Acids And Bases Flashcards
BL acid definition
proton donor
BL base definition
proton acceptor
pH =
-log[H+] (2dp)
[H+] from pH =
10^-pH
Kw =
[H+][OH-]
Kw at 25°c
10^-14 mol2dm-6
steps to calculate pH of a strong acid
calculate conc of H+
apply pH formula
steps to calculate pH of strong base
calculate conc of OH-
calculate conc of H+ using Kw equation
apply pH formula
Ka=
[H+][A-]/[HA]
larger Ka = ______ acid
stronger
pKa=
-logKa
2 assumptions made when calculating the pH of a weak acid
[H+] = [A-] because they dissociated in a 1:1 ratio
As the amount of dissociation is small, assume that the initial conc of the acid remains constant so [HA(initial)] = [HA(eqm)]
Ka = (2 assumptions ver)
[H+]^2/[HA]
steps to calculate pH of weak acid solution with conc and Ka given
use modified Ka formula
rearrange to find H+ conc
apply pH formula
Steps to calculate the concentration of a weak acid solution with pH and Ka given
calculate the [H+] of the solution
use modified Ka equation to find [HA]
Finding pH with strong acid and strong base neutralisation steps
If XS acid:
[H+] = moles XS H+/total volume dm3
pH equation
if XS base:
[OH-] = moles XS OH-/total volume dm3
[H+] = Kw / [OH-]
pH equation
Finding pH with weak acid and strong base neutralisation steps
if XS acid:
[HA] = initial mol HA - mol OH-/ total volume dm3
[A-] = moles OH- added/ total volume dm3
rearrange normal Ka equation for [H+]
pH equation
if XS base:
find OH- conc
use Kw to find H+ conc
calculate pH with H+ conc
Working out pH of a weak acid at half equivalence steps
- Ka = [H+][A-]/[HA]
- Assume [HA]=[A-]
- [H+] = Ka
- pH = pKa
what is the half equivalence point for a weak acid
weak acid reacts with exactly half the neutralisation volume of base
pH of diluted acid
[H+]=[H+] x old vol/new vol
use pH equation
pH of diluted base
[OH-]=[OH-] x old vol/new vol
use Kw to find [H+]
use pH equation
buffer solution meaning
pH doesn’t change significantly if small amounts of acid or base is added
what’s an acidic/basic buffer solution made of?
weak acid
salt of that weak acid
or
weak base
salt of that weak base
What happens if small amounts of acid is added to an acidic buffer eg in an ethanoic acid buffer
- the ethanoic acid equillibria shifts to the left using nearly all H+ added
- large salt ion conc means ratio of weak acid to its ion stays almost constant so pH stays fairly constant
What happens if small amounts of base is added to an acidic buffer eg in an ethanoic acid buffer
- OH- ions will react with any H+ present to form water
- eqm of ethanoic acid shifts to the right to produce more H+ ions
- ratio of weak acid to its ion stays almost constant, so pH stays fairly constant
What happens if small amounts of acid is added to a basic buffer
- H+ reacts with OH- ions in solution
- These can be reproduced from a high concentration of water to counteract the change
- eqm shifts to replace OH- ions
What happens if small amounts of base is added to a basic buffer
- OH- ions react with NH4+ ions in solution
- high conc of NH4+ from salt
- NH3 and H2O is produced so eqm shifts to the left
ammonia and ammonium chloride buffer equations
NH3 + H2O <=> NH4+ + OH-
NH4+ + Cl- <=> NH4Cl
steps to calculate a buffer’s pH given acid conc, salt mol and volume, and Ka
- write out standard Ka expression (eqm concs only)
- Assume salts dissociate fully and weak acids poorly so [salt]=[A-] and [HAinitial]=[HAeqm]
- Rearrange Ka to get [H+]
- calculate salt value and use in Ka equation
- pH equation
why can’t the modified Ka expression be used with buffers?
Cannot confirm [H+]=[A-]
Calculating pH change of a buffer steps when given vol and conc of acid and vol of buffer and concentration of weak acid and salt in the buffer, and find pH and Ka for the weak acid
- calculate mol of weak acid, salt and acid in buffer before they are mixed using c=n/v
- increase mol of HA by mol of H+ present
- decrease mol of A- by mol of H+ present
- find conc of HA and A- by dividing new mol by total volume (acid + buffer volume)
- use Ka formula
- use pH formula
3 uses of buffers
shampoo (mildly acidic) protecting hair that becomes damaged in alkaline conditions
washing powder has enzymes that need optimum pH
blood needs a 7.4 pH for enzymes of metabolic reactions
Strong acid strong base titration curve (pH vs cm3 of base added to acid)
has long steep part from 3-9
initial pH 1
final pH 13
strong acid weak base titration curve
initial pH 1
end pH 10
steep part from 4-7
weak acid strong base titration curve
initial pH around 3
end pH around 13
steep part between 7-9
weak acid weak base titration curve
just a lumpy upwards slope
when will an indicator work?
when the indicator’s pH range lies on the steep part of the titration curve
the indicator will change colour rapidly
when to use phenolphthalein and colour change
strong bases NOT weak bases
colourless to pink alkali
when to use methyl orange and colour change
strong acids NOT weak acids
red acid to yellow alkali (orange end point)
