Definition of buffer
solutions that resist pH change when small amounts of either strong acid
or base are added to them (Note some change but not much)
What is buffer made of
weak acid and it stong conjugate base pair
or
weak base and strong conjugate acid
Reason for why it must be weak acid and conjugate base pair vice versa
As if it is strong then it will be a complete reaction hence cant counteract the change in pH.
CH3COOH(aq) + H2O(l) ⇌ H3O + (aq) + CH3COO- (aq)
Adding HCl
(acid) - USE LCP PREDICT Behaviour
− Increase [H3O+]
− LCP predicts shift of equilibrium to the left partially
counteract change by removing excess H3O+
− Maintain constant pH by preventing [H3O+] from rising
significantly until CH3COO runs out
AK:
Conc of H3O+ increases
Reverse raction is favoured hence eq postion will shift left so system can partially oppose this change hence decrease conc of H3O+ therefore most of added H3O+ is removed
therfore change in conc of H3O+ ions is negligible
hence pH depends on conc of H3O+ ions (show equation of ph)
therefore change in ph is negligible until the ch3coo- runs out, hence pH is kept in a narrow range.
CH3COOH(aq) + H2O(l) ⇌ H3O + (aq) + CH3COO- (aq)
Adding NaOH
(base) - Behaviour of buffer predicted using Le Chatelier’s Principle:
− Decrease [H3O+], OHreact with H3O+
to produce H2O
− LCP predicts shift of equilibrium to the right partially
counteract change by producing and replacing lost H3O+
− Maintain constant pH by preventing [H3O+] from falling
significantly until CH3COOH runs out
AK:
Added NaOH will increase OH- conc hence OH- will be neutralised by H3O+ ion, OH- + H3O+ –><— 2H2O
Hence conc of H3O+ will decrease
hence forward reaction favoured hence equilibrium position will shift right to partially oppose this change in H3O+ conc. until ch3cooh runs out
pH depends on H3O+ conc therefore change in pH is negligible.
CH3COOH(aq) + H2O(l) ⇌ H3O + (aq) + CH3COO- (aq)
Adding HCl
(acid) - qualitative explanations
− Increase [H3O+]
− Conjugate base, CH3COO- (aq) acts as a base reacting with H3O+, consuming it
− CH3COOis consumed, decrease rate of reverse
reaction relative to forward
− Reducing H3O+,
thus pH only decreases slightly, until CH3COOruns out
CH3COOH(aq) + H2O(l) ⇌ H3O + (aq) + CH3COO- (aq)
Adding NaOH
(base) - qualitative explanation
− Weak acid CH3COOH acts as an acid, reacts with OH- ,generating more conjugate base CH3COO- and
consuming extra OH-
− CH3COOH is consumed, decrease the forward reaction
relative to reverse
− [OH-] is reduced, pH is only increased slightly, until
CH3COOH runs out, [H3O+] lower than before NaOH
added
Buffer Capacity
The ability of buffer solution to neutralise excess acid or base without a significant change to its pH
What is buffers made of?
made of weak acid/base and its conjugate base/acid respectfully
What is a requirement to have a good buffer
same concentration of acid and its conjugate base
or same conc of base and conjugate acid
Explain the chemistry of buffer of reaction acetate and H2O reaction when NaOH is added.
Added NaOH will increase OH- conc hence OH- will be neutralised by H3O+ ion, OH- + H3O+ –><— 2H2O
Hence conc of H3O+ will decrease
hence forward reaction favoured hence equilibrium position will shift right to partially oppose this change in H3O+ conc. until ch3cooh runs out
pH depends on H3O+ conc therefore change in pH is negligible.
Explain what the term ‘buffering capacity’ means at the molecular level.
Ability of buffer solution to neutralise added acid or base without a significant change in pH
Explain the chemistry of buffer of reaction acetate and h20 reaction when HCL is added.
Conc of H3O+ increases
Reverse raction is favoured hence eq postion will shift left so system can partially oppose this change hence decrease conc of H3O+ therefore most of added H3O+ is removed
therfore change in conc of hydronium ions is negligible
hence pH depends on conc of hydronium ions (show equation of ph)
therefore change in ph is negligible until the ch3coo- runs out, hence pH is kept in a narrow range.
List three factors that affect buffering capacity.
Exact concentrations of each solution used to make buffer
Relative concentration of each solution used to make buffer
Type of chemical used to make buffer
