Topic 13 - ENERGETICS II
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Definition of lattice energy
The energy released when 1 mole of an ionic compound is formed from gaseous ions
Is lattice energy endo/exothermic?
Always EXOthermic (-ve)
What is the approximate range for lattice energy and what does it signify?
-500 -> -4000 - (-500) shows weaker ionic bonding - (-4000) shows stronger ionic bonding
Compare the theoretical vs experimental LE for CsF if: experimental = -739 kJmol-1 theoretical = -737 kJmol-1
• Theoretical and experimental very similar • Very little covalent character in the bonding • Because Cs+ polarises F- very little
What is the definition of enthalpy of atomisation in formulae and give its sign?
Element -> 1 mol atoms (g) Endothermic (+ve)
What is electron affinity?
1 atom (g) + e- -> ion- (g)
What does it mean if the standard enthalpy of formation of a substance is exothermic?
• Lots of energy is released • Therefore MgCl2 is stable
What is the definition of standard enthalpy of hydration?
The enthalpy change when 1 mole of gaseous ions are hydrated by excess water
What is the definition of standard enthalpy of solution?
The enthalpy change when 1 mole of ionic solid dissolves in excess water
How can we use standard enthalpy of solution to predict solubility?
• If the result is exothermic, the substance is probably soluble (Only insoluble if highly endothermic)
What is entropy?
Entropy (s) is a measure of the amount of disorder
What conditions have an effect on entropy?
• No. of moles • Temperature increase • State
What is standard entropy change of the system?
Total enthalpy change of products - total enthalpy change of reactants
What is standard entropy change of the surroundings?
-△H/T
What is △S total?
△S system + △S surroundings
Why does △S total matter?
According to the 2nd law of thermodynamics, a reaction is only feasbile if △S total > 0
Why might △S total be negative?
• Although △S system is positive, △S surrounding is more negative • Therefore △system is not positive enough to outweigh the negative △S surrounding
What happens when T increases?
• As △H is positive, △S system is positive • As T increases, △S surroundings becomes less negative • △S total becomes less negative
What is the definition of kinetic stability?
• When a feasible reaction doesn’t happen • Because the energy (at that temp) is not enough to overcome the activation energy
What is thermodynamic stability?
When something just isn’t feasibly because △S total < 0
What is △G and what does it show?
△G = △H - T△S system - Reaction is feasible if △G < 0
What is the relation between △G and Kc?
△G = -RT ln(Kc) or Kc = e^(-△G/RT)