JAMB Chemistry 1990 Past Questions & Explanations

A physical change is one where the identity of the substance remains unchanged; only its physical state or form changes. (A) Chlorine reacting with water: $\mathrm{Cl_2 + H_2O \rightarrow HCl + HClO}$ is a chemical change. (B) Chlorine reacting with hydrogen: $\mathrm{Cl_2 + H_2 \rightarrow 2HCl}$ is a chemical change. (C) Dissolution of sodium chloride in water is a physical change—NaCl dissociates into ions but the compound itself is not chemically transformed; it can be recovered by evaporation. (D) Steam passing over heated iron: $\mathrm{3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2}$ is a chemical change. The correct answer is C.

To find the oxidation number of chromium in $\mathrm{Cr_2O_7^{2-}}$: Let the oxidation number of Cr be $x$. We have $2x + 7(-2) = -2$, so $2x - 14 = -2$, giving $2x = 12$, thus $x = +6$. In the product $\mathrm{Cr^{3+}}$, the oxidation number is +3. Therefore, chromium changes from +6 to +3. The correct answer is B.

The oxidizing agent is the substance that gets reduced (gains electrons). In $\mathrm{IO_3^-}$, iodine has an oxidation number of +5. In $\mathrm{I_2}$, iodine has an oxidation number of 0. Since iodine in $\mathrm{IO_3^-}$ goes from +5 to 0, it is reduced. Simultaneously, $\mathrm{I^-}$ (oxidation number -1) is oxidized to $\mathrm{I_2}$ (oxidation number 0). The substance that is reduced is the oxidizing agent, so the answer is $\mathrm{IO_3^-}$ (C).

The enthalpy change for a reaction is calculated as: $\Delta H_{\text{rxn}} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$. Products: $\mathrm{Al_2O_3}$ has $\Delta H_f = -1670$ kJ/mol; $\mathrm{Fe}$ has $\Delta H_f = 0$ (element). Reactants: $\mathrm{Fe_2O_3}$ has $\Delta H_f = -822$ kJ/mol; $\mathrm{Al}$ has $\Delta H_f = 0$ (element). Therefore: $\Delta H_{\text{rxn}} = [(-1670) + 2(0)] - [(-822) + 2(0)] = -1670 - (-822) = -1670 + 822 = -848$ kJ. The correct answer is C.

In galvanic corrosion protection, zinc acts as a sacrificial anode because it has a more negative (less positive) oxidation potential than iron. This means zinc is more easily oxidized than iron, so zinc oxidizes preferentially, protecting the iron underneath. Cathodic protection works when the protecting metal (zinc) is more readily oxidized (less noble) than the metal being protected (iron). The correct answer is B: zinc has a less positive oxidation potential than iron.

The rate of a chemical reaction is affected by two main factors: surface area and temperature. Powdering increases surface area, allowing more particles to come into contact with the acid, increasing reaction rate. Raising temperature increases the kinetic energy of reactant molecules, leading to more frequent and energetic collisions. Option D combines both factors: powdered form (larger surface area) AND higher temperature ($50^\circ C$ vs $25^\circ C$). Therefore, option D will react fastest.

This is an endothermic reaction ($\Delta H = +10$ kJ, positive). According to Le Chatelier's principle, when temperature increases, the equilibrium shifts in the direction that absorbs heat (endothermic direction) to counteract the increase. Here, raising the temperature favors the forward reaction, producing more $\mathrm{I_2}$. (A) Raising pressure: the forward reaction has 2 moles of gas → 2 moles of gas (no change in mole number), so pressure has no effect. (C) A catalyst speeds up the reaction but does not shift equilibrium. (D) Lowering pressure would shift away from the side with more moles, but both sides have equal moles. The correct answer is B.

A gas can be collected by upward displacement of air if it is less dense than air (molecular weight < 29). (A) NO: molecular weight = 30, slightly denser than air—cannot be collected by upward displacement. (B) $\mathrm{H_2}$: molecular weight = 2, much less dense than air, but it rises so quickly that it cannot be easily collected by simple upward displacement; it's typically collected by downward displacement. (C) $\mathrm{NH_3}$: molecular weight = 17, significantly less dense than air (29), and it collects well by upward displacement of air. (D) $\mathrm{Cl_2}$: molecular weight = 71, much denser than air—collected by downward displacement. The correct answer is C.

When nitric acid ($\mathrm{HNO_3}$) is heated, it decomposes according to the reaction: $\mathrm{4HNO_3 \rightarrow 4NO_2 + O_2 + 2H_2O}$. The brown fumes are primarily $\mathrm{NO_2}$ (nitrogen dioxide, which is brown). The decomposition produces three gaseous products: $\mathrm{NO_2}$ (brown), $\mathrm{O_2}$ (colorless), and $\mathrm{H_2O}$ (steam/water vapor). Therefore, the fumes consist of all three products: $\mathrm{NO_2}$, $\mathrm{O_2}$, and $\mathrm{H_2O}$. The correct answer is C.

Let's analyze each test for the four gases: $\mathrm{SO_3}$, $\mathrm{H_2}$, $\mathrm{CO_2}$, $\mathrm{NO}$. (A) Water + litmus: $\mathrm{SO_3}$ and $\mathrm{CO_2}$ dissolve and turn litmus red; $\mathrm{H_2}$ and $\mathrm{NO}$ do not dissolve. This does not distinguish all four. (B) Lime water: $\mathrm{CO_2}$ turns lime water cloudy; $\mathrm{SO_3}$, $\mathrm{H_2}$, $\mathrm{NO}$ do not react. This does not distinguish all four. (C) Expose to atmospheric air: $\mathrm{NO}$ reacts with $\mathrm{O_2}$: $\mathrm{2NO + O_2 \rightarrow 2NO_2}$ (brown fumes); $\mathrm{H_2}$ burns (pale blue flame); $\mathrm{CO_2}$ shows no change; $\mathrm{SO_3}$ reacts slowly with moisture in air. Each gas shows a distinct behavior. (D) Concentrated $\mathrm{H_2SO_4}$: $\mathrm{SO_3}$ dissolves in it; others do not, but this doesn't distinguish the remaining three well. The best answer is C because NO specifically produces visible brown fumes with air.

The Haber process synthesizes ammonia from nitrogen and hydrogen: $\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}$. The catalyst used in the industrial Haber process is finely divided (or precipitated) iron. Iron is chosen because it is cost-effective, readily available, and highly effective at catalyzing this reaction at high temperatures and pressures. Platinum (B) is too expensive for large-scale industrial use, though it would be effective. Vanadium (A) and copper (D) are not typically used in the Haber process. The correct answer is C.

An amphoteric oxide is one that reacts with both acids and bases. The key phrase is "reacts with both HCl and NaOH to give salt and water only." For example, aluminium oxide ($\mathrm{Al_2O_3}$) is amphoteric: $\mathrm{Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O}$ (with acid) and $\mathrm{Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O}$ (with base). (A) An acidic oxide reacts only with bases. (B) A basic oxide reacts only with acids. (C) A neutral oxide does not react with either acids or bases. The correct answer is D.

A metal liberates hydrogen from steam or dilute acid if it is more reactive than hydrogen (i.e., it appears above hydrogen in the reactivity series). (A) Copper is less reactive than hydrogen and does not liberate hydrogen from acids or steam. (B) Iron is more reactive than hydrogen and can liberate hydrogen: $\mathrm{Fe + H_2SO_4 \rightarrow FeSO_4 + H_2}$ and $\mathrm{3Fe + 4H_2O(steam) \rightarrow Fe_3O_4 + 4H_2}$. (C) Lead is below hydrogen in the reactivity series and does not readily liberate hydrogen. (D) Mercury is less reactive than hydrogen and does not liberate hydrogen. The correct answer is B.

The primary hazard of coal fires in poorly ventilated rooms is carbon monoxide (CO) accumulation. When coal burns with incomplete combustion (due to poor oxygen supply in a poorly ventilated room), it produces CO: $\mathrm{2C + O_2 \rightarrow 2CO}$ instead of complete combustion $\mathrm{C + O_2 \rightarrow CO_2}$. Carbon monoxide is a colorless, odorless, toxic gas that binds to hemoglobin in the blood, preventing oxygen transport and causing suffocation/asphyxiation. (A) While $\mathrm{CO_2}$ accumulation can occur, it primarily causes fatigue rather than deep sleep, and $\mathrm{CO_2}$ is the secondary concern. (B) Heat is not the primary safety issue. (D) Coal fires require oxygen, so they do not remove most gases. The correct answer is C.

In the blast furnace production of iron, slag is formed from the reaction of limestone (CaCO₃) with impurities in the ore, primarily silica ($\mathrm{SiO_2}$). The limestone decomposes: $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$, and the CaO then reacts with silica: $\mathrm{CaO + SiO_2 \rightarrow CaSiO_3}$ (calcium silicate or calcium trioxosilicate(IV)). Slag is a mixture of silicates that floats on top of molten iron and is tapped off. (A) Slag is not an alloy of calcium and iron. (B) Coke is the fuel, not slag. (C) Impure iron is the main product, not slag. The correct answer is D.

The key observations are: (1) neutral to litmus — this rules out alkanoates (which are acidic). (2) Gives a colourless gas with metallic sodium — this gas is hydrogen (H₂), produced by the reaction R-OH + Na → R-ONa + ½H₂. This reaction is characteristic of compounds with active hydrogen atoms bonded to oxygen, i.e., alcohols (alkanols). Alkenes and alkanes do not react with sodium in this way. Therefore, the liquid is an alkanol.

Volatility is inversely related to intermolecular forces. Alkanoic acids (R-COOH) have lower volatility (higher boiling points) than alkanols (R-OH) of similar molecular weight because carboxylic acids form extensive hydrogen bonding through their carboxyl group. Each carboxylic acid molecule can form two hydrogen bonds (one as a donor through the O-H, and one as an acceptor through the C=O), while alkanols form only one hydrogen bond per molecule (through their single O-H group). This stronger intermolecular bonding in alkanoic acids results in lower volatility.

The octane number is defined as the percentage by volume (or mass in this case, since they're mixed) of 2,2,4-trimethylpentane (octane number = 100) in a mixture with n-heptane (octane number = 0). Given a mixture of 55 g of 2,2,4-trimethylpentane and 45 g of n-heptane, the total is 100 g. The fraction of 2,2,4-trimethylpentane is 55/100 = 55%. Therefore, the octane number is 55.

Maltose is a disaccharide (a carbohydrate containing multiple carbon, hydrogen, and oxygen atoms). When concentrated sulphuric acid (tetraoxosulphate VI) is added to carbohydrates, it acts as a dehydrating agent, removing water molecules and leaving behind pure carbon. This process is known as carbonization or charring. The result is a black, porous carbon solid called charcoal. Carbon (IV) oxide would be formed by combustion, coal tar is not produced from this reaction, and while toxic fumes may evolve, the main solid product is charcoal.