Bronsted acids and pKa Flashcards
What is a bronsted acid?
Source of protons
What is a lewis acid?
Electron pair acceptor
Show dissociation of an acid HX
HX H+ + X-
What is the conjugate base of nitric acid?
NO3-
How many ionisable protons are there in nitric acid?
1
What is the conjugate base of sulfuric acid?
HSO4-
How many ionisable protons are there in sulfuric acid?
2
In the acids, which protons are ionisable?
The ones attached to the oxygen as this forms an acidic O-H bond
What is the conjugate base of phosphonic acid?
H2PO3-
How many ionisable protons are there in phosphonic acid?
2
What is the conjugate base of phosphoric acid?
H2PO4-
How many ionisable protons are there in phosphoric acid?
3
How is the strength of an inorganic acid measured?
the value of pKa
What is the equation for pKa?
pKa = -log10 (Ka)
How is Ka calculated?
Ka = [H+][X-] / HX
Lower pKa …
Stronger acid
Higher Ka …
Stronger acid
What is the position of prolytic equilibria determined by?
pKa(HX) and pKa(HY)
What happens to acidity when you go down a group?
Increases
Why do molecules become a stronger acid when they move down a group?
Orbital overlap
The H-X bond energy decreases
The X atom gets larger as you move down the group so the orbital overlap is worse, meaning the hydrogen atom is lost more easily
Why do compounds become a stronger acid when they move down a group?
Orbital overlap
The H-X bond energy decreases
The X atom gets larger as you move down the group so the orbital overlap is worse, meaning the hydrogen atom is lost more easily
What happens to acidity as you go along a period?
Increases
Why does acidity increase as you go along a period?
Electronegativity
There is decreasing electronegativity of the conjugate base which means there is increasing bond energy
How does the pKa of H3C-H relate to F-H?
H3C-H = 48 F-H = 3.5
What are compounds with pKa values -1.7
Water
What are compounds where pKa>16 quoted in?
Cant be quoted in water so alternative solvents must be used
What are two common solvents to quote the pKa of basic compounds in?
MeCN (acetonitrile)
DMSO (dimethylsulfoxide)
Why are those two alternative solvents used?
The protons are more acidic than the protons on H2O
Why can compounds with pKa < -1.7 not be quoted in water?
Strong acids react with water
H-Cl + H2O H-OH2+ + Cl-
How do you work out the strength of polyprotic inorganic acids?
The greater degree of ionisation, the weaker the acid
How do the pKa values of H2S and HS- relate?
H2S = 7.0 HS- = 19
Why does the degree of ionisation affect the strength of the acid?
If it is greater, there is an increasing difficulty of removing H+ from the anion
What is Paulings rule used for?
To calculate oxoacid stregnths
What is Pauling’s rule?
OpE(OH)q
p = number of oxo groups
How do you calculate the pKa from Pauling’s rule?
pKa = 8-5p
For Pauling’s rule what do you have to do for polyprotic acids?
Increase pKa by 5 units
How would you write H2SO4 so you could use Pauling’s rule? Do the calculation.
O2S(OH)2 (2 oxogroups)
= 8 - (5x2) = -2 which is the same as the experimental pKa
More oxo groups …
More stabilised base
More stabilised base …
More acidity
More acidity …
Greater delocalisation
Which takes priority in terms of acidity; more oxo groups of the size of the ion?
More oxo groups
What will be the pKa for a monoprotic acid with 0 oxogroups?
8
What will be the pKa for a monoprotic acid with 1 oxogroup?
3
What will be the pKa for a monoprotic acid with 2 oxogroups?
-2
What will be the pKa for a monoprotic acid with 3 oxogroups?
-7
What is the problem with HASO4 2-? Use the example of P and As
Paulings rule predicts pKa = 13
As is bigger than p which means longer A-O bonds which causes delocalisation of charge to be easier, lowering the pKa
How would you use the pKa values to predict reactivity?
If the pKa of the reactant is smaller than the pKa of the product, there is higher acidity of the reactant and lower basicity of the conjugate base.
The reaction is favoured equilibrium lies to the right
What is the acidity like in aprotic solvents? Why?
Much weaker
There is a problem solvating anions in an aprotic solvent
How does the pKa of CH3COOH compare in protic and aprotic solvents?
Protic = 4.8 Aprotic = 22.3
When does solvation of anions occur?
In protic solvents not aprotic solvents
What will an anion associate with?
Any protic source it can find
