5.3 - Acids, bases and buffers Flashcards
Define Bronsted Lowry acid
proton donor
Define Bronsted Lowry base
proton acceptor
Define conjugate base pair
2 species interconnected by transfer of a proton
Write an equation for the neutralisation of an alkali
H3O+ + OH- → 2H2O
or H+ + OH- → H2O
Define buffer solution
system that minimises pH change on addition of small amounts of an acid or base
Define monobasic acid
acid that can donate 1 H atom per molecule of acid
Define dibasic acid
acid that can donate 2 H atoms per molecule of the acid
Define tribasic acid
acid that can donate 3 H atoms per molecule of acid
Define end point
the point during a titration when the indicator changes colour
Define equivalence point
point during a titration when the amount of acid is exactly equal to the amount of base
What is ±1 pH equal to in [H+]?
x10 [H+]
Which is the only factor to change Ka?
temperature
What does a stronger acid have?
- larger Ka
- smaller pKa
- equilibrium right
What does a weaker acid have?
- smaller Ka
- larger pKa
- equilibrium left
How do dicarboxylic acids and alcohols dissociate?
carboxylic acids: one [H+] at a time
alcohol groups do not dissociate
How can Ka be determined experimentally?
prepare a standard solution of weak acid of known concentration, then measuring pH using pH meter
When is the dissociation of water significant?
when pH>6
Equation for whole number pH and [H+] and [OH-] concentrations
p[H+] + p[OH-] = 14
Describe the two ways a buffer solution can be prepared
- weak acid and its salt
- excess of weak acid and a strong alkali
How does adding acid affect position of equilibrium?
- [H+] increases
- H+ reacts with conjugate base A-
- equilibrium shifts left
- removes most H+ ions
How does adding alkali affect position of equilibrium?
- [OH-] increases
- H+ + OH- -> H2O
- HA dissociates
- equilibrium shifts right
- [H+] ions restored and alkaline removed
When is a buffer most effective?
when [HA] = [A-]
Write an equation for the dissociation of carbonic acid
H2CO3 ⇌ H+ + HCO3-
Define indicator
weak acid that changes colour with changing pH due to an equilibrium shift between HA and A-
Describe recrystallisation method
- dissolve in min. quantity of hot solvent
- cool, filter and dry
Effect of temperature on Kw
increases = Kw increases
(and converse)
Kw for pure water
[H+]²
pH of pure water at temperatures above 298K
<7
What does an equilibrium shift left mean for acid strength?
- further left: weaker acid
- so less it dissociates
- so stronger its conjugate base
Assumptions for weak acid calculations
- [H+] = [A-] and ignore any H+ from H2O
- equilibrium [HA] = initial [HA]
- [HA] much greater than equilibrium [H+] (only if Ka<0.001 or not dilute solution)
Why might a measured pH not be true value?
- no concordant results
- pH probe not calibrated
- measuring cylinder used instead of graduated pipette and filler
When is pH = pKa?
half way to end point
(more accurate for one experiment)
