enthalpy changes

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  • Created by: Jasmine
  • Created on: 04-10-16 15:01
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  • Enthalpy Changes
    • Enthalpy of Atomisation
      • is the standard enthalpy change that accompanies the formation of one mole of gaseous atoms
      • endothermic process (positive) because the atoms are attracted to each other and need to be separated
    • Ionisation Enthalpy
      • standard enthalpy change for the removal of an electron from a species in the gas phase to form a positive ion and an electron, both also in the gas phase
      • endothermic process (positive) as electrons need to be removed from shells
      • consider how close the electron is to the nucleus and whether it is paired or not
    • Bond Dissociation Enthalpy
      • standard enthalpy change that accompanies the breaking of a covalent bond in a gaseous molecule to form two free radicals also in the gaseous phase
      • eg Cl2 > 2Cl
      • endothermic (positive) because it requires energy in order to break a covalent bond
    • Electron Affinity
      • standard enthalpy change when an electron is added to an isolated atom in the gas phase
      • exothermic (negative) as there can be a strong attraction to pull them together
        • if more than one electron affinity is required- second and third electron affinities-then the reaction is endothermic (positive)
          • energy is required to overcome the increasing amount of repulsion against a further electron going into the shell
      • occurs with atoms that have a high electronegativity so the higher the electronegativity then the stronger the force of attraction for electrons
    • Enthalpy of Lattice Dissociation
      • stand enthalpy change that accompanies the separation of one mole of solid ionic lattice into its gaseous ions
      • endothermic reaction (positive) as the ionic lattice is separated into ions
    • Enthalpy of Formation
      • enthalpy change involved in the production of one mole of a compound from its elements under standard conditions, reactants and products being in their standard state
      • exothermic (negative) as the compound is formed to be stable. Does not require energy to form


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