O Flashcards
lattice enthalpy
enthalpy change when one mole of solid is formed by its gaseous ions (always large negative values - energy released)
lattice enthalpy becomes more negative
for ions with greater charge density (increased ionic charge, decreased ionic radii)
ions with smaller radius can come closer together giving stronger ionic attraction, ions with higher charges have increased electrostatic interactions
increased electrostatic interaction = more energy released
dissolving ionic substances
water molecules are polar and the charges on the molecules are attracted to the charges on the ions
form ion-dipole interactions
this causes ions to become separated from the lattice and be surrounded by water molecules
energy released from this interaction
hydration enthalpy
enthalpy change for the formation of a solution of ions from one mole of gaseous ions (ALWAYS NEGATIVE - energy released)
enthalpy change of solution =
enthalpy change of hydration of cation + enthalpy change of hydration of anion - lattice enthalpy
if enthalpy change of solution is large and positive
a solute will not dissolve
if enthalpy change of solution is negative
a solute will dissolve
enthalpy change of solution
when one mole of a solute dissolves to form a dilute solution
hydration enthalpy becomes more negative
for ions with greater charge density (increased ionic charge, decreased ionic radii)
small, highly charged ions can get close to water molecules and form stronger ion-dipole interactions
dissolving ionic solids in non-polar solvents
ionic solids are insoluble in non-polar solvents
no positive/negative regions in solvent molecules so unable to interact strongly with ions
large +VE enthalpy change of solution
strong intermolecular bonds in the solute generally indicates
more likely to dissolve
greenhouse effect
- radiation from the sun reaches the Earth, mainly consists of visible and UV radiation
- Earth absorbs some of the solar radiation so warms up and emits IR
- greenhouse gases such as CO2 and CH4 are present in troposphere and they absorb IR
- vibrational energy of bonds increases and molecules in the atmosphere collide
- kinetic energy of some molecules increases as energy is transferred between molecules
- causes temperature of atmosphere to rise
- greenhouse gases emit some of the absorbed radiation and some is also absorbed by the Earth
increased concentrations of greenhouse gases leads to
an enhanced greenhouse effect
IR window
wavelegth of IR that water vapour doesn’t absorb
CO2 absorbs IR at the same wavelength as IR window
Bronsted-Lowry theory
states that an acid is a proton donor, and a base is a proton acceptor
conjugate base
HA»_space; A- + H+
(A-) behaves as a base in the presence of acid
conjugate acid
HA»_space; A- + H+
(HA) behaves as an acid in the presence of base
water can act as
an acid or a base
strong acids
easily and fully dissociate in water to form protons
weak acids
have a weak tendency to dissociate to form protons, so only partially dissociate in solution
assumptions when using acidity constant Ka equation for weak acids
[H+] = [A-]
equilibrium concentration of HA = initial concentration of HA in solution
higher the pKa
weaker the acid
buffers
solutions that can resist changes in pH, despite the addition of acids or alkali
types of buffer:
weak acid and one of its salts
weak base and one of its salts
assumptions with buffers
all the A- ions come from the salt
[the weak acid (HA) supplies very few A- ions in comparison with the fully ionised salt]
almost all the HA molecules of the weak acid remain unchanged
adding acid to buffer
when protons are added, some A- ions from the salt react with the extra protons to form HA and water
this removes the added protons from the solution
pH is reestablished
adding alkali to buffer
protons are removed from the solution
buffer solution counteracts this because protons can be regenerated from the acid HA
pH is reestablished
Ka of buffers
= [H+] x [salt] / [acid]
ratio of [salt] : [acid] in buffer
changing ratio from about 3:1 to about 1:3 alters the pH by roughly 1 unit
this is because it changes [H+] by a factor of about 9 or 10
(increase salt > [H+] lower > pH higher)
Ksp
solubility product
if product of concentrations of ions is less than Ksp value then no precipitate will form
entropy
measure of the number of ways of arranging molecules and distributing energy
entropy change = Joules per Kelvin per mole
greater entropy
gas > liquid > solid heavier larger atoms more particles straight compared with branched energy levels closer together
if entropy change of a process is
positive, reaction will occur spontaneously
negative, reaction is unlikely to occur spontaneously
