acid base equilibria Flashcards
what is a bronsted-lowry acid and base?
a bronsted-lowry acid is a proton donor
a bronsted-lowry base is a proton acceptor
what is a conjugate acid-base pair?
a conjugate acid-base pair constitutes 2 species which differ from each other by one proton
what is a strong and weak bronsted acid?
a strong bronsted acid is a proton donor that dissociates completely in water to give H3O+.
a weak bronsted acid is a proton donor that dissociates partially in water to give H3O+
what is a strong and weak bronsted base?
a strong bronsted base is a proton acceptor that dissociates completely in water to give OH-
a weak bronsted base is a proton acceptor that dissociates partially in water to give OH-
what is pH?
pH = -log[H+]
hence [H+] = 10⁻ᵖᴴ
what is pOH?
pOH = -log[OH-]
hence, [OH] = 10⁻ᵖᴼᴴ
what is Kw?
Kw = [H+] [OH-]
hence, pKw = pH + pOH
Kw = Ka x Kb
hence, pKw = pKa + pKb
pKw = pH + pOH = 14 ONLY at 25C
what is Ka?
Ka is the acid dissociation constant. it indicates the strength of the acid and is constant at a constant temperature
for HA + H2O ⇌ A- + H3O+ ,
Ka = [H+eqm]x[A-eqm]/[HAeqm]
pKa = -lgKa
larger Ka (smaller pKa) = greater extent of dissociation = stronger acid
what is Kb?
Kb is the base dissocation constant. it indicates the strength of the base is constant at a constant temperature.
for BOH + H2O ⇌ BH+ + OH- ,
Kb = [BH+]x[OH-]/[BOH]
what is degree of dissociation, α?
degree of dissociation is the fraction of molecules which is ionised into ions
α = amt ionised/initial amt = [acid]dissociated/[acid]initial
when does hydrolysis occur?
hydrolysis occurs when the cation/anion is a stronger acid/base than water.
conjugate acid/bases of WEAK bases/acids are STRONG -> stronger acids/bases than water -> tend to hydrolyse
what is a buffer solution?
a buffer solution is one that is able to resist pH changes upon addition of a small amount of acid or base
a buffer usually consists of 2 species which make up a conjugate acid-base pair of a weak acid or base
how does a buffer consisting of large reservoirs of CH3CO2H & its conjugate base CH3CO2- resist pH changes?
- when a small amount of H+ is added, the H+ ions react with the large reservoir of CH3CO2-: CH3CO2- + H+ -> CH3CO2H
- the added H+ ions are removed and [CH3CO2H] increases slightly and [CH3CO2-] decreases slightly
- as the original amounts of CH3CO2H and CH3CO2- are large compared to the amount of H+ ions added, the ratio [CH3CO2-]/[CH3CO2H] remains almost constant
- since Ka = [H+] [CH3CO2-]/[CH3CO2H] and Ka is constant at constant T, [H+] remains almost constant, hence pH remains almost constant
for addition of small amount of OH-, change all H+ in the FIRST 3 POINTS ONLY to OH-
how to find pH of buffer?
buffer of weak acid HA & conjugate base A-
Ka = [H+]eqm[A-]initial/[HA]initial = [H+]eqm[conj base]/[acid]
[H+] = Ka x [acid]/[conj base]
buffer of weak base and conjugate acid
[OH-] = Kb x [base]/[conj acid]
what is maximum buffering capacity and what will give the most effective buffer?
maximum buffer capacity is when the buffer can most effectively resist a change in pH in either direction. (when [base]=[conj acid] or [acid]=[conj base])
the most effective buffer is when pH=pKa or when pOH=pKb
descsribe the buffer system in human blood
the following equilibria operates in the H2CO3/HCO3- buffer system in blood:
H+ + HCO3- ⇌ H2CO3 ⇌ H2O + CO2
when H+ is added to blood, the following occurs:
H+ + HCO3- -> H2CO3
amount of HCO3- decreases, but the amount of the change is small compared to the large amount of HCO3- present in the blood
when OH- is added, the following occurs:
OH- + H2CO3 -> HCO3- + H2O or
OH- + CO2 -> HCO3-
what is end-point and equivalance point in a titration?
end-point is the point in a titration at which the indicator changes colour when 1 drop of excess titrant is added.
equivalence point is the point in a titration at which the exact number of moles of acid and base react completely.
how to decide what indicator is suitable for a titration?
the indicator must have a distinct colour change.
the working pH range of the indicator must coincide with the rapid pH change at the equivalence point for the titration.
