Buffer Solutions Flashcards
What is a buffer solution
Solution which resists changes in pH when small amounts of acid/alkali are added, used in shampoo/water in swimming pools eg
What does a typical buffer mixture consist of and how does this work
Solution of a weak acid and it’s salt/CB eg ethanoic acid and sodium ethanoate, must be plenty of both the acid and it’s salt so that any change does not affect pH as concen of HA and A- so high will remain fairly constant
What happens when a little H+ added to buffer
Temporarily increases concentration of H+ so equilibrium shifts towards left hand side of equation to reduce H+ ion concen which counteracts the change as backward reaction favoured and plenty of base ions from the salt to react with H+ ions to produce acid. So increases [HA] and decreases [A-], however such large reservoir that change in ratio negligible and pH remains almost the same
What happens when a little OH- added
OH- concen increases, primarily reacts with acid to make water and salt, also reacts with H+ to make water- as hydrogen ion concen decreased equilibrium will move to right to restore the H+ lost, the acid required for this is readily available due to large excess as so little of the ethanoic acid dissociated in first place. So decreases [HA] and increases [A-] however such large reservoirs that ratio change negligible and pH remains almost the same
Ionic equations for buffer solution of ethanoic acid and sodium ethanoate
CH3COOH reversible CH3COO- + H+
CH3COONa reversible CH3COO- + Na+
Lots of CH3COOH which barely dissociates and CH3COO- from sodium ethanoate which fully ionises
How to calculate pH of buffer
When do all the negative ions come from in buffer
From the added salt as it fully ionises, weak acid only slightly
Does [A-]= [H+] for buffer solutions
No as [A-] coming from weak acid very small compared to that coming from its conjugate base
In pH of buffer solutions where does concen of A- come from
Same as conjugate base concen as it completely ionises
In pH of buffer solutions where does the concentration of HA come from
Same as original acid as so little dissociates
Formula of potassium ethanoate
KCH3COO
What happens to number of moles of HA and A- when hydroxide added
HA decreases by number of moles of sodium hydroxide as reacts to form salt and A- increases by number of moles of sodium hydroxide as equilibrium moves to the right as concen of H+ decreases
How to calculate pH of buffer solution with weak acid and strong base
[H+]= ka no moles of HA - OH- over total volume / [OH-] using total volume
When is the pH of the buffer equal to the pKa of the weak acid
If the concens of weak acid and it’s salt are the same
Why is buffer used in blood
To maintain pH needed for proper metabolic function, used a bicarbonate buffer system between carbon dioxide, water and hydrogencarbonate ions
What occurs at half way to equivalence with weak acid and strong base
pH=pKa halfway to end point as [HA]=[A-]
How to get pH of buffer solution using Henderson-Hasselbalch equation
pH= pKa + log ([A-]/[HA])
How to get [H+] from Henderson-Hasselbalch equation
[H+]= ka [HA]/[A-]
What is buffer zone of strong base weak acid buffer
Horizontal line where adding more of strong base has little effect on pH
When drawing titration curve how many units does vertical region have to take up
At least 3
How can buffer be made with weak acid and strong base
All the base reacts with the acid and the weak acid must be in excess so there’s still some left in the solution once all the base has reacted which slightly dissociates so there’s a large reservoir of the weak acid and it’s conjugate base
What is the buffer system in the blood
Carbonic acid-hydrogencarbonate buffer system, carbonic acid (H2CO3) dissociates into H+ and HCO3- ions
-if concen of H+ rises will react with HCO3- and equilibrium will shift to the left reducing the H+ concen which stops the pH of blood from dropping
-if concen of H+ ions falls more H2CO3 molecules will dissociate and equilibrium will shift to the right which will increase H+ concen
How is level of H2CO3 controlled
-levels of H2CO3 are controlled by respiration as by breathing out CO2 the level of H2CO3 is reduced
How is level of HCO3- controlled
By the kidneys, excess is excreted in the urine
