WAEC Chemistry 2020
Questions & Answers

Distractor check: A student might choose A or B, mistaking industrial bulk chemicals like NaOH and HCl for fine chemicals. Reasoning to the answer: Unlike bulk industrial chemicals, fine chemicals are highly purified substances produced in small quantities for specialized uses such as pharmaceuticals or research. Ethylene serves as a key organic intermediate that fits this category among the choices. Common mistake: Confusing high-volume industrial chemicals with specialty purified chemical products.

Distractor check: A student might select B or C, missing proper IUPAC prefix conventions for numbering and locants. Reasoning to the answer: The structure contains double bonds between positions 1-2 and 3-4, and the chlorine atom is situated at carbon 3 (or carbon 2 depending on numbering, specifically matching chloroprene CH₂=CH-CCl=CH₂, numbered as 1 for CH₂=, 2 for CH-, 3 for CCl=, 4 for CH₂), making the correct systematic designation 2-chloro but-1,3-diene. Common mistake: Incorrectly numbering the carbon chain from the wrong end.

Distractor check: A student might pick A or B, confusing complex carbohydrates with nitrogenous organic polymers. Reasoning to the answer: Starch and glycogen are polysaccharides, and fats are lipids. A polypeptide is a chain of amino acids linked together by peptide bonds, which forms the building blocks of proteins. Common mistake: Grouping structural carbohydrates and lipids together with protein polymers.

Distractor check: A student might choose B or D, incorrectly assuming that elemental hydrogen is a product of hydrocarbon combustion. Reasoning to the answer: Incomplete combustion of hydrocarbons in limited oxygen produces carbon as soot and carbon(II) oxide. Hydrogen does not emerge as a free gas because it combines with oxygen to form water instead, leaving only I and III. Common mistake: Forgetting that hydrogen atoms in hydrocarbons react to form water rather than releasing hydrogen gas.

Distractor check: A student might pick C, confusing the change in oxidation number with the total number of moles of electrons transferred. Reasoning to the answer: The oxidation half-reaction Fe²⁺ → Fe³⁺ + e⁻ involves the loss of 1 electron per individual iron ion. Therefore, exactly 1 mole of electrons is lost when oxidizing 1 mole of iron(II) ions. Common mistake: Confusing the charge magnitude with the stoichiometric mole quantity of electrons.

Distractor check: A student might pick B or C by misapplying Faraday's formula or using an incorrect valency for silver. Reasoning to the answer: Using Faraday's law, mass = (M × Q) / (n × F), where n=1 for the reduction of silver ions (Ag⁺ + e⁻ → Ag), Q is 24,125 C, and F is 96,500 C/mol. Multiplying 108 by 24,125 and dividing by 96,500 yields a mass of 27 g. Common mistake: Forgetting to divide by the Faraday constant or miscalculating the atomic weight ratio.

Distractor check: A student might choose D, thinking that powder at a higher temperature slows down the reaction. Reasoning to the answer: Reaction rate increases with a larger surface area, such as powder over chips, and a higher temperature, such as 40°C over 20°C. Therefore, using calcium trioxocarbonate(IV) chips at 20°C provides both the smallest surface area and the lowest temperature, making the reaction the slowest. Common mistake: Confusing the conditions that maximize reaction speed with those that minimize it.

Distractor check: A student might select C, incorrectly assuming that non-polar covalent compounds dissolve well in water. Reasoning to the answer: Despite possessing a non-polar hydrocarbon chain, ethanol (CH₃CH₂OH) forms strong hydrogen bonds with water molecules via its hydroxyl (-OH) functional group, resulting in high solubility. Common mistake: Overlooking the role of polar functional groups like hydroxyl in determining aqueous miscibility.

Distractor check: A student might choose D, selecting an alkali metal that reacts violently rather than slowly. Reasoning to the answer: While potassium reacts explosively and calcium reacts more readily, iron reacts very slowly with cold water to form rust (Fe + H₂O + O₂ → Fe₂O₃·nH₂O), and silver shows no reaction at all. Common mistake: Assuming all metals react with water at the same rate regardless of their reactivity series position.

Distractor check: A student might pick A or C, forgetting that reactivity increases down Group 1. Reasoning to the answer: Down Group 1, atomic size increases, causing first ionization energy to decrease. Additionally, the metallic bonding weakens down the group, which systematically lowers melting points, such as dropping from lithium at 181°C to cesium at 28°C. Common mistake: Confusing trends in reactivity (which increases down Group 1) with trends in melting point and ionization energy (which decrease).

Distractor check: A student might choose A, thinking that cooling the solution will separate pure water crystals. Reasoning to the answer: Distillation separates the volatile water, which has a boiling point of 100°C, from the non-volatile sugar, since sugar does not vaporize during heating. Common mistake: Confusing physical separation techniques like crystallization with distillation when recovering a liquid solvent.

Distractor check: A student might select A, confusing the name of the uncombined elements with their combined ionic states. Reasoning to the answer: Group 17 elements are known as halogens in their elemental forms, but when they exist in their combined ionic states within compounds, such as sodium chloride, they are called halides. Common mistake: Using the term 'halogens' interchangeably for both the free elements and their combined ionic forms.

Distractor check: A student might pick A, assuming that ionic bonds involve electron sharing like covalent bonds. Reasoning to the answer: Ionic bond breaking, such as dissociating Na⁺Cl⁻ into neutral atoms, involves the more electronegative atom capturing the electron to revert to a neutral state. Common mistake: Conflating ionic bond dissociation mechanisms with covalent bond cleavage.

Distractor check: A student might choose A or B by miscalculating the net charge contribution of the oxygen atoms. Reasoning to the answer: In the chlorate ion ClO₃⁻, letting the oxidation state of chlorine be x yields x + 3(-2) = -1, which simplifies to x - 6 = -1, solving for x = +5. Common mistake: Forgetting to account for the overall ionic charge of the polyatomic group when calculating oxidation states.

Distractor check: A student might pick B, confusing mass conservation with the law of constant composition. Reasoning to the answer: Balancing chemical equations ensures an equal number of atoms on both sides, which directly reflects Lavoisier's fundamental law stating that mass is conserved in all chemical reactions. Common mistake: Confusing stoichiometry conservation laws with laws regarding fixed elemental ratios.

Distractor check: A student might select B, assuming chlorine has a higher electronegativity difference with sodium than fluorine does. Reasoning to the answer: Sodium has an electronegativity of 0.9 while fluorine has 4.0, yielding the greatest difference of 3.1, compared to chlorine at 2.1, bromine at 1.9, and iodine at 1.6. Common mistake: Overlooking fluorine as the most electronegative element when calculating bond polarity differences.

1) Distractor check: A student might mistakenly select nearness to raw materials by assuming it represents a general industrial clustering effect, or choose favourable climate conditions or availability of storage facilities, but these are secondary concerns compared to the primary economic driver. 2) Reasoning to the answer: Locating chemical manufacturing plants close to necessary material inputs—such as placing the Haber process near natural gas sources—significantly cuts down transportation expenses. This financial efficiency makes minimizing haulage distance the paramount factor for industrial siting. 3) Common mistake: Assuming all utilities and environmental factors carry equal weight when choosing a factory location.

1) Distractor check: A student might mistakenly pick carbon-carbon single bonds are stronger than carbon-hydrogen bonds by confusing bond strength with intermolecular interactions, or select pentane does not burn easily as propane by mixing up combustibility with boiling points. 2) Reasoning to the answer: Because pentane possesses a greater number of electrons and a more expansive molecular surface area than propane, it experiences much more robust London dispersion forces. These enhanced intermolecular attractions require more thermal energy to overcome, resulting in a higher boiling point of 36 degrees Celsius compared to minus 42 degrees Celsius for propane. 3) Common mistake: Confusing intramolecular covalent bond strength with intermolecular forces when comparing physical properties of alkanes.

1) Distractor check: A student might mistakenly pick ammonium chloride by overlooking the fact that it undergoes sublimation upon heating, or select lead (II) trioxonitrate (V) due to its noticeable visual changes during thermal breakdown. 2) Reasoning to the answer: Potassium trioxocarbonate (IV) remains thermally stable and refuses to decompose when heated. In contrast, ammonium chloride sublimes, lead (II) trioxonitrate (V) breaks down to yield lead (II) oxide, and sodium hydrogen trioxocarbonate (IV) decomposes into sodium trioxocarbonate (IV). 3) Common mistake: Forgetting that certain salts like ammonium chloride undergo phase changes rather than chemical decomposition.

1) Distractor check: A student might mistakenly select oxygen or ethene by confusing multiple covalent bonding with single linkages, or choose carbon (IV) oxide because of its complex linear structure. 2) Reasoning to the answer: While oxygen contains a double bond and carbon (IV) oxide features two double bonds, and ethene incorporates a carbon-carbon double bond, water consists entirely of single covalent bonds linking hydrogen to oxygen. 3) Common mistake: Overlooking the single covalent bond structure of water in favor of molecules containing unsaturated double bonds.