AQA A Level Chemistry - Acids and Bases

define acid (in bronsted lowry terms)

proton donor

define base

proton acceptor

what is a strong acid?

one where the H+ ions fully dissociate

what are the strong acid dissociation equations?

• HA → H+ + A-

• HA + H2O → H3O+ + A-

• diprotic: H2SO4 → HSO4 - + H+

  • HSO4 - → SO4 - + H+

what are the common strong acids?

• HCl

• HNO3

• H2SO4

what are acids commonly composed of?

• always hydrogen

• bonded to an electronegative atom like O, F, Cl

• if a compound has multiple hydrogen atoms, only the atoms directly bonded to an electronegative atom will dissociate (i.e. those bonded to C cannot)

what are weak acids?

those where the H+ ions are only partially dissociated

what are the weak acid dissociation equations?

• CH3COOH/HA + (aq) ←→ H+ + CH3COO- (aq)

• HA ←→ H+ + A-

• HA + H2O ←→ H3O+ + A-

what are the common weak acids?

• CH3COOH (ethanoic acid)

• other carboxylic acids

what is the difference between a base and an alkali?

• base - accepts protons

• alkali - base that donates OH- ions in aqueous solution

• all alkalis are bases, not all bases are alkalis

what are strong bases?

those that are fully dissociated in aqueous solution (OH- fully)

what are some examples of strong bases?

• KOH

• NaOH

what is the base dissociation equation for a strong base (NaOH)?

NaOH → Na⁺ + OH^-

what are weak bases?

those that partially dissociate in aqueous solution

what is the most common weak base?

NH3

what is the base dissociation equation for NH3?

NH₃ + H₂O ←→ NH₄⁺ + OH^-

what is the general equation for a neutralisation reaction?

acid + base → salt + water

how will neutralisation reactions of weak and strong acids/bases differ?

• those with one weak element will be slower than with one strong

• however, if it is the same concentration and volume of a strong/weak acid reacting with constant base, they will both neutralise it completely as the weak acid will just continually slowly dissociate

define pH

• logarithmic scale of H+ concentration

• 10x increase in concentration when you go down by 1 pH

what is the equation for pH?

• pH = -log [H⁺]

• [H⁺] = 10^-pH

how does water dissociate?

• H2O ←→ H⁺ + OH^-

• slight dissociation - compared to concentration of water (which is very concentrated) it is negligible/basically 0

• always equal of H+ and OH- in water

what is the ionic product of water?

• equilibrium constant for dissociation of water

• Kw = [H+][OH-]

• units of mol2dm-6

• holds true for solutions of acids/bases in water

how can the ionic product of water be represented in pure water?

• Kw = [H+]²

• as pure water is always neutral (but not always pH 7) so [H+] = [OH-]

how does the pH of pure water vary?

• varies with temperature

• at 25C, pH is 7

• as temperature increases, pH decreases and Kw increases but this is still neutral pH

• because dissociation of water is endothermic

what is Kw at 25C?

1×10-14

how would you calculate the pH of a strong base?

• use Kw and [OH-] to calculate [H+] as dissolved in water

• and [alkali] = [OH-] if monobasic

• then sub into pH equation

• also pKw = -logKw

what is Ka?

acid dissociation constant

what is the equation for Ka?

Ka = [H+][A-]/[HA] moldm-3

how can the Ka equation be simplified for weak acids?

• at equilibrium Ka = [H+]_eq[A-]_eq/[HA]-[H+]_eq

• assume H+ and A- are formed in equal amounts, H+ produced by dissociation of water is negligible; the dissociation of weak acids is so small and [HA] so great that [HA]eq = [HA]start

• therefore Ka = [H+]² /[HA]

how would you work out pH of weak acids and pKa?

• use Ka equation; Ka= [H+]² /[HA]

• then pH equation

• pKA = -logKa

how is Ka found for a specific acid?

experimentally using standard solutions of HA and a pH meter

what is the equation to help with pH dilution calculations?

C1V1 = C2V2

what is a titration curve?

graph of pH against volume of acid/base added - shape differs depending on whether it’s a strong or weak acid/base

explain the shape of this general titration curve

• 1 - slight rise in pH; as acid is in great excess, so increases in [OH-] do not alter pH much and pH rises slower

• 2 - sharp rise in pH: pH increases rapidly when a small amount of base is added as it swings from excess of acid to excess of base

• in this region, the equivalence point occurs - when [H+]=[OH-] - midpoint of vertical section

• can be calculated as volume of substance being added required to neutralise substance of known volume

• 3 -slight rise: little additional change in pH as base is in great excess

what does a strong acid/strong base titration curve look like?

vertical range 3-10

what does a strong acid/weak base titration curve look like?

vertical range 3-8

what does a weak acid/strong base titration curve?

vertical range 6-10

what does a weak acid/weak base titration curve look like?

no vertical range

what are pH indicators?

• weak acids where HIn (acid) is 1 colour and In- is another

• HIn ← → H+ + In-

• when [HIn] = [In-], end point has been reached

• most indicators change colour over roughly 2pH, with the endpoint being in the middle

• this means that different indicators are used for different titrations as the equivalence point for titration needs to be in the pH range/close to indicator endpoint

what are the respective colours of phenolpthalein?

• acid - colourless

• alkali - pink

what are the respective colours of methyl orange?

• acid - red

• alkali - yellow

what are the respective colours of thymol blue?

• acid - red

• alkali - yellow

what are the respective colours for bromothymol blue?

• acid - yellow

• alkali - blue

what are the respective colours of methyl red?

• acid - red

• alkali - yellow

what is the endpoint pH range for phenolpthalein?

8.3-10

what is the endpoint pH range for methyl orange?

3.1-4.4

what is the endpoint pH range for thymol blue?

1.2-2.8

what is the endpoint pH range for bromothymol blue?

6-7.6

what is the endpoint pH range for methyl red?

4.4-6.2

what types of titrations is phenolphthalein suitable for?

• strong strong

• weak acid strong base

what types of titrations is methyl orange suitable for?

• strong strong

• strong acid weak base

what types of titration is thymol blue suitable for?

none

what types of titration is bromothymol blue suitable for?

• strong strong

• strong acid weak base

what types of titration is methyl red suitable for?

• strong strong

• strong acid weak base

how would you measure pH in a weak- weak titration?

pH probe as no vertical region

what is a buffer solution?

a solution that can resist changes in pH when small amounts of acid or base are added, or there is slight dilution

what is an acidic buffer made of?

• weak acid and conjugate base

• conjugate base either added directly via salt, or NaOH added to react with acid to produce it

how does an acidic buffer work if a small amount of acid is added?

• extra H+ ions have been added

• the position of equilibrium in HA ←→ H+ + A- shifts to the left to remove the added H+ ions

• maintains constant pH

• large amount of conjugate base means decrease in A- is insignificant

how does an acidic buffer work if a small amount of base is added?

• extra OH- have been added

• these react with H+ ions in solution removing some

• the position of the HA ←→ H+ + A- equilibrium shifts to the right

• replaces H+

• maintains pH

• HA + OH- → A- + H2O

how is a basic buffer solution formed?

• weak base and its salt

• almost always ammonia

how does an basic buffer work if a small amount of acid is added?

• added H+ are removed by reacting with NH3

• forming ammonium

• NH3 + H+ ←→ NH4+

• equilibrium shifts right

how does an basic buffer work if a small amount of base is added?

• OH- ions react with some of the H+ ions in the buffer to form water

• H+ + OH- → H2O

• equilibrium shifts left to replace the H+

how do you find pH of an acidic buffer?

• use Ka = [H⁺][A^-]/[HA]

• where H+ is what you are finding

• Ka is of weak acid

• A- is from fully ionised salt

• HA is weak acid

what are some considerations when finding pH of acidic buffers?

• if finding H+, can use moles of A- and HA as it is the ratio that is important not concentration

• may have volume and solid, volume and volume - in this case volumes dilute each other so must be included/considered

• extra equation for buffer - pH = pKa + log (X-/HX)

how do you calculate new pH when acid/base added to buffer solution?

find:

• mol HA start

• mol A- start

• mol H+ added (or OH)

• mol HA after

• mol A- after

• [H+]

• pH

what are some uses of buffers?

• blood is a pH buffer and removes dissolve CO2

  • H+ + HCO3- ←→ CO2 + H2O

  • extra H+/CO2 is removed by equilibrium moving

• shampoos have buffers to prevent being too acidic/basic and damaging hair/skin