Thermodynamics; Enthalpy changes.

Notes;
The overall enthalpy change in a reaction is often the sum of several endothermic, and exothermic processes.
example the formation of sodium chloride:

-Na atoms in solid state seperate to form (g) atoms
-(g) atoms form Na+ (g) ions
-Cl2 (g) molecules split into Cl (g) atoms
-Cl (g) atoms form Cl-(g) ions
-Positive and negative (g) ions attract each other and form a solid crystal lattice.

Releases a lot of energy (overall exothermic), and the heat from this can be measured and it is the sum of  number of different enthalpy changes.
The standard enthalpy of formation when one mole of NaCl is formed from its elements in their standard states under standard conditions.

Standard enthalpy changes:
the size of any enthalpy change will depend on the amounts and conditions, so we use the agreed standard conditions and amounts with all the substances in their standard states.
This way comparisons can be made between different measurements.


Using NaCl as a continous example

Enthalpy of atomisation:
stage 1-Energy is supplied to the solid Na metal to form (g) atoms, the energy required to vapourise one mole of solid atoms is known as the enthalpy of atomisation. (or sublimation for metallic/atomic lattice)
It is an endothermic process because energy needs to be supplied to break the bonds between the atoms, due to the attractive forces.

Ionisation: 
stage 2-Involves removing the outer electron from each (g) atom to form a positive ion and an electron.
It is endothermic because energy needs to be supplied to remove the electron due the attraction between the nucleus and the outer shell electrons.
Many metals can lose more than one electron from their outer shell, which results in the 2nd and 3rd ionisation energy's, these are always greater due the increased attraction between the electron and the nucleus, because this electron is closer to the nucleus.

Thermodynamics; Enthalpy change definitions

Enthalpy change: for a system is the heat taken in or given off during a chemical or physical change that takes place at a constant pressure.


Standard conditions:
-1 mole, mol-1
-100kpa
-298K
(if other temperature is used, will be stated next to the H in the sign for enthalpy change e.g. H1000)


Enthalpy of formation: the enthalpy change when one mole of a molecule is formed from its elements in their standard states. 

Enthalpy of atomisation: is the standard enthalpy change that accompanies the formation of one mole of gaseous atoms. 

Ionisation enthalpy: the standard change for the removal of one electron from a species in the gas phase to form a positive ion and an electron both also in the gas phase.  Mg --> Mg+ +e- 


Atomisation: change needed to produce 1 mole of atoms in their standard states, (half the value for Cl-Cl bond enthalpy) (g) molecules to (g) atoms.

Bond dissociation enthalpy: is the standard enthalpy change that accompanies the breaking of a covalent bond in a gaseous molecule to form two free radicals also in their gaseous phase.

Electron affinity: is the standard enthalpy change when an electron is added to an isolated atom in the gas phase.

Lattice enthalpy: the standard enthalpy change that accompanies the separation of one mole of a solid ionic lattice into its gaseous ions. 


Standard enthalpy of formation: is the enthalpy change involved in the production of one mole of a compound from its elements under standard conditions, with all products and reacants in their standard states.


Hess's Law: overall enthalpy change in a complete cycle must/ will be equal to 0.


First law of thermodynamics: energy can not be created or destroyed, but stored and transferred.