Chemistry 2018 Jamb Examination

Carbon dioxide (CO₂) and water (H₂O) are already fully oxidised compounds and therefore cannot react further with oxygen. Carbon monoxide (CO) can react with oxygen to form carbon dioxide, while sulphur dioxide (SO₂) can react with oxygen to form sulphur trioxide (SO₃).
Thus, the pair of substances that can react further with oxygen to form higher oxides are sulphur dioxide (SO₂) and nitric oxide (NO).

Manganese(IV) oxide (MnO₂) acts as a catalyst in the thermal decomposition of potassium chlorate (KClO₃) to produce oxygen gas. A catalyst functions by lowering the activation energy required for the reaction to occur, thereby increasing the reaction rate without itself being consumed.
This allows the decomposition to occur at a lower temperature than it normally would.

The correct method to obtain pure water from a mixture of sand, water, and methanoic acid is Neutralisation with sodium hydroxide followed by distillation. Therefore, the correct method is filtration followed by distillation.

From Faraday’s laws of electrolysis:\[\text{Mass liberated} \propto \dfrac{E \times Q}{F}\]For the same charge \(Q\):\[\dfrac{m_{Al}}{m_{Ag}} = \dfrac{E_{Al}/n_{Al}}{E_{Ag}/n_{Ag}}\]\[\dfrac{m_{Al}}{3.6} = \dfrac{27/3}{108/1} = \dfrac{9}{108} = \dfrac{1}{12}\]\[m_{Al} = \dfrac{3.6}{12} = 0.3g\]Hence, 0.3g of aluminium will be liberated by the same quantity of electricity.

Nitrogen gas can be prepared in the laboratory by heating a mixture of ammonium chloride \((NH_4Cl)\) and sodium dioxonitrate(III) \((NaNO_2)\).
The reaction proceeds as:
NH₄Cl + NaNO₂ → NaCl + 2H₂O + N₂
This reaction produces nitrogen gas without any other gaseous impurities, making it suitable for laboratory use.

The element with atomic number 14 is silicon (Si). Its electronic configuration is \(1s^2, 2s^2, 2p^6, 3s^2, 3p^2\).
The outermost shell (n = 3) contains two electrons in the 3s orbital and two in the 3p orbital, giving a total of four valence electrons.
Therefore, silicon has four electrons in its valence shell.

The balanced equation for the reaction is:\[2H_2 + O_2 \rightarrow 2H_2O\]From the equation, 2 volumes of hydrogen react with 1 volume of oxygen.
Given 8cm³ of hydrogen, the required oxygen is half of that: \(8 \div 2 = 4cm³\).
Oxygen available = 20cm³, oxygen used = 4cm³.
Oxygen remaining = \(20 - 4 = 16cm³\).

Magnesium oxide (MgO) is a highly stable compound due to the strong ionic bond between magnesium and oxygen. It cannot be reduced by carbon or hydrogen under ordinary bunsen burner conditions.
In contrast, oxides like lead(II) oxide, copper(II) oxide, and tin(IV) oxide can all be reduced by hydrogen or carbon to their respective metals.
Hence, magnesium oxide is the oxide that is not reduced under these conditions.

The given compound structure represents a branched-chain alkane with four carbon atoms in the longest chain and a methyl group attached to the second carbon atom.
Hence, the correct IUPAC name is 2-methylbutane.
This is because the parent chain (butane) is numbered from the end nearest the substituent, and the methyl group appears on carbon 2.

A white precipitate that dissolves in excess sodium hydroxide and also dissolves in excess ammonia is characteristic of zinc(II) ions (Zn²⁺).
Calcium (Ca²⁺) and lead (Pb²⁺) hydroxides are insoluble in excess reagents, while aluminium hydroxide dissolves only in excess NaOH but not in excess ammonia.
Therefore, the cation in M is zinc.