Physical 12: Acids and Bases

Question 1

What is a Bronsted-Lowry acid?

Answer 1

A substance that donates protons

Question 2

What is a Bronsted-Lowry base?

Answer 2

A substance that accepts protons

Question 3

How does water act as a base?

Answer 3

When it accepts protons
e.g.
HCl + H2O –> H3O+ +Cl-

Question 4

How does water act as an acid?

Answer 4

When it donates protons
e.g.
H2O + NH3 –> NH4+ + OH-

Question 5

What is the ionic product of water?

Answer 5

Kw = [H+][OH-]

Question 6

How do you calculate pH using [H+]?

Answer 6

Question 7

How do you calculate [H+] from pH?

Answer 7

Question 8

How does concentration of [H+] change as pH increases?

Answer 8

As pH increases, the concentration of H+ ions decreases

Question 9

How does Kw change with temperature?

Answer 9

As temperature increases, Kw increases
Because the equilibrium reaction of water is endothermic in the forward direction
So the reaction shifts in the endothermic direction (forward)

Question 10

What is a strong acid?

Answer 10

One that completely dissociates into its ions in water

Question 11

What is a strong base?

Answer 11

One that completely dissociates into its ions in water

Question 12

What is a weak acid?

Answer 12

One that only partially dissociates into its ions in water

Question 13

What is a weak base?

Answer 13

One that only partially dissociates into its ions in water

Question 14

What is the acid dissociation constant?

Answer 14

Question 15

Do strong acids have a smaller or larger acid dissociation constant?

Answer 15

The stronger the acid, the larger the value of Ka
Because the equilibrium will lie further to the right if the acid is more dissociated

Question 16

What is pKa?

Answer 16

Often used to express Ka for a weak acid
Gives a measure of how strong a weak acid is
Small pKa = stronger acid

Question 17

How do you calculate pKa?

Answer 17

Question 18

Give three examples of strong acids.

Answer 18

Hydrochloric acid HCl
Nitric acid HNO3
Sulfuric acid H2SO4
Chloric acid HClO3

Question 19

Give three examples of weak acids.

Answer 19

Ethanoic acid
Citric acid
Benzoic acid

Question 20

Give three examples of strong bases.

Answer 20

Sodium hydroxide NaOH
Potassium hydroxide KOH
Calcium hydroxide Ca(OH)2
Barium hydroxide Ba(OH)2

Question 21

Give two examples of weak bases.

Answer 21

Ammonia NH3
Ammonium hydroxide (NH4OH)

Question 22

What is an acid-base titration used for?

Answer 22

To determine the amount of acid/alkali needed to neutralise a given amount of acid/alkali

Question 23

How do you calibrate a pH meter?

Answer 23

Place the probe in a buffer solution of known pH

Question 24

Draw the titration curve of a strong acid and strong base.

Answer 24

Question 25

Draw the titration curve of a strong acid and weak base.

Answer 25

Question 26

Draw the titration curve of a weak acid and strong base.

Answer 26

Question 27

Draw the titration curve of a weak acid and weak base.

Answer 27

Question 28

What is the equivalence point?

Answer 28

The point on a titration curve at which sufficient acid/alkali has been added to neutralise the solution The number of moles of H+ is the same as the number of moles of OH-

Question 29

What is the end point?

Answer 29

The point in a titration where the sufficient amount of acid/alkali has been added to make the indicator change colour

Question 30

What are the three properties of a suitable indicator?

Answer 30

1) The colour change must be sharp rather than gradual at the end point 2) The end point must be the same as the equivalence point 3) The indicator should give a distinct colour change

Question 31

What is the half-equivalence point?

Answer 31

The point half way between zero and the equivalence point Half of HA has been converted into A- So [HA] = [A-]

Question 32

Prove that pKa = pH at the half equivalence point.

Answer 32

Question 33

What is a buffer solution?

Answer 33

A solution that resists small changes to pH when small volumes of acid or alkali are added They are based on an equilibrium reaction which will move in the right direction to either remove additional H+ or OH-

Question 34

How do you make an acidic buffer?

Answer 34

Weak acid + soluble salt of that acid

Question 35

How do acidic buffers work?

Answer 35

The dissociation of a weak acid is an equilibrium reaction So if alkali is added, the OH- will react with HA to produce more H+ and A-, making the solution more acidic again

Question 36

How do you make a basic buffer?

Answer 36

Weak base + soluble salt of that base

Question 37

How do basic buffers work?

Answer 37

If acid is added, it reacts with the aqueous base