21 - buffers and neutralisation Flashcards
what is a buffer
System that minimises pH changes when small amounts of an acid or base are added
what is a buffer made from
o Weak acid, HA = removes added alkali
o Conjugate base, A- = removes added acid
how to prepare a buffer using two solutions
resulting solution of ethanoate buffer - needed for calculations
relatively high concentrations of CH3COOH (due to partial ionisation of ethanoic acid) and CH3COO- (due to full ionisation of sodium ethanoate)
Preparation by partial neutralisation of the weak acid
- adding an aqueous solution of an alkali - NaOH - to EXCESS weak acid. (needed for calculations)
- Weak acid is partially neutralised by alkali, forming conjugate base
- Some of weak acid is left over, forming weak acid component
Resulting solution of partial neutralisation - needed for calculations
mixture of salt of weak acid and unreacted weak acid
how does a buffer remove added acid
- If you add more acid, H+:
- [H+] increases
- H+ ions react with conjugate base A-
- Equilibrium position shifts to the left removing most of H+ ions
how does a buffer remove added alkali
- If you add more alkali, OH-:
- [OH-] increases
- Small concentration of H+ ions react with OH- ions to form water - H+ conc decreases
- HA dissociates shifting equilibrium to right to restore most of H+ ions
when is a buffer most effective
when there are equal concentrations of weak acid and its conjugate base
pH of buffer solution
same as pKa value of HA
operating pH of buffer
two unit range across pKa of HA
why is calculating pH of buffer different to calculating pH of weak acid
with the weak acid calculation we assume that [H+] = [A-], but for a buffer solution, this isn’t true for buffer as A- has been added as one of the components of the buffer.
how to work out pH of buffer
calculating pH of buffer made from weak acid + salt - first method
- find number of moles of acid and conjugate base
- calculate concentrations in total buffer solution = would need total volume
- use Ka formula = rearrange for H+
calculating pH of buffer made from partial neutralisation - second method
- find number of moles of added base and use that to find number of moles for conjugate base
- find number of moles added of acid
- minus number of moles of added base from number of moles of acid as acid was in excess
- now have number of moles left for weak acid and moles of conjugate base
ESSENTIALLY
- find number of moles of acid and conjugate base
- calculate concentrations in total buffer solution = would need total volume
- use Ka formula = rearrange for H+
buffers in body
carbonic acid-hydrogen carbonate H2CO3/HCO3-
H2CO3 dissociates into H+ and HCO3-
Carbonic acid-hydrogen carbonate buffer system - added acid
Carbonic acid-hydrogen carbonate buffer system - added alkali
draw and label neutralisation curve
describe neutralisation curve
- when base first added, acid in excess and pH increases very slightly, as vertical section approached, pH starts to increase as acid is used up more quickly by base.
- pH increases rapidly during addition of very small volume of base, hence vertical bit formed on graph, only drops of solution needed for this part
- After this, pH will rise slightly as bas now in great excess
equivalence point
volume of one solution that exactly reacts with volume of the other solution
how does methyl orange work
- Weak acid HA = red
- Conjugate base A- = yellow
- At end point of titration, equal concentrations of HA and A- hence end point colour is orange
if indicator added to an initially acidic solution and base is added later
if indicator added to an initially basic solution and acid is added later
what happens to pH at end point
how to choose an indicator
must use indicator thats colour change that would occur over the vertical section of the graph
End point should coincide with equivalence point
explain 4 assumptions that you make when calculating the pH of a buffer when adding a weak acid + salt of that acid
The volume of solution does not expand when the sodium methanoate is added.
The H+ from the water can be ignored.
At equilibrium the concentration of the methanoate ions is the same as the initial concentration of the sodium methanoate.
At equilibrium the concentration of the methanoic acid is the same as its initial concentration.
describe the procedure that is required to use a pH meter
The probe of the pH meter must be calibrated using (buffer) solutions of known pH.
The temperature setting of the pH meter must match the temperature of the solution whose pH is to be measured.
The probe must be placed vertically in the solution and the pH should not be read until it is steady.
The probe should be washed with de-ionised (distilled) water before another measurement is made
how to draw a titration curve
- concentration of the acid
- concentration of the base
- the volume that is needed to exactly neutralise one solution by the other
