AS Chemistry Definitions 1. Relative atomic mass – the weighted average mass of one atom of the element relative to one-twelfth the mass of one atom of 12C 2. Relative isotopic mass – the mass of one atom of the isotope relative to one-twelfth the mass of one atom of 12C 3. Relative molecular mass – the mass of a molecule measured on a scale in which an atom of the 12C isotope has a mass of exactly 12 units 4. Relative formula mass – the mass of one formula unit of a compound measured on a scale in which an atom of the 12C isotope has a mass of exactly 12 units 5. Mole – the amount of substance that has the same number of particles (atoms, ions, molecules or electrons) as there are atoms in exactly 12g of the 12C isotope 6. Avogadro constant (L or NA) – is the number of atoms (6.02 × 1023 atoms) in exactly 12g of the 12C isotope 7. Empirical formula – the simplest whole number ratio of the elements present in one molecule or formula unit of the compound 8. Molecular formula – the actual number of atoms of each element in one molecule of a compound 9. Nucleon number – sum of the numbers of protons and neutrons 10. Isotopes – are atoms of the same element with different nucleon (mass) numbers 11. First ionisation energy – the energy needed to remove one electron from each atom in one mole of atoms of the element in the gaseous state to form one mole of gaseous 1+ ions 12. Atomic orbitals – a region of space around the nucleus of an atom that can be occupied by one or two electrons 13. Electronegativity – the ability of a particular atom, which is covalently bonded to another atom, to attract the bond pair of electrons towards itself 14. Bond energy – the energy needed to break 1 mole of a particular bond in 1 mole of gaseous molecules 15. Bond length – the mean distance between the nuclei of two atoms that are bonded together in a molecule 16. Bond polarity –the unequal distribution of electrons in a covalent bond 17. Co-ordinate bond – a covalent bond in which both electrons in the bond come from the same atom 18. Ideal gas – a gas whose volume varies in proportion to the temperature and in inverse proportion to the pressure 19. Melting – change of state from solid to liquid 20. Vaporisation – change of state from liquid to gas 21. Vapour pressure – the pressure exerted by a vapour in equilibrium with a liquid 22. Alloy – a mixture of two or more metals or a metal with a non-metal 23. Standard enthalpy change of reaction – the enthalpy change when the amounts of reactants shown in the equation react to give products under standard conditions 24. Standard enthalpy change of formation – the enthalpy change when one mole of a compound is formed from its elements under standard conditions 25. Standard enthalpy change of combustion – the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions 26. Standard enthalpy change of hydration (of an anhydrous salt) – the enthalpy change when one mole of a hydrated salt is formed from one mole of the anhydrous salt under standard conditions 27. Standard enthalpy change of solution – the enthalpy change when one mole of solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions 28. Standard enthalpy change of neutralisation – the enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions 29. Standard enthalpy change of atomisation – the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions 30. Hess’ law – the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same 31. Oxidation – gain of oxygen by an element OR removal of hydrogen from a compound OR loss of electrons OR an increase in oxidation number 32. Reduction – loss of oxygen by an element OR addition of hydrogen to a compound OR gain of electrons OR a decrease in oxidation number 33. Oxidising agent – a reactant that increases the oxidation number of (or removes electrons from) another reactant 34. Reducing agent – a reactant that decreases the oxidation number of (or adds electrons to) another reactant 35. Redox reaction – a reaction in which oxidation and reduction take place at the same time 36. Reversible reaction – a reaction in which products can be changed back to reactants by reversing the conditions 37. Dynamic equilibrium – a state of balance in which the rate of the forward reaction is equal to the rate of the backward reaction 38. Closed system – a system in which matter or energy is not lost or gained 39. Le Chatelier’s principle – if the conditions of a system in equilibrium are changed, the position of equilibrium shifts to the direction that reduces the change 40. Acid – a substance that neutralises a base 41. Base – a substance that neutralises an acid 42. Alkali – a base that is soluble in water 43. Brønsted-Lowry acid – a proton (H+ ion) donor 44. Brønsted-Lowry base – a proton (H+ ion) acceptor 45. Strong acids – acids that completely dissociate in solution 46. Weak acids – acids that are only partially dissociated in solution 47. Strong bases – bases that completely dissociate in solution 48. Weak bases - bases which dissociate to only a small extent in solution 49. Rate of reaction – the rate of change in the amount of reactants or products 50. Activation energy – the minimum energy that colliding particles must possess for a successful collision that results in a reaction to take place 51. Catalyst – a substance that increases the rate of a reaction but remains chemically unchanged itself at the end of the reaction 52. Catalysis – the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst 53. Homogeneous catalysis – the type of catalysis in which the catalyst and reactants are in the same phase 54. Heterogeneous catalysis – the type of catalysis in which the catalyst is in a different phase from the reactants 55. Enzyme – a protein molecule that acts as an enzyme 56. Periodicity – the repeating patterns in the physical and chemical properties of the elements across the periods of the Periodic Table 57. Thermal decomposition – the breakdown of a compound into two or more different substances using heat 58. Volatility – how easily a substance can evaporate 59. Disproportionation – the simultaneous reduction and oxidation of the same species in a chemical reaction 60. Hydrocarbons – are compounds of carbon and hydrogen only 61. Functional group – the atom or group of atoms which gives an organic compound its properties 62. Structural isomers – compounds with the same molecular formula but different structural formulae 63. Stereoisomers – compounds whose molecules have the same atoms bonded to each other but with different arrangements of the atoms in space 64. Optical isomers – stereoisomers that exist as two nonsuperimposable mirror images 65. Chiral centre – a carbon atom with four different groups attached 66. Homolytic fission – the breaking of a covalent bond to make two uncharged atoms or groups of atoms, each with an unpaired electron 67. Free radical – very reactive atom or molecule that has a single unpaired electron 68. Heterolytic fission – the breaking of a covalent bond to make a positive ion and negative ion 69. Carbocation – an alkyl group carrying a single positive charge on one of its carbon atoms 70. Electrophile – an acceptor of a pair of electrons 71. Nucleophile – a donator of a pair of electrons 72. Hydrolysis – breakdown of a compound by water, which is often speeded up by reacting with acid or alkali 73. Condensation reaction – a reaction in which two organic molecules join together and in the process eliminate a small molecule e.g. water or hydrogen chloride 74. Saturated hydrocarbon – compounds of hydrogen and carbon only in which the C-C bonds are all single covalent bonds, resulting in the maximum number of hydrogen atoms in their molecules 75. Unsaturated hydrocarbon – compounds of hydrogen and carbon only whose molecules contain carbon-to-carbon double bonds or triple bonds 76. Monomer – a small, reactive molecule that reacts to make long-chain molecules called polymers 77. Polymer – a long-chain molecule made up of many repeating units 78. Infra-red spectroscopy – a technique for identifying compounds based on the change in vibrations of particular atoms when infra-red radiation of specific frequencies is absorbed Made by u/Rakki_Yajirushi GOOD LUCK FOR YOUR EXAMS!
