| Term | Definition |
|---|
| Empirical formula | The simplest whole number ratio of atoms of each element present in a compound |
| Molecular formula | The actual number of atoms of each element in a molecule |
| General formula | The simplest algebraic formula of a member of a homologous series |
| Homologous series | Series of organic compounds having the same functional group but with each successive member differing by CH₂ |
| Structural isomers | Compounds with the same molecular formula but different structural formula |
| Stereoisomers | Compounds with the same structural formula but with a different arrangement in space |
| Homolytic fission | The breaking of a covalent bond with one of the bonded electrons going to each atom, forming two radicals |
| Heterolytic fission | The breaking of a covalent bond with both of the bonded electrons going to one of the atoms, forming a cation and an anion |
| Radical | A species with an unpaired electron |
| Nucleophile | An electron pair donor |
| Electrophile | An electron pair acceptor |
| Addition reaction | A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule |
| Substitution reaction | A reaction in which an atom or group of atoms is replaced by a different atom or group of atoms |
| Elimination reaction | The removal of a molecule from a saturated molecule to make an unsaturated molecule |
| Atom economy | (Molecular mass of desired products/ Molecular mass of all products) x 100 |
| Percentage yield | (Actual mass of desired product/ Max possible mass of desired product) x 100 |
| Hydrocarbon | A compound of hydrogen and carbon only |
| Sigma bond (σ bond) | Bond formed directly between two atoms by the head on overlap of orbitals |
| Pi bond (π bond) | Bond formed above and below the plane of an atom by the sideways overlap of p-orbitals |
| Polymer | Long chain molecule built up from monomer units |
| Monomer | Small molecule that combines with many other monomers to form a polymer. |
| Activation energy | The minimum energy required to start a reaction by breaking bonds |
| Standard conditions | 100kPa (1 atm), 298K (25*C) and a concentration of 1 mol dm^-3 |
| Enthalpy, H | The heat content that is stored in a chemical system |
| Exothermic ΔH = - ve | Reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants |
| Endothermic ΔH = + ve | Reaction in which the enthalpy of the products is greater than the enthalpy of the reactants |
| Standard Enthalpy change of reaction ΔHrΘ | The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states |
| Enthalpy change of formation ΔHfΘ | The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions |
| Enthalpy change of combustion ΔHcΘ | The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states |
| Average bond enthalpy | The average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given type of bond in the molecules of a gaseous species |
| Hess' law | If a reaction can take place by more than one route, the total enthalpy change is independent of the route taken |
| Specific heat capacity | The energy required to raise the temperature of 1g of a substance by 1 degree C |
| Catalyst | Increases the rate of a reaction without being consumed by the overall reaction |
| Heterogeneous catalysis | The catalyst is in a different physical state to the reactants |
| Homogeneous catalysis | The catalyst is in the same physical state as the reactants |
| Dynamic equilibrium | The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction |
| le Chatelier's principle | When a system in dynamic equilibrium is subjected to a change, the position of the equilibrium will shift to minimise the change |
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