Topic 14 Flashcards
Acid definition
Proton donor
Base definition
Accept proton
Acid and its conjugate base
HNO3 + HNO2 –> NO3- + H2NO2+
HNO3: Acid 1
NO3: Base 1
HNO2: Base 2
H2NO2+: Acid 2
Calculation of pH
pH = -log[H+]
[H+] = 10^-pH
Strong acid
Complete dissociation
[H+] = [acid]
Weak acid
Partially dissociate
Aq solution and pure water equilibrium
H2O <–> H+ + OH-
Kw = [H+][OH-]
pKw at 25=14
(can calculate [H+] or [OH-] if we know one of those)
Neutral definition
[H+]=[OH-]
Weak acid dissolve in water
HA + H2O <–> H3O+ + A-
HA <–> H+ + A-
Weak acid calculation (and assumptions)
Ka = [H+][A-]/[HA]
Ka = [H+]^2/[HA]
Assumptions
1. [H+]eq.=[A-]eq. because they have dissociated according to 1:1 ratio
2. As the amount of dissociation is small, initial concentration of undissociated acid has remained constant
Half equivalence/Half neutralisation
stage of the titration at which exactly half the amount of weak acid has been neutralised
Half neutralisation calculation
[HA] = [A-]
pH = pKa
Buffer solution
pH does not change significantly if small amounts of acid / alkali are added to it
Buffer solution work
Acid <–> base + H+
If small amount of acid is added, the equilibrium (need to write it out) will shift to the left removing all the H+ ions added. As there is a large concentration of salt ion in the buffer, the ratio of [acid]/[salt] stays almost constant, so pH stays fairly constant.
If small amount of alkali is added to the buffer, OH- will react with H+ ions forming water.
H+ + OH- –> H2O
equilibrium will shift to the right producing more H+ ions.
Acid <–> base + H+
Some acid molecules are changed to salt ions but as there are large concentration of salt ion in the buffer, the ratio [acid]/[salt] stays almost constant, so pH stays fairly constant
Calculating pH of buffer solutions
treat the [A-] is due to added salt
[HA] = concentration of weak acid
[H+] = Ka*([HA]/[A-])
*find the concentration of salt and weak acid in solution
if strong base added –> calculate the number of moles of weak acid neutralised (直接用 mole of weak acid - strong base added), calculate the remaining moles of weak acid –> turn into concentration and do substitution to calculate [H+]
Calculating change in pH of buffer on addition of small amount of acid or alkali
If small amount of alkali added to buffer –> moles of buffer acid would reduce by number of moles of alkali and mole of salt would increase by the same amount –> calculate new pH
If small amount of acid is added to buffer –> moles of buffer acid would increase by the number of moles of acid added and moles of salt would decrease by the same amount –> calculate new value
Indicator
indicator acts as a weak acid
HIn <–> H+ + In-
when acidic, more H+ so equilibrium position push to the left, colour will be at left and vice versa
special case
bromomythol blue at neutralisation is the colour mixture of blue and yellow = green
when choose indicator, check the data with vertical part of the graph. Equivalence point
Diprotic acid / Diprotic base
calculation need to times 2
titration have 2 vertical point –> need to use 2 indicators
