3.9 Acid-base Equilibria

Question 1

Acids are

Answer 1

Proton donors

Question 2

Bases are

Answer 2

Proton acceptors

Question 3

A strong acid

Answer 3

Dissociates fully in water and at equilibrium will only have ions present

Question 4

A weak acid

Answer 4

Dissociates only partly in water and at equilibrium has some ions but predominantly acid molecules

Question 5

A dilute acid has

Answer 5

A low number of particles per unit volume

Question 6

A concentrated acid has

Answer 6

A high number of particles per unit volume

Question 7

A conjugate acid

Answer 7

Acts as an acid in the backward reaction

Question 8

A conjugate base

Answer 8

Acts as a base in the backward reaction

Question 9

Acid equilibrium constant is

Answer 9

Ka

Question 10

Ka is

Answer 10

[H+][A-] / [HA]

Question 11

The Ka of a strong acid is

Answer 11

High

Question 12

The ionic product of water is

Answer 12

Kw=[H+][OH-]

Question 13

pH=

Answer 13

-log10[H+]

Question 14

How to calculate the pH of a strong acid

Answer 14

Since a strong acid completely dissociates the [H+] = [acid] and so pH = -log[acid]

Question 15

How to calculate the concentration of a strong acid

Answer 15

Using [acid] = [H+] and [H+] = lnvlog(-pH)

Question 16

pH of weak acids

Answer 16

pH = -log10(ailisradd Ka[HA])

Question 17

pH of a strong base

Answer 17

[base] = [OH-]
[H+] = Kw/[OH-]

Question 18

The pH at half-neutralisation is

Answer 18

pKa

Question 19

What does a buffer solution do

Answer 19

Resists a change in pH when small amounts of acid or alkali are added

Question 20

What does a buffer solution consist of

Answer 20

A weak acid and its salt

Question 21

What happens when an acid is added to a buffer solution

Answer 21

• the concentration of H+ initially increases
• this causes the reversible reaction to shift left so that the H+ ions are removed by a reaction with the anion

Question 22

What happens when an alkali is added to a buffer solution

Answer 22

• OH- ions combine with H+ ions to form water
• this reduces the concentration of H+ and shifts the reversible reaction right
• the acid dissociates to release more H+ ions

Question 23

What assumptions are done during the calculation of the pH of buffers

Answer 23

• assume that all the salt dissociates
• assume none of the acid dissociates

Question 24

When are buffer solutions usedv

Answer 24

• in many biochemical experiments where a constant pH is needed
• when enzymes are used or stored
• in the preparation of synthetic and processed food so they can be eaten and digested without undue change in pH

Question 25

What is at the equivalence point of pH curves

Answer 25

A solution of the salt formed from the acid and base The pH of this solution is the mid-point of the vertical part of the curve

Question 26

Equivalence point of a strong acid/strong base

Answer 26

• pH=7 • The solution of the salt is neutral • water is slightly dissociated but there is no tendency for any of the ions yo interact with the ions from the water

Question 27

Equivalence point of a strong acid/strong base

Answer 27

• pH = 4-6 • the solution and therefore the salt is acidic • the cation present can dissociate to release H+ ions in a reversible reaction

Question 28

Equivalence point of weak acid/strong base

Answer 28

• pH= 8-10 • the solution and therefore the salt is basic • the salt releases all its anions and these react to remove H+ ions from the solution

Question 29

How is a suitable indicator selected

Answer 29

The pH range of the indicator must lie on the vertical part of the curve