MITIHANI POPOTE EXAMINATIONS SERIES FORM FOUR CHEMISTRY EXAMINATION SERIES 10 (With Marking Guide)

FORM FOUR CHEMISTRY EXAMINATION SERIES 10

Chemistry Examination Answers
Series 10

Section A (15 Marks)

Question 1(i)

Splints are among the first aid kit components. Which of the following sentences best describes its use?

C. Used to support broken bones and is tied using bandages

Splints serve crucial medical purposes:

• Immobilization: Prevent movement of fractured bones to avoid further injury
• Pain reduction: Minimize pain by stabilizing the injury
• Healing: Maintain proper bone alignment during healing
• Types: Can be rigid (wood/metal) or flexible (inflatable)

First aid splints are temporary until professional medical care is available.

Question 1(ii)

A simple proof that some chemical reactions take place in our bodies is that

D. The food we eat or the drinks we take are quite different from the waste products from our bodies

This demonstrates metabolic transformations:

• Digestion: Complex carbohydrates → simple sugars
• Respiration: Glucose + O₂ → CO₂ + H₂O + energy
• Protein metabolism: Amino acids → urea
• Evidence: Chemical analysis shows food (input) and excretory products (output) have completely different compositions

Question 1(iii)

The reaction, 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g), is an example of a

C. Redox reaction

This is the Ostwald process for nitric acid production:

• Oxidation: N changes from -3 (NH₃) to +2 (NO)
• Reduction: O changes from 0 (O₂) to -2 (H₂O)
• Simultaneous: Both oxidation and reduction occur
• Industrial importance: First step in nitric acid manufacture

NH₃ oxidation: -3 → +2 (loss of 5e⁻ per N atom)

O₂ reduction: 0 → -2 (gain of 2e⁻ per O atom)

Question 2 - Matching

List A	List B	Explanation
(i) Lime water turns milky when air is pumped	E. Presence of carbon dioxide	CO₂ reacts with Ca(OH)₂ to form insoluble CaCO₃
(ii) Condition for iron to rust	B. Oxygen and water/moisture	Both required for electrochemical corrosion
(iii) Oxygen, Heat and Fuel	D. Fire triangle	Three components needed for combustion
(iv) Coating iron with zinc	G. Galvanization	Zinc sacrificially protects iron from rusting
(v) Flammable liquids	H. Class B fire	Classification for liquid fuel fires

Section B (70 Marks)

Question 3 7 marks

(a) Water hardness analysis:

(i) Harder water brand: Uhai

Hardness is primarily caused by Ca²⁺ and Mg²⁺ ions:

• Uhai: 3.05 mg/L Ca²⁺ + 4.15 mg/L Mg²⁺ = 7.2 mg/L total hardness
• Dasani: 2.69 mg/L Ca²⁺ + 0.11 mg/L Mg²⁺ = 2.8 mg/L total hardness
• Higher mineral content = greater hardness

(ii) Calcium benefits:

1. Essential for bone and teeth development
2. Required for muscle function and nerve transmission
3. Helps blood clotting

(iii) Water treatment: Chlorine is added as disinfectant

(b) Hydrogen vs. Phosphorus:

(i) More electropositive: Hydrogen

Electropositivity decreases across periods:

• Hydrogen (Group 1) more likely to lose electrons
• Phosphorus (Group 15) more electronegative

(ii) Compound formed: PH₃ (Phosphine)

P + 3H → PH₃

Question 4 7 marks

Oxygen preparation:

2H₂O₂(l) → 2H₂O(l) + O₂(g)

(a)(i) MnO₂ role: Catalyst

Manganese dioxide:

• Provides alternative reaction pathway
• Lowers activation energy
• Not consumed in reaction

(ii) Without catalyst: Reaction would be extremely slow

(b) Oxygen test:

1. Insert glowing splint into gas
2. Positive test: splint reignites brightly
3. Confirms presence of oxygen (supports combustion)

Question 7 7 marks

Molecular formula determination:

Given: 86% C, 14% H; 3.2 dm³ = 6g at STP

Step 1: Empirical formula

Element	%	Moles	Ratio
C	86	86/12 = 7.17	7.17/7.14 ≈ 1
H	14	14/1 = 14	14/7.14 ≈ 2

Empirical formula: CH₂ (Mass = 14 g/mol)

Step 2: Molar mass

3.2 dm³ = 0.143 mol (3.2/22.4)

Molar mass = 6g/0.143mol = 42 g/mol

Step 3: Molecular formula

n = 42/14 = 3 → (CH₂)₃ = C₃H₆

Molecular formula: C₃H₆

Section C (15 Marks)

Question 13 15 marks

Water pollution prevention methods:

1. Proper waste disposal:
   • Treat industrial effluents before discharge
   • Implement sewage treatment systems
2. Agricultural management:
   • Controlled fertilizer/pesticide use
   • Establish buffer zones near water bodies
3. Oil spill prevention:
   • Proper maintenance of oil tankers
   • Quick response protocols
4. Public education:
   • Promote proper chemical disposal
   • Encourage water conservation
5. Legislation:
   • Enforce water quality standards
   • Penalize polluters
6. Ecosystem protection:
   • Preserve wetlands (natural filters)
   • Maintain riparian vegetation