Overview
This lecture provides a comprehensive AQA A-Level Chemistry revision summary, covering atomic structure, bonding, periodic trends, practical skills, organic chemistry, analytical techniques, and key definitions essential for exams.
Atomic Structure & Isotopes
- Atoms consist of protons (mass 1, charge +1), neutrons (mass 1, charge 0), and electrons (tiny mass, charge -1).
- The nucleus is tiny compared to the whole atom and contains protons and neutrons.
- Atomic number (Z) is the number of protons; mass number (A) is protons plus neutrons.
- Isotopes are atoms with the same number of protons but different numbers of neutrons, affecting physical properties but not chemical behavior.
Electronic Structure & Ionisation
- Electrons occupy shells, subshells (s, p, d, f), and orbitals; electron configuration fills lower energy levels first.
- Ions form by gaining or losing electrons to achieve noble gas configuration; group trends dictate typical ion charges.
- First ionisation energy is the energy to remove one electron from each atom in a mole of gaseous atoms; affected by atomic radius, nuclear charge, and shielding.
- Successive ionisation energies show evidence for shell structure.
Chemical Calculations & Practical Skills
- Relative atomic/molecular mass (Ar, Mr) are weighted averages compared to C-12.
- Moles = mass/Mr; Number of particles = moles × Avogadro's constant (6.02 ×10²³).
- Ideal gas law: PV=nRT; Concentration = mass/volume.
- Balancing equations and using correct state symbols are essential.
- Percentage yield = (actual yield/theoretical yield) × 100; Atom economy = (mass of useful product/total mass of reactants) × 100.
- Standard solutions and titrations require careful technique and attention to concordant results.
- Percentage error = (uncertainty/measurement) × 100.
Bonding & Structure
- Ionic bonds: transfer of electrons between metals and nonmetals, resulting in oppositely charged ions.
- Covalent bonds: sharing of electron pairs between nonmetals; dative bonds are when one atom donates both electrons.
- Metallic bonding: delocalized electrons between positive metal ions.
- Giant structures (ionic, metallic, covalent) have high melting/boiling points; simple molecules have low melting/boiling points due to weak intermolecular forces.
- VSEPR theory explains molecular shapes; bond angles depend on lone and bonding pairs.
Periodicity & Groups
- Elements arranged by atomic number; periodicity refers to repeating trends.
- Group 2: atomic radius and reactivity increase down the group; solubility trends for hydroxides and sulfates.
- Group 7: electronegativity decreases and boiling point increases down the group; displacement reactions and halide tests.
- Period 3: trends in atomic radius, ionisation energy, and oxide chemistry.
Redox, Energetics & Equilibrium
- Oxidation is loss of electrons; reduction is gain (OIL RIG).
- Standard enthalpy changes: combustion, formation, neutralisation, atomisation, hydration, etc.
- Hess’s law: total enthalpy change is independent of the pathway.
- Bond enthalpy calculations: ΔH = bonds broken – bonds formed.
- Dynamic equilibrium shifts to oppose changes (Le Chatelier); Kc and Kp expressions measure equilibrium position.
Organic Chemistry & Mechanisms
- Functional groups: alcohols, alkanes, alkenes, haloalkanes, carbonyls, carboxylic acids, esters, amines, etc.
- Isomerism: structural (chain, position, functional group), stereoisomerism (E/Z, optical).
- Nucleophilic substitution (OH–, CN–, NH₃), elimination, and electrophilic addition (alkenes).
- Alcohols: produced by fermentation or hydration; oxidation depends on primary/secondary/tertiary structure.
- Synthetic routes: use summary charts and learn common mechanisms.
Analysis & Practical Techniques
- Mass spectrometry: fragmentation patterns reveal structure; molecular ion peak gives Mr.
- IR spectroscopy: characteristic absorption regions for functional groups (OH, C=O, COOH).
- NMR: chemical shift, peak integration, and spin-spin splitting reveal hydrogen/carbon environments.
- Chromatography (TLC, column, GC): separates mixtures; Rf values for identification.
- Tests for ions: flame tests, precipitate formation, ammonia production, etc.
Transition Metals & Complexes
- Transition elements: incomplete d subshells; variable oxidation states, colored ions, act as catalysts.
- Complex ions: central metal ion surrounded by ligands; shapes depend on coordination number.
- Ligand exchange can change color, shape, and coordination.
- Catalysis: heterogeneous (different phase) and homogeneous (same phase).
Biochemistry & Polymers
- Amino acids: general structure, zwitterions, peptide bonds.
- Proteins: levels of structure, bonding, and enzyme activity.
- DNA: nucleotide structure and base pairing.
- Polymers: addition vs. condensation, biodegradability, disposal methods.
Key Terms & Definitions
- Isotope — atoms with the same number of protons but different numbers of neutrons.
- Mole — amount of substance containing Avogadro’s number (6.02×10²³) of particles.
- Enthalpy change (ΔH) — heat energy change at constant pressure.
- First ionisation energy — energy required to remove one electron from each atom in a mole of gaseous atoms.
- Electronegativity — ability of an atom to attract shared electrons in a bond.
- Kc/Kp — equilibrium constants for concentrations/partial pressures.
Action Items / Next Steps
- Review and memorize key definitions, equations, and periodic trends.
- Practice balancing equations, calculations, and drawing mechanisms.
- Complete recommended practice questions and past papers.
- Attend live revision workshops and use online multiple-choice quizzes.
- Use summary charts and flashcards for organic synthetic routes and mechanisms.