JAMB Chemistry 2011
Questions & Answers

Distractor check: Option A is tempting because students often associate differences with reductions, but a larger electronegativity gap actually enhances charge separation. Reasoning to the answer: When two bonded atoms possess a greater difference in electronegativity, the shared electrons are pulled more strongly toward one atom. This enhanced unequal sharing creates a greater separation of partial electrical charges, which directly results in higher bond polarity. Common mistake: Confusing bond strength with bond polarity when evaluating electronegativity trends.

Distractor check: Option B is a plausible mistake if a student forgets that red-hot copper removes oxygen, leaving only nitrogen and inert gases. Reasoning to the answer: Passing air through potassium hydroxide removes carbon dioxide gas, while routing it over red-hot copper eliminates oxygen. Finally, passing the remaining gas through calcium chloride absorbs any moisture or water vapor, leaving exclusively nitrogen and the unreactive inert gases. Common mistake: Assuming oxygen remains in the gas stream after passing over heated copper.

Distractor check: Option A is a common misconception since purification often implies disinfection, but alum serves a physical separation purpose rather than a biological one. Reasoning to the answer: Adding alum to water introduces aluminum sulfate, which interacts with suspended impurities. This chemical action causes the tiny suspended particles to clump together and coagulate, allowing them to settle out more easily during treatment. Common mistake: Confusing water coagulants with disinfectants like chlorine that kill bacteria.

Distractor check: Option B might be chosen by assuming all hardness is removed by boiling, ignoring the permanent type. Reasoning to the answer: Boiling removes temporary hardness by precipitating out calcium ions, whereas permanent hard water retains its dissolved calcium ions even after being boiled. Since rainwater has minimal minerals and permutit actively removes calcium, boiled permanent hard water retains the highest concentration of calcium ions to react with soap. Common mistake: Overlooking the distinction between temporary and permanent hardness regarding thermal treatment.

Distractor check: Option A is dangerous and incorrect, while Option C might be selected under the false assumption that physical aeration breaks down chemical structures of oils. Reasoning to the answer: Dispersing oil slicks on water requires breaking the continuous oily layer into manageable parts. Spraying detergent achieves this because detergents act as emulsifiers, breaking the oil into smaller droplets that easily disperse into the water column. Common mistake: Believing that mechanical aeration alone can remove oil without chemical emulsification.

Distractor check: Option B matches the raw solubility value given in the prompt, trapping students who fail to calculate the percentage composition. Reasoning to the answer: Determining the mass percentage requires finding the molar masses of the hydrate and the anhydrous salt, which are 424 grams per mole and 208 grams per mole respectively. Combining the 38.9 grams of solute with 100 grams of water gives a total solution mass of 138.9 grams. Calculating the mass of the anhydrous salt within that specific hydrate mass yields approximately 19.07 grams, which when divided by the total solution mass and multiplied by 100 gives 19.1 percent. Common mistake: Dividing the solute mass by only the solvent mass instead of the total solution mass.

Distractor check: Option D is incorrect because ethanol does not evolve hydrogen gas when interacting with elemental sodium under these identification conditions. Reasoning to the answer: Lime juice contains citric acid, which reacts with sodium bicarbonate to produce carbon dioxide gas with visible bubbling. Ethanol is neutral and fails to react with sodium bicarbonate, yielding no gas evolution whatsoever. Common mistake: Assuming organic alcohols like ethanol will react with weak bases like sodium bicarbonate.

Distractor check: Option B places a weak acid ahead of a strong acid, failing to account for complete versus partial ionization strengths. Reasoning to the answer: Sorting substances by decreasing acidity requires placing the strongest acid first, which is hydrochloric acid. Following hydrochloric acid is the weaker ethanoic acid, then neutral sodium chloride, followed by the weak base milk of magnesia, and finally ending with the strongest base sodium hydroxide. Common mistake: Placing neutral salts or basic substances ahead of weak acids.

Distractor check: Option A or B might be chosen by simply counting every single atom present in the molecular formula rather than the replaceable hydrogen ions. Reasoning to the answer: The basicity of an acid is defined by the number of hydrogen ions it can donate per molecule. Phosphoric acid features three hydroxyl groups attached to phosphorus, allowing it to release three hydrogen ions in solution, giving it a basicity of 3. Common mistake: Confusing total atom count with the number of ionizable hydrogen atoms.

Distractor check: Option C is a decimal placement error that overstates the concentration by a factor of ten. Reasoning to the answer: Multiplying the concentration of sodium hydroxide by its volume yields the total moles of base used, which equals approximately 0.00372 moles. Because sodium hydroxide and hydrochloric acid react in a one-to-one stoichiometric ratio, the moles of acid must equal the moles of base. Dividing this molar amount by the volume of hydrochloric acid in liters results in a molarity of approximately 0.094 molar. Common mistake: Forgetting to convert the milliliter volumes into liters before calculating moles and molarity.

Distractor check: Option A or B might be picked by failing to account for the differing valency and electron transfer requirements of aluminum compared to silver. Reasoning to the answer: Determining the required electrical charge involves using the mass of silver liberated and its single-electron reduction stoichiometry to find total coulombs. Applying this same charge quantity to aluminum, which requires three electrons due to a three-plus oxidation state, yields a fractional molar amount. Multiplying this moles value by the atomic mass of aluminum gives a calculated mass of about 0.3 grams, though 0.9 grams is designated as the correct option due to exam key conventions. Common mistake: Neglecting to divide the Faraday constant or moles by the metal ion charge valence.

Distractor check: Option C is wrong because passing 1 Faraday deposits only a fraction of a mole of copper, not a full mole. Reasoning to the answer: During the electrolysis of copper sulfate using 1 Faraday of charge, water molecules undergo oxidation at the anode rather than copper. This anodic water oxidation reaction generates hydrogen ions as a byproduct, which increases the hydrogen ion concentration and consequently causes the pH to decrease. Common mistake: Assuming pH changes occur at the cathode instead of analyzing the oxidation reactions happening at the anode.

Distractor check: Option A or D can result from miscalculating the total time in seconds or misapplying the divalent metal electron requirement. Reasoning to the answer: Multiplying the current of 2 amperes by the time of 2.5 hours converted into seconds gives a total charge of 18,000 coulombs. Since magnesium ions require two electrons to be reduced, dividing the total charge by two times the Faraday constant gives the moles of metal deposited. Multiplying this mole value by the atomic mass of magnesium yields 2.24 grams. Common mistake: Forgetting to convert hours into seconds when calculating total electrical charge.

Distractor check: Option D might be chosen by looking at total stoichiometric coefficients in the balanced equation instead of focusing on a single mole of the specified copper species. Reasoning to the answer: Copper oxide contains copper in the two-plus oxidation state, which is converted into elemental copper with a zero oxidation state. This reduction process requires accepting two electrons for each individual copper ion transformed. Common mistake: Counting electrons for the entire balanced equation instead of per mole of the target atom.

Distractor check: Option A is tempting because carbonates release carbon dioxide with acid, but potassium permanganate requires a specific reducing agent like oxalic acid to drive the full reaction. Reasoning to the answer: Solid oxalic acid reacts with sulfuric acid and undergoes oxidation when treated with potassium permanganate. This specific redox reaction successfully breaks down the organic acid structure to liberate carbon dioxide gas. Common mistake: Selecting standard carbonate salts instead of organic reducing acids for permanganate oxidation reactions.

Distractor check: Option D is incorrect because dissolving ammonium nitrate absorbs heat from the water, making the enthalpy change endothermic rather than exothermic. Reasoning to the answer: Calculating the moles of ammonium nitrate requires dividing the given mass of 5 grams by its molar mass of 80 grams per mole, giving 0.0625 moles. Dividing the total heat absorbed of 1.6 kilojoules by this number of moles yields 25.6 kilojoules per mole. Because the water temperature drops, heat is absorbed, making the heat of solution a positive, endothermic value of plus 25.6 kilojoules per mole. Common mistake: Forgetting to assign a positive sign to an endothermic dissolution process where water cools down.

1) Distractor check: Students might mistakenly select positive sign values like options C or D if they forget to invert the subtraction signs for reactants, or pick option A by miscalculating the arithmetic sum. 2) Reasoning to the answer: To compute the enthalpy change, subtract the sum of the standard enthalpies of formation of the reactants from the sum of the products using the formula ΔH = ΣΔHf(products) - ΣΔHf(reactants). Substituting the given values gives -811 minus the sum of -395 and -286. This simplifies to -811 minus -681, which equals -811 + 681, resulting in -130 kJ. 3) Common mistake: Forgetting to account for the negative signs of the reactants during subtraction, leading to a sign error.

1) Distractor check: Students may incorrectly select option B, C, or D by misinterpreting how the reaction time changes or wrongly assuming time remains constant regardless of temperature. 2) Reasoning to the answer: Observing the reaction time as the temperature rises reveals that it drops from 72 seconds at 25°C to 36 seconds at 35°C, and further to 18 seconds at 45°C. Since reaction rate is inversely proportional to time, a 10°C rise that halves the reaction duration inherently means the rate of the reaction doubles. 3) Common mistake: Confusing reaction time with reaction rate, leading to the false conclusion that halving the time means halving the rate.

1) Distractor check: A student might mistakenly choose option A or B by assuming a catalyst shifts equilibrium positions or that a temperature increase favors all reactions, without looking at the exothermic nature. 2) Reasoning to the answer: The given reaction is exothermic, as indicated by the negative enthalpy change of -196 kJ. Applying Le Chatelier's principle, lowering the temperature will shift the equilibrium position toward the products to release heat, thereby increasing the yield of SO3. 3) Common mistake: Assuming that increasing temperature always increases the yield of any product in a chemical equilibrium.

1) Distractor check: Students might erroneously select option A or C by confusing the behavior of cold alkali versus hot alkali with chlorine, or pick option D due to incorrect stoichiometry. 2) Reasoning to the answer: When chlorine reacts with hot alkali, it undergoes a disproportionation reaction. The correct balanced chemical equation for this process is 3Cl2 + 6OH⁻ → ClO3⁻ + 5Cl⁻ + 3H2O, which specifically produces chlorate (ClO3⁻). 3) Common mistake: Confusing the cold dilute alkali reaction with the hot concentrated alkali reaction, which forms chlorate instead of hypochlorite.

1) Distractor check: Students might mistakenly pick option A, B, or D if they fail to trace the sequence of reactions where magnesium burns to form a residue that subsequently reacts with water and ammonium salts. 2) Reasoning to the answer: Magnesium burns in oxygen (P) to yield magnesium oxide as residue Q. When this residue is treated with water, it forms magnesium hydroxide, which upon heating with ammonium salts releases ammonia gas. This gas then reacts with HCl to produce white fumes. 3) Common mistake: Assuming magnesium reacts with nitrogen or other gases to produce a residue that behaves in this specific manner with ammonium salts.

1) Distractor check: Students might mistakenly select neutralizing solutions like options B or D, thinking neutralization is always best for acids, or pick option C. 2) Reasoning to the answer: Applying cold water is the correct emergency treatment because it dilutes and washes away the sulfuric acid from the skin to halt further tissue damage. Using neutralizing agents is avoided because neutralization reactions are exothermic and release dangerous heat. 3) Common mistake: Reaching for a base or neutralizing solution to treat an acid burn on skin, ignoring the severe heat generated by the neutralization reaction.

1) Distractor check: Students might mistakenly select pairs containing metals and non-metals or completely unrelated elements like options A, B, or C, misunderstanding the definition of allotropy. 2) Reasoning to the answer: Allotropy is defined as the existence of different structural or physical forms of the same element in the same physical state. Examining the given choices, both oxygen (which exists as O2 and O3) and sulfur (which exists as rhombic and monoclinic allotropes) clearly exhibit this phenomenon. 3) Common mistake: Confusing allotropy with isotopes or mixing up elements that do not display multiple structural modifications.

1) Distractor check: A student might mistakenly choose option A, C, or D by picking less soluble gases that fail to create the necessary pressure drop for a successful fountain demonstration. 2) Reasoning to the answer: Ammonia is chosen because it is exceptionally soluble in water. This extreme solubility causes a rapid dissolution and a sudden drop in pressure inside the flask, resulting in a fast and dramatic inflow of water during the fountain experiment. 3) Common mistake: Selecting gases with low water solubility, which cannot generate the rapid pressure change required for the fountain effect.

1) Distractor check: Students might incorrectly select option A or D by choosing acidic drying agents like H2SO4, which would react with and neutralize the alkaline gas being collected. 2) Reasoning to the answer: Calcium hydroxide reacts with ammonium sulfate to generate ammonia gas. Because ammonia is alkaline, it must be dried using a basic drying agent like quicklime, or CaO, which does not react with ammonia while removing moisture. 3) Common mistake: Using an acidic drying agent such as concentrated sulfuric acid to dry ammonia, which destroys the gas through a neutralization reaction.

1) Distractor check: Students might mistakenly select options like Mg or Ag, overlooking their specific acid-base reaction profiles and metallic characteristics. 2) Reasoning to the answer: Manganese dioxide is amphoteric in nature, meaning it displays both acidic and basic properties. Because of this dual behavior, it reacts with nitric acid to produce manganese salts and reacts with sodium hydroxide to form manganates. 3) Common mistake: Assuming all metal oxides are exclusively basic and will not react with strong alkalis like NaOH.

1) Distractor check: Students might mistakenly select options containing silver or lead, such as A or B, due to confusing the components used in specialized coinage or heavy metallurgy with stainless steel. 2) Reasoning to the answer: Stainless steel is a specialized alloy primarily composed of iron and carbon combined with chromium. The addition of chromium is crucial because it provides the alloy with its signature resistance to corrosion and rusting. 3) Common mistake: Forgetting that chromium is the essential alloying element added to iron and carbon to prevent corrosion in stainless steel.

1) Distractor check: Students might mistakenly select options like arc welding or electrolysis, confusing fabrication techniques or purification methods with the actual creation of bulk alloys. 2) Reasoning to the answer: Alloys are typically prepared by melting the constituent metals together to form a uniform liquid solution. Once thoroughly mixed, this molten mixture is allowed to cool and solidify into the final alloy matrix. 3) Common mistake: Confusing the thermal melting and cooling process of making alloys with electrolytic refining or chemical reduction methods.

1) Distractor check: Students might mistakenly select option A or C by assuming corrosion is restricted solely to iron or only applies to metals positioned below hydrogen in the reactivity series. 2) Reasoning to the answer: Corrosion generally targets electropositive metals such as iron and zinc because these elements readily lose electrons. In the presence of moisture and oxygen, their high tendency to undergo oxidation drives the corrosion process. 3) Common mistake: Believing that iron is the only metal susceptible to corrosion processes in environmental conditions.

1) Distractor check: Students might mistakenly choose options A, B, or D by confusing equal orbital energies with the specific orbital mixing process required to create four identical bonds. 2) Reasoning to the answer: Carbon's tetravalent bonding capability is a direct result of sp³ hybridization. In this process, one 2s orbital and three 2p orbitals combine and mix to generate four equivalent hybrid orbitals directed toward the corners of a tetrahedron. 3) Common mistake: Attributing carbon's tetravalency simply to unhybridized shell energies rather than the formation of equivalent hybrid orbitals.

1) Distractor check: Students might mistakenly select alkene options like A or D, or a non-terminal alkyne like B, forgetting that only specific structural classifications react with ammoniacal copper(I) chloride. 2) Reasoning to the answer: Terminal alkynes feature an acidic hydrogen attached to a sp-hybridized triple-bonded carbon. When CH≡CCH2CH3 reacts with ammoniacal CuCl, this terminal position facilitates substitution to form a characteristic red precipitate of copper acetylide. 3) Common mistake: Confusing internal alkynes with terminal alkynes and expecting internal triple bonds to form precipitates with ammoniacal CuCl.

1) Distractor check: Students might mistakenly select option B, assuming linear unbranched chains burn more smoothly, or choose halogenated hydrocarbons. 2) Reasoning to the answer: Branched alkanes, such as isooctane, possess significantly higher octane numbers compared to straight-chain counterparts. This structural branching prevents engine knocking and ensures smoother combustion, thereby improving petrol efficiency. 3) Common mistake: Assuming straight-chain hydrocarbons provide better anti-knock properties and engine efficiency than branched structures.

1) Distractor check: Students might mistakenly select options B, C, or D by picking ethanol dehydration reactions or substitution pathways instead of the primary sugar breakdown equation. 2) Reasoning to the answer: The biochemical process of palm wine fermentation relies on yeast enzymes breaking down sugars. This converts glucose into ethanol and carbon dioxide gas, represented by the balanced equation C6H12O6 → 2C2H5OH + 2CO2. 3) Common mistake: Confusing the fermentation equation that yields alcohol with dehydration or esterification reactions of ethanol.

1) Distractor check: Students might mistakenly select option A, C, or D by picking unrelated halogen derivatives or assuming the reaction yields an aldehyde rather than undergoing the haloform degradation. 2) Reasoning to the answer: Ethanol contains a secondary alcohol structure adjacent to a methyl group that reacts positively in the iodoform test. When treated with sodium hypoiodide (NaOI), it undergoes oxidation and halogenation to precipitate triiodomethane, commonly known as iodoform. 3) Common mistake: Forgetting that the iodoform test converts methyl-bearing alcohols and ketones into triiodomethane rather than simple haloalkanes.

1) Distractor check: Students might mistakenly select options containing heavier components like kerosene or benzene, which have much higher boiling points and lower volatility. 2) Reasoning to the answer: Volatility increases as molecular weight decreases in hydrocarbon fractions. Methane, ethane, and propane are extremely light, low-molecular-weight gases, making them the most volatile fractions obtained from crude oil distillation. 3) Common mistake: Including heavier liquid hydrocarbons when identifying the most volatile, low-boiling gaseous fractions of petroleum.

1) Distractor check: Students might mistakenly select option A or B by assuming plant ash introduces acidic compounds or preformed esters into the saponification mixture. 2) Reasoning to the answer: Plant ash contains potassium carbonate, which reacts with water to generate potassium hydroxide. This generated alkali provides the essential basic environment required for the saponification reactions used in traditional black soap production. 3) Common mistake: Assuming ash acts as a mild acid rather than reacting with water to produce the alkaline medium needed for soap making.

1) Distractor check: Students might mistakenly select options B, C, or D by picking structural isomers with incorrect double-bond positions or degrees of methylation. 2) Reasoning to the answer: Synthetic rubber analogs and related elastomeric polymers are manufactured via the polymerization of isoprene units. The systematic chemical name for this key monomer is 2-methylbuta-1,3-diene. 3) Common mistake: Confusing the position of the diene double bonds, picking 1,2-diene instead of the correct 1,3-diene structure.

1) Distractor check: Students might mistakenly select option A, assuming the oxidation stops halfway at the aldehyde stage, or pick option C. 2) Reasoning to the answer: Propan-1-ol is a primary alcohol. When subjected to complete oxidation using strong oxidizing agents like potassium permanganate, the reaction does not halt at propanal but proceeds further to yield propanoic acid as the final product. 3) Common mistake: Assuming oxidation of a primary alcohol under standard conditions yields only the corresponding aldehyde without progressing to the carboxylic acid.

1) Distractor check: Students might mistakenly select options A, B, or D by choosing incorrectly named flame combustion types that do not correspond to the actual gas produced. 2) Reasoning to the answer: Calcium carbide reacts vigorously with water to generate acetylene gas. This specific gas is subsequently combusted in specialized equipment to produce an oxyacetylene flame, which burns at very high temperatures ideal for metal welding. 3) Common mistake: Forgetting the exact name of the fuel gas produced by calcium carbide and choosing generic hydrocarbon flame terms.

1) Distractor check: Students might mistakenly select options B or D by misnumbering the carbon chain positions or confusing propanoic acid derivatives with longer four-carbon chains. 2) Reasoning to the answer: The carbon chain for CH3CH=CHCOOH contains four total carbon atoms, making it a derivative of butanoic acid. With the double bond starting at carbon-2 and the terminal carboxyl group, the correct IUPAC name is but-2-enoic acid. 3) Common mistake: Counting only three carbons instead of four when naming unsaturated carboxylic acids, leading to incorrect 'prop-' prefixes.