Acid-base Equilibria Flashcards
Brønsted–Lowry acid
proton donor
Brønsted–Lowry base
proton acceptor
acid-base reactions involve the transfer of
protons
pH
-log10[H+]
[H+] from pH
10^-ph
strong acid
fully dissociated
weak acid
partially dissociated
pH of a strong acid
no ka
[H+]=[HA]
pH of a weak acid
ka=[H+]^2/[HA]
assumptions of a weak acid
- [A-]=[H+] as no H+ from the dissociation of water only from the acid
- [HA] equilibrium = [HA] original as dissociation is negligable
Kw
[H+][OH-]=[H+]^2
=1x10^-14
so pH of water is 7
pH of strong base
using kw
[H+]=kw/[OH-]
pKa
-log10(Ka)
pKw
-log10(Kw)
dilution of strong acids
for each dilution of x10 pH increases by 1
dilution of weak acids
Weak acids are in equilibrium
as they are diluted
some of the undissociated acid molecules split up
pH does not increase as fast as strong acid
each dilution of 10x, the pH increases by 0.5 unit, and for each dilution of 100x, the pH increases by 1 unit
numbers for pH curves
13 11
8 7 6
3 1
how to select a suitable indicator
needs to change colour in the vertical region,
pH changes sharply so accurate
pH range within vertical region
end point
point when the indicator changes colour
should be in the middle of the vertical section of the line
buffer solution
resists the changes in pH if you add a small amount of acid or base
how does a buffer solution work
contains high of both [HA] and [A-] adding small H+: H+ + A- > HA adding small OH-: HA + OH- > A- + H2O [HA] : [A-] stays aprox constant [H+] and pH stays aprox constant
pH of a buffer
Ka= [H+][A-] / [HA]
at 1/2 equivalence
pKa = pH as
[HA]=[A-] so
Ka=[H+] as they cancel out
expression for Kin / Ka of indicator
=[H+][in-]/[Hin]
at end point, Kin
[in-]=[Hin] so [in-]/[Hin]=1 and [H+] = Kin -log[H+]=-log[Kin] pH=pKin
difference in enthalpy changes of neutralisation values for strong and weak acids
strong acids: HA>H+ + A- all H+ the add alkali to neutralise
weak acids: HA <>H+ + A- add alkali to neutralise
equi shifts right to replace H+
HA dissociation is endothermic
some energy is reabsorbed so enthalpy is less -ve
pH of blood
7.4
buffers of the blood
carbonate/hydrogencarbonate base pairs
H2CO3(aq) >HCO3-(aq) + H+(aq) to decrease H+
H2CO3(aq) >CO2(aq) + H2O(l) to increase H+
