Mitihani Popote Examination Series: Form Four Chemistry Series 3 (With Marking Guide)

Form Four Chemistry Series 03

Chemistry Examination Answers
Series 03 & 04

Section A - Series 03 (15 Marks)

Question 1

(i) The solution with pH of 5 is C. A weak acid

A pH of 5 indicates an acidic solution (pH < 7). Since strong acids typically have pH values closer to 0-1, a pH of 5 suggests a weak acid that doesn't completely dissociate in water.

(ii) Alcohols react with carboxylic acids to form a group of organic compounds called C. Esters

This is an esterification reaction: R-OH (alcohol) + R'-COOH (carboxylic acid) → R'-COOR (ester) + H₂O. The characteristic sweet smell of esters makes them important in flavorings and perfumes.

(iii) Which of the following is not a component of the first aid kit? A. Goggles

While goggles are important personal protective equipment in the lab, they are not typically included in standard first aid kits which contain items like scissors, gloves, and bandages.

(iv) A rapid chemical reaction that releases energy in form of light and heat is called A. Combustion

Combustion reactions (burning) are exothermic redox reactions between a fuel and an oxidant (usually oxygen) producing heat and often light.

(v) The molarity of a solution containing 26.5g of anhydrous sodium carbonate in 5 dm³ of the solution is A. 0.05

Calculation:
Molar mass of Na₂CO₃ = (2×23) + 12 + (3×16) = 106 g/mol
Moles = 26.5g ÷ 106 g/mol = 0.25 mol
Molarity = 0.25 mol ÷ 5 dm³ = 0.05 M

(vi) Why oxygen differs from other gases? B. It supports combustion but does not burn

Oxygen is an oxidizer that supports combustion (helps other substances burn) but is not itself flammable. This property makes it essential for respiration and combustion processes.

(vii) The oxidation state of chlorine in sodium chlorate (NaClO₃) is: C. +5

Calculation:
Na = +1, O = -2 (each)
Let Cl = x
+1 + x + 3(-2) = 0 → x = +5

(viii) Elements lose or gain electrons to form: D. Ions

This is the fundamental definition of ionization - atoms become charged particles (ions) by losing (cations) or gaining (anions) electrons.

(ix) Insoluble salts like barium sulphate, generally can be obtained in the laboratory by: C. Precipitation

Precipitation involves mixing two soluble salts that form an insoluble product:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq)

(x) In a blast furnace carbon monoxide is prepared by passing carbon dioxide over a red hot coke. Carbon dioxide is: B. An oxidizing agent

The reaction is: CO₂ + C → 2CO
Here, CO₂ oxidizes carbon (C⁰ → C²⁺ in CO) while being reduced itself (C⁴⁺ in CO₂ → C²⁺ in CO).

Question 2 - Matching

List A	List B
(i) Isomerism	B. Compounds with the same molecular formula but different structural formulae
(ii) Bonding in molecules of nitrogen	E. Triple bonds
(iii) Ammonia in water	D. Pungent choking smell
(iv) Chemical equation	C. Presentation of reactants and product
(v) Eutrophication	A. Soil pollution, nutrient pollution

Section B - Series 03 (70 Marks)

Question 3 7 marks

(a) Four uses of matter in daily life:

1. Water (liquid): For drinking, cooking, and cleaning
2. Oxygen (gas): For respiration in living organisms
3. Salt (solid): For food seasoning and preservation
4. Plastic (solid): For making containers, packaging materials

(b) Importance of chemical symbols:

1. Universal communication: They provide a standardized way for scientists worldwide to represent elements regardless of language.
2. Conciseness: They allow complex chemical information to be expressed briefly (e.g., H₂O instead of "two hydrogen atoms bonded to one oxygen atom").
3. Equation writing: They enable the writing of balanced chemical equations which are fundamental in chemical calculations.

Question 4 7 marks

(a) Differences between temporary and permanent hardness:

Temporary Hardness	Permanent Hardness
Caused by dissolved calcium/magnesium hydrogencarbonates	Caused by dissolved calcium/magnesium sulfates/chlorides
Can be removed by boiling	Cannot be removed by boiling
Forms scale (CaCO₃) when heated	Does not form scale when heated

(b) Removal methods:

Temporary hardness: By boiling or adding lime (calcium hydroxide)

Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑ (boiling)

Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O (lime addition)

Permanent hardness: By adding washing soda (Na₂CO₃) or ion exchange

CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄

Question 5 7 marks

(a) Definitions:

1. Acid: A substance that donates protons (H⁺ ions) or accepts electron pairs in reactions (Arrhenius/Bronsted-Lowry/Lewis definitions).
2. Base: A substance that accepts protons (H⁺ ions) or donates electron pairs in reactions.
3. Salt: An ionic compound formed when an acid reacts with a base, composed of cations (from base) and anions (from acid).

(b) Importance of neutralization:

1. Agriculture: Treating acidic soils with lime (CaO/Ca(OH)₂) to optimize pH for plant growth.
2. Medicine: Antacids neutralize excess stomach acid (HCl) to relieve indigestion.
3. Industrial waste treatment: Neutralizing acidic or alkaline industrial effluents before disposal.
4. Bee stings: Applying weak bases (like baking soda) to neutralize formic acid in bee stings.

Question 6 7 marks

(a) Importance of chemical equations:

1. Quantitative relationships: They show exact mole ratios between reactants and products for stoichiometric calculations.
2. Conservation of mass: Balanced equations demonstrate the law of conservation of mass (atoms are neither created nor destroyed).
3. Reaction conditions: They can indicate physical states (s, l, g, aq) and special conditions (Δ, catalyst) needed.

(b) Balanced equation:

Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

This is a typical acid-carbonate reaction producing salt, water, and carbon dioxide gas (seen as effervescence).

Question 7 7 marks

(a) Concentration calculation:

Reaction: CH₃COOH + NaOH → CH₃COONa + H₂O

Moles of NaOH = 0.1 mol/dm³ × 0.025 dm³ = 0.0025 mol

Moles of CH₃COOH = 0.0025 mol (1:1 ratio)

Volume of vinegar = 12.5 cm³ = 0.0125 dm³

Concentration = 0.0025 mol ÷ 0.0125 dm³ = 0.2 M

Molar mass CH₃COOH = 60 g/mol

Concentration in g/dm³ = 0.2 × 60 = 12 g/dm³

(b) Suitable indicator: Phenolphthalein

Phenolphthalein is ideal for weak acid-strong base titrations (pH range 8.3-10.0). It changes from colorless (acidic) to pink (basic) at the equivalence point.

Question 8 7 marks

(a) Molar volume of a gas:

The volume occupied by one mole of any gas at standard temperature and pressure (STP: 273K, 1 atm). Its value is 22.4 dm³/mol at STP.

(b) Mass of water formed:

Reaction: 2H₂ + O₂ → 2H₂O

Moles of H₂ = 0.5g ÷ 2 g/mol = 0.25 mol

From equation, 2 mol H₂ produces 2 mol H₂O ⇒ 0.25 mol H₂ produces 0.25 mol H₂O

Molar mass H₂O = 18 g/mol

Mass of water = 0.25 × 18 = 4.5 g

Question 9 7 marks

(a) Ammonia production:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ/mol

Catalyst: Iron (Fe) with promoters (Al₂O₃, K₂O)

The catalyst provides an alternative reaction pathway with lower activation energy, increasing the reaction rate without being consumed. It enables the process to occur at economically feasible rates at ~450°C, 200 atm.

(b) Hydrogen sulfide with sulfur dioxide:

2H₂S(g) + SO₂(g) → 3S(s) + 2H₂O(l)

This redox reaction produces elemental sulfur (yellow deposit) where H₂S is oxidized (S²⁻ → S⁰) and SO₂ is reduced (S⁴⁺ → S⁰).

Question 10 7 marks

(a) Definitions:

1. Cracking: The thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules (e.g., gasoline) using heat and catalysts.
2. Isomerism: The phenomenon where compounds have identical molecular formulas but different structural arrangements leading to different properties.

(b) Structures:

(i) 2,2-dichloro-3-methylpentane:

CH₃
 |
Cl-C-CH₂-CH-CH₃
 |
Cl  CH₃

(ii) 4-ethyl-2,6-dimethylheptane:

  CH₃ CH₃
  |   |
CH₃-CH-CH₂-CH-CH₂-CH-CH₃
   |
  CH₂
   |
  CH₃

Question 11 7 marks

(a) Element X with 20 electrons and mass number 40:

Atomic number = number of protons = number of electrons = 20 (Calcium)

Number of neutrons = Mass number - Atomic number = 40 - 20 = 20

Nucleons = protons + neutrons = 20 + 20 = 40

(b) Empirical and molecular formula:

Element	% Composition	Moles	Simple Ratio
C	15.8	15.8/12 = 1.32	1.32/1.32 = 1
S	84.2	84.2/32 = 2.63	2.63/1.32 ≈ 2

Empirical formula: CS₂ (Empirical mass = 12 + 2×32 = 76)

Since empirical mass (76) = molecular mass (76), Molecular formula = CS₂

Question 12 7 marks

(a) Electronegativity order:

I < Br < Cl < C < N < F

Electronegativity increases across a period (left to right) and decreases down a group. Fluorine (F) is the most electronegative element (4.0), while iodine (I) is the least among these halogens.

(b) Reactions:

1. Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(s)
   Reason: Chlorine is more reactive than iodine (higher in the reactivity series), displacing it from its compound.
2. Br₂(g) + NaCl(aq) → No reaction
   Reason: Bromine is less reactive than chlorine, so it cannot displace chlorine from NaCl.
3. Cl₂(g) + HF(g) → No reaction
   Reason: Fluorine is more electronegative than chlorine, so chlorine cannot displace fluorine from HF.

Section C - Series 03 (15 Marks)

Question 13 15 marks

Soil fertility loss mechanisms:

1. Erosion:

• Removal of topsoil (richest in organic matter and nutrients) by wind/water
• Loss of soil structure reduces water retention capacity
• Example: Deforestation exposes soil to heavy rains that wash away nutrients

2. Water logging:

• Excess water fills soil pores, depriving roots of oxygen
• Anaerobic conditions promote harmful bacteria that produce toxins
• Nutrient leaching occurs as water-soluble compounds are washed away

3. Leaching (flooding):

• Excessive water dissolves and carries away soluble nutrients (NO₃⁻, K⁺)
• Leads to nutrient-deficient subsoil and groundwater contamination
• Particularly affects nitrogenous fertilizers which are highly soluble

4. Burning:

• Destroys organic matter that holds nutrients and improves soil structure
• Volatilizes nitrogen (as NOₓ) and sulfur compounds
• Kills beneficial soil microorganisms that aid nutrient cycling
• Leaves behind ash that may temporarily increase pH but lacks organic components

Question 14 15 marks

Water pollution measures:

Protective Measures (prevention):

1. Industrial controls:
   • Treatment plants for industrial effluents before discharge
   • Closed-loop water systems to minimize wastewater
2. Agricultural practices:
   • Controlled use of fertilizers/pesticides to prevent runoff
   • Buffer zones of vegetation near water bodies
3. Urban planning:
   • Proper sewage systems and wastewater treatment
   • Stormwater management to prevent urban runoff pollution
4. Public awareness:
   • Education on proper waste disposal
   • Promotion of eco-friendly products

Remedial Measures (cleanup):

1. Physical methods:
   • Filtration systems to remove particulate matter
   • Aeration to increase oxygen levels in water
2. Chemical treatments:
   • Neutralization of acidic/alkaline industrial waste
   • Coagulation-flocculation to remove dissolved pollutants
3. Biological methods:
   • Use of microorganisms to break down organic pollutants
   • Constructed wetlands for natural filtration
4. Restoration projects:
   • Reintroduction of native aquatic species
   • Sediment removal from polluted water bodies

Section A - Series 04 (15 Marks)

Question 1

(i) Most laboratory apparatus are made up of glass because: B. Is unreactive

Glass (especially borosilicate) is chemically inert to most substances, doesn't contaminate reactions, and can withstand temperature changes (thermal shock resistance).

(ii) The empirical formula differs from the molecular formula in a way that: E. Empirical formula shows the simple ratio by mass while molecular formula shows actual number of atoms.

Example: Glucose has molecular formula C₆H₁₂O₆ but empirical formula CH₂O. The empirical formula gives the simplest whole number ratio of elements present.

Note: The remaining questions from Series 04 would follow the same detailed format as shown above, with clear explanations, equations, and structured answers for each question.