20 - Acids, bases and pH Flashcards
What is the Arrhenius model of acids?
-> acids dissociate and release H+ ion in (aq) solution
-> alalis dissociate and release OH- ions in (aq) solutions
-> H+ ions are neutralised by OH- to form water
H+(aq) + OH-(aq) —-> H2O (l)
What is the Bronsted-Lowry model for acids and bases?
-> Bronsted-Lowry acid is a proton donor
-> Bronsted-Lowry base is a proton acceptor
What is a conjugate acid-base pair?
Are two species that can be interconverted by the transfer of a proton
ie HC- and Cl-
In the reaction what is the conjugate acid and base?
HCl(aq) <—-> H+(aq) + Cl-(aq)
-> in the forward reaction HCl releases a proton to form Cl-, its conjugate base
-> in the reverse direction Cl- accepts a proton to form HCl, its conjugate acid
HCl(aq) + OH-(aq) <—–> H2O(aq) + Cl-(aq)
What are the conjugate base pairs?
Acid 1 : HCl
Base 1 : Cl-
Acid 2 : H2O
Base 2 : OH-
Reaction between HCl and H2O, what are its conjugate pairs?
HCl + H2O <—-> H3O+ + Cl-
A1 B2 A2 B1
Why is the hydronium ion important
it is the active ingredient in any aqueous acid
Monobasic, dibasic and tribasic
refers to the number of H+ that cen be replaced per molecule in an acid-base reaction
acid + metal
-> salt + hydrogen
redox
What are carbonates?
Acid + Carbonate
-> they are a base which can neutrlise acids
—> salt + water + CO2
mathmatical relationship between [H+] and pH
pH = -log[H+]
[H+] = 10^-pH
on a logarithmic scale, what dies a change in one pH represent?
a 10 times difference
what happens when a strong acid is in (aq) solution?
what is this equation?
a strong monobasic acid completely dissociates
HA(aq) —> H+(aq) + A-(aq)
what is the relationship of [H+] and [HA] for a strong acid
[H+] = [HA]
what is a weak acid?
give an equation
partially dissociates
HA <—> H+ + A-
What is the acid dissociation constant Ka?
compare Ka pf a strong and weak acid
strong acids have a large Ka
-> the value of [H+][A-] is larger than [HA]
mathmatical relationship between Ka and pKa
pKa = -logKa
Ka - 10^pKa
what is the Ka equation of a weak acid, why is it this?
Ka= [H+]^2 / [HA]
-> when HA molecules dissociate H+ and A- are formed in equal quantities
What are the approximations that have led to the simplification of the Ka equation?
Approximation 1
-> that the [H+] and [A-] are equal. Because the acid is in aqueous solution there is a small concentration due to the dissociation of water. However this is very small and can be neglected
[H+]eqm = [A-]eqm
Approximation2
-> the equilibrium concentration of HA is smaller than the undissociated concentration
[HA]eqm = [HA]start - [H+]eqm
-> as the dissociation of a weak acid is small you can assume that [HA]start> [H] and you can neglect any decrease in concentration of HA from dissociation
(Dissociation of HA is negligible )
[HA]eqm = [HA]start
approximation in calculation involving weak acids
1) assumes that dissociation of water is negligible
-> [H+]eqm = {A-}eqm
-> at 25’c dissociation of water can be considered negligible if pH<6.
-> this approximation falls for very weak acids or dilute solutions
2) assumes that the concentrations of acid is much greater than the H= concentration at equilibrium
[HA]start» [H+]eqm
[HA]eqm = [HA]start - [H+]eqm approximate to
[HA]eqm = [HA]start
-> it holds for weak acids with a small Ka value
-> it breaks down wen [H+] becomes significant and there is a real difference between [HA]eqm and [HA]start - [H+]eqm
- it is not justified for stronger weak acids and for very dilute solutions
how can the Ka of a weak acid be determined?
- prepare a standard solution of the weak acid of a known concentration
- measuring the pH of the standard solution using a pH meter
Explain the ionisation of water
H2O + H2O
- water ionies slightly acting as both an acid and a base
H2O + H2O <—> H3O- + OH-
A1 B2 A2 B1
How do find the ionic product or water?
Ka = [H+][OH-] / [H2O]
Ka x [H2O] = [H+][OH-]
Ka x [H2O] = Kw
Kw = [H+][OH-]
what is Kw
the ionic product of water
it is the ions in the water multiplied together
what is the value of Kw at 298K
1 x 10^-14 mol2dm-6
in an acidic solution of pH 3 [H+] = 10-3 moldm-3
what is the [OH-]
10-11
