1.12 acid base equilibria Flashcards
what is a bronsted-lowry acid ?
- a substance that can donate a proton
what is a bronsted-Lowry base
- a substance that can accept a proton
how can we calculate pH?
pH = - log [ H + ]
how many d.p should we give pH values to?
- two decimal places
what do strong acids do?
- completely dissociate
how can we find concentration of H+ from pH?
1 x 10 ^ - pH
what is the expression for Kw?
Kw = [ H+ (aq) ] [ OH- (aq) ]
why can we assume [ H20 (l) ] is constant
- concentration is much bigger than the concentration of the ions
what is the value for Kw at 298K
1.00 x 10 ^ -14
why are pure water solutions neutral?
[ H+] = [ OH- ]
concentration of H+ is equal to the concentration of OH-
what is the equation of Kw for pure water solutions?
Kw = [ H+ (aq) ] ^2
what effect does increasing the temperature on the dissociation of water have on the pH?
- forwards reaction is endothermic
- increasing temp causes equilibrium to shift to RHS, to oppose the increase in temp
- this increases concentration of [ H + ]
- so lower pH
what do strong bases do
- completely dissociate into their ions
how can we calculate the pH of a strong base?
- we are normally given the concentration of hydroxide ion
- rearrange with Kw equation, to find H
- use pH equation
what do weak acids do
- only slightly dissociate in water
- give an equilibrium mixture
what is the expression for weak acids dissociation?
Ka = [ H+ ] [ A- ] / [ HA ]
whats the relationship between Ka and acids
- the larger the Ka the stronger the acid
how can we calculate Ka from pKa?
Ka = 10^-pKa
what two assumptions are made when calculating pH of a weak acid
[ H+ ] = [ A- ] , because they have dissociated in a 1:1 ratio
- as the amount of dissociation is small, assume that initial concentration of the undissociated acid has remained constant
how can we calculate pH from strong acid and strong base neutralisations?
- work out moles of original acid, so moles of H+
- work out moles of base added, so moles of OH-
- work out which one is in excess
in strong acid and strong base neutralisations, what happens if excess acid is present?
new conc of H+ = moles excess / total volume (dm3)
pH = - log [ H+ ]
in strong acid and strong base neutralisations what happens if base is in excess
[ OH ] = moles excess OH / total volume
[ H+ ] = Kw / [ OH- ]
pH = - log [ H+ ]
what happens if the bases / acids are diprotic ?
- double the moles of the acid
- double the moles of the base
in weak acids and strong base neutralisations what happens if excess alkali is present
- use same method as strong acid strong base
in weak acid and strong base neutralisations what happens if excess acid is present?
[ HA ] = inital moles of HA - moles of OH / tv
[ A- ] = moles of OH added / total volume
rearrange Ka = [H] [A] / [HA]
pH = - log [ H ]
how do you work out the pH of a weak acid at half equivalence
[ H + ] = Ka
- we can assume [ HA ] = [ A- ]
how can we calculate the pH of a diluted strong acid ?
[ H+ ] = [ H+ old ] x old volume / new volume
pH = -log[H]
remember to add the volumes for the new volume
what is a buffer solution?
- a solution where the pH does not change significantly if small amounts of acid or alkali are added to it
how can i calculate the pH of a diluted base?
new OH = OH x old volume / new volume
H+ = Kw / OH
pH = -log[H+]
what is an acidic buffer solution made from?
- a weak acid and a salt of that weak acid
what is a basic buffer solution made from?
- a weak base and a salt of that weak base
explain why the pH stays fairly constant when small amounts of alkali/acid are added
- are there is a large concentration of salt ion in the buffer, the ratio or acid / salt stays almost constant
how can we calculate the pH of making a buffer by adding salt solution / solid salt
- find moles of both solutions
- use given Ka
[H+] = Ka [ HA ] / [ A- ]
we can enter moles straight away as they have same volume
how can we calculate the pH of a buffer made by adding sodium hydroxide
- calculate moles of solutions
- find moles in excess
- moles excess / total volume = conc of acid
- rearrange Ka
- moles of OH added / total volume = conc of salt
whats the steps for constructing a pH curve
- transfer 25cm^3 of acid to a conical flask
- measure initial pH
-add alkali in small amounts - stir mixture to equalise pH
- measure and record the pH
- repeat steps 3-5
- add until alkali in excess
what are the 4 main types of pH curves
- strong acid strong base
- strong acid weak base
- weak acid strong base
- weak acid weak base
what is the pH at equivalent point of a strong acid and strong base curve
pH = 7 at equivalence point
what are the key points to sketching a curve
- inital and final pH
- volume at neutralisation
- general shape
what is the pH at equivalence point for weak acid strong base
pH > 7
what is the pH at equivalence point for strong acid weak base curves
pH < 7
what is an indicator
- changes colour from Hln to ln- over a narrow range.
- different indicators change colours over different ranges
what is the equilbrium for indicators?
Hln <> ln- + H+
what happens at low pH
low pH means high concentration of [H+]
so equilbrium favors left hand side of equation ( colour A ) is present
what happens at high pH
at high pH, low concentration of [H+]
equilbrium favors right hand side, so colour B is present
what happens at transition point
[Hln] ≈ [ln-]
both colours may be visible as a mixture
what is the colour change for methyl orange and over what pH range?
red to yellow
around 3.1 - 4.4
what is the colour change of phenolpthalein and over what pH range?
colourless to pink
around 8.3 - 10.0
