Chapter 22 Enthalpy And Entropy

• Entropy is a measure of the dispersal of energy in a system, which increases as the system is more disordered • /\ S represents the change In entropy in J K -1 mol -1.

• At 0 K there is no energy in a system so Entropy = 0 • All systems above 0K have Entropy > 0

• Entropy increases during changes of state that give MORE random arrangements of particles

• Standard Entropy is the entropy, of one mole of a substance under standard conditions

• Lattice enthalpy (/\ LE H) is the enthalpy change due to the formation of 1 mole of ionic lattice from gaseous ions (under standard conditions) • ALWAYS EXOTHERMIC

Standard enthalpy change of formation (/\ f H) is the enthalpy change due to one mole of a substance forming from its constituent elements under standard conditions

• lattice enthalpy - a measure of the strength of ionic bonding in a giant ionic lattice • lattice enthalpy - the enthalpy change due to the formation of 1 mole of ionic lattice from gaseous ions (under standard conditions) • it is always exothermic

• the enthalpy change due to 1 mole of a compound forming from its constituent elements under standard conditions

• the enthalpy change by forming 1 mole of gaseous atoms from the element in its standard state under standard conditions • always endothermic (bond breaking) • if the element is a gas - the value is related to bond enthalpy

• it is the opposite of ionisation energy • it measures the energy required to gain electrons

• the enthalpy change when an electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of -1 gaseous ions • always negative (exothermic)

• lattice enthalpy cannot be measured directly, must be calculated indirectly using known energy changes in Born-Hager cycles

• as ionic radius increases the lattice enthalpy becomes less negative as the attraction between ions will decrease • as ionic charge increases, lattice enthalpy becomes more negative as the attraction between ions will increase

• standard enthalpy change of solution - the enthalpy change due to 1 mole of a solute dissolving in a solvent • can be endo or eco thermic

The enthalpy change due to dissolving gaseous ions in water to form 1 mole of aqueous ions

• as ionic radius increases the hydration enthalpy becomes less negative as the attraction between ion and water decreases • as ionic charge increases the hydration enthalpy becomes more negative as the attraction between ion and water increases • if the hydration enthalpy is greater than the lattice enthalpy dissolving is likely if enthalpy change of solution is negative

The feasability of a reaction is used to describe if a reaction will be able to happen and is energetically feasible/spontaneous

The free energy during a reaction is the overall energy change caused by the: 1. Heat transfer between the system and surroundings (delta H, enthalpy change) 2. Dispersal of energy within the system at the temperature of the reaction (TdeltaS, entropy change at the temperature of the reaction)

• The Gibbs equation - change in free energy = change in enthalpy - change in entropy • a reaction is feasible when change in free energy is negative.