thermodynamics Flashcards
def: hess’s law
the enthalpy change for a chemical reaction is the same, regardless of the route taken from reactants to products
def: the standard enthalpy of formation ∆Hɵf
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
def: the standard enthalpy of combustion ∆Hɵc
the enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states
def: the standard enthalpy of atomisation ∆Hɵat
the enthalpy change when one of gaseous atoms is formed from an element in its standard state
def: mean bond enthalpy ΔHBEΘ
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
def: first ionisation enthalpy 1st∆Hɵi
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
def: second ionisation enthalpy 2nd∆Hɵi
the standard enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to give one mole of gaseous ions each with a 2+ charge
def: first electron affinity 1st∆Hɵea
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
def: second electron affinity 2nd∆Hɵea
the standard enthalpy change when one mole of electrons is added to a mole of gaseous ions each with a single negative charge, to form a mole of ions each with a two negative charge
def: lattice formation enthalpy ΔHLFΘ
the standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
def: lattice dissociation enthalpy ΔHLDΘ
the standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions
def: standard enthalpy of hydration ΔHhydΘ
the standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions
def: standard enthalpy of solution ΔHsolΘ
the standard enthalpy change when one mole of solute dissolves in enough solvent to form a solution in which the ions are far enough apart not to interact with each other
if the enthalpy change arrow points up, it is
positive
if the enthalpy change arrow points down, it is
negative
what two factors determine how exothermic a lattice enthalpy will be?
- charge on the ions
- size of the ions (ionic radius)
(the bigger the charge and the smaller the ion, the greater the charge density)
charge on the ions:
the greater the charge an ion has the greater it’s attraction to an oppositely chsrged ion
size of the ions (ionic radius):
the smaller the ion, the greater the attraction to an oppositely charged ion
the more negative the enthalpy of lattice formation, the …
stronger the ionic bonds
CRAM:
charge on the ions
radius/size of the ions
attraction between the ions
more exothermic/endothermic
the theoretical value of enthalpy of lattice formation and the experimentally determined value match very closely, this indicates
[compound] shows almost purely ionic bonding
the theoretical value of enthalpy of lattice formation lower than the experimentally determined value, this means the bonding is stronger than theoretical value predicted, this indicates…
[compound] has some covalent character present
which compounds will have covalent character?
a positive ion which is small and highly charged - very polarising
a negative ion which is large and highly charged - very polarised
theoretical:
model name, type of ions and nature of bonding?
perfect ionic model;
ions are point charges, perfect spheres which cannot be distorted;
ions show purely ionic bonding
experimental:
model name, type of ions and nature of bonding?
born haber cycle;
ions are polarisable;
covalent character
if an ionic compound has no covalent character:
enthalpy of lattice formation calculated by perfect ionic model is (almost) equal to enthalpy of lattice formation calculated by experimental model
if there is covalent character in an ionic compound:
enthalpy of lattice formation calculated by perfect ionic model is less exothermic than the enthalpy of lattice formation calculated by experimental model
experimental born haber model allows for covalent character and predicts stronger bonding
to dissolve an ionic compound:
break the ionic bonds;
forms bonds between the water molecules and the ions
soluble =
negative sign
not soluble =
positive sign
three factors which determine whether a chemical reaction is feasible:
temperature
enthalpy - feasible when exothermic because reactants want to release energy to environment, so products lower in energy and more stable
entropy
entropy
the amount of disorder within a system
symbol ‘S’ - always a positive number
entropy change is delta S, can be pos or neg
JK-1mol-1 = units
different types of ways entropy increases:
solid to aqueous (dissolving molecules or ions in solution)- particles go from neatly arranged to more disordered
when gases combine - more disordered
evaporating a liquid - particles in water are closely packed and particles in gas are spread out
gas produced
more particles of products than reactants and same state
to calculate change in entropy:
sum of products - sum of reactants
if change makes a system more random = negative delta S
if change makes a system more ordered = positive delta S
gibbs free energy equation
deltaG = deltaH - TdeltaS
delta G = gibbs free energy = kJmol-1
delta H = enthalpy change = kJmol-1
T = temperature = Kelvin
delta S = entropy change = kJK-1mol-1
a reaction is feasible if delta G is less than or equal to zero
gibbs free energy graphs
y = mx + c
y axis is gibbs free energy
x axis is temperature
m is minus entropy change
c is enthalpy change (intercept)
slope increase = entropy change is negative
slope decrease = entropy change is positive
y = mx + c
deltaG = -deltaS T + deltaH
what does ɵ mean ?
standard conditions
standard pressure 100kPa or 1atm
standard temperature 298K
1 mol dm-3 concentration for all solutions
