MITIHANI POPOTE EXAMINATION SERIES: FORM FOUR CHEMISTRY SERIES 01 (With Marking Guide)

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Chemistry Examination Answers

Section A

Question 1

i. B. 0.2

To deposit calcium from molten calcium chloride (CaCl₂), the reaction is:
Ca²⁺ + 2e⁻ → Ca
Moles of calcium = mass/molar mass = 4g/40g/mol = 0.1 mol
Since 1 mole of Ca requires 2 faradays, 0.1 mol requires 0.2 faradays.

ii. D. Tetracloromethane

In the presence of excess chlorine, methane undergoes successive substitution reactions, ultimately forming carbon tetrachloride (CCl₄).

iii. B. Alloys

An alloy is a uniform mixture of two or more metals (or a metal and a non-metal).

iv. A. An endothermic reaction

The positive ΔH value (X KJ/mol) indicates that heat is absorbed, making it an endothermic reaction.

v. B. 2:8:8

Chlorine (atomic number 17) gains one electron to form Cl⁻, resulting in the electron configuration 2:8:8.

vi. C. water is solvent, glucose is solute and the product is solution

In a glucose-water solution, water is the solvent, glucose is the solute, and the resulting mixture is the solution.

vii. A. Absorbs water vapour

Anhydrous copper(II) sulfate is white but turns blue when it absorbs water vapor to form the hydrated form (CuSO₄·5H₂O).

viii. D. 20cm³ of 1M Sulphuric acid solution

Sulfuric acid is a strong acid and provides more H⁺ ions per unit volume compared to acetic acid, leading to faster hydrogen production.

ix. D. Sulphur trioxide, carbon dioxide and Nitrogen dioxide

These oxides react with water to form acids (acidic oxides).

x. A. It should be rinsed off with large quantities of running water.

Concentrated sulfuric acid is highly corrosive. Immediate rinsing with plenty of water dilutes and removes the acid, minimizing damage.

Question 2

List A	List B
i. Biogas	D. Gases fuel derived from decomposition of biological waste
ii. Biomass	E. Renewable source of energy
iii. Natural gas	B. Non-renewable gaseous fuel
iv. Producer gas	C. Industrial gases fuel which is a mixture of nitrogen gas, hydrogen and carbon monoxide
v. Water gas	A. Industrial gas which is a mixture of hydrogen and carbon monoxide

Section B

Question 3

a)

i. A laboratory is a controlled environment designed for scientific experiments, research, and analysis.

ii. Potassium chlorate (KClO₃) and hydrogen peroxide (H₂O₂).

iii. Hydrogen gas is less dense than air, so it is collected by downward displacement of air (upward delivery).

b)

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

ii. NH₄Cl(aq) + NaOH(aq) → NaCl(aq) + NH₃(g) + H₂O(l)

iii. Ca(OH)₂(aq) + H₂SO₄(aq) → CaSO₄(aq) + 2H₂O(l)

iv. Mg(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + H₂(g)

Question 4

a)

- Element R (atomic number 20) is calcium (Ca), and element S (atomic number 17) is chlorine (Cl).

- The compound formed is CaCl₂, with an ionic bond.

b)

Properties of CaCl₂:

1. High melting and boiling points due to strong ionic bonds.
2. Soluble in water.
3. Conducts electricity in molten or aqueous state.

Question 5

a)

- At STP, 1 mole of any gas occupies 22.4 dm³.

- Number of moles in 20 cm³ (0.02 dm³) of Cl₂ gas:

Moles = 0.02/22.4 = 8.93 × 10⁻⁴ mol

- Number of molecules:

8.93 × 10⁻⁴ × 6.02 × 10²³ = 5.37 × 10²⁰ molecules

b)

- Molar mass of Cu(NO₃)₂ = 63.5 + 2(14 + 3(16)) = 187.5 g/mol.

- Moles in 5 g:

5/187.5 = 0.0267 mol

- Each formula unit of Cu(NO₃)₂ produces 3 ions (1 Cu²⁺ and 2 NO₃⁻).

- Total ions:

0.0267 × 3 × 6.02 × 10²³ = 4.82 × 10²² ions

Section C

Question 13

a)

How can you tell water is polluted? Give two ways:

1. Visual indicators: Presence of algae, discoloration, or floating debris.
2. Odor: Unpleasant smells indicate pollution.

b)

Warnings concerning likely effects of coal use:

1. Air pollution (release of SO₂, NOₓ, and particulate matter).
2. Health risks (respiratory diseases).
3. Acid rain formation (damage to ecosystems).
4. Contribution to climate change (CO₂ emissions).
5. Water pollution (ash disposal contaminates water sources).

Question 14

a)

i. A functional group is a specific group of atoms responsible for the characteristic reactions of a compound.

ii. A homologous series is a family of organic compounds with the same functional group and similar chemical properties, differing by a CH₂ unit.

iii. Isomerism is the phenomenon where compounds have the same molecular formula but different structural arrangements.

b)

Isomers of C₄H₁₀ (Butane):

1. Butane:

CH₃-CH₂-CH₂-CH₃

2. 2-Methylpropane (Isobutane):

CH₃-CH(CH₃)-CH₃

c)

i. Ethene to Ethane:

- Reagent: Hydrogen gas (H₂).

- Condition: Nickel catalyst at 150°C.

CH₂=CH₂ + H₂ → CH₃-CH₃

ii. Ethene to Chloroethane:

- Reagent: Hydrogen chloride (HCl).

CH₂=CH₂ + HCl → CH₃-CH₂Cl

iii. Ethene to 1,2-Dibromoethane:

- Reagent: Bromine water (Br₂).

CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br

iv. Ethene to Ethanol:

- Reagent: Steam (H₂O).

- Condition: Phosphoric acid catalyst at 300°C and 60 atm.

CH₂=CH₂ + H₂O → CH₃-CH₂OH