Comprehensive AS and A2 Chemistry Definitions Guide

Atomic Structure and Fundamental Quantities

Isotope: Atoms of the same element with the same number of protons and electrons but different number of neutrons and different masses.
Relative atomic mass ( $A_r$ ): The weighted mean mass of an atom of an element compared with $1/12^{th}$ the mass of one atom of carbon-12.
Relative isotopic mass: Mass of an isotope compared to $1/12^{th}$ the mass of an atom of carbon-12.
Mole: Unit for amount of substance. It contains Avogadro’s constant of $6.02 \times 10^{23}$ particles.
Avogadro’s constant ( $N_A$ ): Number of atoms per mole = $6.02 \times 10^{23} \, mol^{-1}$ .
Orbital: A region around the nucleus that can hold up to $2$ electrons with opposite spins.
First ionisation energy: Energy needed to remove $1 \, mol$ of electrons from $1 \, mol$ of gaseous atoms to form $1 \, mol$ of gaseous $1+$ ions.

Chemical Composition and Formulae

Empirical formula: The simplest whole number ratio of atoms of each element present in a compound.
Molecular formula: The actual number and type of atoms of each element in a molecule.
Hydrated: A crystalline compound containing water of crystallisation.
Anhydrous: Containing no water of crystallisation.
Water of crystallisation: Water molecules that are bonded into a crystalline structure of a compound.
$.xH_2O$ : Number of water(s) of crystallisation.

Acids, Bases, and Salts

Acid: A substance which releases $H^+$ ions in aqueous solution.
Base: A substance which accepts $H^+$ ions from an acid.
Alkali: A substance that releases $OH^-$ ions in aqueous solution.
Salt: $H^+$ ions have been replaced by the metal ions or ammonium ion.
Neutralisation: $H^+$ and $OH^-$ reacting to form $H_2O$ .

Redox Reactions and Oxidation Numbers

Oxidation: Loss of electrons. Oxidation number increases. E.g. $0$ to $+1$ .
Reduction: Gain of electrons. Oxidation number decreases. E.g. $0$ to $-1$ .
Disproportionation: Is the oxidation and reduction of the same element.
Oxidising agent: Accepts electrons.
Reducing agent: Donates electrons.

Atomic and Molecular Bonding

Ionic bonding: Strong electrostatic attraction between positive and negative ions.
Giant ionic lattice: Repeating patterns of oppositely charged ions attracted strongly in all directions.
Covalent bonding: Strong electrostatic attraction between a shared pair of electrons and the nuclei of bonded atoms.
Giant Covalent Lattice: Network of atoms bonded by strong covalent bonds.
Dative covalent bond (Coordinate bond): A shared pair of electrons where both electrons have been donated by one atom.
Metallic bonding: Strong electrostatic attraction between positive ions and delocalised electrons.
Sigma bond ( $\sigma$ ): Overlap of orbitals directly between the bonding atoms.
Pi bond ( $\pi$ ): Sideways overlap of adjacent $p$ -orbitals above and below the bonding carbon atoms.

Periodicity and Intermolecular Forces

Electronegativity: Ability of an atom to attract the bonding electrons in a covalent bond.
Polar: Large difference in electronegativity. Molecule is non-symmetrical and dipoles do not cancel out.
Periodicity: Is the repeating trends in physical and chemical properties of elements.
Hydrogen bond: Bond between $H^{\delta+}$ atom and lone pair of $F$ , $O$ or $N$ .

Enthalpy and Chemical Energetics

Enthalpy change of formation ( $\Delta H_f$ ): The enthalpy change when $1 \, mole$ of a compound is formed from its elements under standard conditions ( $298 \, K$ and $100 \, kPa$ ), with all reactants and products in their standard states.
Enthalpy change of combustion ( $\Delta H_c$ ): The enthalpy change when $1 \, mole$ of a substance reacts completely with oxygen under standard conditions ( $298 \, K$ and $100 \, kPa$ ), with all reactants and products in their standard states.
Enthalpy change of neutralisation ( $\Delta H_{neut}$ ): The formation of $1 \, mole$ of water from reaction of an acid/ $H^+$ with an alkali/ $OH^-$ . The reaction is defined by the following equation: - $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$ ( $1 \, mole$ )
Enthalpy change of reaction ( $\Delta H_r$ ): The enthalpy change that occurs for a reaction in molar quantities/stoichiometric ratio as shown/stated/given/in equation.
Average bond enthalpy: Breaking of $1 \, mol$ of bonds in gaseous molecules.
Lattice enthalpy ( $\Delta H_{LE}$ ): Formation of $1 \, mol$ of ionic lattice from gaseous ions.
Enthalpy of solution ( $\Delta H_{sol}$ ): Dissolving of $1 \, mol$ of solute.
Enthalpy of hydration ( $\Delta H_{hyd}$ ): Dissolving of $1 \, mol$ of gaseous ions in water.
Entropy ( $\Delta S$ ): Measure of the dispersal of energy in a system.

Kinetics and Catalysis

Activation energy ( $E_a$ ): Minimum energy required for a reaction to take place.
Catalyst: Is a substance that increases the rate of a reaction by providing an alternative route with a lower activation energy and is not used by the overall reaction.
Homogeneous catalyst: Catalyst is in the same physical state as the reactants.
Heterogeneous catalyst: Catalyst is in a different physical state from the reactants.
Rate of reaction: Change in concentration per unit time.
Order: The power to which the concentration of a reactant is raised in a rate equation.
Overall order: Sum of the individual orders of the reactants in a rate equation.
Rate-determining step: The slowest step in a multi-step reaction.
Rate constant ( $k$ ): Calculated as $k = \frac{\ln(2)}{t_{1/2}}$ .
Half-life ( $t_{1/2}$ ): The time taken for the concentration to decrease by a half.

Chemical Equilibrium

Dynamic equilibrium: Exists in a closed system and is when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of the reactants and products do not change.

Foundation Organic Chemistry

Homologous series: A series of organic compounds having the same functional group but with each successive member differing by $CH_2$ .
Hydrocarbon: Contains carbon and hydrogen only.
Saturated Hydrocarbon: Contains $C-C$ and $C-H$ single bonds only (alkanes).
Unsaturated: Contains $C=C$ bonds.
Stereoisomers: Compounds with the same structural formula but with a different arrangement in space.
Structural isomers: Compounds with the same molecular formula but different structural formulae.
Optical isomers/enantiomers: Non-superimposable mirror images of around a chiral centre.

Organic Reaction Mechanisms

Homolytic fission: Breaking a covalent bond in which each bonding atom receives one electron from the bonded pair to form two radicals.
Heterolytic fission: Breaking a covalent bond in which one bonding atom receives both electrons from the bonded pair to form a positive ion and a negative ion.
Radical: A species with an unpaired electron.
Curly arrow: Movement of an electron pair.
Electrophile: An electron pair acceptor.
Nucleophile: An electron pair donor.
Hydrolysis: Breaking of a $C-X$ bond by adding water.
Carbocation: A carbon with a full positive charge that is bonded to one alkyl group.

Advanced Acid-Base Theory and Amino Acids

Bronsted-Lowry acid: Proton donor.
Bronsted-Lowry base: Proton acceptor.
Conjugate acid-base pair: A pair of species linked to each other by the gain or loss of a proton.
Buffer: A system that minimises pH changes on addition of small amounts of an acid or base.
Isoelectric point: pH at which amino acid exists as a zwitterion.

Electrochemistry and Transition Elements

Standard Electrode Potential: The e.m.f of a cell comprising of a half cell compared with a hydrogen/ $H^+$ ion half cell at a temperature of $298 \, K$ , solution concentrations of $1.0 \, mol \, dm^{-3}$ and pressure of $100 \, kPa$ .
Transition elements: D-block elements that have an ion with an incomplete $d$ -sub-shell.
Ligand: A molecule or ion that donates a pair of electrons to a central metal ion to form a coordinate bond.
Coordination number: Number of coordinate bonds attached to the central metal ion.

Polymerisation

Condensation polymerisation: Joining monomers to form a polymer with the loss of a small molecule. E.g. $H_2O$ , $HCl$ .