acid base equilibria

Question 1

what is a bronsted-lowry acid and base?

Answer 1

a bronsted-lowry acid is a proton donor
a bronsted-lowry base is a proton acceptor

Question 2

what is a conjugate acid-base pair?

Answer 2

a conjugate acid-base pair constitutes 2 species which differ from each other by one proton

Question 3

what is a strong and weak bronsted acid?

Answer 3

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+

Question 4

what is a strong and weak bronsted base?

Answer 4

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-

Question 5

what is pH?

Answer 5

pH = -log[H+]

hence [H+] = 10⁻ᵖᴴ

Question 6

what is pOH?

Answer 6

pOH = -log[OH-]

hence, [OH] = 10⁻ᵖᴼᴴ

Question 7

what is Kw?

Answer 7

Kw = [H+] [OH-]
hence, pKw = pH + pOH

Kw = Ka x Kb
hence, pKw = pKa + pKb

pKw = pH + pOH = 14 ONLY at 25C

Question 8

what is Ka?

Answer 8

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

Question 9

what is Kb?

Answer 9

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]

Question 10

what is degree of dissociation, α?

Answer 10

degree of dissociation is the fraction of molecules which is ionised into ions
α = amt ionised/initial amt = [acid]dissociated/[acid]initial

Question 11

when does salt hydrolysis occur?

Answer 11

hydrolysis occurs when the cation/anion derived from a weak base/acid is a stronger acid/base than water.

conjugate acid/bases of WEAK bases/acids are STRONG -> stronger acids/bases than water -> tend to hydrolyse

Question 12

what is a buffer solution?

Answer 12

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

Question 13

how does a buffer consisting of large reservoirs of CH3CO2H & its conjugate base CH3CO2- resist pH changes?

Answer 13

• 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-

Question 14

how to find pH of buffer?

Answer 14

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]

Question 15

what is maximum buffering capacity and what will give the most effective buffer?

Answer 15

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

in a titration, when half the equivalence point titre is added, maximum buffer capacity is achieved. eg when 30cm3 of NaOH is needed to neutralise HCl. when 15cm3 of NaOH is added, maximum buffer capacity is achieved.

Question 16

descsribe the buffer system in human blood

Answer 16

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-

Question 17

what is end-point and equivalance point in a titration?

Answer 17

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.

Question 18

how to decide what indicator is suitable for a titration?

Answer 18

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.

METHYL ORANGE

• working range: 3.1-4.4
• low pH colour: yellow
• high pH colour: red

PHENOLPHTHALEIN

• working range: 8.2-10.0
• low pH colour: colourless
• high pH colour: pink