.
- Concentration: g/dm³ or mol/dm³; 1 dm³ = 1000 cm³.
Chemical Changes & Reactivity
- Reactivity series ranks metals by tendency to lose electrons; more reactive metals displace less reactive ones.
- Extraction: Metals less reactive than carbon can be displaced from oxides by carbon (reduction).
- Oxidation = loss of electrons; reduction = gain (OIL RIG).
- Acids react with metals to produce salts and hydrogen; neutralization produces salts and water.
Acids, Bases, and Titrations
- pH is a logarithmic scale: Each unit = tenfold change in H⁺ ion concentration.
- Strong acids fully dissociate; weak acids partially dissociate.
- Titrations (triple only): Used to determine concentration using a burette, pipette, and indicator; calculations involve moles and volumes.
Electrolysis
- Electrolysis uses electricity to decompose ionic compounds; cations go to cathode (gain electrons, reduced), anions to anode (lose electrons, oxidized).
- Only less reactive metals deposit at cathode in aqueous solutions; otherwise, hydrogen is produced.
- Anode produces oxygen unless halide ions are present.
Energy Changes in Reactions
- Exothermic reactions release more energy than needed to break bonds (temperature rises).
- Endothermic reactions absorb net energy (temperature falls).
- Activation energy is the initial energy needed to start a reaction.
- Bond energy calculations: Net energy change = energy in – energy out.
Key Terms & Definitions
- Atom — basic unit of an element.
- Compound — substance of different atoms chemically bonded.
- Ion — charged particle formed by losing/gaining electrons.
- Isotope — atom of same element with different neutrons.
- Cation — positively charged ion.
- Anion — negatively charged ion.
- Mole — amount of substance containing Avogadro’s number of particles.
- Relative Atomic Mass (RAM) — average mass of atoms of an element.
- Electrolysis — decomposition of compounds using electricity.
- Exothermic — reaction that releases energy.
- Endothermic — reaction that absorbs energy.
Action Items / Next Steps
- Review example chemical equations and practice balancing.
- Memorize group properties, reactivity series, and key formulas (moles, concentration).
- For triple science: Study titration and atom economy calculations.
- Complete any assigned homework or practice questions.