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
Entropy: The change in disorder in a system. Entropy change is positive when a liquid change from a solid to a liquid, or a liquid to 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.
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Bond Dissociation Energy
The energy released in any reaction is the difference in energy between the bonds of the reactants and the bonds of the products. Breaking bonds is always endothermic whilst making a bond is always exothermic.
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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.
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.
Enthalpy of Solution
Whenever an ionic compound is dissolved in a solvent there are two steps that need to take place.
These are:
- Separate particles of the solute from each other
- Combine separated solute and solvent particles to make the solution
The enthalpy change for this reaction can be worked out as:
The lattice dissociation enthalpy is the breaking down of a solid into its gaseous ions, and this is always positive.
When looking at trends down a group the lattice energy would decrease as the radius of the ion would increase. This means that the lattice dissociation enthalpy would become less endothermic.
The hydration enthalpy is the energy released when water surrounds the gaseous ions. The interaction between the water solvent molecule and the ion is bigger the smaller the ion is and therefore it becomes less exothermic as the ions get bigger.
Apart from the enthalpy of solution the solubility of solution is also dependant on the entropy change of the solution.
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:
