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.

1) Distractor check: A student might mistakenly pick positional isomerism by confusing spatial arrangement differences with shifts in functional group locations, or select functional isomerism due to structural variations. 2) Reasoning to the answer: Cis and trans arrangements represent a specific subclass of geometrical isomerism. This phenomenon arises because restricted rotation around a double bond locks atoms or groups into distinct spatial configurations. 3) Common mistake: Confusing positional isomers with geometrical isomers that rely on restricted bond rotation.

1) Distractor check: A student might mistakenly select C2H5OH or CH3COOH by assuming larger organic molecules always have lower boiling points, ignoring the presence of stronger intermolecular forces. 2) Reasoning to the answer: Butane contains only weak van der Waals forces and boils at minus 0.5 degrees Celsius, making it the most volatile among the choices. The remaining compounds display significantly higher boiling points due to intermolecular hydrogen bonding, with acetic acid boiling at 118 degrees Celsius, water at 100 degrees Celsius, and ethanol at 78 degrees Celsius. 3) Common mistake: Neglecting the profound impact of hydrogen bonding when comparing boiling points across different organic and inorganic families.

1) Distractor check: A student might mistakenly select C4H9OH because they miscalculate the higher mass percentage, or pick CH3OH due to a failure to balance the atomic ratios correctly. 2) Reasoning to the answer: For C3H7OH, the carbon mass accounts for 36 out of a total molar mass of 60, yielding exactly sixty percent carbon. Other options fail this mass percentage check, as CH3OH yields thirty-seven point five percent, C2H5OH gives fifty-two point two percent, and C4H9OH reaches sixty-four point six percent. 3) Common mistake: Incorrectly computing the mass percentage of carbon by misapplying atomic weights in the molecular formula.

1) Distractor check: A student might mistakenly select I and III only by assuming all organic derivatives containing oxygen react identically with alkali metals, or pick III only through a misinterpretation of ester reactivity. 2) Reasoning to the answer: Both carboxylic acids and alkanols react directly with sodium metal to evolve hydrogen gas, as demonstrated by the reaction of two moles of alcohol with two moles of sodium to form sodium alkoxide and hydrogen. Esters, however, lack the active hydrogen required for this specific displacement reaction. 3) Common mistake: Assuming esters possess reactive hydroxyl protons similar to alcohols and carboxylic acids.

1) Distractor check: A student might mistakenly choose carbon or carbon monoxide by confusing reducing species with oxidizing agents during chemical reactions. 2) Reasoning to the answer: Oxygen acts as an oxidizing agent because it readily accepts electrons during redox processes. Conversely, carbon, carbon monoxide, and atomic hydrogen function as reducing agents by readily donating electrons to other species. 3) Common mistake: Classifying oxygen as a reducing agent because it participates actively in combustion reactions.

1) Distractor check: A student might mistakenly pick all reactions cease to occur by assuming a system at equilibrium has stopped all microscopic activity, or select the reaction has gone to completion. 2) Reasoning to the answer: A dynamic equilibrium is established when the rate of the forward reaction matches the rate of the reverse reaction precisely, leaving macroscopic concentrations constant without halting molecular processes. 3) Common mistake: Believing that chemical equilibrium implies a static state where all molecular reactions have stopped.

1) Distractor check: A student might mistakenly choose decomposition because they associate all breakdown processes with heat release, or select all three choices through a general misconception about thermal changes. 2) Reasoning to the answer: Neutralization reactions and combustion processes always release thermal energy into their surroundings. Decomposition, however, can be strongly endothermic, such as the thermal breakdown of calcium carbonate into calcium oxide and carbon dioxide. 3) Common mistake: Assuming every type of chemical decomposition process releases heat like combustion and neutralization.

1) Distractor check: A student might mistakenly pick standard temperature is 298 K or standard pressure is 1 atm by confusing common thermodynamic parameters with incorrect choices. 2) Reasoning to the answer: Delta H zero represents the standard enthalpy change of a reaction, not the activation energy, which refers instead to the energy barrier that reactants must overcome to proceed. Standard conditions correctly include a temperature of 298 Kelvin, a pressure of 1 atmosphere, and solution concentrations of 1 mole per cubic decimetre. 3) Common mistake: Confusing thermodynamic enthalpy changes with the kinetic activation energy barrier.

1) Distractor check: A student might mistakenly select 0.200 mol dm minus 3 by forgetting to account for the dilution volume change, or choose 0.020 mol dm minus 3 due to a decimal placement error. 2) Reasoning to the answer: First, determine the initial moles of sodium carbonate by multiplying zero point two hundred moles per cubic decimetre by five cubic centimetres divided by one thousand, which yields zero point zero zero one moles. Dividing this total amount by the final volume of zero point two hundred and fifty cubic decimetres gives a resulting concentration of zero point zero zero four moles per cubic decimetre. 3) Common mistake: Failing to scale the initial moles by the new total volume after dilution.

1) Distractor check: A student might mistakenly pick 3P by assuming the pressure remains unchanged when volume increases, or select 2P through an incorrect inverse proportionality setup. 2) Reasoning to the answer: According to Boyle's law, the product of initial pressure and initial volume equals the product of final pressure and final volume, written as P one V one equals P two V two. Substituting three P for the initial pressure and two V for the new volume yields an expression where P two equals three P times V divided by two V, simplifying to three halves P. 3) Common mistake: Applying direct proportionality instead of inverse proportionality when solving gas law problems involving volume and pressure changes.

1) Distractor check: A student might mistakenly select 3.75 cubic decimetres by halving the volume ratio incorrectly, or pick 7.50 cubic decimetres by assuming a one-to-one stoichiometric requirement. 2) Reasoning to the answer: Based on the balanced chemical equation, one mole or volume of methane requires two moles or volumes of oxygen for complete combustion. Therefore, burning seven point five cubic decimetres of methane demands twice that amount, resulting in fifteen point zero cubic decimetres of oxygen at standard temperature and pressure. 3) Common mistake: Neglecting the stoichiometric coefficients from the balanced equation when calculating reacting gas volumes.

1) Distractor check: A student might mistakenly pick 6.40 g dm minus 3 by converting mass incorrectly, or select 0.031 mol dm minus 3 by omitting the volume division step. 2) Reasoning to the answer: Calculate the moles of copper (II) sulfate by dividing the mass of five point zero grams by its molar mass of one hundred and sixty, giving zero point zero three one two five moles. Dividing this mole quantity by the solution volume of zero point two fifty cubic decimetres yields a solubility of zero point one two five moles per cubic decimetre. 3) Common mistake: Forgetting to convert the solution volume from cubic centimetres to cubic decimetres when calculating molar solubility.

1) Distractor check: A student might mistakenly choose covalent or electrovalent by assuming standard bonding categories apply without checking electron pair origins, or pick hydrogen due to the presence of hydrogen atoms. 2) Reasoning to the answer: In the ammonium ion, the nitrogen atom supplies a lone pair of electrons to bond with a hydrogen ion, establishing a coordinate or dative covalent linkage. 3) Common mistake: Classifying the bond in the ammonium ion as a standard shared-pair covalent bond without recognizing the origin of the electron pair.

1) Distractor check: A student might mistakenly select Al2O3 or Na2O by confusing ionic metal oxides with network solids, or pick P4O10 because of its complex molecular formula. 2) Reasoning to the answer: Silicon dioxide forms a robust tetrahedral giant covalent lattice structurally similar to diamond. In contrast, sodium oxide is ionic, phosphorus pentoxide is molecular, and aluminium oxide exhibits mixed ionic and covalent characteristics. 3) Common mistake: Assuming all high-melting-point oxides share a giant covalent network structure.

1) Distractor check: A student might mistakenly pick NaOH or KOH by assuming all metal hydroxides share identical high solubility profiles in water. 2) Reasoning to the answer: Calcium hydroxide is only sparingly soluble, dissolving at a low rate of zero point one seven grams per one hundred millilitres. Conversely, ammonium hydroxide, sodium hydroxide, and potassium hydroxide dissolve readily to form highly concentrated aqueous solutions. 3) Common mistake: Assuming all common group hydroxides dissolve easily in water regardless of their alkaline earth metal classification.

1) Distractor check: A student might mistakenly choose a salt whose solubility increases with temperature would not crystallize easily on cooling, ignoring the principle of temperature-dependent solubility separation. 2) Reasoning to the answer: Salts exhibiting a high degree of solubility variance with temperature—such as potassium nitrate—yield efficient separation because lowering the temperature drastically reduces their capacity to stay dissolved, forcing them to crystallize out. 3) Common mistake: Believing that salts with high temperature-dependent solubility make poor candidates for crystallization purification.

1) Distractor check: A student might mistakenly select 5.4 times 10 to the power of minus 2 by misplacing decimal points during the volume conversion, or pick 5.4 times 10 to the power of minus 1 through a calculation order error. 2) Reasoning to the answer: Determine the number of moles by multiplying the molar concentration of zero point one zero eight moles per cubic decimetre by the volume expressed in cubic decimetres, which is zero point zero fifty litres. This multiplication yields zero point zero zero five four moles, or five point four times 10 to the power of minus three moles. 3) Common mistake: Forgetting to convert the given volume in cubic centimetres to cubic decimetres before multiplying by concentration.

1) Distractor check: A student might mistakenly select 0.010 mol dm minus 3 by ignoring the stoichiometric mole ratio in the balanced chemical equation, or pick 0.100 mol dm minus 3 due to a multiplication error. 2) Reasoning to the answer: According to the balanced equation for the reaction of sulfuric acid with sodium hydroxide, one mole of acid neutralizes two moles of base. Multiplying the acid concentration of zero point zero one moles per cubic decimetre by zero point zero twenty cubic decimetres gives two times 10 to the power of minus four moles of acid, requiring four times 10 to the power of minus four moles of sodium hydroxide, which results in a concentration of zero point zero twenty moles per cubic decimetre when divided by zero point zero twenty cubic decimetres. 3) Common mistake: Neglecting the stoichiometric factor of two when calculating neutralization concentrations for diprotic acids.

1) Distractor check: A student might mistakenly select Hund's rule by confusing orbital energy filling rules with spin multiplicity guidelines, or pick Pauli Exclusion Principle. 2) Reasoning to the answer: The Aufbau principle establishes that electrons must fill available atomic orbitals in order of increasing energy, entering the lowest empty energy level first to build the ground state configuration. 3) Common mistake: Confusing the energy-filling sequence dictated by the Aufbau principle with Hund's rule of maximum multiplicity.

1) Distractor check: A student might mistakenly pick sodium or potassium by assuming alkali metals fail to react vigorously with cold water. 2) Reasoning to the answer: While lithium, potassium, and sodium react violently with cold water to generate hydrogen gas, zinc requires high-temperature steam rather than cold water to undergo a similar displacement reaction. 3) Common mistake: Assuming transition metals like zinc share the high reactivity of alkali metals with cold water.