3.1.12 Acids and Bases

Question 1

What is an acid?

Answer 1

A proton donor

Question 2

What is a base?

Answer 2

A proton acceptor

Question 3

What is the only compound that only acts as an acid?

Answer 3

HF

Question 4

What is pH?

Answer 4

The logarithmic scale used as a measure of hydrogen ion concentration (2.d.p)

Question 5

What is a strong acid?

Answer 5

An acid that fully dissociates

Question 6

What is a weak acid?

Answer 6

An acid that partially dissociates

Question 7

What is a monoprotic acid?

Answer 7

An acid that donates one mole of protons per mole of acid

Question 8

What is a diprotic acid?

Answer 8

An acid that donates two moles of protons per mole of acid

Question 9

What is an alkali?

Answer 9

A soluble base (in water)

Question 10

What is a conjugate acid?

Answer 10

The species formed from a Brønsted-Lowry base by addition of a proton

Question 11

What is a conjugate base?

Answer 11

The species formed from a Brønsted-Lowry acid by loss of a proton

Question 12

How is pH related to hydrogen ion concentration?

Answer 12

pH = -log[H+]

Question 13

What is Kw?

Answer 13

Water naturally dissociates: H2O ⇌ H+ + OH-
Kw = [H+] [OH-]
At room temperature = 1x10^-14

Question 14

What happens to Kw at higher temperatures? (2)

Answer 14

Equilibrium shifts to right (endothermic direction)

Kw increases

Question 15

What is Ka? (2)

Answer 15

The dissociation constant for a weak acid

Weak acids dissociate only slightly: HA ⇌ H+ + A-

Question 16

What assumptions does the weak acid form of the Ka equation rely on? (2)

Answer 16

All [H+] comes from HA dissociating (not H2O)

The equilibrium lies far to the left so the initial and equilibrium concentrations of HA are equal

Question 17

How is pKa calculated?

Answer 17

pKa = -log(Ka)

Question 18

What is a buffer solution? (4)

Answer 18

A mixture of a weak acid/base and a solution of its salt
Resists small changes in pH

Acidic: weak acid ⇌ salt + H+
Basic: base + H+ ⇌ salt

Question 19

How is the pH of a buffer solution changed?

Answer 19

By changing the ratio of acid to salt

Question 20

Action of acidic buffers - adding acid (2)

Answer 20

[H+] increases

Equilibrium shifts to the left to restore Kc

Question 21

Action of acidic buffers - adding base (3)

Answer 21

OH- + H+ > H2O
[H+] decreases
Equilibrium shifts to right to restore Kc

Question 22

Action of basic buffers - adding acid (2)

Answer 22

[H+] increases

Equilibrium shifts right to restore Kc

Question 23

Action of basic buffers - adding base (3)

Answer 23

OH- + H+ > H2O
[H+] decreases
Equilibrium shifts left to restore Kc

Question 24

What is true at half neutralisation?

Answer 24

pKa = pH

Question 25

What are the applications of buffer solutions? (3)

Answer 25

Shampoo (maintains pH whilst hair is washed)
Biological washing powder (keeps pH at right level for enzymes)
Biological buffer system in blood

Question 26

What colour change does methyl orange undergo?

Answer 26

Acid - red
Neutral - peach
Alkaline - yellow

Question 27

What range does methyl orange undergo a colour change in?

Answer 27

3.2 < pH < 4.4

Question 28

What colour change does phenolphthalein undergo?

Answer 28

Colourless to pink (alkaline)

Question 29

What range does phenolphthalein change colour in?

Answer 29

8.2 < pH < 10

Question 30

What are indicators?

Answer 30

Weak acids that dissociate:

HA (colour 1) ⇌ H+ + A- (colour 2)