chapter 21 Flashcards
what are buffer solutions?
a system that minimises pH changes when small amounts of an acid or alkali are added
contains weak acid and conjugate base
what is needed to make a buffer solution?
a larger reservoir of both HA and A-
what happens when you add an acid to a buffer system?
[H+] increases initially
large reservoir of A- ions reacts with excess H+
equilibrium moves to the left
[H+] and pH are restored
what happens when you add alkali to a buffer system?
OH- ions react with H+ ions
[H+] decreases
large reservoir of HA molecules dissociate
equilibrium position moves to the right
[H+] and pH are restored
how do you prepare a buffer solution?
- mix solution containing weak acid with a solution that contains a salt of the weak acid
or - add aqueous solution of an alkali to an excess of a weak acid, then partially neutralise weak acid, solution then contains unreacted HA and A- salt
how do you calculate the pH of a buffer?
[H+] = Ka x [HA]/[A-]
what do you change for coarse tuning of pH?
Ka
what do you change for fine tuning of pH?
[HA]/[A-]
what does the blood plasma pH need to be maintained at?
between 7.35 and 7.45
what is the pH in the body controlled by?
the carbonic acid - hydrogencarbonate buffer system
what happens when their is a drop in pH of blood plasma?
[H+] increases
H+ ions react with conjugate base HCO3-
equilibrium moves to the left
[H+] and pH are restored
what happens when their is an increase in pH of blood plasma?
[OH-] increases
OH- ions react with H+ ions
[H+] decreases
H2CO3 dissociates
equilibrium moves to the right
[H+] and pH are restored
what is the purpose of titrations?
to find equivalence point for a reaction (volume of one solution that exactly reacts with the volume of the other solution)
what does the titration curve look like for strong acid and strong base?
vertical section centred at pH 7
what does the titration curve look like for strong acid weak base?
vertical section centred at pH<7
what does the titration curve look like for weak acid strong base?
vertical section centred at pH> 7
what does the titration curve look like for weak acid weak base?
no vertical section
what do acid-base indicators do?
enable us to find equivalence point in a titration without curve
causes colour change which coincides with vertical section of curve
what are examples of acid-base indicators?
methyl orange
phenolphthalein
what are the colour changes for methyl orange?
red —> yellow
HA —-> A-
what are acid-base indicators?
weak acid
distinctly different colour from conjugate bases
what are the colour changes for phenolpthalein?
colourless —> pink
HA ——> A-
what happens when you add a strong base to a strong acid with methyl orange?
initially red due to high [H+]
base added so OH- reacts with H+
weak acid dissociates
equilibrium shifts right
end point of titration when HA=A- and colour is in between two extremes (orange)
what happens when you add acid to a basic solution with methyl orange?
methyl orange initially yellow due to low [H+]
acid is added so H+ ions react with A- ions
equilibrium shifts left
end point when HA=A- (orange colour)
what is true when [HA]=[A-]?
[H+]=Ka
pH=pKa
what is the equivalence point?
when the volume of one solution has reacted exactly with the volume of the second solution
what is the end point of a titration?
when indicator colour changes
[HA]=[A-]
what should you consider when choosing an indicator for a titration?
choose one that has a colour change that coincides with the vertical section of the titration curve
