3.1.12 Acids & Bases Flashcards
what is meant by a ‘Bronsted-Lowry acid’?
proton donor
what is meant by a ‘Bronsted-Lowry base’?
proton acceptor
what makes an acid strong? what equation can be used to show this?
- strong acid fully dissociates/ionises in solution to release a high [H+]
- HA + H2O -> H3O+ + A-
(H3O+ = H+; can be used interchangeably)
what makes an acid weak and what equation can be used to show this?
- weak acids partially dissociate in solution to release low [H+]
- HA + H2O ⇌ H3O+ + A-
how to name conjugate acids and bases?
HA + H2O -> H3O- + A-
- HA = acid
- H2O = base
- H3O- = conjugate acid
- A- = conjugate base
how to calculate pH or [H+]?
pH = -log[H+]
[H+] = 10 to the power of -pH
when calculating the pH of strong acids, what do you need to remember?
[H+] = HA because there is full dissociation
when calculating the pH of weak acids, what assumptions are made?
- initial conc. of acid = equilibrium conc. of acid
- @ equilibrium, [H+] = [A-]
how do you calculate ka (acid dissociation constant)?
[H+][A-]/[HA]
- when it is a weak acid [H+] = [A-]
what is ‘pka’ and how do you calculate it?
pka determines the strength of an acid
- pka = -log(ka)
- ka = 10 to the power of -pka
how do you use ka and pka to find strengths of weak acids?
- the higher the ka value, the stronger the weak acid because the equilibrium lies more to the right so there is a higher [H+]
- pka is the inverse of ka, so the higher the pka value the weaker the acid
what is the formula for kw (ionic product of water)?
kw = [H+][OH-]
what is the kw of water at standard temp. & pressure?
1x10 to the power of -14
units: mol²dm^-6
why does the pH of water change with temperature?
H2O ⇌ H+ + OH-
- when temperature increases, equilibrium position shifts to the endothermic side (forward reaction is favoured)
- [H+] increases, so pH decreases
why is water neutral at all pHs?
because [H+] = [OH-]
how is an acid buffer made?
- react excess acid with strong alkali e.g. NaOH
- all the NaOH is neutralised so the remaining acid and salt is left in the buffer
what is a buffer solution?
a solution that resists changes in pH when small amounts of acid and alkali are added
what does a buffer consist of?
a mixture of a weak acid and the salt of its conjugate base
how do buffer solutions work? use the symbol equation as an example:
CH3COOH ⇌ CH3COO- + H+
- **added OH- **-> pH will increase
- the added OH- will react with the H+ (H+ and OH- -> H2O)
- the equilibrium position will shift to the right so [H+] increases so pH decreases back down.
- added H+ -> pH will decrease
- equilibrium position will shift to the left so [H+] decreases, so pH increases back up.
how do you obtain a pH curve? (explain the calibration process)
- rinse pH probe w/ distilled water
- dip pH probe into a buffer solution
- wash pH probe in distilled water AGAIN
- dip pH probe in buffer solution w/ a different pH
- calibrate a pH probe and use to measure the initial pH of the alkali in the conical flask
how do you obtain a pH curve? (explain the steps post-calibration)
- add acid from the burette in 2cm³ increments and swirl the mixture
- record pH after every addition; reduce size of the portions of the increments close to the endpoint because the pH would change suddenly
- repeat until the acid is in excess
- plot a graph of pH vs. volume (of acid/alkali)
what does a strong acid + strong base pH curve look like? (e.g. HCl and NaOH)
- very large vertical section
- the pH at the equivalence point is 7 because there is full dissociation for both acid + alkali so [H+] = [OH-]
what does a strong acid + weak base pH curve look like? e.g. HCl and NH3
- relatively large vertical section
- pH at equivalence point is LESS than 7. acid has full dissociation and base has partial dissociation, so [H+] > [OH-]
what does a strong base + weak acid pH curve look like? e.g. NaOH + HCOOH
- equivalence point pH >7
- [OH-] = full dissociation; [H+] = partial dissociation
- vertical section is relatively small
what does a weak acid + weak base pH curve look like? e.g. NH3 and HCOOH
- no vertical section due to little dissociation
how does an indicator work in the presence of an acid?
H(In) ⇌ H+ + In-
(colour A) (colour B)
- equilibrium position shifts to the left because added H+ reacts with In-
- colour A is observed
how does an indicator work in the presence of an alkali?
H(In) ⇌ H+ + In-
(colour A) (colour B)
- the OH- reacts w the H+ so equilibrium position shifts to the right
- colour B is observed
when does the endpoint occur?
- indicator contains equal [H(In)] and [In-].
- the colour between the 2 extreme colours of the indicator are observed
what is meant by ‘equivalence point’?
volume of one solution that exactly reacts with the volume of another solution
when should an indicator change colour?
it should change colour at a pH range that lies within the vertical section of the pH curve.
- they change colour within a range of minimum of 2 pH units.
why is there no indicator for a weak acid + weak base?
there is no vertical section.
how do you use pH curves to determine ka/pka of a weak acid?
- at 1/2 equivalence point, [A-] = [HA], meaning ka = [H+]
(4) explain how a pH curve can be used to determine the ka of an acid.
- find equivalence point on the pH curve and read off hte volume added.
- half this value to find 1/2 equivalence point
- read off the pH at 1/2 equivalence point - this is the pka
- ka = 10 to the power of -pka
what are the uses of buffers?
- cosmetic industries - prevents changes in pH so prevents skin irritation
- blood buffer (made up of H2CO3 + hydrogencarbonate)
explain how the blood maintains a pH of 7.4 after vigorous exercise.
H2CO3 ⇌ H+ + HCO3-
- H+ will increase (build up of lactic acid), resulting in a fall in pH, so equilibrium position will shift to the left to oppose the change
- [H+] will decrease, which increases pH back up to 7.4
explain why the concentration of water is not used to calculate ka.
[H2O] is always constant
