Titration Curves Flashcards
How to find titrations curves?
Titrate the acid/alkali together: add 2cm3nof alkali in the burette to a conical flask with 25cm3 weak acid
Measure pH using meter each time, stir mixture to ensure solution has same pH
Until reaching the endpoint of colour change and measure pH when this is reached
Continue till in excess
Plot a graph to show variation
Strong acid/strong base graph character
Straight line at very low acid pH then sharp increase which levels off to a straight line at very high pH
Vertical part = midpoint if this is equivalence point when [H+] = [OH-] at 7
Vertical part on titration curve
All the [H+] has been neutralised as it equals the concentration of OH- so adding very small volumes of OH- rapidly increases the pH of the solution
Aka end point
Strong acid weak base titration curve characteristics
Starts at very low strong acid pH
Smaller end point (vertical line) that does not level off at a very high pH because a weak base is being added (favours the bottom of the graph)
Equivalence point in strong acid strong base
7
Equivalence point in strong acid weak base
Under 7
Weak acid/strong base curve characteristics
Not a very low acidic starting pH
Small end vertical point that levels off at a very high pH as a strong acid is used
Equivalence point is over 7
Equivalence point of weak acid/strong base
Over 7
What is formed when strong base is added to a weak acid?
A buffer region: resists change in pH as the strong base reacts with weak acid to form the weak acids salt
Which dissociates to form carboxylate ion: large concentration
So when more base is added, the position of equilibrium shifts so more of the weak acid dissociates to produce more H+ to neutralise added OH- (until too much added and no longer works)
The volume at half the equivalence point
Is called the half neutralisation point because half of the acid has reacted with OH-
So half of weak acid is still remaining: half that does react is the weak acid’s salt formed
The pH at the volume of the half neutralisation point is equal to…
pKa value
Because [HA] = [A-] So these cancel in Ka expression
Ka = [H+] and -log both sides
pKa = pH
Weak acid weak base
Very small equivalence point starts at weak acid pH and ends at weak acid pH
Indicators equilibrium equation
HIn <——> In- + H+
When is a colour change seen in an indicator?
When [HIn] = [In-]
So we need an indicator which this point will coincide with the neutralisation point (vertical line) of the titration
What can titration curves be used for?
To find the value of Ka of a weak acid
Finding the Ka of a weak acid-strong base titration
The pH at the half neutralisation point is equal to pKa
So 10^-pka = Ka value
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