Thermodynamics

• The second law of thermodynamics states that the entropy of an isolated system always increases over time.
• The first law of thermodynamics states that energy cannot be created or destroyed, only transferred from one form to another.
• The zeroth law of thermodynamics states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
• Thermodynamic systems can be classified as open (energy and matter exchange with surroundings), closed (no mass transfer but energy exchanges occur) or isolated (neither energy nor matter transfers to/from environment).
• Born-Haber cycles allow enthalpy changes to be determined that cannot be measured directly.
• Lattice dissociation enthalpy is the energy required to break an ionic lattice into its constituent ions in a gaseous state under standard conditions.
• Lattice formation enthalpy is the energy required to form an ionic lattice from its constitutient ions in a gaseous state under standard conditions.
• Atomisation enthalpy is the energy required for the formation of one mole of gaseous atoms under standard conditions.
• Enthalpy of electron affinity is the enthalpy change when one mole of electrons is added to one mole of gaseous atoms under standard conditions.
• Enthalpy of solution is the enthalpy change when one mole of ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions.
• Enthalpy of hydration is the enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions under standard conditions.
• When dissolved, lattice enthalpy equals the sum of change of enthalpy of hydration minus the change of enthalpy of solution.
• In the perfect ionic model, it is assumed all ions are perfectly spherical and display no covalent character.
• Covalent character occurs when two joined ions have varying sizes or charges meaning the charge distribution is not even.
• Entropy increases as temperature increases because the particles gain more energy and move further apart.
• Gases have the greatest entropy then liquids then solids.
• When a substance melts or evaporates, there is a sudden increase in entropy. The entropy change of vaporisation is much greater than that of fusion as a gas is much more disordered than a liquid or a solid.
• The overall entropy change (delta S) for a reaction is measured in J/Kmol and is the entropy of the products minus the entropy of the reactants.
• Since all things tend to a state of disorder, all spontaneous reactions have a positive entropy value.
• A negative entropy change means that the system has become more ordered.
• Gibbs free energy allows entropy change to be found without needing to measure the effects on the surroundings.
• Gibbs free energy (delta G) is measured in KJ/mol and is delta H-T delta S.
• Gibbs free energy is negative for all spontaneous reactions.
• The temperature at which a reaction becomes feasible is when Gibbs free energy equals zero.