5a. Acids

Question 1

Acid

Definition

Answer 1

An acid is a substance which dissociates in water/aqeous solution to give hydrogen ions.

Question 2

Describe what happens when a pure acid is dissolved in an organic solvent vs water

Answer 2

In an organic solvent (methylbenzene/propanone):
- In the absence of water, pure acid (e.g. hydrogen chloride) still exists as simple molecules
- The acid molecules do not dissociate to form hydrogen ions.
- No hydrogen ions are formed.
- As the presence of hydrogen ions are responsible for the acidic property, the solution is neutral.

In water:
- In the presence of water, acid molecules (e.g. hydrogen chloride) completely/partially dissociate to give hydrogen ions (and chloride ions).
- As the presence of hydrogen ions are responisble for the acidic property, the solution is acidic.

Question 3

Basicity of an Acid

Definition

Answer 3

Basicity of an acid is the maximum number of hydrogen ions produced by one moleule of the acid when the acid molecule dissociates in water.

Monobasic acids: hydrochloric acid HCl, nitrc acid HNO₃, ethanoic acid CH₃COOH
Dibasic acids: sulfuric acid H₂SO₄, carbonic acid H₂CO₃
Tribasic acids: phosphoric acid H₃PO₄

Question 4

Strength of an Acid

Definiton

Answer 4

Strength of an acid depends on the extent of dissociation of the acid molecules in water.

Question 5

Strong Acid

Definition

Answer 5

A strong acid is one that completely dissociates (→) in water to give hydrogen ions.

e.g. hydrochloric acid HCl, sulfuric acid H₂SO₄, nitric acid HNO₃
All acid molecules form ions.

Question 6

Weak Acid

Definition

Answer 6

A weak acid is one that partially dissociates (⇌) in water to give hydrogen ions.

e.g. ethanoic acid CH₃COOH, carbonic acid H₂CO₃, phosphoric acid H₃PO₄
Most acid molecules remain unchanged; few ions are formed.

Question 7

Concentration of a Solution

Definition

Answer 7

Concentration of a solution is a measure of how much solute has dissolved in 1 dm³ of the solution.

mol/dm³ is the unit for concentration

Question 8

Explain why aqueous acids can conduct electricity

Answer 8

In the presence of water, acid molecules completely/partially dissociate to form ions. These ions can move freely in water to conduct electricity.

Question 9

Why must blue litmus paper be moist to test for acidic gases such as sulfur dioxide?

Answer 9

SO₂(g) + H₂O(l) → H₂SO₃(aq)
Sulfur dioxide molecules react with water to form sulfurous acid.
H₂SO₃(aq) ⇌ 2H⁺(aq) + SO₃²⁺(aq)
Sulfurous acid partially dissociates in water to form hydrogen ions which turn blue litmus paper red.

Question 10

Physical and Chemical Properties of Acids

Answer 10

Physical properties of acids:

• Sour
• Turn moist blue litmus paper red
• pH < 7
• Can conduct electricity (are electrolytes)

Chemical properties of acids:

• Acids react with metals above hydrogen in the reactivity series to form salt and hydrogen gas only.
• Acids react with metal carbonates / metal hydrogen carbonates to form salt, carbon dioxide, and water.
• Acids react with bases (metal oxides / metal hydroxides) to form salt and water only (neutralisation).

Question 11

Why do some reactive metals (e.g. lead) have no apparant reaction with acids?

Answer 11

Lead metal appears not to react with hydrochloric acid or sulfuric acid.
The initial reaction produces a layer of lead(II) chloride or lead(II) sulfate. This layer is insoluble in water/aqueous solution and coats onto the surface of the lead metal, which prevents further reaction between the acid and the lead metal.

Question 12

Uses of Acids

Answer 12

1. Manufacture of fertilisers - Sulfuric acid and nitric acid are important starting materials for the prodcution of ammonium sulfate and ammonium nitrate, which are active ingredients in fertilisers.
2. Battery acid in cars - Dilute sulfuric acid reacts with lead plates and lead(IV) oxide plates in the battery to release electrical energy.
3. Pickling of metals - Hydrochloric acid is used to remove rust which consists mainly of iron(III) oxide. Hydrochloric acid will react with ruse to form iron(III) chloride.
4. Food production - Phosphoric acid is added to food and beverages to give them a sour taste. Carbonic acid is used to make fizzy drinks. Ethanoic acid acts as a food preservative and flavour enhancer.