C21 Buffers And Neutralisation Flashcards
What does the pH of a buffer depend on?
pKa of weak acid
Ratio of conc of weak acid and its conjugate base
Define buffer solution
Mixture of weak acid and its conjugate base which minimises changes in pH when small quantities of acid or alkali are added.
What does a buffer contain?
Weak acid and conjugate base
Define conjugate acid-base pair
Contains 2 species that can be interconverted by the transfer of a proton
When is a buffer most effective?
[HA] = [A-]
pH of buffer solution = pKa of HA
How can a buffer be prepared?
Mixing weka acid with solution of its salt
Partially neutralising solution of weak acid
What are the 2 ways in which buffers can be made
Weak acid and its conjugate base
Weak acid and a strong alkali
In which direction does the equilibrium shift when an acid is added to a buffer solution and why
Equilibrium shifts to the left because [H+] increases and the conjugate base reacts with H+ to remove most of the H+
In which direction does the equilibrium shift when an alkali is added to a buffer solution and why
Equilibrium shifts to the right, because [OH-] increases and the small conc of H+ reacts with OH- to restore the H+ ions HA dissociates shifting the equilibrium
Which buffer system maintains blood pH at 7.4? What happens when acid/alkali is added
H^+ + HCO3^- <==> CO2 + H2O
Add OH- —> reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost, add H+ —> equilibrium shifts to the right, removing excess H+
What is a titration
The addition of an acid/base of known concentration to a base/acid to determine the concentration. An indicator is used to show that neutralisation has occurred, as is a pH meter.
Define the term equivalence point
The point at which the exact volume of base has been added to just neutralise the acid, or vice-versa
What’s the end point
The point at which pH changes rapidly
What are the properties of a good indicator for a reaction
Sharp colour change (not gradual) - no more than one drop of acid/alkali needed for colour change
End point must be the same as the equivalence point otherwise titration gives wrong answer
Distinct colour change so it is obvious when the end point has been reached
What indicator would you use for a strong acid-strong base titration
Phenolphthalein or methyl orange, but phenolphthalein is usually used as clearer colour change
What indicator would you use for a strong acid-weak base titration
Methyl orange
What indicator would you use for a strong base-weak cid titration
Phenolphthalein
What indicator would you use from a weak acid-weak base titration
Neither methyl orange or phenolphthalein is suitable, as neither give a sharp change at the end point
What colour is methyl orange in acid? In alkali
Red in acid
Yellow in alkali
What colour is phenolphthalein in acid? In alkali?
Colourless in acid
Red in alkali
What colour is bromothymol blue in acid? In alkali?
Yellow in acid
Blue in alkali
Describe how to use a pH metre
Remove the pH probe from storage solution and rinse with distilled water
Dry with probe and place it into the solution with unknown pH
Let the probe stay in the solution until it gives a settled reading
