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 an alkali?

Answer 3

A soluble base

Question 4

How do you find the pH of a solution? (equation)

Answer 4

-log₁₀[H⁺]

Question 5

What is [H⁺]?

Answer 5

The concentration of hydrogen ions in the solution.

Question 6

What is a strong acid?

Answer 6

When H⁺ fully dissociates from the acid.

Question 7

What is a weak acid?

Answer 7

When H⁺ only partially dissociates from the acid.

Question 8

What is a monoprotic acid?

Answer 8

Releases one H⁺ per molecule of acid.

Question 9

What is a diprotic acid?

Answer 9

Releases two H⁺ per molecule of acid.

Question 10

What is the equation used to find concentration or volume post-dilution?

Answer 10

C₁V₁ = C₂V₂

Question 11

What is the ionic product of water, based on?

Answer 11

The fact that water only dissociates partly.

Question 12

What is the dissociation of water equation?

Answer 12

H₂O ⇌ H⁺ + OH¯

H₂O + H₂O ⇌ H₃O⁺ + OH¯

Question 13

What is the ionic product of water formula?

Answer 13

Kw = [H⁺][OH¯]

Question 14

Why is the concentration of water not in the ionic product of water?

Answer 14

It is in so much excess, that the water concentration is pretty much constant.

Question 15

What is Ka?

Answer 15

The weak acid dissociation constant.

Question 16

What is the weak acid dissociation formula?

Answer 16

[HA]

Question 17

What is pKa?

Answer 17

The pH of a weak acid solution.

Question 18

How do you find pKa?

Answer 18

-log₁₀Ka

Question 19

When does pKa = pH?

Answer 19

When the weak acid is at the half neutralisation point where [A¯] = [HA]

Question 20

What is Ka when the concentration of [H⁺] = [A¯]?

Answer 20

[HA]

Question 21

Why does Kw vary with temperature?

Answer 21

• The forward’s reaction is endothermic.
• So increasing temperature shifts the equilibrium in favour of the forward’s reaction.
• This, therefore, increases the Kw.

Question 22

How do you find the pH of a reaction between a weak acid & strong base? (when acid is in excess)

Answer 22

1. Find acid (HA) moles and OH¯ (base) moles.
2. Find out how much HA is in excess?
3. Find A¯ moles and concentration.
4. Find the excess HA concentration.
5. Use Ka to find [H⁺] and then pH.

Question 23

How do you find the pH of a reaction between a weak acid & strong base? (when the base is in excess)

Answer 23

1. Find acid (HA) moles and OH¯ (base) moles.
2. Find out how much OH¯ is in excess.
3. Use Kw to find [H⁺] and then find the pH.

Question 24

Outline what a titration reaction is?

Answer 24

• Used to find the concentration of a solution by gradually adding a second solution to react with.
• You need to know the equation of the reaction.
• You need to know the concentration of the second solution.
• It is all based on neutralisation between the two solutions.

Question 25

How do you know which indicator to use?

Answer 25

- Must show the endpoint of the reaction with a distinct colour change. - The colour change must be rapid. - Must lie between the vertical part of the acid/base graph.

Question 26

What is the equivalence point between an acid and a base?

Answer 26

When [H⁺] = [OH¯]

Question 27

When is half neutralisation?

Answer 27

When the reaction is halfway between zero and the equivalence point.

Question 28

What does a buffer do?

Answer 28

Maintains an approximately constant pH, and combats the addition/dilution of small amounts of acid & base.

Question 29

What is an acidic buffer made of?

Answer 29

- A weak acid | - A salt of that same weak acid

Question 30

How do you make an acidic buffer? (2 ways)

Answer 30

1. Add a weak acid and a weak acid salt. | 2. The half neutralisation point of a reaction between a weak acid & alkali.

Question 31

What is the pH of a buffer made from half neutralisation?

Answer 31

Because [HA] = [A¯] pH = pKa

Question 32

How does the acid in an acidic buffer oppose the change and keep the pH constant?

Answer 32

The acid, HA, opposed changes when OH is added. The HA reacts with OH to form H₂O + A¯.

Question 33

How does the weak salt in an acidic buffer oppose the change and keep the pH constant?

Answer 33

The initial weak acid dissociation provides a small amount of A¯ so can't oppose all the change from extra H⁺. The salt provides enough A¯, so when excess H⁺ is added it can react with the A¯ and use it up (by forming HA).

Question 34

How do you find the pH of an acidic buffer? (half neutralisation)

Answer 34

1. Find HA & OH¯ moles 2. Find HA & A¯ moles 3. Find [HA] & [A¯] 4. Use Ka to find [H⁺] and the pH.

Question 35

How do you find the pH of an acidic buffer? (acid + salt)

Answer 35

2. Find HA & A¯ moles 3. Find [HA] & [A¯] 4. Use Ka to find [H⁺] and the pH.

Question 36

How do you find the pH after adding some acid/base to the buffer solution?

Answer 36

1. Find original HA & A¯ moles 2. Find the moles of what is added. - If you add acid (H⁺), add these moles to HA, and take them away from A¯. - If you add a base (OH¯), take these away from A¯, and take them away from HA.