Acids, Bases and Buffers

Question 1

Define a Brønsted-Lowry acid?

Answer 1

Proton donor

Question 2

Define a Brønsted-Lowry base?

Answer 2

Proton acceptor

Question 3

Define Lewis acid?

Answer 3

Electron pair acceptor

Question 4

Define Lewis base?

Answer 4

Electron pair donor

Question 5

What ion causes a solution to become acidic? (2 answers)

Answer 5

H+
H3O+

Question 6

Write an equation for the ionisation of water.

Answer 6

H2O(l) —> H+ (aq) + OH- (aq)

Question 7

Give an example of a monobasic acid.

Answer 7

HCl

Question 8

Give an example of a dibasic acid.

Answer 8

H2SO4

Question 9

Give an example of a tribasic acid.

Answer 9

H3PO4

Question 10

Define strong acid.

Answer 10

An acid that dissociates completely

Question 11

Give some examples of strong acids

Answer 11

• HCl
• H2SO4
• HNO3

Question 12

Define weak acids.

Answer 12

Acids that only partially dissociate

Question 13

Give some examples of weak acids.

Answer 13

• methanoic acid
• any organic acid

Question 14

Write the acid dissociation constant expression.

Answer 14

For acid HA, HA —> H(+) + A-
Ka = [H+][A-]/[HA]

Question 15

What does a larger Ka value mean?

Answer 15

Larger the Ka — greater the extent of dissociation

Question 16

Write the equation used to convert Ka into pKa

Answer 16

pKa = -log10(Ka)

Question 17

What is the relationship between pKa and strength of the acid?

Answer 17

smaller the pKa the stronger the acid

Question 18

Write the equation used to convert the concentration of H+ into pH.

Answer 18

pH = -log[H+]

Question 19

Why is a pH scale useful compared to the concentration of H+?

Answer 19

pH scale allows a wide range of H+ concentration to be expressed as simple positive values.

Question 20

[H+] of a strong acid is equal to what?

Answer 20

[H+] = [HA]

Question 21

What is the assumption made when calculating pH of weak acids?

Answer 21

It is assumed that the concentration of acid at equilibrium is equal to the concentration of acid after dissociation. This is because only very little of the acid dissociates.

Question 22

Write the expression for the ionic product of water, Kw.

Answer 22

Kw = [H+][OH-]

Question 23

What are the units for Kw?

Answer 23

mol^2 dm^-6

Question 24

What physical factors affect the value of Kw? How?

Answer 24

Temperature only — if temp is increased, the equilibrium moves to the right so Kw increases and the pH of pure water decreases.

Question 25

Define the term strong base.

Answer 25

Base that dissociates 100% in water.

Question 26

Give some examples of strong bases.

Answer 26

- NaOH - KOH - Ca(OH)2

Question 27

Give an example of a weak base.

Answer 27

Ammonia

Question 28

Write the equation uses to calculate [H+] of strong bases.

Answer 28

[H+] = Kw/ [OH-]

Question 29

Define a buffer solution

Answer 29

A mixture that minimises pH change on addition of small amounts of an acid or a base

Question 30

What are the 2 ways in which buffers can be made?

Answer 30

- weak acid and its conjugate base - weak acid and a strong alkali

Question 31

In which direction does the equilibrium shift when an acid is added to a buffer solution? Why?

Answer 31

Equilibrium shifts to the left because [H+] increases and the conjugate base reacts with the H+ to remove most of the H+

Question 32

In which direction does the equilibrium shift when an alkali is added to a buffer solution? Why?

Answer 32

Equilibrium shifts to the right, because [OH-] increases and the small concentration of H+ reacts with OH-. To restore the H+ ions HA dissociates shifting the equilibrium.

Question 33

Write the equation used to calculate [H+] of buffer solution.

Answer 33

[H+] = Ka x [HA]/[A-]

Question 34

Which buffer system maintains blood pH at 7.4? What happens when an acid/alkali is added?

Answer 34

H+ + HCO3- —> CO2 + H2O Add OH- —> reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost Add H+ —> equilibrium shifts to the right, removing excess H+

Question 35

What is a titration?

Answer 35

The addition of an acid/base of known concentration to a base/acid to determine the concentration. An indicator is used to show that neutralisation has occurred, as is a pH meter

Question 36

Define the term equivalence point.

Answer 36

The point at which the exact volume of base has been added to just neutralise the acid, or vice versa.

Question 37

What is the end point?

Answer 37

The point at which pH changes rapidly

Question 38

What are the properties of a good indicator for a reaction? (3)

Answer 38

- sharp colour change - end point must be the same as the equivalence point - distinct colour change so it is obvious when the end point has been reached

Question 39

What indicator would you use for a strong acid-strong base titration?

Answer 39

Phenolphthalein

Question 40

What colour is methyl orange in acid? In alkali?

Answer 40

Red in acid; yellow in alkali.

Question 41

What colour is phenolphthalein in acid? In alkali?

Answer 41

Colourless in acid; red in alkali

Question 42

What colour is bromothymol blue in acid? In alkali?

Answer 42

Yellow in acid and blue in alkali

Question 43

Describe how to use a pH meter

Answer 43

- remove the pH probe from storage solution and rinse with distilled water - dry the probe and place it into the solution with unknown pH - let the probe stay in the solution until it gives a settled reading