Buffers Flashcards
What is a buffer solution?
A system that minimises pH changes when small amounts of an acid or base are added.
What are the 2 components of a buffer solution?
A weak acid (HA)
Its conjugate base (A-)
What are the 2 ways to prepare weak acid buffer solutions?
Preparation from a weak acid and its salt.
Preparation by partial neutralisation of the weak acid.
How do you prepare a buffer solution from a weak acid and it’s salt?
Mix a solution of weak acid (eg ethanoic acid) and one of its salts (eg CH3COONa).
When the acid is added to water, the acid partially dissociates. Weak acid component of the buffer.
CH3COOH (aq) H+(aq) + CH3COO- (aq)
Salt of the weak acid is the conjugate base part of the buffer. When added to water, they completely dissolve.
CH3COONa(s) + aq -> CH3COO-(aq) + Na+(aq)
How do you prepare a buffer solution from partial neutralisation of the weak acid?
Adding an aqueous solution of alkali (eg NaOH) to an excess of weak acid. Weak acid partially neutralised by alkali forming the conjugate base. Some weak acid is left over unreacted. Resulting solution contains mixture of salt of the weak acid and any unreacted weak acid.
How can a buffer act as 2 reservoirs to act independently to remove added acid or alkali?
HA(aq) H+(aq) + A-(aq)
Position of equilibrium can shift left to remove acid (H+) and right to remove alkali (OH-).
How does changing the weak acid effect the buffer solution it produces?
It changes the pH ranges that buffer works at
When is a buffer most effective?
When there are equal concentrations of weak acid and it’s conjugate base.
[HA] = [A-]
When [HA] = [A-] what is true about the pH?
pH = pKa value of HA
How many pH units is the operating pH usually over?
2, centred at the pH/pKa value (pH=pKa)
How do you find the ratio of concentrations of the weak acid and it’s conjugate base?
[HA] / [A-]
How do you work out Ka?
([H+] X [A-]) / [HA]
How do you work out pH using the Ka expression?
Rearrange to [H+] = Ka X ([HA] / [A-])
pH = -log [H+(aq)]
What pH does blood need to be at?
Between pH 7.35 and 7.45
How is pH in the blood maintained?
A mixture of buffers such as the Carbonic acid - hydrogencarbonate buffer system.
What happens if the pH of the blood drops below 7.35?
Called acidosis, causes shortness of breath, fatigue, maybe shock or death.
What happens if the pH of blood rises above 7.45?
Called alkalosis, causes muscle spasms, lightheadedness and nausea.
How does the carbonic acid - hydrogencarbonate buffer system work?
( H2CO3(aq) H+(aq) + HCO3-(aq) )
H+ added, react with conjugate base HCO3-(aq), equilibrium shifts left to remove most H+.
OH- added, small [H+] react to form H2O, H2CO3 dissociates shifting equilibrium right to replace H+ lost
What are the 2 ways to measure changes in pH during an acid-base titration?
Using an indicator
pH meter
What are the key features of a pH titration curve?
When the base is first added, the acid is in excess so pH rises very slightly. As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quickly.
pH increases quickly (vertically) only drops are needed for the whole vertical section.
After vertical section, pH only increases slowly again as base is in excess.
How is the graph different if you add an acid to a base?
The shape of the curve is the same it just starts from a high pH and gets lower.
What is the equivalence point?
The volume of one solution that exactly reacts with the volume of another solution.
The vertical section of the graph.
What is the end point?
When the indicator contains equal concentrations of HA and A- and the colour will be in between the 2 extremes of colour.
What is an acid-base indicator?
A weak acid, HA, that has a distinctly different colour from its conjugate base, A-.
