acids and bases part 2 Flashcards
what is added in an acid-base titration
a solution of a base of known concentration from a burette, to a measured amount of acid
what is used to find the end point in acid-titration
an indicator
how can the concentration of the acid be calculated in an acid base titration
from the volume of base required
what is a titration curve
a plot of pH against volume of base added
how is the pH measured
pH meter
what does the shape of a titration curve depend on
the strength of the acid and base used
key features of titration curve: strong acid and strong base
- low starting pH
- long vertical sector
- high end pH
key features of titration curve: strong acid weak base
- low starting pH
- shorter vertical sector
- lower end pH
key features of titration curve: weak acid and strong base
- higher starting pH
- shorter vertical sector
- high end pH
pH at equivalence point: strong acid and strong base
7
pH at equivalence point: strong acid weak base
5
pH at equivalence point: weak acid strong base
9.5
equivalence point
the point at which the acid and base are in the correct molar ratio
an indicator for an acid base titration
weak acid (HIn)
why do different indicators change colour at different pH values
they have different Ka values
when is the end point of a titration
the point at which the indicator changes colour
for a successful titration, which two points must correspond
equivalence and end point- so that indicator changes sharply at equivalence point
what must the pH range of the chosen indicator correspond to
the region of rapid pH change (vertical portion of pH curve)
buffer
a solution that maintains an approximately constant pH on addition of small amounts of acid/base
applications of buffers
shampoos and biological washing powder
what is an acidic buffer a mixture of
a weak acid and one of its salts
what does an acidic buffer maintain a pH of
below 7
in what two ways can an acidic buffer be made
add together HA and its salt, A-
partly neutralise HA with NaOH
buffer action (HA>
A- reacts with added H+ to create HA
equilibrium shifts to left hand side
buffer action (HA>
OH- reacts with H+ to create H2O- equilibrium shifts to RHS
what concentrations are much greater in buffer solutions
[HA] and [A-] much greater than [H+]
what does the pH of a buffer solution depend on
the Ka of the acid and the [HA]:[A-]
how to work out [H+] in a buffer solution
Ka[HA]/[A-]
why doesn’t volume need to be taken into account in buffer solution calculations
cancel eachother out
the buffer range
the range of pH values which a buffer solution using a particular weak acid can have
what is the buffer range usually for a weak acid
pKa +/- 1
Required practical: pipette 25.0 cm3 of the…
Acid into a tall form beaker and add a few drops of the appropriate indicator
Required practical: place the tall form beaker on top of a
Magnetic stirrer and place the magnet in the beaker
Required practical: fill the bursts with
The base
Required practical: rinse the electrode with distilled water and dip it into the acid in the
Beaker
Required practical: record the pH in a
Results table
Required practical: results table should include
pH of solution and colour of indicator
Required practical: switch on the stirrer and make sure
That the magnet doesn’t strike the electrode when in position
Add 5cm of the
Base from the burette and note both the pH and the indicator colour
Required practical: continue adding the base, taking readings every
5cm3 intervals for first 20cm3
Then every 1cm3 until 23.0cm3
Around equivalence you should add base in 0.5cm3 portions
Return to 1cm3 portions until 30.0cm3
Then 5cm3 portions until 50.0cm3 base added
Required practical: plot a graph of pH against
Volume of base added