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.