JAMB Chemistry 2023 Past Questions & Answers

According to Le Chatelier's principle, if a change is made to a system in equilibrium, the system will shift to oppose that change. An exothermic reaction releases heat, so when the temperature is decreased (heat is removed), the equilibrium will shift to the right to produce more heat. This shift to the right favours the formation of more products.

Common mistake: Students often mistakenly choose option A because they confuse the rule for exothermic reactions with that of endothermic reactions, where a decrease in temperature would cause a shift to the left.

Radon (Rn) is the correct answer because it is a noble gas, located in Group 18 of the periodic table. Unlike the other noble gases listed, Radon is radioactive because its nucleus is unstable. It is formed naturally in the environment from the radioactive decay of heavier elements like uranium and thorium, which are present in rocks and soil.

Common mistake: Students may mistakenly choose Argon because it is the most abundant and well-known noble gas in the Earth's atmosphere, but they forget that it is not radioactive.

Entropy is a measure of the degree of disorder or randomness in a system. Gas particles move freely and randomly, making them the most disordered state of matter, while solids are the most ordered. The change from a liquid to a gas involves a very large increase in volume and particle freedom, leading to the greatest increase in disorder. Therefore, this process has the highest positive entropy change.

Common mistake: Students might choose option B (Solid → Liquid) because it also represents an increase in disorder, but they forget that the increase in disorder from liquid to gas is much greater.

A neutral compound has no overall electrical charge. Oxidation numbers are assigned to atoms in a compound to represent their apparent charge. For the entire compound to be neutral (have a charge of 0), the sum of all the positive and negative oxidation numbers of the atoms must cancel each other out, resulting in a total of zero.

Common mistake: Students may mistakenly choose -1 or 1 because they confuse the rule for a neutral compound with the rule for a polyatomic ion, where the sum of oxidation numbers equals the charge on the ion.

The kinetic theory of gases describes the behaviour of gases based on the motion of their particles. A key principle of this theory is that gas particles are in constant, random motion. Gas pressure is the force created when these fast-moving particles repeatedly collide with the inner walls of their container.

Common mistake: Students might mistakenly choose option A, confusing the random translational motion of gas particles with the vibrational motion that is characteristic of particles in a solid.

Eutrophication is a specific environmental problem that occurs when bodies of water like rivers and lakes become overly rich in nutrients, such as nitrates and phosphates from fertilizers and sewage. These excess nutrients act like food for algae, causing them to grow uncontrollably in a process called an algal bloom. Therefore, the correct definition is the excessive growth of algae due to this nutrient enrichment.

Common mistake: Students often mistakenly choose option A because eutrophication is a form of water pollution, but option A is a general definition for water pollution, not the specific definition of eutrophication.

The Haber process, also called the Haber-Bosch process, is the specific name for the industrial method used to synthesize ammonia. In this process, nitrogen gas from the air is reacted with hydrogen gas, which is typically sourced from natural gas. The reaction is carried out under conditions of high temperature $(around 400-450°C)$, high pressure, and in the presence of an iron catalyst to produce ammonia.

Common mistake: Students might mistakenly choose 'Nitrification' because it sounds related to nitrogen, but this is a biological process done by bacteria in the soil to convert ammonia to nitrates, not an industrial process to make ammonia.

Niels Bohr proposed the model of the atom where electrons travel in fixed, circular orbits or shells around the nucleus. This model is often called the planetary model because it resembles planets orbiting the sun. Bohr's model was an important improvement on Rutherford's earlier nuclear model and helped explain why atoms are stable.

Common mistake: Students often mistakenly choose Ernest Rutherford because he discovered the nucleus, but it was Niels Bohr who refined the model by placing electrons in specific, fixed orbits.

An endothermic reaction is a chemical process that absorbs heat from its surroundings, making the surroundings feel cooler. Options A and B are combustion (burning) reactions, which always release a large amount of heat and are therefore exothermic. Option C, the formation of table salt from its elements, is also highly exothermic. The decomposition of hydrogen peroxide (Option D) is the best answer, as energy is absorbed to break the bonds within the hydrogen peroxide molecules, a feature of many decomposition reactions.

Common mistake: Students often mistakenly choose combustion reactions like A or B, forgetting that burning always releases heat, which is the definition of an exothermic reaction, the opposite of an endothermic one.

The equilibrium constant (Kc) is calculated as the concentration of products divided by the concentration of reactants. When a reaction is reversed, the substances that were products become the new reactants, and the old reactants become the new products. This means the original Kc expression is flipped upside down, making the new equilibrium constant the reciprocal (1/Kc) of the original value.

Common mistake: Students often mistakenly choose C, thinking the constant is a fixed property of the substances involved, but they forget that reversing the reaction inverts the ratio of products to reactants.