Topic 8 & 18 - Acids and bases

Question 1

Define Bronsted-Lowry acid and base

Answer 1

A Bronsted-Lowry acid donates a proton (BADP)

A Bronsted-Lowry base accepts a proton (BBAP)

Question 2

Define Lewis acid and base

Answer 2

A Lewis acid accepts (a pair of) electrons (LAAE)

A Lewis base donates (a pair of) electrons (LBDE)

Question 3

What are the requirements for a substance to be a Bronsted-Lowry acid or base?

Answer 3

An acid must be able to dissociate and release H⁺

A base must be able to accept H⁺ (must have a lone pair of electrons)

Question 4

Deduce the conjugate acid and base pairs of a Bronsted-Lowry base and acid

Answer 4

Question 5

What can we say about the relationship between Bronsted-Lowry and Lewis acids?

Answer 5

All Bronsted-Lowry acids are Lewis acids but not vice versa

Question 6

What are alkalis?

Answer 6

Bases that dissolve in water to form OH^-1

Question 7

List four types of bases that are not hydroxides

Answer 7

1. Metal oxides
2. Ammonia
3. (soluble) Carbonates
4. Hydrogencarbonates

Question 8

What is a universal indicator?

Answer 8

A mixture of several indicators that can be used to identify acids and bases on the whole pH scale

Question 9

Define salt

Answer 9

The compound formed when the hydrogen of an acid is replaced by a metal or another positive ion

Question 10

What are spectator ions in acid-base reactions?

Answer 10

Species that do not change during the reaction (can be cancelled out)

Question 11

What is the reaction between acids and metals? Give three examples

Answer 11

Acid + metal → salt + hydrogen

2 HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)

H₂SO₄(aq) + Fe(s) → FeSO₄(aq) + H₂(g)

2 CH₃COOH(aq) + Mg(s) → Mg(CH₃COOH)₂(aq) + H₂(g)

Question 12

What is the reaction between acids and bases? Give three examples

Answer 12

Acid + base → salt + water

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

HNO₃(aq) + NH₄OH(aq) → NH₄NO₃(aq) + H₂O(l)

2 CH₃COOH(aq) + CuO(s) → Cu(CH₃COO)₂(aq) + H₂O(l)

Question 13

What is the reaction between acids and carbonates? Give three examples

Answer 13

Acid + carbonate → salt + water + carbon dioxide

2 HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)

H₂SO₄(aq) + Na₂CO₃(s) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)

CH₃COOH(aq) + KHCO₃(s) → KCH₃COO(aq) + H₂O(l) + CO₂(g)

Question 14

Define effervescence

Answer 14

A reaction that gives off gas and produces visible bubbles (acid + metal, acid + carbonate)

Question 15

Distinguish between strong and weak acids and bases

Answer 15

Strong acids and bases dissociate almost completely in solution.

Weak acids and bases only partially dissociate in solution.

Question 16

What are three examples of strong acids?

Answer 16

1. Hydrochloric acid, HCl
2. Nitric acid, HNO₃
3. Sulfuric acid, H₂SO₄

Question 17

What are three examples of weak acids?

Answer 17

1. Ethanoic acid, CH₃COOH
2. Carbonic acid, H₂CO₃
3. Phosphoric acid, H₃PO₄

Question 18

What are four examples of strong bases?

Answer 18

1. Lithium hydroxide, LiOH
2. Sodium hydroxide, NaOH
3. Potassium hydroxide, KOH
4. Barium hydroxide, Ba(OH)₂

Question 19

What are two examples of weak bases?

Answer 19

1. Ammonia, NH₃
2. Ethylamine, C₂H₅NH₂

(other amines)

Question 20

When are comparisons between the strength of acids and bases valid?

Answer 20

When the concentration and the temperature of the solutions are the same

Question 21

List and describe three ways of distinguishing between strong and weak acids and bases

Answer 21

1. Electrical conductivity
   - Depends on the concentration of mobile ions
   - Strong show higher conductivity than weak
   - Can be measured by a conductivity meter or probe
2. Rate of reaction
   - Strong will produce H⁺ ions at a faster rate than weak
3. pH
   - The higher the H⁺ concentration, the lower the pH

Question 22

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

Answer 22

pH = –log[H⁺]

[H⁺] = 10^–pH

Question 23

What is the change in [H⁺] when the pH changes by one unit?

Answer 23

A 10-fold change

Question 24

What are values of pH for solutions that are acidic, neutral, and alkaline?

Answer 24

**Acidic **= pH < 7

**Neutral **= pH = 7

Alkaline (basic) = pH > 7

Question 25

What is the expression for the ionic product constant of water (K_w)?

Answer 25

K_w = [H⁺][OH^–]

Question 26

K_w is dependent on?

Answer 26

Temperature

Question 27

What is the relationship between the concentrations of H⁺ and OH^–?

Answer 27

Inversely proportional

Question 28

What is the pOH scale used for?

Answer 28

Describing the OH^– concentration of solutions

Question 29

What is the relationship between pOH and [OH^–]?

Answer 29

pOH = –log[OH^–] [OH^–] = 10^–pOH

Question 30

What is the relationship between pH and pOH at 25°C?

Answer 30

pH + pOH = 14

Question 31

What is the relationship between pK_w and K_w?

Answer 31

pK_w = –log(K_w) K_w = 10^–pK_w

Question 32

What is the relationship between pH, pOH, and pK_w?

Answer 32

pH + pOH = pK_w

Question 33

What are dissociation constants used for?

Answer 33

Expressing the strength of acids and bases

Question 34

Define the acid dissociation constant

Answer 34

The greater the value of K_a, the greater the dissociation, and so the stronger the acid

Question 35

Define the base dissociation constant

Answer 35

The greater the value of K_b​, the greater the dissociation, and so the stronger the base

Question 36

What is the relationship between K_a, K_b, and K_w?

Answer 36

K_a X K_b = K_w

Question 37

What are the relationships between K_a and pK_a and between K_b and pK_b?

Answer 37

pK_a = –logK_a pK_b = –logK_b K_a = 10^–pK_a K_b = 10^–pK_b The relationship between K_a and pK_a and between K_b and pK_b is inverse → The larger the pK_a, the weaker the acid → The larger the pK_b, the weaker the base

Question 38

What is the relationship between pK_a, pK_b, and pK_w?

Answer 38

pK_a + pK_b = pK_w = 14

Question 39

What is a buffer solution?

Answer 39

A solution that resist the changes in pH on the addition of *small amounts* of acid or alkali

Question 40

What are the two types of buffer solution?

Answer 40

1. Acidic buffers that maintain the pH below 7 2. Basic buffers that maintain the pH above 7

Question 41

How is an acidic buffer made? Give an example

Answer 41

By mixing an aqueous solution of a weak acid with a solution of its salt of a strong alkali CH₃COOH(aq) + NaCH₃COO(aq) The mixture will contain high concentration of CH₃COOH and CH₃COO^– (acid and conjugate base) that are ready to react with OH^– and H⁺

Question 42

Describe the reaction of CH₃COOH + NaCH₃COO buffer solution with acid

Answer 42

H⁺ combines with the base CH₃COO^– to for CH₃COOH, decreasing the acidity

Question 43

Describe the reaction of CH₃COOH + NaCH₃COO buffer solution with base

Answer 43

OH^– combines with the acid CH₃COOH to form CH₃COO^– and H₂O₂, removing the OH^– ions.

Question 44

How is a basic buffer made? Give an example

Answer 44

By combining a weak base with its salt of a strong acid NH₃(aq) + NH₄Cl(aq) Basic buffers work exactly the same way as acidic except the other way round

Question 45

How are pH and pOH of a buffer solution determined?

Answer 45

Question 46

How does dilution influence buffers?

Answer 46

Does not change the pH but lowers its buffering capacity

Question 47

What factors can influence buffers?

Answer 47

Temperature and dilution

Question 48

What happens to the pH in anion hydrolysis?

Answer 48

It increases

Question 49

What happens to the pH in cation hydrolysis?

Answer 49

It decreases

Question 50

Outline the possible hydrolysis combinations and the nature of their salts

Answer 50

1. Strong acid + strong base = neutral salt 2. Weak acid + strong base = basic salt 3. Strong acid + weak base = acidic salt

Question 51

What is the equivalence point?

Answer 51

The point when stoichiometrically equivalent amounts of acid and base have been reacted together. At this point the solution contains only salt and water.

Question 52

Give an example and draw the titration curve of the reaction of a strong acid and strong base

Answer 52

HCl + NaOH → NaCl + H₂O

Question 53

Give an example and draw the titration curve of the reaction of a weak acid and strong base

Answer 53

CH₃COOH + NaOH ⇔ NaCH₃COO + H₂O

Question 54

Give an example and draw the titration curve of the reaction of a strong acid and weak base

Answer 54

HCl(aq) + NH₃(aq) ⇔ NH₄Cl(aq)

Question 55

Give an example and draw the titration curve of the reaction of a weak acid and weak base

Answer 55

CH₃COOH(aq) + NH₃(aq) ⇔ CH₃COONH₄(aq)

Question 56

Describe the action of an acid-base indicator

Answer 56

HIn(aq) ⇔ H⁺(aq) + In^–(aq) - Weak acids or weak bases in which the undissociated and dissociated forms have different colours - HIn has colour A and In^– has colour B - H⁺ concentration determines to which side the equilibrium shifts and thus changes colour

Question 57

When do indicators change colour?

Answer 57

When the pH is equal to their pK_a (the end point) **K_a = [H⁺] or pK_a** **= pH**

Question 58

What can indicators be used for in titrations?

Answer 58

To identify the equivalence point