acids and bases [P2] PAPER 1

Question 1

what is an acid?

Answer 1

proton donor

Question 2

what is a base?

Answer 2

proton acceptor

Question 3

what is a strong acid?

Answer 3

an acid that completely dissociates in water to form H+ ions

Question 4

what is a weak acid?

Answer 4

an acid that slightly dissociates in water to form H+ ions

Question 5

how to calculate pH?

Answer 5

pH = – log [H+]

Question 6

how to calculate the concentration of hydrogen ions, [H+] ?

Answer 6

Question 7

what is the value of Kw at room temperature (25°C) ?

Answer 7

10^–14

Question 8

Kw = ?

Answer 8

Kw = [H+] [OH–]

[H+] = [OH–] => Kw = [H+]^2

Question 9

Why does [H2O] not appear in the Kw equation?

Answer 9

[H2O] is very high, so [H2O] is effectively constant

Question 10

Explain why the value of Kw increases as the temperature increases.

Answer 10

• equilibrium is endothermic in the forwards direction
• equilibrium shifts to the right to oppose the increase in temperature

Question 11

Why is water neutral?

Answer 11

[H+] = [OH-]

Question 12

Suggest why the ph probe is washed with distilled water between each of the calibration measurements.

Answer 12

different solutions must not contaminate each other

Question 13

The calibrated pH meter is used to monitor the pH during a titration of hydrochloric acid with sodium hydroxide.
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.

Answer 13

to avoid missing the end point

Question 14

how to determine if an indicator is suitable using a pH curve?

Answer 14

has a colour change within the vertical part of the pH curve

Question 15

Ka = ?

Answer 15

[H+] is NOT equal to [A–] in buffer calculations

Question 16

pKa = ?

Answer 16

– log Ka

Question 17

at temperatures above 298K, the pH of water is ___ than 7

Answer 17

less than

Question 18

Answer 18

• add fixed volume of alkali in a beaker
• add acid in small portions from a burette
• stir and use a pH meter to record the pH after each addition of acid

Question 19

Suggest a suitable piece of apparatus that could be used to measure out a solution to then produce a pH curve from the results obtained.

Answer 19

burette as it can deliver variable volumes

Question 20

Use information from the curve in the figure above to explain why the end point of this reaction would be difficult to judge accurately using an indicator.

Answer 20

• the change in pH is gradual
• an indicator would change colour over a range of volumes of [whatever is on the x axis]

Question 21

colour of methyl orange at a low pH?

Answer 21

red

Question 22

colour of methyl orange at a high pH?

Answer 22

yellow

Question 23

colour of phenolphthalein at a low pH?

Answer 23

colourless

Question 24

colour of phenolphthalein at a high pH?

Answer 24

pink

Question 25

pH range of colour change for methyl orange?

Answer 25

3.2 – 4.4

Question 26

pH range of colour change for phenolphthalein?

Answer 26

8.2 – 10.0

Question 27

pH curves for
• strong acid / strong base
• strong acid / weak base
• weak acid / strong base
• weak acid / weak base

Answer 27

Question 28

what is a buffer?

Answer 28

solution that is able to resist changes in pH when small volumes of acid or alkali are added

Question 29

pH at half-neutralisation = ?

Answer 29

pKa

Question 30

Answer 30

B

Question 31

Explain how a solution of [acid] and [alkali] can act as a buffer when small amounts of acid/alkali are added.

Answer 31

• acid increases the concentration of H+ ions, so equilibrium shifts to the left to oppose the increase in H+ ions
• the OH– ions from the alkali reacts with the H+ ions, so equilibrium shifts to the right to replace the H+ ions
• concentration of H+ remains almost constant

Question 32

explain why pH = –logKa when [A–] = [HA]

Answer 32

• if [A–] = [HA], then Ka = [H+]
• pH = –log [H+] = –logKa

Question 33

suggest one reason why there may be an anomalous point in a pH curve, assuming the pH meter reading is correct.

Answer 33

ineffective swirling of the mixture

Question 34

Suggest how the procedure of an experiment that results in a pH curve could be slightly modified in order to give a more reliable value for the end-point.

Answer 34

• add smaller volumes of (x-axis thing) around near the end-point
• take more pH readings around the end-point

Question 35

State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement.

Answer 35

after storage, the meter doesn’t give accurate readings

Question 36

Describe how you would obtain the pH curve for a titration.

Answer 36

• measure pH of the acid/alkali
• add acid/alkali from burette in known small volumes
• stir mixture, then measure the pH
• repeat until the acid/alkali is in excess
• add in smaller increments near the end-point

Question 37

A mixture, formed by adding 50 cm3 of 1.00 mol dm–3 salicylic acid solution to 25 cm3 of 1.00 mol dm–3 sodium hydroxide solution, can be used to determine the pKa of salicylic acid. State one measurement that must be made for this mixture and explain how this measurement can be used to determine the pKa of salicylic acid.

Answer 37

• pH
• [HA] = [A–], so pH = pKa

Question 38

Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of sodium hydroxide.

Answer 38

• added OH– removed when reacting with H+ in the acidic buffer
• HX ⇌ H+ + X–
• equilibrium shifts to the right to oppose the decrease in H+
• ratio of [HX] to [X–] remains constant (hence pH and [H+] remain constant)

Question 39

Other than by using a different pH meter, state one way in which the accuracy of the pH readings could be improved.

Answer 39

calibrate meter

Question 40

Describe briefly how you would ensure that a reading from a pH meter is accurate.

Answer 40

• calibrate pH meter with solution of known pH
• plot calibration curve

Question 41

Suggest why the concentration of sodium hydroxide in a solution slowly decreases when left open to air.

Answer 41

NaOH reacts with carbon dioxide in the air

Question 42

Concentrated sulfuric acid reacts with solid sodium chloride.
Give the observation you would make in this reaction.
State the role of the sulfuric acid.

Answer 42

• steamy fumes
• acid

Question 43

Answer 43

• lower pH
• magnesium hydroxide is less soluble

Question 44

Some sodium sulfate is dissolved in a sample of a solution of NaHSO4.
Explain why this increases the pH of the solution.

Answer 44

Question 45

A student completed an experiment to determine the enthalpy of neutralisation for the reaction between ethanedioic acid solution (HOOCCOOH) and potassium hydroxide solution, and found it was –33.6 kJmol-1 per mole of water formed.
In a similar experiment, the enthalpy of neutralisation for the reaction between sulfuric acid and potassium hydroxide solution was found to be –57.0 kJmol-1 per mole of water formed.
Suggest an explanation for the difference between the two values.

Answer 45

• HOOCCOOH is a weak acid
• more energy needed to break bonds / complete dissociation

Question 46

how to make a buffer solution using a weak acid and strong base?

Answer 46

add excess [weak salt] to [strong base]

e.g. add excess ethanoic acid to potassium hydroxide