1.12 Acids & Bases

Question 1

What is a Bronsted-Lowry acid?

Answer 1

Proton donor

Question 2

What is a Bronsted-Lowry base?

Answer 2

Proton acceptor

Question 3

What ion causes a solution to be acidic?

Answer 3

Hydrogen ions (H⁺)

Question 4

What ion causes a solution to be alkaline?

Answer 4

Hydroxide ions (OH⁻)

Question 5

Write an equation for the ionisation of water.

Answer 5

H₂O (l) ⇌ H⁺ (aq) + OH⁻ (aq)

Question 6

Derive Kw using the equation for the ionisation of water.

Answer 6

Keq = [H⁺][OH⁻] / [H₂O]
[H₂O] Keq = [H⁺][OH⁻]
[H₂O] is so large compared to [H⁺] and [OH⁻] that [H₂O] Keq can be considered to be constant
[H₂O] Keq = Kw
∴ Kw = [H⁺][OH⁻]

Question 7

What is the value of Kw at 298K?

Answer 7

1.0 x 10⁻¹⁴

Question 8

What physical factors affect the value of Kw?

Answer 8

Temperature only

Question 9

How does temperature affect the value of Kw?

Answer 9

If temperature is increased, the equlibrium moves to the right so Kw increases and the pH of pure water decreases

Question 10

Why is pure water still neutral, even if pH does not equal 7?

Answer 10

[H⁺] = [OH⁻]

Question 11

Give an expression for pH in terms of H⁺.

Answer 11

pH = - log₁₀[H⁺]

Question 12

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

Answer 12

Lower pH = higher concentration of H⁺

Question 13

If two solutions have a pH difference of 1, what is the difference in [H⁺]?

Answer 13

A factor of 10

Question 14

How is [H⁺] found from pH?

Answer 14

[H⁺] = 10⁻ᵖᴴ

Question 15

How is [OH⁻] found from pH?

Answer 15

Find [H⁺]
Use Kw (1.0 x 10⁻¹⁴ ) to calculate [OH⁻]

Question 16

What is different when finding [H⁺] from the concentration of diprotic and tripotic acids?

Answer 16

Need to multiply the concentration of the acid by the number of protons to find [H⁺]

Question 17

What is a diprotic acid?

Answer 17

A polyprotic acid that can donate two protons or hydrogen atoms per molecules to an aqueous solution

Question 18

What is a polyprotic acid?

Answer 18

Acids capable of losing more than a single proton per molecule in acid-base reactions

Question 19

What is a tripotic acid?

Answer 19

A polyprotic acid that can donate three protons or hydrogen atoms per molecules to an aqueous solution

Question 20

How is the pH of a strong alkaline solution calculated?

Answer 20

Use Kw to calculate [H⁺] from [OH⁻]
Use pH = -log[H⁺]

Question 21

What is a strong acid?

Answer 21

An acid which fully dissociates in water to release H⁺ ions
(HX -> H⁺ + X⁻)

Question 22

What is a strong base?

Answer 22

A base which fully dissociates in water to release OH⁻ ions
(XOH -> X⁺ + OH⁻)

Question 23

What is the difference between concentration and strength?

Answer 23

Concentrated = many mol per dm³
Strong = amount of dissociation

Question 24

What is a weak acid?

Answer 24

An acid that only partially dissociates in water to release H⁺ ions

Question 25

What is a weak base?

Answer 25

A base that only partially dissociates in water to release OH⁻ ions

Question 26

Give 3 examples of strong acids.

Answer 26

Hydrochloric acid (HCl) Sulfuric acid (H₂SO₄) Phosphoric acid (H₃PO₄)

Question 27

Give 3 examples of strong bases.

Answer 27

Sodium hydroxide (NaOH) Calcium Carbonate (CaCO₃) Sodium Carbonate (Na₂CO₃)

Question 28

Give examples of weak acids.

Answer 28

Ethanoic acid (CH₃COOH) Any organic acid

Question 29

Give an example of a weak base.

Answer 29

NH₃

Question 30

What is Ka?

Answer 30

For acid HA: HA ⇌ H⁺ + A⁻ Ka = [H⁺][A⁻] / [HA]

Question 31

How is the pH of a weak acid found?

Answer 31

Use the equation for Ka, subbing in values for [A⁻] and [HA] USe pH = - log[H⁺] equation to find pH

Question 32

What is a titration?

Answer 32

The addition of an acid/base of known titration to a base/acid of unknown titration to determine the concentration An indicator is used to show that neutralisation has occurred

Question 33

Define the term equilavence point.

Answer 33

The point at which the exact volume of acid/base has been added to just neutralise the base/acid

Question 34

What generally happens to the pH of a solution around the equivalence point?

Answer 34

A large and rapid change in pH Not in weak-weak titration

Question 35

How is the unknown concentration calculated in a titration?

Answer 35

Calculate mols of one reactant Balanced equation to work out mols of other Use conc = mol/vol

Question 36

What is the end point of a titration?

Answer 36

The volume of acid/alkali added when the indicator just changes colour

Question 37

Name the 3 properties of a good indicator for a titration.

Answer 37

Sharp colour change End point same as equivalence point Distinct colour change

Question 38

What indicator is used for a strong acid-strong base titration?

Answer 38

Phenolphthalein (or methyl orange)

Question 39

What indicator is used for a strong acid-weak base titration?

Answer 39

Methyl orange

Question 40

What indicator is used for a weak acid-strong base titration?

Answer 40

Phenolphthalein

Question 41

What indicator is used for a weak acid-weak base titration?

Answer 41

Neither methyl orange or phenolphthalein give sharp colour change

Question 42

What colour is methyl orange?

Answer 42

Red in acid Yellow in alkali

Question 43

What pH does methyl orange change colour?

Answer 43

4-5

Question 44

What colour is phenolphthalein?

Answer 44

Colourless in acid Red in alkali

Question 45

What pH does phenolphthalein change colour?

Answer 45

9-10

Question 46

What is the half neutralisation point of a titration?

Answer 46

Volume = half volume added at equivalence point

Question 47

What is a buffer solution.

Answer 47

A solution that resists changes in pH when small amount of acid/alkali are added

Question 48

What do acidic buffer solutions contain?

Answer 48

A weak acid and a soluble salt of that acid that fully dissociates

Question 49

Write a reaction for an acidic buffer with added acid.

Answer 49

H⁺ + A⁻ -> HA

Question 50

Write a reaction for an acidic buffer with added acid.

Answer 50

HA + OH⁻ -> H₂O + A⁻

Question 51

What do basic buffer solutions contain?

Answer 51

Weak base and soluble salt of that weak base

Question 52

How can the pH of buffer solutions be calculated?

Answer 52

Use Ka of weak acid Sub in [A⁻] and [HA] Calculate [H⁺] to pH

Question 53

How can the new pH of a buffer solution be calculated after an acid or base is added?

Answer 53

Calculate moles of H⁺, A⁻ & HA before acid/base added Use equations to work out new moles of A⁻ & HA -> find [H⁺] -> pH

Question 54

What products are buffers found in?

Answer 54

Shampoos Detergents Maintain pH to avoid damage to skin, hair, fabrics