Chapter 22 ENTHALPY & ENTROPY

• What is enthalpy change of formation, $\Delta_fH^{\theta}$?


  The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
• What is enthalpy change of atomisation, $\Delta_{at}H^{\theta}$?


  The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
• What is the enthalpy change of first ionisation energy, $\Delta_{IE}H^{\theta}$?


  The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
• What is the enthalpy change of first electron affinity, $\Delta_{EA}H^{\theta}$?


  The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.
• What is lattice enthalpy, $\Delta_{LE}H^{\theta}$?


  The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
• What is entropy?

  Entropy is a measure of the dispersal of energy in a system
• What causes entropy to increase?

  • Change of state (gas has highest disorder)
  • If number of moles increases
• What is the standard entropy $S^{\theta}$of a substance?


  The entropy of one mole of substance, under standard conditions (100kPa, 298K) which is ALWAYS positive.
• What are the units of entropy?

  $JK^{-1}mol^{-1}$
• What does Gibbs Free Energy tell us?

  Whether a reaction is feasible or not. It is feasible if $\Delta G\le0$. However, even if a reaction is calculated to be feasible, an reaction may not be observed due to high activation energy or rate of reaction being very slow.
• When will a reaction be feasible at any temperature?

  When$\Delta H$ is negative and $\Delta S$ is positive, $\Delta G$ is always negative
• When will a reaction be feasible at low temperatures?

  When $\Delta H$ is negative and $\Delta S$ is also negative, $\Delta G$ will only be negative at low values of T
• When will a reaction be feasible at high temperatures?

  When $\Delta H$ is positive and $\Delta S$ is positive, $\Delta G$ will only be negative at high values of T
• When will a reaction never be feasible?

  When $\Delta H$ is positive and $\Delta S$ is negative, $\Delta G$will always be positive therefore not feasible
• What is the enthalpy change of solution, $\Delta_{sol}H^{\theta}$?


  The enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions.
• What is the enthalpy change of hydration, $\Delta_{hydr}H^{\theta}$?


  The enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions.
• What factors affect lattice enthalpy and enthalpy of hydration?

  • a larger charge on the ion
  • a smaller ionic radius
• How does a smaller ionic radius increase enthalpy of hydration?

  A smaller ionic radius means stronger electrostatic forces of attraction to water molecules and hydration energy becomes more exothermic
• How does a smaller ionic radius increase lattice enthalpy?

  Smaller ionic radius, electrostatic forces of attraction increases therefore lattice energy is more exothermic
• How does increasing ionic charge increase lattice enthalpy?

  Ionic charge increases, electrostatic forces of attraction increases and lattice energy becomes more exothermic
• How does increasing ionic charge affect enthalpy of hydration?

  Ionic charge increases, electrostatic forces of attraction between water molecules increases and hydration energy becomes more exothermic.