Chapter 21 - Buffers and Neutralisation Flashcards
Buffer solutions, Buffer solutions in the body, Neutralisation.
What is a buffer solution?
A system that minimises pH changes when small amounts of an acid or a base are added.
how do the two components of an acid buffer solution work?
1) the weak acid HA removes added alkali
2) The conjugate base A- removes added acid
What two components does an acid buffer solution contain?
An acid buffer solution contains a weak acid and its conjugate base.
HA = H+ + A- weak acid conjugate base
How can a weak acid buffer solution be prepared from a weak acid and its salt?
A buffer solution can be prepared by mixing together
a solution of a weak acid
a solution of a salt of the weak acid
How can a weak acid buffer solution be prepared by partial neutralisation?
A buffer solution can be formed by adding togther
an excess of a weak aqueous acid
and a solution of aqueous alkali
In a buffer solution, what occurs to increase pH when acid is added?
[H+] increases
H+ ions react with conjugate base A-
Equilibrium shifts to the left removing most H+ ions
H+ + A- = HA
In a buffer solution, what occurs to decrease pH when alkali is added?
[OH-] increases
H+ ions react with OH-
HA dissociates, shifting equilibirum to the right to restore H+ ions
When is a buffer solution most effective?
Equal concentrations of the weak acid and its conjugate base.
A buffer solution is most effective with equal concentrations of weak acid and its conjugate base
Why is this the case?
The pH of the buffer solution is the same as the pKa value of HA.
Operating pH is typically over two pH units, centred at pH of pKa value.
whats the expression of Ka for a weak acid buffer solution?
Ka= [H+(aq)][A-(aq)] / [HA(aq)]
How can the pH of a buffer solution be calculated?
When Ka, [HA] and [A-] are known:
Ka=[H+][A-]/[HA]
How is pH calculated if [A-]=[HA]?
Ka=[H+] as [A-] and [HA] cancel out in the equation
pKa=pH
What is the Henderson-Hasselbalch equation for calculating pH?
pH=pKa + log([A-]/[HA])
Where does blood pH operate between?
7.35 and 7.45
What happens if the blood pH falls out of the right range?
Below - acidosis causing fatigue, shortness of breath and in extreme, death.
Above - alkalosis causing musle spasms and nausea.
What are the significant points on a pH titration curve?
At first, excess of acid, pH increases slowly.
Vertical section, pH increases rapidly on addition of very small volume of base.
Equivalence point, centre of the vertical section.
At the end, excess of base, pH increases slowly.
What is an acid-base indicator?
An acid-base indicator is a weak acid, HA, that has a distinctively different colour from its conjugate base, A-.
what does a strong acid vs strong base titration curve look like? what indicators are suitable?
it starts low cause it a strong acid and ends high cause its a strong base
what does a weak acid vs strong base titration curve look like? what indicators are suitable?
starts higher because its a weak base and it ends higher cause its a strong alkali
what does a strong acid vs weak base titration curve look like? what indicators are suitable?
starts low as its a strong acid and ends low as its a weak base
what does a weak acid vs weak base titration curve look like? what indicators are suitable?
its starts high as its a weak acid and ends low as its a weak base and there’s not much change
