Bronsted acids and pKa

Question 1

What is a bronsted acid?

Answer 1

Source of protons

Question 2

What is a lewis acid?

Answer 2

Electron pair acceptor

Question 3

Show dissociation of an acid HX

Answer 3

HX H+ + X-

Question 4

What is the conjugate base of nitric acid?

Answer 4

NO3-

Question 5

How many ionisable protons are there in nitric acid?

Answer 5

1

Question 6

What is the conjugate base of sulfuric acid?

Answer 6

HSO4-

Question 7

How many ionisable protons are there in sulfuric acid?

Answer 7

2

Question 8

In the acids, which protons are ionisable?

Answer 8

The ones attached to the oxygen as this forms an acidic O-H bond

Question 9

What is the conjugate base of phosphonic acid?

Answer 9

H2PO3-

Question 10

How many ionisable protons are there in phosphonic acid?

Answer 10

2

Question 11

What is the conjugate base of phosphoric acid?

Answer 11

H2PO4-

Question 12

How many ionisable protons are there in phosphoric acid?

Answer 12

3

Question 13

How is the strength of an inorganic acid measured?

Answer 13

the value of pKa

Question 14

What is the equation for pKa?

Answer 14

pKa = -log10 (Ka)

Question 15

How is Ka calculated?

Answer 15

Ka = [H+][X-] / HX

Question 16

Lower pKa …

Answer 16

Stronger acid

Question 17

Higher Ka …

Answer 17

Stronger acid

Question 18

What is the position of prolytic equilibria determined by?

Answer 18

pKa(HX) and pKa(HY)

Question 19

What happens to acidity when you go down a group?

Answer 19

Increases

Question 20

Why do molecules become a stronger acid when they move down a group?

Answer 20

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

Question 21

Why do compounds become a stronger acid when they move down a group?

Answer 21

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

Question 22

What happens to acidity as you go along a period?

Answer 22

Increases

Question 23

Why does acidity increase as you go along a period?

Answer 23

Electronegativity

There is decreasing electronegativity of the conjugate base which means there is increasing bond energy

Question 24

How does the pKa of H3C-H relate to F-H?

Answer 24

H3C-H = 48 
F-H = 3.5

Question 25

What are compounds with pKa values -1.7

Answer 25

Water

Question 26

What are compounds where pKa>16 quoted in?

Answer 26

Cant be quoted in water so alternative solvents must be used

Question 27

What are two common solvents to quote the pKa of basic compounds in?

Answer 27

MeCN (acetonitrile) | DMSO (dimethylsulfoxide)

Question 28

Why are those two alternative solvents used?

Answer 28

The protons are more acidic than the protons on H2O

Question 29

Why can compounds with pKa < -1.7 not be quoted in water?

Answer 29

Strong acids react with water | H-Cl + H2O H-OH2+ + Cl-

Question 30

How do you work out the strength of polyprotic inorganic acids?

Answer 30

The greater degree of ionisation, the weaker the acid

Question 31

How do the pKa values of H2S and HS- relate?

Answer 31

``` H2S = 7.0 HS- = 19 ```

Question 32

Why does the degree of ionisation affect the strength of the acid?

Answer 32

If it is greater, there is an increasing difficulty of removing H+ from the anion

Question 33

What is Paulings rule used for?

Answer 33

To calculate oxoacid stregnths

Question 34

What is Pauling's rule?

Answer 34

OpE(OH)q | p = number of oxo groups

Question 35

How do you calculate the pKa from Pauling's rule?

Answer 35

pKa = 8-5p

Question 36

For Pauling's rule what do you have to do for polyprotic acids?

Answer 36

Increase pKa by 5 units

Question 37

How would you write H2SO4 so you could use Pauling's rule? Do the calculation.

Answer 37

O2S(OH)2 (2 oxogroups) | = 8 - (5x2) = -2 which is the same as the experimental pKa

Question 38

More oxo groups ...

Answer 38

More stabilised base

Question 39

More stabilised base ...

Answer 39

More acidity

Question 40

More acidity ...

Answer 40

Greater delocalisation

Question 41

Which takes priority in terms of acidity; more oxo groups of the size of the ion?

Answer 41

More oxo groups

Question 42

What will be the pKa for a monoprotic acid with 0 oxogroups?

Answer 42

8

Question 43

What will be the pKa for a monoprotic acid with 1 oxogroup?

Answer 43

3

Question 44

What will be the pKa for a monoprotic acid with 2 oxogroups?

Answer 44

-2

Question 45

What will be the pKa for a monoprotic acid with 3 oxogroups?

Answer 45

-7

Question 46

What is the problem with HASO4 2-? Use the example of P and As

Answer 46

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

Question 47

How would you use the pKa values to predict reactivity?

Answer 47

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

Question 48

What is the acidity like in aprotic solvents? Why?

Answer 48

Much weaker | There is a problem solvating anions in an aprotic solvent

Question 49

How does the pKa of CH3COOH compare in protic and aprotic solvents?

Answer 49

``` Protic = 4.8 Aprotic = 22.3 ```

Question 50

When does solvation of anions occur?

Answer 50

In protic solvents not aprotic solvents

Question 51

What will an anion associate with?

Answer 51

Any protic source it can find