Chapter 20 - Acids, Bases and pH

Question 1

What is the Arrenhius model of acids and bases ?

Answer 1

• Acids dissociate and release H+ ions in aqueous solution
• Alkalis dissociate and release OH- ions in aqueous solution
• H+ ions are neutralised by OH- ions to form water

Question 2

What is an alkali ?

Answer 2

An alkali is a soluble base

Question 3

What is a bronsted lowry acid ?

Answer 3

A proton donor

Question 4

What is a bronsted lowry base ?

Answer 4

A proton acceptor

Question 5

What is a conjugate acid-base pair ?

Answer 5

Two species that can be interconverted by transfer of a proton

Question 6

In the dissociation of HCl to H+ and Cl-, identify the conjugate acid and base.

Answer 6

• HCl releases a proton so it is a conjugate acid
• Cl- accepts a proton therefore it is a conjugate base

Question 7

In an aqueous solution what does dissociation require ?

Answer 7

It requires a proton to be transferred from an acid to a base

Question 8

When water is the base, what is formed ?

Answer 8

• The hydronium ion
• H3O+

Question 9

Why is the hydronium ion important ?

Answer 9

It is the active ingredient in any aqueous solution

Question 10

What do the terms monobasic, dibasic and tribasic refer to?

Answer 10

The total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction.

Question 11

How many hydrogens in a monobasic acid ?

Answer 11

1

Question 12

How many hydrogens in a dibasic acid ?

Answer 12

2

Question 13

How many hydrogens in a tribasic acid ?

Answer 13

3

Question 14

How can redox reactions be simplified ?

Answer 14

Remove the spectator ions

Question 15

What is the word equation for the reaction of an acid and a metal ?

Answer 15

Acid + metal → salt + hydrogen

Question 16

What is the word equation for the reaction of an acid and a carbonate ?

Answer 16

Acid + carbonate → salt + water + carbon dioxide

Question 17

What is the word equation for the reaction of an acid and a base ?

Answer 17

Acid + base → salt + water

Question 18

What is the word equation for the reaction of an acid and an alkali ?

Answer 18

Acid + alkali -> salt + water

Question 19

What is the relationship between pH and concentration of H+?

Answer 19

• Low value [H+] = high pH
• High value [H+] = low pH

Question 20

What is the equation for working out pH from [H+]?

Answer 20

pH = -log[H+] (base 10)

Question 21

How is the pH calculated for a strong acid?

Answer 21

Assume it fully dissociates therefore [H+] = [HA]

Question 22

How is the new pH calculated for a strong acid on dilution?

Answer 22

• Work out change in concentration of HA and therefore [H+]
• Then put it back into pH = -log[H+]

Question 23

What does a pH less than 7 show ?

Answer 23

It shows increasing acidity

Question 24

What does a pH more than 7 show ?

Answer 24

It shows increasing alkalinity

Question 25

What is a pH of 7 ?

Answer 25

Neutral

Question 26

What is a change in one pH number equal to ?

Answer 26

It is equal to a 10 times difference in [H+]

Question 27

What is the general formula for the dissociation of a weak acid ?

Answer 27

HA -><- H+ + A-

Question 28

How is the acid dissociation constant calculated ?

Answer 28

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

Question 29

What are the units for the acid dissociation constant ?

Answer 29

moldm^-3

Question 30

What is the equilibrium like the larger the numerical value ?

Answer 30

- The greater the dissociation - The greater the acid strength

Question 31

Why do we use pKa ?

Answer 31

- Because it is difficult to use the Ka values that have a negative indices - We convert the Ka value into a negative logarithm called pKa

Question 32

What is the equation for pKa ?

Answer 32

pKa = -log(10)Ka

Question 33

How do we get from pKa back to Ka ?

Answer 33

Ka = 10^-pKa

Question 34

What are the pKa and Ka values like for a strong acid ?

Answer 34

- Large Ka value - Smaller pKa value

Question 35

What are the pKa and Ka values like for a weak acid ?

Answer 35

- Small Ka value - Large pKa value