2025 - JAMB Chemistry Past Questions and Answers - page 1

The condition for spontaneity of any reaction is determined using the Gibbs free energy relationship:
\[\Delta G = \Delta H - T\Delta S\]
A reaction will occur spontaneously when \(\Delta G\) is negative.

In this reaction, one mole of gaseous PCl\(_5\) produces one mole of gaseous Cl\(_2\) and one mole of solid PCl\(_3\). Since a solid has much less disorder than a gas, the overall disorder of the system decreases. This means the entropy change, \(\Delta S\), is negative.

Because \(\Delta S\) is negative, the term \(-T\Delta S\) becomes positive. This makes the entire expression \(\Delta G = \Delta H - T\Delta S\) increase unless \(\Delta H\) is negative enough to compensate.

For \(\Delta G\) to remain negative and allow the reaction to occur spontaneously, the enthalpy change must itself be negative so that it offsets the positive contribution from \(-T\Delta S\).

This means the reaction must release heat, which corresponds to a negative enthalpy change.

Therefore, the reaction will be spontaneous when \(\Delta H\) is negative.

Corrosion of iron involves its gradual reaction with oxygen and water present in the environment. During this process, iron atoms lose electrons and combine with oxygen to form hydrated iron(III) oxide.

This reddish-brown substance that forms on the surface of iron is commonly called rust. The process itself is specifically known as rusting.

Other processes such as burning involve rapid oxidation with flame, galvanizing refers to coating iron with zinc to protect it, and alloying refers to mixing iron with other metals to improve properties.

Therefore, the corrosion process of iron is called rusting.

The total number of electrons can be obtained by adding the superscripts in the electron configuration.

\[2 + 2 + 6 + 2 = 12\]

This means the atom has atomic number 12. The element with atomic number 12 is magnesium.

The outermost shell contains two electrons in the 3s orbital. Elements with two valence electrons belong to Group 2 of the periodic table.

The highest principal energy level present is 3, which means the element is in Period 3.

Therefore, the atom belongs to Group 2 and Period 3.

Molecules inside a liquid attract each other equally in all directions, but molecules at the surface experience a net inward pull because there are no molecules above them.

This inward attraction causes the surface of the liquid to contract and minimize its exposed area. The property responsible for this behavior is called surface tension.

A sphere has the smallest possible surface area for a given volume. Because surface tension acts to reduce surface area, the liquid naturally forms a spherical shape.

Therefore, water drops are spherical because of surface tension.

Pressure changes affect equilibrium only when there is a difference in the number of moles of gaseous reactants and gaseous products.

If the volume does not change, this implies that the number of gaseous molecules remains the same on both sides of the reaction equation.

When pressure increases or decreases under these conditions, the rate of the forward and reverse reactions changes equally because both sides contain the same number of gas molecules.

Since both directions are affected equally, there is no shift in equilibrium.

Therefore, pressure has no effect on the equilibrium position.

Amines are classified according to the number of carbon groups directly attached to the nitrogen atom.

In this compound, the nitrogen atom is part of an NH\(_2\) group. This means nitrogen is bonded to one carbon atom and two hydrogen atoms.

An amine in which nitrogen is attached to only one carbon group is called a primary amine.

If nitrogen were attached to two carbon groups, it would be secondary, and if attached to three carbon groups, it would be tertiary.

Since only one carbon is attached to nitrogen here, the compound is a primary amine.

2-methylpropene contains a carbon–carbon double bond, which can react with hydrogen chloride through addition.

During the reaction, the hydrogen atom attaches to one carbon atom of the double bond, while the chlorine attaches to the other carbon atom.

The hydrogen atom preferentially attaches to the carbon atom that already has more hydrogen atoms. This results in the formation of a more stable intermediate where the positive charge is on the more substituted carbon atom.

The chlorine atom then attaches to this more substituted carbon atom, producing the final compound.

The resulting product is 2-chloro, 2-methylpropane.

The empirical formula is obtained by dividing all subscripts in the molecular formula by their greatest common divisor.

For C\(_6\)H\(_{12}\)O\(_6\), dividing each subscript by 6 gives:

\[CH_2O\]

The empirical mass is the sum of the atomic masses of the elements in the empirical formula.

Carbon contributes 12.
Hydrogen contributes \(2 \times 1 = 2\).
Oxygen contributes 16.

Adding these values gives:

\[12 + 2 + 16 = 30\]

Therefore, the empirical mass is 30.

Antifreeze is added to water in vehicle radiators to lower the freezing point and raise the boiling point of the liquid.

This ensures the cooling system works efficiently in both cold and hot conditions.

One suitable substance used for this purpose is propanetriol, which mixes well with water and prevents freezing.

Other substances listed do not provide the same stable antifreeze properties required in radiators.

Therefore, antifreeze used in radiators is a mixture of water and propanetriol.

Concentrated sulphuric acid has a very strong attraction for water molecules. Because of this property, it removes water from compounds that contain hydrogen and oxygen.

For example, when it is added to sugar, it removes water from the sugar molecules and leaves behind black carbon. This shows clearly that it extracts water from substances.

A substance that removes water from other substances is known as a dehydrating agent.

Therefore, the primary function of concentrated sulphuric acid is that it acts as a dehydrating agent.