Thermodynamics Flashcards
(39 cards)
The standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states
Na(s) + 1/2 Cl2(g) -> NaCl
First ionisation enthalpy
The standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge
The standard enthalpy of atomisation
The enthalpy change when one mole of gaseous atoms are formed from an element in its standard state
1/2 Br2 (l) -> Br(g)
Mean bond enthalpy
The enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals averaged over a range of compounds
Br2 (l) -> 2Br (g)
First electron affinity
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single negative charge under standard conditions
O(g) + e- -> o- (g)
Lattice formation enthalpy
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions
Mg2+ (g) + 2Br- (g) -> MgBr2 (s)
Lattice dissociation enthalpy
The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions
MgBr2 (s) -> Mg2+ (g) + 2Br- (g)
Standard enthalpy of hydration
The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions
Cl-(g) + aq -> Cl- (aq)
Standard enthalpy of solution
The standard enthalpy change when one mole of solute dissolves to form its aqueous ions
NaCl(s) + aq -> Na+ (aq) + Cl-
What can effect the strength of an ionic bond
The size of the ions- smaller ions have a strange attraction
The charge of the ions- higher charged ions have a stronger attraction
What do stronger ions lattices have
More exothermic lattice formation enthalpy
More endothermic lattice dissociation enthalpy
Compare different techniques for measuring ionic bond strength
One method is theoretical also known as the perfect ionic model it assumes that ions are perfect spheres. The nature of the bonding is purely ionic
One method is experimental also known as the born hater cycle it assumes that ions are polarisable. The nature of the bonding is ionic plus some covalent character
what are the two key factors that determine how exothermic a lattice enthalpy will be
charge on ions -the greater the charge on an ion has the greater it’s attraction to the oppositely charged ion
size of ions (ionic radius) -the smaller the ion the greater the attraction to an oppositely charged ion
CRAM
Charge on the ions
Radius of the ions
Attraction between ions
More exothermic/endothermic
which compound will have more exothermic lattice formation enthalpy MgO or Na2O
Mg2+ has a greater charge than Na +
Mg 2+ is smaller than Na +
Mg 2+ is more strongly attracted to O2-
MgO has the more exothermic lattice formation enthalpy
describe covalent character using Zn 2+ and Se 2-
the Zn2+ ion is quite small with quite a strong +ve charge
the Se 2- ion is large with a diffuse electron cloud
the 2 with ionically bond because of electrostatic attraction
the Zn 2+ ion is strongly polarising
the electron cloud about the Se 2- ion becomes distorted
some the electron density is shared
this is covalent character
what characteristics will compounds with covalent character have
a +ve ion which is small and highly charged (very polarising)
a -ve ion which is very large and highly charged (very polarisable)
the theoretical lattice enthalpy model
perfect ionic model
ions are point charges
and the bonding is perfectly ionic
the experimental model for lattice enthalpy
born haber
ions are polarisable
covalent character
what’s the ^H LF calculated by perfect ionic model like in comparison to ^H LF calculated by experimental model
when there is NO COVALENT CHARACTER
equal
what’s the ^H LF calculated by perfect ionic model like in comparison to ^H LF calculated by experimental model
when there IS COVALENT CHARACTER
less exothermic as the born haber model allows for covalent character and predicts bonding
what happens in terms of bonds when you dissolve an ionic compound
1.break the ionic bonds
2. form bonds between the water molecules and the ions
What two factors affect how exothermic a lattice enthalpy will be
Charge of ions
Size of ions
How does the charge of ions affect the how exothermic a lattice enthalpy will be
The greater the charge on an ion has the greater it’s attraction to me oppositely charge ion
