Thermodynamics

• Atomisation is the standard enthalpy change which accompanies the formation of one mole of gas atoms from its element in its standard state
• Ionisation is the standard enthalpy change which takes place when one mole of electrons is removed from one mole of gas atoms to form one mole of positive ions in the gas phase.
• Bond dissociation is the standard enthalpy change which takes place when one mole of a covalent bond in the gas phase is broken to form two gas atoms
• Electron affinity is the standard enthalpy change which accompanies the addition of an electron into one mole of atoms in the gas phase to form one mole of negative gas ions
• Lattice formation is the enthalpy change which takes place when one mole of solid ionic lattice is formed from its gas ions
• Lattice dissociation is the standard enthalpy change which takes place when one mole of solid ionic lattice is broken into its gas ions
• Lattice formation is always exothermic (negative)

  Lattice dissociation is always endothermic (positive)
• Solution is the standard enthalpy change when one mole of solid ionic lattice is dissolved completely in enough water to form aqueous ions that do not interact with each other
• Hydration is the standard enthalpy change when one mole of gas ions is dissolved completely in water to form one mole of aqueous ions
• Dissolving NaCl

  ∆Hsol (NaCl) = ∆HLdiss (NaCl) + ∆Hhyd (Na) + ∆Hhyd (Cl)
• Why does Na have a higher ∆hat than K?

  Na+ is a smaller ion than K+

  Hence Na+ has a higher charge density and attracts delocalised electrons more strongly. More energy is required to break metallic bonds
• Why is the second ionisation energy of Mg higher than the first ionisation energy?

  Mg+ ion has a positive charge hence it has a stronger attraction between the protons in the nucleus and the outer electrons. More energy needed to remove outer electron.
• Which has a higher ∆hdiss, Cl2 or Br?

  Cl is a smaller atom than Br. The attraction between the shared pair of electrons and the protons in the nucleus is stronger in Cl2. More energy required to break covalent bond in Cl2, hence higher bond dissociation
• The first electron affinity of oxygen is exothermic because there is a net attraction between protons in the nucleus of the O atom and incoming electrons (bond making)
• The second electron affinity of oxygen is endothermic as the negative charge on the O- ion causes repulsion (bond breaking)
• Does CaO or MgO have higher ∆HL(diss)?

  MgO because Mg2+ has a higher charge density and attracts O2- more strongly.
• Compare ∆HL(form) of NaF and NaCl

  F- has higher charge density. Stronger attraction between Na+ and F-. NaF has more exothermic lattice formation
• Enthalpy of hydration is exothermic as energy is released when positive and negative ions attract the water molecules
• spontaneous/feasible change - a change that has a natural tendency to occur without any external influences. when no energy put in overall
• a reaction is feasible when ∆H is exothermic
• a reaction is feasible when ∆S is positive as there is an increase in disorder
• entropy change ∆S is the measure of disorder in a system - unit is kJ/K/mol
• we can see if entropy is increasing by looking at changes of state and the number of moles of the same state.
• solids have low entropy and low disorder while gases have high entropy
• gibbs free energy

  ∆G = ∆S - T∆S

  a reaction is feasible when ∆G less than or equal to 0 unit for ∆G is kJ/mol (divide ∆S value by 1000 to convert from J to kJ)
• the minimum or maximum temperature for feasibility can be calculated when ∆G = 0, hence T = ∆H/∆S
• a reaction is always feasible when ∆S is positive and ∆H is negative
• a reaction is never feasible when ∆S is negative and ∆H is positive
• both ∆H and ∆S are negative = feasible at low temperatures

  both ∆H and ∆S are positive = feasible at high temperatures