AS.3 Acids, Bases And Buffers Flashcards
Acid
Proton donor
Base
Proton acceptor
Strong acid
Proton donor that dissociates fully
Weak acid
Proton donor that dissociates partially
Alkali
Soluble base that contains OH- ions
Exception -> NH3 + H2O ——> NH4 + + OH-
Acid base pair
Two molecules which interconvert between each other with the loss or gain of a proton
Acid base pair example
HA + B ————-> A- + HB+
HA - acid 1
A- - base 1
Conjugate
B - base 2
HB+ - acid 2
Conjugate
Acid + metal ( ionic equation )
Salt + H2
Na + H+ ———> Na+ + H2
Acid + metal oxide (ionic equation)
Salt + water
MgO + 2H+ ————> Mg2+ + H2O
Acid + alkali (ionic equation)
Salt and water
OH- + H+ ——-> H2O
Acid and carbonate
Salt + water + carbon dioxide
Group I carbonate - soluble
CO3 2- + 2H+ ———-> CO2 + H2O
Group II carbonate - insoluble
MgCO3 + 2H+ ———> Mg2+ + CO2 + H2O
pH equation
-log [H+]
[H+] equation
10^-pH
pH of strong acids
[monobasic acid] = [H+]
pH of weak acids
HA <———-> H+ + A-
Ka = [H+] [A-]/ [HA]
Ka approximates
[HA] eq = [HA] undissociated
Amount of HA considered constant because equilibrium is so far to left
[H+] = [A-]
assume only source of ions is from dissociation of HA
Rearrange Ka to calculate pH
[H+] = square root of (Ka x [HA])
pKa
-log(Ka)
The larger the Ka, the ——- the pKa
The larger the Ka, the more dissociated the acid is, therefore the stronger the weak acid
The larger the Ka, the smaller the pKa
The smaller the pKa, more dissociated the acid is and therefore the stronger the weak acid
pH of pure water
Kw = [H+] [OH-]
Kw at 298K
1 x 10^ -14
Rearranging Kw
Square root of Kw = H+
pH of a strong base
Kw
——— = [ H+]
[NaOH]
Kw = [H+] x [OH-]
[OH-] = [NaOH]
Buffer solution
A solution that resists the change to its pH despite the addition of small amounts of acid or alkali
How to make a buffer - method 1
Weak acid + salt of weak acid
CH3COOH + CH3COONa
Don’t react but coexist
How to make a buffer - method 2
Excess of weak acid + limited amount of alkali
Buffer - add small amount of acid
Acid introduces more H+
Weak acid equilibrium will oppose this change and shift to left
The CH3COO- reacts with added H+ to restore amount
[H+] maintained and pH remains unchanged
Buffer - adding small amount of alkali
OH- from alkali will react with H+ ions in the buffer
Weak acid equilibrium will oppose this change and shift to right
Weak acid will dissociate more to restore amount of H+
[H+] maintained and pH remains unchanged
Buffer calculation
Ka = [H+] [A-]
————-
[A-]
Buffers in blood
Carbonic acid acts as weak acid
Hydrogencarbonate acts as conjugate base
H2CO3
HCO3-
Blood buffers - increase in H+
Weak acid equilibrium shifts to left
Blood buffers - increase in OH-
Removed by reaction with H+ from equilibrium
Equilibrium then shifts to right to restore [H+]
Blood buffers - production of more H2CO3
Removed by action of enzyme
H2CO3 ——-> H2O + CO2 (aq)
CO2 transported to lungs
CO2 (aq) ————> CO2 (g)
Normal blood pH
7.35 - 7.45
Graph line shape of neutralisation
pH increase slightly - large excess of H+
Point of neutralisation - rapid as small conc of H+ remaining is used up and replaced by OH-
Selecting suitable indicator
Indicator - Very weak acid
Colour change must be within pH range of equivalence point of titration
Suitable indicator for SA/SB
3———->11
Methyl orange
Phenolphthalein
Suitable indicator for WA/SB
7 ———> 11
Phenolphthalein only (8.4 - 10)
Suitable indicator for SA/WB
3————->7
Methyl orange only (2.8 - 4)
Suitable indicator for WA/WB
No vertical section of curve
No suitable indicator
