thermodynamics

• enthalpy of formation

  enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions (100kPa and 298K)

  eg. Na(s) + 0.5Cl2(g) ---> NaCl(s)
• enthalpy of ionisation 1st

  enthalpy change when one electron is removed from each atom in a mole of gaseous atoms to form a mole of gaseous ions with a 1+ charge under standard conditions

  eg. Mg(g) ---> Mg+(g) + e-
• enthalpy of atomisation

  enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions

  eg. Na(s) ---> Na(g)
• bond disassociation enthalpy

  enthalpy change when one mole of covalent bonds is broken into 2 gaseous atoms (or free radicals) under standard conditions

  eg. Cl2(g) ---> 2Cl(g)
• 1st electron affinity

  enthalpy change when each atom in a mole of gaseous atoms gains one electron to produce one mole of gaseous 1- ions under standard conditions

  eg. Mg(g) + e- ---> Mg-(g)
• lattice dissociation enthalpy

  enthalpy change when one mole of ionic crystal is broken into its constituent gaseous ions under standard conditions

  eg. NaCl(s) ---> Na+(g) + Cl-(g)
• lattice formation enthalpy

  enthalpy change when one mole of ionic crystal is formed by its constituent gaseous ions under standard conditions

  eg. Na+(g) + Cl-(g) ---> NaCl(s)
• enthalpy of hydration

  enthalpy change when one mole of gaseous ions becomes aqueous ions

  eg. Na+(g) + aq ---> Na+(aq)
• enthalpy of solution

  enthalpy change when one mole of ionic substance dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another

  eg. NaCl(s) + aq ---> Na+(aq) + Cl-(g)

  -can be exo or endo
• Born-Haber cycle

  -specific application of hess' law for ionic compounds

  -up arrow - endothermic

  -down arrow - exothermic
• Theoretical lattice enthalpies

  -possible to calculate a theoretical value if you know;

  -geometry of the ionic solid

  -charge on the ions

  -distance between ions

  -our theoretical is close to the real value of NaCl because assumed that is is a highly ionic compound with electrostatic attraction between cations and anions

  -however zinc sulfide value is off - suggests not purely ionic - possesses some covalent character
• zinc sulfide - covalent character

  -zinc is a smaller ion with greater charge

  -zinc ions attract electron density towards themselves distorting e- cloud which makes bond slightly covalent

  -as you move left to right across periodic table, lattices become less ionic and more covalent
• factors affecting lattice enthalpy

  -charge of ion

  -radius of ion
• factors effecting lattice enthalpy: ionic radius

  -lattice energy becomes less exothermic as ionic radius decreases

  -charge on ions is more spread out when ion is larger

  -ions are also further apart from each other in lattice

  -electrostatic forces between oppositely charged ions are weaker
• factors effecting lattice enthalpy: ionic charge

  -lattice energy gets more exothermic as ionic charge increases

  -greater ionic charge means higher charge density

  -results in stronger electrostatic attraction
• Entropy (S)

  -measure of how disordered or chaotic a system is

  -more disorder = increase in entropy

  -increased entropy = energetically more stable

  -in order of entropy: solid, liquid, gas
• Feasible or spontaneous reactions

  -reactions can take place of their own, they are energetically favourable

  -outcome of the second law of thermodynamics - entropy in the universe is always increasing

  -feasibility takes no account of rate of reaction, it states only what is possible - a feasible reaction could be extremely slow like rusting
• calculating entropy changes

  Delta S= Sum of Sproducts- Sum of Sreactants

  -JK -1 mol-1
• Gibbs free energy

  delta G = delta H - T delta S

  -delta G - kJmol-1

  -delta H - kJmol-1

  -T - K

  -delta S - Jk-1mol-1 (must be converted to kJ-1mol-1 by /1000)
• reaction feasibility

  -gibbs is used to find whether a reaction is feasible or not

  -when gibbs is +ve reaction is not feasible

  -when gibbs is -ve reaction is feasible