THERMODYNAMICS - PHYSICAL 2 Flashcards
What does size of enthalpy change depend on
-strength / energy of bond or force
-stronger the bond, stronger the enthalpy change
enthalpy change of formation
enthalpy change when 1 mole of substance if formed from its component elements in their standard states
eg of enthalpy change of formation
Na (s) + 1/2Cl2 (g) > NaCl(s)
enthalpy change of lattice formation
enthalpy change when 1 mole of ionic compound id formed from its component ions in gaseous state
eg of enthalpy change of lattice formation
Na+ (g) + Cl- (g) > NaCl (s)
type of energy change of enthalpy change of lattice formation and formation
-exothermic
-ionic bonds
-with strong electrostatic forces of attraction being formed
Factors affecting strength of enthalpy change of lattice formation
-Charge on ion > greater charge then strong electrostatic force of attraction between ions > more exothermic lattice formation enthalpy
-size of ion > smaller ionic radius > stronger electrostatic forces between ions > more exothermic lattice formation
enthalpy change of atomisation
enthalpy change when 1 mole of gaseous atom are formed
eg of enthalpy change of formation
Na (s) > Na (g)
1/2Cl(g) >Cl (g)
Type of energy change of enthalpy change of atomisation
endothermic > breaking bonds of attraction
1st ionisation enthalpy
enthalpy change to remove 1 mole of e- from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
eg of 1st ionisation enthalpy
Na (g) > Na+ (g) + e-
type of energy change of 1st ionisation enthalpy
endothermic > overcoming electrostatic forces of attraction between nucleus and outer e-
factors that affect strength of 1st ionisation enthalpy
-nuclear charge
-e- sheilding
-size of ion
bond dissociation enthalpy
enthalpy change when 1 mole of covalent bonds are broken into gaseous state
eg of bond dissociation enthalpy
Cl2 (g) > 2Cl (g)
1st e- affinity
enthalpy change to add 1 mole of e- to 1 mole of gaseous atoms to make 1 mole of gaseous ions with 1- charge
eg of 1st e- affinity
Cl(g) + e- > Cl- (g)
type of energy change for 1st e- affinity
exothermic >forming electrostatic forces of attraction between nucleus and an e-
factors that affect strength of 1st e- affinity
-nuclear charge
-e- shielding
-size of atom
2nd e- affinity
enthalpy change of 1 mole of e- added to 1 mole of gaseous ions with 1- charge to make 1 mole of gaseous ions with 2- charge
eg of 2nd e- affinity
O- (g) + e- > O2-(g)
type of energy change in 2nd e- affinity
endothermic >repulsion between ion and e-
Factors affecting lattice enthalpy
-stronger ionic bonds> stronger electrostatic forces between ions
-ionic charge > highly charged ions form stronger ionic bonds so electrostatic attraction
-ionic radius > smaller ions have greater charge density > form stronger ionic bonds so stronger electrostatic forces
perfect ionic model
-perfect spheres
-no covalent interaction
ionic compounds with some covalent character
-some ionic compounds have distorted ions that are no longer spheres and some covalent interaction
-e- clouds of ions overlap and form weak covalent bond
-covalent bond stronger than ionic bond and lattice enthalpy will be more exothermic
Theoretical lattice enthalpy
-assumes ions as point charges and calculates the energy of attraction between them
-assumes ionic compound has perfect ionic model
experimental lattice enthalpy
-calculated using born-haber cycle
-enthalpy changes in cycle have ultimately been found using experiments
comparing theoretical and experimental lattice enthalpy if values are very similar
-compound has only ionic bonds and obeys perfect ionic model
-perfect spheres and no covalent character
comparing theoretical and experimental lattice enthalpy if experimental value > theoretical value
-actual bonding is stronger (more exothermic)
-compound has some covalent character and ions have been distorted so no longer spheres
Why do some ionic compounds have covalent character
if + ion is polarising it pulls e- cloud of the - ion towards itself, distorting ion and creates overlap of clouds creating weak covalent bond
factors affecting polarising ability of positive ion
-ionic radius > smaller ions have greater charge density > more polarising
-ionic charge > more highly charged ions have greater charge density > more polarising
factors affecting polarising ability of negative ion
-ionic radius > larger ionic are easier to distort
-ionic charge > lower charged ions re easier to distort
