Chapter 22

• Hess's law

  The total enthalpy change of reaction is the same independent of the route taken - conservation of energy
• Standard Lattice enthalpy

  The enthalpy change that accompanies the formation of one mole of an ionic lattice from its gaseous ions under standard conditions and states
• Lattice enthalpy is a measurement of...

  Ionic bond strength
• The more exothermic a lattice enthalpy value...

  The stronger the ionic bond - more energy released when ionic lattice is made
• What effects lattice enthalpy?

  1. Ionic size - smaller radii means a higher charge density, the ions can also sit closer together meaning electrostatic attraction is stronger so more negative/exothermic lattice enthalpy value
  2. Ionic charge - the higher the charge on the ions, the stronger the electrostatic attraction and therefore the more exothermic the lattice enthalpy value
• What 2 things happen when an ionic lattice dissolves in water?

  1. The bonds between the ions break as the ionic lattice dissociates - this is the opposite of lattice enthalpy and is endothermic
  2. Bonds form between gaseous ions and water. This is exothermic and the enthalpy change here is called the enthalpy of hydration
• The enthalpy change of solution is the overall effect on the enthalpy of lattice dissociation and enthalpy of hydration ( what happens when an ionic lattice dissolves)
• Exothermic enthalpy of solution profile

  
• Endothermic enthalpy of solution profile

  A) Enthalpy of hydration

  B) Enthalpy of solution

  C) Lattice enthalpy

  D) Gaseous ions

  E) aqueous ions

  F) Ionic lattice

  
• Substances are more energetically stable when...

  They are more disordered
• What is it called when a substance reaches maximum entropy state (lowest energy state)?

  Thermodynamically stable
• The standard entropy change of a substance is...

  The entropy change of one mole of a substance under standard conditions. Standard entropies are always positive.
• Feasibility table

  When enthalpy change is negative and entropy change is positive = negative free energy change at any temp so always feasible

  When enthalpy change is positive and entropy change is negative = positive free energy change at any temp so never feasible

  When enthalpy change is negative and entropy change is negative = negative free energy change but only at low temperatures

  When enthalpy change is positive and entropy change is positive = negative free energy change but only at high temperatures