Acids and bases Flashcards
acidic ion
H+
alkaline ion
OH-
acid definition
proton donor
base definition
proton acceptor
weak acid definition
partially dissociates
strong acid definition
dissociates fully
pH definition
pH= - log [H+]
acid dissociation equation for ethanoic acid
Ka= [CH3COO-][H+]
[CH3COOH]
[H+] from pH
[H+] = 10-pH
dissociation of water equation
H2O ⇌ H+ + OH-
Kw of pure water
1 x10-14 mol2dm-6 (at 298K)
when is Kw used
to find [H+] from [OH-]
why does Kw increase with temp.
the dissociation of water is endothermic so increasing temp. increases dissociation
neutrality of water
pure water is neural at all temperatures by definition so pH scale shifts with temp.
buffer solution definition
a mixture which resists change in pH when small amounts of acid or base are added
pKa
-log(Ka)
strong acid pH ~
1
weak acid pH ~
4-5
strong base pH ~
13
weak base pH ~
9-10
methyl orange
colour change acid > alkali
red > yellow
methyl orange
end point
3.0 - 4.5
phenolphthalein
colour change acid > alkali
colourless > pink
phenolphthalein
end point
8.3 - 10.0
lower Ka value
less dissociation
weaker acid
why is pKa calculated
to more easily see the strength of acids
higher pKa value
weaker acid
choosing an indicator
end point within ± 1 pH of equivalence point
equation linking weak acid conc. and Ka
Ka = [H+]2
[HA]
why is chloroethanoic acid stronger than ethanoic acid
the electronegative Cl atom pulls electon density away in the anion increasing its stabilty so chloroethanoic acid dissociates more readily
why is propanoic acid weaker than ethanoic acid
greater alkyl induction due to longer carbon chain pushes e- density down the chain reducing stability of anion, therefore less dissociation
conjugate pair
an acid-base pair that differs by one proton in their formulas
amphoteric
a substance which can act as either an acid or a base
diprotic acids
donate 2 protons
indicators
a weak acid whose conjugate pair is a different colour
equivalence point in an acid base titration
where an equal number of moles of H+ and OH- have been mixed
the reaction mixture is neutral
acidic buffer
solution of a weak acid and a salt of that acid
acidic buffer example
propanoic acid + sodium propanoate
basic buffer
mixture of a weak base and a salt of that base
basic buffer example
ammonia + ammonia chloride
how do acidic buffers work when acid is added
extra H+ ions react with A- to form HA
[H+] = Ka x [acid]/[salt]
Ka is constant
minimal change in [acid]/[salt] therefore minimal change in pH
how do acidic buffers work when alkali is added
extra OH- ions react with HA to form A- and H2O
[H+] = Ka x [acid]/[salt]
Ka is constant
minimal change in [acid]/[salt] therefore minimal change in pH
strong acid + strong base pH calculation
moles of each
excess formed
conc.
if OH- in excess use Kw to find [H+]
-log[H+]
weak acid + strong base pH calculation
find moles of each
find [acid] and [salt] formed using table
use [H+] = Ka x [acid]/[salt]
weak acid + salt pH calculation
use [H+] = Ka x [acid]/[salt]
Kw equation
Kw = [H+] [OH-]
addition of acid to acidic buffer calculation
add acid moles to HA
take away acid moles from A-
Ka = [H+] x [salt]/[acid]
2 tests to distinguish between strong and weak acids
pH paper
Na2CO3 (disappearing cross)
