chapter 21 Flashcards
what is a buffer solution?
a system that minimises pH changes when small amounts of acid/alkali are added.
components
contain 2 components- a weak acid and its conjugate base, the weak acid HA removes added alkali, the conjugate base A- removes added acid
preparation from a weak acid & its salt
mixing a solution w/ ethanoic acid CH3COOH w/ sodium ethanoate CH3COONa
CH3COOH H+ + CH3COO-
CH3COONa(s) + aq –> CH3COO-(aq) + Na+(aq)
preparation by partial neutralisation of the weak acid
adding aq sol of an alkali eg NaOH, weak acid is partially neutralised forming the conjugate base– mix of salt & unreacted acid
action of a buffer solution
conjugate base removes added acid
1 [H+] increases
2 H+ ions react w/ conjugate base
3 equilibrium shifts to left removing H+ ions
HA H+ + A-
H2O
3 HA dissociates shifting eq position to the right
added alkali
—————->
HA H+ + A-
choosing components for a buffer solution
most effective when equal concs of acid and conjugate base
when [HA]=[A-]
-pH of buffer solution is the same as the pKa value of HA
-operating pH is over 2 units on pH of pKa value
calculating pH of a buffer solution
[H+]= Ka x [HA]/[A-]
buffers in the body- carbonic acid-hydrogencarbonate system
on addition of H+
- [H+] increases
- H+ ions react w/ conjugate base HCO3-
- equilibrium position shifts to left, removing H+ ions
added acid
H+ + HCO3-
on addition of OH- -[OH-] increases -small conc of H+ reactions w/ OH- H+ + OH- --> H2O -H2CO3 dissociates shifting equilibrium to right to restore H+ ions
added alkali
—————————–>
H2Co3 H+ + HCO3-
henderson- Hasselbatch equation
pH= pKa + log [A-]/[HA]
pH titration curve
when base is first added, acid is in excess and pH increases slightly
as vertical section is approached, pH starts to increase more quickly as acid is used up
pH increases rapidly during addition of small amount of base- vertical section
equivalence point- vol of one solution that exactly reacts w/ volume of another- centre of vertical section
acid-base indicators- the end point
an acid- base indicator is a weak acid that has a distinctively different colour from its conjugate base eg methyl orange- weak acid is red, conjugate base is yellow
at end point, indicator contains equal concs of HA and A– in between 2 colours
choosing the indicator
use an indicator that has a colour change which coincides w/ vertical section of curve
