Chemistry 2023 Jamb Examination

Chlorine exists as a diatomic molecule (Cl₂) and is a greenish-yellow gas at room temperature and standard atmospheric pressure.
It has a boiling point of approximately −34°C, which is well below room temperature, hence it remains gaseous under normal laboratory conditions.

When a molecule has three bonding pairs and no lone pairs around the central atom, the electron pairs arrange themselves as far apart as possible to minimise repulsion, forming a trigonal planar geometry.
This arrangement gives bond angles of approximately 120°. An example of such a molecule is boron trifluoride (BF₃).

Iron is extracted mainly from its oxide ores. The two most important ores of iron are hematite (Fe₂O₃) and magnetite (Fe₃O₄).
Hematite is considered the principal ore because it is more abundant and easier to process through reduction in a blast furnace.
When heated with carbon in the furnace, hematite is reduced to metallic iron, releasing carbon dioxide in the process.

Calcium (Ca) is found in Group 2 of the periodic table, which is known as the alkaline earth metals.
Elements in this group, such as magnesium and barium, are characterised by having two valence electrons, forming divalent cations (e.g., Ca²⁺).
They are reactive metals, but less reactive than alkali metals, and commonly form basic oxides and hydroxides.

Crude oil is a complex mixture of hydrocarbons that includes alkanes, cycloalkanes, and aromatic compounds.
Methane (CH₄) is the simplest hydrocarbon and is a major component of natural gas, which is often found alongside crude oil.
It is considered one of the primary constituents because it represents the simplest form of hydrocarbon derived from petroleum sources.

In urban environments, the incomplete combustion of fossil fuels is the principal source of carbon monoxide (CO).
Vehicle engines, especially those running on petrol, often burn fuel under conditions of limited oxygen, producing CO instead of CO₂.
Although industrial activities and forest fires also emit CO, vehicle emissions are the dominant source in cities.

To find the empirical formula, assume 100 g of the compound.
Then, masses: C = 40 g, H = 6.67 g, O = 53.33 g.
Convert to moles:
\( \text{C: } 40/12 = 3.33 \) mol, \( \text{H: } 6.67/1 = 6.67 \) mol, \( \text{O: } 53.33/16 = 3.33 \) mol.
Divide by the smallest (3.33): C : H : O = 1 : 2 : 1.
Therefore, the empirical formula is CH₂O.

Chromatography is a separation technique that relies on the different affinities of substances towards a stationary phase (such as paper or silica gel) and a mobile phase (such as a solvent).
As the mobile phase moves, components that are more soluble in it move faster and farther than those with stronger attraction to the stationary phase.
This method is commonly used to separate and identify pigments, inks, and other mixtures.

Ethanoic acid (CH₃COOH) is the IUPAC name for the compound commonly known as acetic acid.
It is a weak carboxylic acid responsible for the sour taste and pungent smell of vinegar.
Its name ‘acetic’ is derived from the Latin word ‘acetum,’ meaning vinegar.
Therefore, ethanoic acid and acetic acid refer to the same chemical compound.

The formation of alkanols (alcohols) from alkenes occurs by adding water (H₂O) across the double bond of the alkene.
This process is called hydration and represents an addition reaction since new atoms are added to the carbon–carbon double bond.
The reaction is typically catalysed by acids such as sulphuric acid (H₂SO₄).
Hence, the process is classified as an addition reaction.