Thermodynamics

Thermodynamics is the study of the energy in a chemical system, and in order to understand this better a number of definitions have to be cleared up first.

Definitions
Standard temperature and pressure: The conditions used for the reaction. This has to be 1 atm and 25 0C

Enthalpy: The energy released or absorbed during the reaction

EntropyThe change in disorder in a system. Entropy change is positive when a liquid change from a solid to a liquid, or a liquid to a gas because more disorder is obtained. It is therefore negative when a gas condenses to a liquid or a liquid freezes to a solid since less disorder is produced.

Standard enthalpy of combustion: The energy released when 1 mole of compound reacts with air at standard temperature and pressure.

Standard enthalpy of formation: The energy released or absorbed when 1 mole of a compound is formed from its components at standard temperature and pressure.

Standard enthalpy of atomisation: The energy absorbed when 1 mole of gaseous atoms are formed from its original state at standard temperature and pressure.

Standard enthalpy of bond dissociation: The energy required to break 1 mole of bonds at standard room temperature and pressure.

Standard enthalpy of ionisation: The energy required for 1 mole of atoms to lose 1 mole of electrons from its outer shell at standard temperature and pressure.

Standard enthalpy of 1st electron affinity: The energy released when 1 mole of atoms gain 1 mole of electrons to form a uni-negative ion at standard temperature and pressure.

Standard enthalpy of 2nd electron affinity: The energy required when 1 mole of uni-negative atoms gain 1 mole of electrons to form a di-negative ion at standard temperature and pressure.

Lattice enthalpy: The energy released when 1 mole of a compound is formed by reacting the ions at gaseous state at standard temperature and pressure.

Hess’s Law

Hess’s Law states that no matter the route taken for a reaction, the enthalpy change for the reaction will be the same.

Born Haber Cycle
The Born Haber Cycle is a complex Hess’s law which is used to find the lattice enthalpy since this cannot be found experimentally.

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Neutralisation of acids and bases
The standard enthalpy of neutralisation is the energy required to produce one mole of water from an acid-base reaction.

Since the enthalpy change involves the preparation of 1 mole of water this should be irrespective of the acids and bases used, and the enthalpy is more or less the same for different acids and bases reactions.

For a weak acid, a weak base reaction the value can go down to 10kJ mol-1 depending on how weak the acid and base are. The weaker they are, the more energy is needed to release H+ and OH- and therefore the less energy will be released

Gibbs free energy

The free energy is one of the most important equations in chemistry since this will define whether a reaction takes place, or not. For a reaction to take place ∆G must be negative.
There are possible cases that need to be analysed for this equation:

Problems on Thermodynamics