Acid base intro, pH, pKa, speciation

Question 1

What is an acid?

Answer 1

A Bronsted acid is a molecule that can donate a proton to a base - any molecule containing H joined to a more electronegative atom

Question 2

What happens to a H proton when placed in water?

Answer 2

Form H3O+

Question 3

What determines the strength of an acid?

Answer 3

The strength of the -H bond

Question 4

What are the factors that affect strength of -H bond?

Answer 4

1) Charge of molecule
2) Charge stabilisation
- inductive effects
- delocalisation

Question 5

How does the charge of a molecule affect the strength of acid?

Answer 5

A more positive or less negative charge means the -H bond is weaker and therefore the acid is stronger

Question 6

How do inductive effects affect strength of acid?

Answer 6

More electronegative groups pull electrons towards itself meaning -H bond is weakened and therefore acid is stronger

Question 7

How does delocalisation affect strength of acid?

Answer 7

Delocalisation of electrons weakens the -H bond meaning the acid is stronger

Question 8

What is a base?

Answer 8

A bronsted acid is a molecule that a base can take a proton from - a molecule that can accept (pick-up) a proton

Bases must have a lone pair of electrons

Question 9

What does an acid form when it loses its proton?

Answer 9

The conjugate base

Question 10

What is Kw?

Answer 10

Kw = [H3O+][OH-] = 10^-14

Question 11

What is pH?

Answer 11

Number indicating the acidity of a compound

pH = - log [H3O]+

therefore [H3O]+ = 10 ^ -pH

Question 12

What is Ka?

Answer 12

Acid dissociation constant - provides information on how much an acid dissociates in water

Ka = [A-][H3O+] / [HA]

therefore pKa = -log [Ka]

Question 13

How does Ka relate to acid strength?

Answer 13

Higher Ka means more dissociation occurring therefore stronger acid

Question 14

How does pKa relate to acid strength?

Answer 14

Higher Ka = Lower pKa therefore lower pKa indicates more association and therefore stronger acid

Question 15

What is a strong acid?

Answer 15

Dissociates greatly in water

pKa smaller or more negative than 0

Question 16

What is a weak acid?

Answer 16

Dissociates partially in water

pKa between 0 and 14

Question 17

What happens when a weak acid dissociates in water?

Answer 17

It forms a conjugate base that is also a weak base

Question 18

What is Kb?

Answer 18

Kb = [H:B+][OH-] / [:B]

therefore pKb = - log [Kb]

Question 19

What is the relationship between Ka, Kb and Kw?

Answer 19

Ka x Kb = Kw

Question 20

What is speciation?

Answer 20

The idea that there are different forms of species in a solution at different pH levels

Question 21

What does it mean when Ka = [H3O+]

Answer 21

By Ka, this means that the concentration of acid and its conjugate base is equal

Therefore pKa = pH

Question 22

What does it mean when pKa = pH?

Answer 22

The amount of acid and its conjugate base in the solution are equal

Question 23

What does it mean when pKa > pH?

Answer 23

Acid conditions - therefore the acid form is more present than its conjugate base

Question 24

What does it mean when pKa < pH?

Answer 24

Basic conditions - the conjugate base is more present that the acid

Question 25

What is a zwitterion?

Answer 25

Amino acid which is neutral but contains charges