Topic 8: Acids and Bases

Question 1

What is the Bronsted-Lowry theory of Acids and Bases?

Answer 1

Acids are proton/H+ donors. Bases are proton/H+ acceptors.

Question 2

What charges do the conjugate pairs gain?

Answer 2

The conjugate acid gains a positive charge.
The conjugate base gains a negative charge.

Question 3

What colour is Litmus in acids and bases?

Answer 3

Red in acids.
Blue in bases.

Question 4

What colour is Methyl orange in acids and bases?

Answer 4

Red in acids.
Yellow in bases.

Question 5

What colour is phenolphthalein in acids and bases?

Answer 5

Colourless in acids.
Pink in bases.

Question 6

Give the formula for an acid reacting with a base

Answer 6

Acid + base —> Salt + water

Question 7

Give the formula for an acid reacting with a metal oxide

Answer 7

Acid + metal oxide —> Salt + water

Question 8

Give the formula for an acid reacting with a metal

Answer 8

Acid + metal —> Salt + hydrogen

Question 9

Give the formula for an acid reacting with a carbonate

Answer 9

Acid + carbonate —> Salt + water + carbon dioxide

Question 10

What does a change of one pH unit represent?
And if the pH increases how does this affect [H+]?

Answer 10

A ten-fold change in [H+].
Increase in pH = decrease in [H+]

Question 11

Dilute acids vs concentrated acids (+ what ion does so)

Answer 11

Dilute acids demonstrate the typical properties of acids.
i.e. they turn litmus red

Concentrated acids do not demonstrate the typical properties of acids.

*IT IS THE HYDRONIUM ION THAT GIVES ACIDS THEIR ACID PROPERTIES

Question 12

Define strong acids

Answer 12

Strong acids ionize/dissociate almost completely in water. “—>”

Question 13

Define weak acids

Answer 13

Weak acids will transfer a few protons to water. They mostly remain as molecules. “⇌”

Question 14

Give three examples of strong acids and three examples of weak acids

Answer 14

Hydrochloric acid (HCl)
Nitric acid (HNO₃)
Sulfuric acid (H₂SO₄)
——
Phosphoric acid (H₃PO₄)
Ethanoic acid (CH₃COOH)
Carbonic acid (H₂CO₃)

Question 15

Give four examples of strong bases and two examples of weak bases

Answer 15

Lithium hydroxide LiOH
Sodium hydroxide NaOH
Potassium hydroxide KOH
Barium hydroxide Ba(OH)₂
——
Ammonia NH₃
Ethylamine CH₃CH₂NH₂

Question 16

Electrical conductivity, rate of reaction and pH for strong and weak acids / bases

Answer 16

Strong acids and bases
- Good electrical conductors as lots of ions (charged particles)
- Fast rate of reaction
- pH
Acids: Low as many H+ ions
Bases: High as many OH- ions

Weak acids and bases
- Poor electrical conductors as few ions
- Slow rate of reaction
- pH
Acids: Weakly acidic as less H+ ions
Bases: Weakly basic as less OH- ions

Question 17

A weak acid has a …
A strong acid has a …

Answer 17

strong conjugate base.
weak conjugate base (and vice versa)

Question 18

Normal rainwater vs acid rain

Answer 18

Normal rainwater is naturally acidic due to the presence of dissolved carbon dioxide which forms weak carbonic acid. Normal rain has a pH above 5.6.

Acidic rain is caused by sulfur and nitrogen oxides and has a pH below 5.6.

Question 19

Describe how sulfur oxides contribute to acid rain

Answer 19

Sulfur dioxide is produced by the burning of fossil fuels.
It can then produce sulfurous acid (H₂SO₃), sulfur trioxide (SO₃) or sulfuric acid (H₂SO₄).

Question 20

Describe how nitrogen oxides contribute to acid rain

Answer 20

Nitrogen monoxide is produced by internal combustion engines in vehicles.
It can produce nitrogen dioxide (NO₂), nitrous acid (HNO₂) and nitric acid (HNO₃).

Question 21

What are the effects of acid deposition?

Answer 21

• Impact on materials. Corrodes metals and erodes structures. (Nitrous acid and sulfurous acid)
• Is toxic to plant life as it stops plants’ ability to undergo photosynthesis and damages the roots.
• Is toxic to marine life/affects marine ecosystems. Contributes to eutrophication (algae growth) and lake acidification. (Carbonic acid)
• Can cause respiratory issues in humans.

Question 22

How can SO₂ emissions be reduced?

Answer 22

Pre-combustion methods
- Crushing coal and washing with water
- Hydrodesulfurization (reacts sulfur with hydrogen to form hydrogen sulfide)

Post-combustion methods
- Flue gas desulfurization

Question 23

How can NO₂ emissions be reduced?

Answer 23

Pre-combustion methods
- Catalytic converters in vehicles
- Lower temperature of combustion (this reduces the amount of nitrogen monoxide produced)

Question 24

What is the Lewis theory of Acids and Bases?

Answer 24

A Lewis acid is an electron acceptor (it is electron deficient) i.e. an electrophile

A Lewis base is an electron donor (it is electron-rich) i.e. a nucleophile.
(REMEMBER BASE MAKES BANK BC THEY’RE RICH)

Question 25

What is the value of pKw?

Answer 25

14

Question 26

What is the value of Kw?

Answer 26

10^-14

Question 27

What type of relationship does K of acids or bases have with pK of acids or bases? Demonstrate with strong acids.

Answer 27

Inverse relationship. i.e. in strong acids Ka is high (bc lots of products are formed) and the pKa is low.

Question 28

What are the assumptions we make when calculating the pH of weak acids or bases?

Answer 28

The concentration of the products are equal. The concentration of the reactant stays the same because so little has broken down.

Question 29

Define amphiprotic

Answer 29

Able to both donate or accept a proton (act as a Bronsted-Lowry acid or base). Remember 'h' in amphiprotic as H+ is the basis of the Bronsted-Lowry definition.

Question 30

Define amphoteric

Answer 30

Able to act as an acid or a base/display acidic or basic properties.

Question 31

Define a buffer solution

Answer 31

An aqueous solution consisting of a weak acid/base and its conjugate pair that resists changes to pH when small amounts of acid or base are added. This is because the solution is in equilibrium (Le Chatelier's principle - the system will act to minimise the effect of the change).

Question 32

Preparations of a buffer solution

Answer 32

1) Mixing roughly equal proportions of a weak acid/base and its conjugate pair 2) Partially neutralising a weak acid with a strong base

Question 33

At the equivalence point, a strong acid and a strong base give what pH? (pH curves)

Answer 33

pH of 7

Question 34

At the equivalence point, a strong acid and a weak base give what pH? (pH curves)

Answer 34

pH<7 (Buffer region is present. This is where the pH is hardly changing.)

Question 35

At the equivalence point, a weak acid and a strong base give what pH? (pH curves)

Answer 35

pH>7 (Buffer region is present. This is where the pH is hardly changing.)

Question 36

At the equivalence point, a weak acid and a weak base give what pH? (pH curves)

Answer 36

pH of 7

Question 37

Where is the equivalence point on a pH curve?

Answer 37

Halfway up the point of inflexion (the steep line).

Question 38

On a pH curve where is the pKa found?

Answer 38

Half of the equivalence point.

Question 39

Why with a weak acid and strong base or strong acid and weak base is the pH NOT 7?

Answer 39

Because one of the products of the neutralisation reaction will react further in hydrolysis (proton transfer).

Question 40

How do we select an indicator for titrations?

Answer 40

If the pKa is + or - 1 of the equivalence point. i.e. it changes colour at pH values close to the equivalence point and does not interfere with the reaction.

Question 41

What are acid-base indicators?

Answer 41

Acid-base indicators are weak acids or weak bases. The species differs in colour from its conjugates. Hln ⇌ H⁺ + ln⁻ Where Hln is colour 1, ln⁻ is colour 2.