Unit 3

• Enthalpy change


  The heat energy exchange of a process with its surroundings at a constant pressure
• Enthalpy change of atomisation


  The enthalpy change when one mole of gaseous atoms forms from an element in its standard state
• First ionisation energy


  The enthalpy change when one mole of gaseous positive ions forms from one mole of gaseous atoms
• First electron affinity


  The enthalpy change when one mole of gaseous negative ions forms from one mole of gaseous atoms
• Enthalpy change of lattice formation

  The enthalpy change when one mole of an ionic compound forms from its constituent gaseous ions
• Enthalpy change of formation


  The enthalpy change when one mole of a compound forms from its constituent elements in their standard states
• Born Haber cycle


  A type of Hess cycle that shows the formation of ionic compounds
• Why is the second electron affinity endothermic?

  It takes energy to overcome the repulsive force between the negative ion and the electron
• Factors affecting the enthalpy change of lattice formation

  Smaller ions->Stronger attraction->Higher enthalpy change of lattice formation

  Higher charge->Stronger attraction->Higher enthalpy change of lattice formation
• Standard enthalpy of solution


  The enthalpy change when one mole of an ionic compound is dissolved in water to produce aqueous ions.
• Standard enthalpy of hydration


  The enthalpy change when one mole of gaseous ions is converted to one mole of aqueous ions.
• ∆Hsol=∆HLB+∆Hhyd

  (If the enthalpy of solution is negative, the substance will dissolve)
• Factors affecting the standard enthalpy of hydration

  Smaller ions -> Higher standard enthalpy of hydration

  Higher charge -> Higher standard enthalpy of hydration
• Spontaneous reactions


  Reactions that occur without having to input energy.
• Entropy 


  The freedom possessed by the atoms/molecules within the system.
• Effect of state on entropy

  • Solid particles have a low entropy as they vibrate about fixed positions with strong forces of attraction between them
  • Gas particles have a high entropy as they move freely in all directions with few forces of attraction between them
• Effect of energy on entropy

  The more energy quanta a substance has, the more ways they can be arranged and the greater the entropy.
• Effect of the number of particles on entropy

  The more particles a substance has, the more ways they can be arranged and their energy shared between them, meaning the substance will have a greater entropy.
• ∆G=∆H-T∆S

  (If the free energy change is negative, the reaction will be spontaneous)