Enthalpy and Entropy

Lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions. e.g Na+ (g) + Cl- (g) = NaCl (s) gaseous ions solid ionic lattice

lattice enthalpy is Exothermic and hence Enthalpy change is negative because bond forming is exothermic.

A way of indirectly calculating lattice enthalpy from other known energy changes because it cant be calculated directly.

lattice formation is Changing gaseous ions into the solid ionic lattice and its Exothermic. gaseous ions to ionic lattice

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states. Na (s) + 1/2 Cl2 (g) = NaCl (s) reactants to one mole of a product

The standard enthalpy change of atomisation is the enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions. Na (s) = Na (g) 1/2 Cl2 (g) = Cl (g)

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. Na(g) = Na+ (g) + e-

ionisation energy is endothermic because Energy is required to overcome the attraction between a negative electron and a positive nucleus.

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions. Cl (g) + e- = Cl- (g)

Electron being added is attracted in towards the nucleus.

Born-haber cycles for group two compounds require both first and second ionisation energies.

born-haber cycles for oxides and sulfides require first and second electron affinities/ O+ e- = O- O- + e- = O 2-

the second electron affinity is negative because 1) an electron is being gained to a negatively charged atom which repels s there the same charge 2) Energy is needed to force the negatively charged electron onto the ion

The negative ion repels the additional electron, so energy is required to force the electron onto it.

standard enthalpy change of solution is The enthalpy change that takes place when one mole of a solute dissolves in a solvent under standard conditions. Na+Cl- + aq= Na+ (aq) + Cl- (aq) solute to dissolved

when a solid ionic compound dissolves in water the Ionic lattice breaks up Water molecules are attracted to and surround the ions.

When a solid ionic compound dissolves in water, the Ionic lattice is broken up forming separate gaseous ions. gaseous ions interact with polar water molecules forming hydrated aqueous ions. This is Called enthalpy change of hydration.

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions. Na+ + aq = Na + (aq)

as Ionic radius increases Attraction between ions decreases Lattice enthalpy becomes less negative ( less exothermic) Melting point decreases as less energy is needed to overcome the decreased attraction

as Ionic charge increases Attraction between ions increases Lattice enthalpy becomes more negative (more exothermic) Melting point increases as more energy is needed to overcome increased attraction

as Ionic size increases Attraction between ion and water molecules decreases Hydration energy less negative

Ionic charge increases Attraction with water molecules increases Hydration energy becomes more negative

Many compounds with endothermic enthalpy changes of solution are soluble.

entropy is a measure of the dispersal of energy within the chemicals in a chemical system.