Acids Bases Buffers Flashcards
Bronsted-Lowry Acid definition
Proton Donor
Bronsted-Lowry Base definition
proton acceptor
Kw
Kw = conc H+ x conc OH-
value of Kw at 298 K
1.0 x 10-14
what physical factors affect value of Kw
temperature only
if temperature is increased, the equilibrium moves to the right so Kw increases and the pH of pure water decreases
why is pure water still neutral even if pH does not equal 7
conc. H+ = conc. OH-
give an expression for pH in terms of H+
pH = -log10[H+]
what is the relationship between pH and [H+]
lower pH = higher [H+]
if two solutions have a pH difference of one what is the difference in [H+]
factor of 10
how do you find [H+] from pH
[H+] = 10 to the power of -pH
what is the difference when finding [H+] from concentration of diprotic and triprotic acids
need to multiply concentration of acid by number of protons to find [H+]
how do you calculate the pH of a strong alkaline solution
use Kw to calculate [H+] from [OH-]
use pH=-log[H+]
define strong acid
fully dissociates in water
define strong base
fully dissociates in water
what is the difference between concentrated and strong
concentrated means many mol per dm^3
strong refers to amount of dissociation
what is a weak acid and weak base definition
do not fully dissociate in water
partially dissociate
examples of strong acid
HCl, H2SO4, H3PO4
examples of strong bases
NaOH, CaCO3, Na2CO3
examples of weak acids
CH3COOH (any organic acids)
example of weak bases
NH3
what is Ka
Ka= [H+][A-] / [HA]
what is a titration
addition of an acid/base of known titration to base/acid of unknown titration to determine the concentration
indicator used to show neutralisation has occurred, as is a pH meter
equivalence point definition
point at which exact volume of base has been added to just neutralise base
what generally happens to pH of solution around equivalence point
there is a large and rapid change in pH, except in the weak-weak titration
what is the end point
volume of acid or alkali added when the indicator just changes colour. if the right indicator is chosen, equivalence point= end point
3 properties of a good indicator
sharp colour change (not gradual)
end point must be same as equivalence point
distinct colour change
indicators for a strong acid - strong base titration
phenolphthalein or methyl orange
phenolphthalein has a clearer colour change though
indicator for a strong acid - weak base titration
methyl orange
indicator for a strong base-weak acid titration
phenolphthalein
indicator for weak acid - weak base titration
neither methyl orange or phenolphthalein as neither give a sharp change at the end point
methyl orange
- colour in acid
- colour in alkali
- at what pH does the colour change
red in acid
yellow in alkali
changes at p = 4-5 (approx same as pKa value)
phenolphthalein
- colour in acid
- colour in alkali
- at what pH does the colour change
colourless in acid
red in alkali
changes about pH = 9-10 (approx same as pKa value)
what is the half-neutralisation point
when volume = half the volume that has been added at the equivalence point
define buffer solution
solution that can resist changes in pH when small amount of acid/alkali are added
what do acidic buffer solutions contain in general terms
a weak acid and a soluble salt of the acid that fully dissociates
write a reaction for an acidic buffer with added acid
A- + H+ –> HA
opposes addition of H+
write a reaction for an acidic buffer with added alkali
HA + OH- –> H2O + A-
how else can you achieve an acidic buffer solution other than just mixing the constituents
neutralise half of a weak acid (acid must be in excess) with an alkali forming a weak acid/soluble salt mixture
what do basic buffer solutions contain in general terms
weak base and soluble salt of that weak base
how can you calculate the pH of buffer solutions
use the Ka of the weak acid, sub in [A-] and [HA] to calculate [H+] then calculate pH
how can you calculate the new pH of a buffer solution when acid or base is added
calculate number of moles of H+ and A- and HA before acid or base is added. use equations to work out new moles of A- and HA and find [H+] then pH
which buffer system maintains blood pH at 7.4? what happens when acid/alkali is added?
H+ + HCO3 - <—-> CO2 + H2O
add OH- –> reacts with h+ to form H2O, then shifts equilibrium left to restore H+ lost
add H+ –> equilibrium shifts to the right, removing excess H+
