Chemistry general

• Relative atomic mass, Ar

  Ratio of the average mass of one atom of the element to the mass of an atom of 12C isotope, expressed on the 12C scale
• Relative isotopic mass, A

  Ratio of the mass of one atom of the isotope to the mass of an atom of 12C isotope, expressed on the 12C scale
• Relative molecular mass, Mr

  Ratio of the average mass of one molecule of the substance to the mass of an atom of 12C isotope, expressed on the 12C scale
• Relative formula mass of an ionic compound, Mr

  Ratio of the average mass of one formula unit of the compound to the mass of an atom of 12C isotope, expressed on the 12C scale
• Empirical formula


  Simplest formula which shows the ratio of the atoms of the different elements in the compound
• Molecular formula


  Formula which shows the actual number of atoms of each element present in one molecule of a compound
• Mole


  Amount of substance containing a number of particles equal to the Avogadro constant, L (6.02 × 1023)
• Standard enthalpy change of formation, ΔH

  Enthalpy change when one mole of the compound is formed from its elements under standard conditions (at 25 °C, 1 bar)
• Standard enthalpy change of combustion, ΔH

  Enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions (at 25 °C, 1 bar)
• Standard enthalpy change of hydration, ΔHhyd


  Enthalpy change when one mole of the gaseous ions is dissolved in a large amount of water under standard conditions (at 25 °C, 1 bar)
• Standard enthalpy change of solution, ΔHsol

  Enthalpy change when one mole of a substance dissolves in such a large volume of solvent that addition of more solvent produces no further heat change under standard conditions (at 25 °C, 1 bar)
• Standard enthalpy change of atomisation, ΔHat


  Enthalpy change when an element or a compound is converted into one mole of free gaseous atoms under standard conditions (at 25 °C, 1 bar)
• Standard enthalpy change of neutralisation, ΔH


  Enthalpy change when one mole of water is formed in the neutralisation between an acid and an alkali, the reaction being carried out in aqueous solution under standard conditions (at 25 °C, 1 bar)
• 1 bar = 1×105 Pa; 25 °C = 298 K
• Lattice energy of an ionic crystal

  The enthalpy change when one mole of an ionic crystal is formed from its constituent gaseous ions
• Rate of reaction


  The change in the concentration of reactants or products with time