Key Terms

• Bond enthalpy


  The amount of energy required to break one mole of the stated bond in the gas phase
• Born-Haber cycle

  Calculates the lattice enthalpy by applying Hess's law and comparing the standard enthalpy change of formation of the ionic compound to values such as ionisation energy and electron affinity
• Covalent character

  The partial sharing of electrons between atoms that have an ionic bond
• Enthalpy of atomisation


  Enthalpy change when one mole of gaseous atoms is formed from the elements in its standard states. It is always endothermic
• Enthalpy of formation


  Enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
• Enthalpy of hydration


  Enthalpy change when one mole of a gaseous ion is completely dissolved in water under standard conditions
• Enthalpy of lattice dissociation

  Enthalpy change when one mole of a solid ionic compound is converted into its gaseous ions
• Enthalpy of lattice formation

  Enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
• Enthalpy of solution


  Enthalpy change when one mole of ionic solid completely dissolves in water under standard conditions to form an infinitely dilute solution
• Entropy


  A measure of the disorder of a system. The units of entropy are JK–1mol–1. On a molecular level, gases are more disordered than liquids, which are more disordered than solids. A reaction that produces a greater number of molecules than the number of reactants molecules will have a positive entropy change, as there will exist more random arrangements of these molecules, i.e. the system will become more disordered
• Feasible reaction


  For a reaction to be feasible at a given temperature it must occur spontaneously. This means no extra energy is required for the reaction to occur
• First electron affinity


  Enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
• First ionisation energy


  Enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous ions
• Gibbs free-energy change


  A measure of the feasibility of a chemical reaction. ΔG = ΔH – TΔSsystem
• Ionic character


  The amount the electrons are shared between two atoms. Limited electron sharing corresponds with a higher percentage of ionic character
• Concentration-time graph


  A graph which can be used to deduce the rate of reaction by drawing tangents to the curve
• Order of reaction


  Tells you how the reactant's concentration will affect the rate of reaction. In the rate equation, it is the appropriate power to which the concentration of the reagent is raised

  
• Overall order of a reaction


  Sum of all the individual orders of all the reactants in a chemical reaction
• Rate-concentration data


  Rate-concentration data or graphs can be used to deduce the order (0, 1 or 2) with respect to a particular reactant
• Rate constant


  Relates the rate of a chemical reaction at a given temperature to the product of the concentrations of reactants

  
• Rate determining step


  The slowest step in a multi-step reaction. The overall rate is decided by this step since species occurring in the RDS will also occur in the rate equation
• Rate equation


  Describes the relationship between the rate of chemical reaction and the concentrations/pressures of reagents. Rate = k[A]m [B]n, where k is the rate constant and where m and n are the orders of reaction with respect to reactants A and B

  
• Catalyst


  Increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. A catalyst does not affect the equilibrium constant since it increases the rate of the forward and backward reaction equally
• Equilibrium


  A reversible reaction is at equilibrium when the rate of the forward reaction equals the rate of the backward reaction. The concentrations of the reactants and products remains constant

  
• Equilibrium constant (KP)


  A value that expresses the relationship between the amounts of gaseous products and gaseous reactants present at equilibrium

  
• Homogeneous system


  A system where all the reactants and products are in the same phase
• Mole fraction of gas


  Value used to calculate partial pressure

  
• Partial pressures


  The pressure exerted by a particular gas in a mixture in a closed system. Related to mole fraction: Partial pressure = Mole fraction x Total pressure
• Acceleration


  Second stage of TOF spectrometry. The positively charged ions are accelerated by an electric field so that they have the same kinetic energy.
• Reversible reaction


  A reaction in which the products can react together to form the original reactants
• Atom


  The smallest part of an element that can exist. All substances are made up of atoms.
• Total pressure


  Sum of all partial pressures
• Atomic nucleus


  Positively charged object composed of protons and neutrons at the centre of every atom with one or more electrons orbiting it.
• Anode


  Positive electrode where oxidation takes place
• Atomic number


  The number of protons in the nucleus.
• Cathode


  Negative electrode where reduction takes place
• Electron


  Negatively charged subatomic particle which orbit the nucleus at various energy levels. Relative mass of 1/2000.
• Electrochemical cell


  Combination of two half cells, consists of two electron conductors (electrodes) separated by an ionic conductor (electrolyte). Cells are used to measure electrode potentials by reference to the standard hydrogen electrode
• Electron configuration


  The distribution of electrons of an atom in orbitals.
• Electron impact ionisation


  Method of ionisation in TOF spectrometry. The sample is vaporised and an electron gun fires high energy electrons at it, causing an electron to be knocked off each particle to produce 1+ ions.