Chemistry 2011 Jamb Examination

Although the exact image is not reproduced here, the stated correct option is that the diagram illustrates the effect of decreasing concentration. Here is why decreasing concentration lowers reaction rate (collision theory explanation):

1. Collision theory: reaction rate depends on frequency of effective collisions between reactant particles. Effective collisions require proper orientation and sufficient energy (activation energy).
2. If concentration is decreased, there are fewer reactant particles per unit volume, so the frequency of collisions falls proportionally.
3. Fewer collisions → fewer successful collisions per unit time → lower observed rate. Graphically this is often shown as a curve or bar reducing as concentration decreases.
4. Contrast with other factors: decreasing temperature reduces kinetic energy (fewer particles exceed activation energy), decreasing surface area reduces exposed reactive surface (for heterogeneous reactions), and decreasing pressure affects gaseous reactions (by reducing concentration). The most direct and generic explanation for the typical plotted change is concentration.

Therefore the diagram corresponds to a decrease in concentration (option A).

Apply the Bronsted–Lowry acid–base concept:
• An acid is a proton donor; a base is a proton acceptor.
• Acetic acid (CH₃COOH) donates a proton (H⁺) to OH^-, forming CH₃COO^- and H₂O.

By definition, the species that remains after an acid has donated a proton is its conjugate base. Here, CH₃COO^- is the species formed when CH₃COOH loses H⁺, so CH₃COO^- is the conjugate base of acetic acid.

Therefore the correct answer is A (conjugate base).

Interpret the compound name: potassium aluminium tetraoxosulphate(VI) is the systematic name for KAl(SO₄)₂ (the 'tetraoxosulphate(VI)' is SO₄^2-). This salt dissociates in water into its constituent ions.

Write the dissociation (simplified):\[ \mathrm{KAl(SO_4)_2 \rightarrow K^+ + Al^{3+} + 2SO_4^{2-}} \]
Count the cations produced per formula unit: K⁺ (one) and Al³⁺ (one) → total = 2 cations.

Therefore the number of cations produced is 2 (option D).

Definition clarification: 'Alkali' is commonly used to mean a soluble base—specifically a soluble hydroxide of an alkali or alkaline earth metal (e.g., NaOH is an alkali; Ca(OH)₂ and Mg(OH)₂ are basic hydroxides, although Mg(OH)₂ is sparingly soluble).

Ammonia (NH₃) is a weak base because it accepts a proton from water:\[ \mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-} \]But NH₃ itself is not a metal hydroxide and therefore, by the stricter definition, is not classified as an alkali. It is a basic gas (or basic solution when dissolved) but not an 'alkali' in the usual sense used in inorganic chemistry questions.

Hence NH₃ is the one that is NOT an alkali (option A).

Thermal pollution (raising the temperature of a water body, e.g., from discharge of heated industrial effluent) has several ecological effects; a primary physical/chemical consequence is a reduction in dissolved oxygen solubility.

Why dissolved oxygen falls when temperature rises:
1. Gas solubility in liquids generally decreases as temperature increases — warmer water holds less dissolved O₂ than cooler water.
2. Higher temperatures also increase the metabolic rates of aquatic organisms, raising their oxygen demand.
3. Combined, these effects can lead to oxygen depletion (hypoxia), stressing or killing fish and other aerobic organisms.

Therefore the principal listed effect is a reduction in the level of oxygen (option D).

Deliquescent compounds are substances that absorb so much moisture from the air that they eventually dissolve in the absorbed water to form a solution. CaCl₂ is highly hygroscopic and can absorb water until it liquefies. Other options like Na₂CO₃ and CuO may absorb some moisture but not enough to become deliquescent, while hydrated Na₂CO₃.10H₂O already contains water of crystallization and is not deliquescent.

Element X with atomic number 12 is magnesium, a metal with two valence electrons. Element Y with atomic number 17 is chlorine, a non-metal with seven valence electrons. When they react, magnesium loses two electrons to form Mg²⁺, while each chlorine atom gains one electron to form Cl⁻. The electrostatic attraction between the oppositely charged ions forms an ionic or electrovalent bond. This explains why MgCl₂ is an ionic compound.

Water hardness is caused by dissolved salts of calcium and magnesium. Temporary hardness is due to calcium hydrogen carbonate which decomposes upon boiling. Permanent hardness comes from soluble salts such as calcium sulphate. Option B correctly includes calcium carbonate and calcium sulphate, which contribute to water hardness. Sodium salts and hydroxides in the other options do not cause significant hardness.

The rule of solubility states that like dissolves like. Iodine and naphthalene are both non-polar substances. They are insoluble in polar solvents such as water or ethanol but dissolve readily in non-polar solvents such as benzene. Therefore, benzene is a suitable solvent for both.

Among the noble gases, argon is the most abundant in the atmosphere, making up about 0.9% by volume. Helium and neon are present only in trace amounts, while xenon is extremely rare. This abundance is why argon is often used industrially, for example in electric bulbs.