MITIHANI POPOTE EXAMINATIONS SERIES FORM FOUR CHEMISTRY EXAMINATION SERIES 5 (With Marking Guide)

CHEMISTRY EXAMINATION SERIES 5

MITIHANI POPOTE EXAMINATIONS SERIES

FORM FOUR CHEMISTRY EXAMINATION SERIES 5

YEAR: 2022

SECTION A (15 Marks)

Answer all questions in this section

1. For each of the items (i) – (x), choose the correct answer from among the given alternatives and write its letter beside the item number in the answer booklet provided.

1. A chemist is a person who investigates and collects data on changes of chemical substances before making any conclusion. Which important qualities must this person have?
   A. Mastery of the periodic table of elements
   B. Skills in working report in English language
   C. Keen in the use of most of the sense organs
   D. Interact in the use of computer
   E. Familiar with other science subject
   Answer: C
   Explanation: A chemist must be observant, using sense organs (e.g., sight, smell) to detect changes in chemical reactions accurately.
2. A mixture of oil and water is termed as:
   A. Suspension
   B. Residue
   C. Filtrate
   D. Solution
   E. Emulsion
   Answer: E
   Explanation: Oil and water form an emulsion, a mixture of two immiscible liquids where one is dispersed in the other.
3. When a nonmetal ion becomes an atom it:
   A. Gains electron(s)
   B. Gains proton(s)
   C. Loses proton(s)
   D. Loses electron(s)
   E. Gains and loses electron(s)
   Answer: A
   Explanation: Nonmetal ions (anions) gain electrons to achieve a stable electron configuration and become neutral atoms.
4. "Water is referred to as the universal solvent" what does this mean?
   A. Water is neither acidic nor basic
   B. Water exists as three states of matter
   C. Water dissolves both organic and inorganic solvents
   D. Water is used more in domestic
   E. Water dissolves more substances
   Answer: E
   Explanation: Water dissolves a wide range of substances (e.g., salts, sugars) due to its polar nature, earning it the title "universal solvent."
5. The volume of 0.2 M H₂SO₄ required to neutralize completely 25.00 cm³ of 0.05 M KOH is:
   A. 0.626 cm³
   B. 6.125 cm³
   C. 6.315 cm³
   D. 3.125 cm³
   E. 12.5 cm³
   Answer: D
   Explanation: Reaction: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O. Moles of KOH = 0.05 M × 0.025 dm³ = 0.00125 mol. Moles of H₂SO₄ = 0.00125 / 2 = 0.000625 mol. Volume of H₂SO₄ = 0.000625 / 0.2 = 0.003125 dm³ = 3.125 cm³.
6. The property of metals that can be turned into thin sheets is called?
   A. Ductility
   B. Sonorosity
   C. Malleability
   D. Expansibility
   E. Lustrous
   Answer: C
   Explanation: Malleability allows metals to be hammered into thin sheets.
7. One among the following substances is made by application of chemistry in agriculture:
   A. Yeast
   B. PVC
   C. Toothpaste
   D. Pesticides
   E. Alcohols
   Answer: D
   Explanation: Pesticides are chemical compounds used in agriculture to protect crops from pests.
8. Which of the following compounds does not belong to the alkane homologous series?
   A. C₂H₄
   B. CH₄
   C. C₄H₁₀
   D. C₃H₈
   E. C₅H₁₂
   Answer: A
   Explanation: Alkanes have the formula CₙH₂ₙ₊₂. C₂H₄ is an alkene (CₙH₂ₙ), not an alkane.
9. The IUPAC name of H₂SO₄ is:
   A. Sulphuric (IV) acid
   B. Sulphuric acid
   C. Sulphuric (V) acid
   D. Sulphurous acid
   E. Sulphuric (VI) acid
   Answer: B
   Explanation: H₂SO₄ is named sulphuric acid in IUPAC nomenclature, reflecting its composition with sulphur in the +6 oxidation state.
10. Which oxides are gaseous at room temperature?
    A. Carbon dioxide and copper(II) oxide
    B. Sulphur dioxide and Copper (I) oxide
    C. Carbon dioxide and Sulphur dioxide
    D. Copper oxide and Iron oxide
    E. Iron oxide and Carbon dioxide
    Answer: C
    Explanation: Carbon dioxide (CO₂) and sulphur dioxide (SO₂) are gases at room temperature, unlike metal oxides which are solids.

2. Match the item in List A with the responses in List B by writing the letter of the most correct response beside the item number in the answer sheet provided.

List A	List B	Answer
Synthesis reaction	A. A single compound breaks down into two or more simpler substances B. Reaction between acid and base C. Two or more reactants form one compound D. A more reactive element displaces a less reactive element from a compound E. Reaction that involves a transfer of electrons between two species F. Formation of an insoluble salt in which two solutions containing soluble salt are combined G. Reaction between alcohol and carboxylic acid	C
Precipitation reaction		F
Redox reaction		E
Displacement reaction		D
Neutralization reaction		B

SECTION B (70 Marks)

Answer all questions in this section

3. (a) Explain briefly the following terms:

1. Mole
   Answer: A mole is the amount of substance containing 6.02 × 10²³ particles (Avogadro’s number), equivalent to the molar mass in grams.
2. Molar volume
   Answer: Molar volume is the volume occupied by one mole of a gas at standard temperature and pressure (22.4 dm³ at STP).

(b) A current of 0.2 A is passed for exactly 30 minutes resulting in the deposition of 0.4 g of silver.

1. Briefly state the five factors which may affect the amount of silver deposited:
   
   • Current strength (amperes).
   • Time of electrolysis.
   • Concentration of electrolyte (AgNO₃).
   • Nature of the electrode material.
   • Temperature of the electrolyte.
2. Calculate the quantity of electricity passed through the electrolyte:
   Answer: Time = 30 × 60 = 1800 s. Charge (Q) = I × t = 0.2 × 1800 = 360 C.
3. Calculate the number of electrons passed through the electrolyte:
   Answer: Moles of Ag = 0.4 / 108 = 0.003704 mol. Electrons per Ag atom = 1. Total electrons = 0.003704 × 6.02 × 10²³ = 2.23 × 10²¹ electrons.

4. (a) Distinguish bases from acids based on their physical properties. Two points.

• Acids: Sour taste, turn blue litmus red.
• Bases: Bitter taste, slippery feel, turn red litmus blue.

(b) Briefly give one reason for the following uses of metals:

1. Aluminum is used for making window frames.
   Answer: Lightweight and corrosion-resistant due to oxide layer.
2. Iron is used for making bridges.
   Answer: High strength and durability for structural support.
3. Copper for electrical wiring.
   Answer: Excellent electrical conductivity.
4. Lead for roofing.
   Answer: Malleable and resistant to weathering.
5. Zinc for coating of iron.
   Answer: Prevents corrosion via galvanization.

5. (a) The formation of lime scale in water pipes has both an advantage and a disadvantage. Explain:

• Advantage: Lime scale (CaCO₃) forms a protective layer, reducing pipe corrosion.
• Disadvantage: Reduces water flow and efficiency by narrowing pipes.

(b) With the help of chemical equations, explain how you can remove temporary and permanent hardness of water.

• Temporary Hardness: Caused by Ca(HCO₃)₂. Removed by boiling:
  
  Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
• Permanent Hardness: Caused by CaSO₄ or MgSO₄. Removed by adding sodium carbonate:
  
  CaSO₄(aq) + Na₂CO₃(aq) → CaCO₃(s) + Na₂SO₄(aq)

6. The industrial preparation of ammonia in the Haber process is represented by the following equation:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -46.2 kJ/mol

What will happen to the position of equilibrium if:

1. (i) The temperature of the system is increased?
   Answer: Shifts left (towards reactants), as the reaction is exothermic, favoring the reverse reaction at higher temperatures.
2. (ii) More nitrogen gas is added to the equilibrium mixture?
   Answer: Shifts right, producing more NH₃, as per Le Chatelier’s principle.
3. (iii) The formed ammonia gas is removed from the equilibrium mixture?
   Answer: Shifts right to produce more NH₃ to restore equilibrium.

(b)

1. What is the use of a catalyst in the reaction above?
   Answer: Increases reaction rate by lowering activation energy without shifting equilibrium.
2. What is the meaning of the negative sign against the value of heat change -46.2 kJ/mol in the chemical reaction given above?
   Answer: Indicates an exothermic reaction, releasing 46.2 kJ/mol of heat.
3. List down three factors which affect the position of equilibrium in the reversible reaction.
   
   • Temperature
   • Pressure
   • Concentration of reactants/products

7. (a) An atom M has an atomic number 15 and mass number 31.

1. What is the number of protons and neutrons?
   Answer: Atomic number = 15 (protons = 15). Neutrons = 31 - 15 = 16.
2. Write the electronic configuration of atom M.
   Answer: Atom M is phosphorus (P). Configuration: 1s² 2s² 2p⁶ 3s² 3p³.

(b) What amount of current in ampere will be liberated within one hour if 2.24 dm³ of chlorine were produced at S.T.P at the anode, when sodium chloride solution is electrolyzed?

Moles of Cl₂ = 2.24 / 22.4 = 0.1 mol. Reaction: 2Cl⁻ → Cl₂ + 2e⁻. Electrons = 0.1 × 2 = 0.2 mol. Charge = 0.2 × 96500 = 19,300 C. Time = 1 hr = 3600 s. Current = 19,300 / 3600 ≈ 5.36 A.
Answer: Approximately 5.36 Amperes.

8. (a) Using balanced chemical equations, show steps on how methane undergoes complete chlorination in the presence of sunlight.

CH₄ + Cl₂ → CH₃Cl + HCl

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

CH₂Cl₂ + Cl₂ → CHCl₃ + HCl

CHCl₃ + Cl₂ → CCl₄ + HCl

Explanation: Methane undergoes substitution reactions, replacing H atoms with Cl atoms in the presence of sunlight, forming chloromethanes.

(b) Write the structures of the following organic compounds:

1. 1,2-dichloroethane:
   Cl-CH₂-CH₂-Cl
2. 2,2-dimethylpropane:
   (CH₃)₃CCH₃
3. pent-2-yne:
   CH₃-C≡C-CH₂CH₃

9. (a) Give two reasons which verify the statement that "chemists prefer to use molecular equations rather than word equations."

• Molecular equations are precise, showing exact chemical formulas and stoichiometry.
• They are universally understood, unlike word equations which vary by language.

(b) Dilute hydrochloric acid is reacted with zinc granules, the product formed is zinc chloride and hydrogen gas is evolved.

1. Write the balanced chemical equation for the reaction:
   
   Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
2. Write the ionic equation for the reaction:
   
   Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)

10. (a)

1. Write a balanced chemical equation for the laboratory preparation of chlorine gas:
   
   2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂
2. Why is the gas prepared in the fume chamber?
   Answer: Chlorine is toxic and corrosive, requiring a fume chamber to prevent inhalation and exposure.

(b)

1. Why can chlorine be collected over water?
   Answer: Chlorine is sparingly soluble in water, allowing collection by downward displacement of water.
2. List down two uses of chlorine gas:
   
   • Water purification (disinfectant).
   • Production of bleaching agents (e.g., bleach).

11. (a) Write the chemical symbols of the following elements:

• Silver: Ag
• Mercury: Hg
• Lead: Pb
• Beryllium: Be
• Iron: Fe

(b) You are provided with a compound composed of 22.2% zinc, 11.6% sulphur, 22.3% oxygen, and the rest is water of crystallization. Calculate the molecular formula of the compound if its molecular mass is 283.

Percentage of water = 100 - (22.2 + 11.6 + 22.3) = 43.9%. Molar mass = 283 g/mol.
Moles: Zn = 22.2 / 65 = 0.3415; S = 11.6 / 32 = 0.3625; O = 22.3 / 16 = 1.3938; H₂O = 43.9 / 18 = 2.4389.
Ratio: Zn:S:O:H₂O = 0.3415/0.3415 : 0.3625/0.3415 : 1.3938/0.3415 : 2.4389/0.3415 ≈ 1:1:4:7.
Empirical formula: ZnSO₄·7H₂O. Molar mass of ZnSO₄·7H₂O = 65 + 32 + (4×16) + 7×(2×1 + 16) = 287, close to 283.
Answer: ZnSO₄·7H₂O.

12. (a) Write down five characteristics of a good fuel:

• High energy output per unit mass.
• Readily available and affordable.
• Low environmental pollution.
• Easy to store and transport.
• Safe to handle and use.

(b) Why are we advised to use alternative sources of energy such as solar energy, biogas, and wind energy rather than fossil fuels? Give two points.

• Renewable and sustainable, unlike depleting fossil fuels.
• Lower greenhouse gas emissions, reducing environmental impact.

SECTION C (15 Marks)

Answer one (1) question from this section

13. (a) Explain the effects of the following to the environment:

1. Acidic rain:
   Answer: Damages ecosystems, acidifies soil and water, and corrodes buildings.
2. Artificial fertilizers:
   Answer: Cause eutrophication, leading to algal blooms and oxygen depletion in water bodies.
3. Plastic containers:
   Answer: Non-biodegradable, leading to pollution and harm to wildlife.

(b)

1. What control measures should be taken to prevent the destruction of the ozone layer?
   Answer: Reduce CFC emissions, promote ozone-friendly alternatives, and enforce international agreements like the Montreal Protocol.
2. List down three gases that cause greenhouse effects:
   
   • Carbon dioxide (CO₂)
   • Methane (CH₄)
   • Nitrous oxide (N₂O)