Chemistry 2018 Jamb Examination

At the cathode, \(Cu^{2+} + 2e^- \rightarrow Cu\).
Charge passed: \(Q = I \times t = 1.60 \times 3600 = 5760\;C\).
Moles of electrons: \(n_e = \dfrac{Q}{F} = \dfrac{5760}{96500} = 0.0597\;mol\).
Moles of Cu deposited: \(\dfrac{0.0597}{2} = 0.02985\;mol\).
Mass of Cu: \(0.02985 \times 64 = 1.91\;g\).
Hence, 1.91g of copper will be deposited.

Sulphuric acid ionises completely in two steps:
\(H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}\).
Thus, the concentration of \(H^+\) ions is twice that of the acid: \([H^+] = 2 \times 0.05 = 0.10\;mol\,dm^{-3}\).
\(pH = -\log [H^+] =\)
\( -\log(0.10) = 1.00\).
Therefore, the pH of the acid solution is 1.00.

The extraction method of a metal depends mainly on its reactivity, which is indicated by its position in the electrochemical series.
Highly reactive metals such as sodium and calcium are extracted by electrolysis, while less reactive ones like iron and copper are extracted by reduction with carbon or other agents.
Therefore, the determining factor is the position of the metal in the electrochemical series.

Aluminium hydroxide \(Al(OH)_3\) is a weak base and sparingly soluble in water.
It ionises only slightly to produce a small number of ions in solution.
Strong bases like NaOH and KOH dissociate completely, while Ca(OH)₂ dissociates moderately.
Hence, \(Al(OH)_3\) produces relatively few ions in solution.

Half-life helps determine the rate of radioactive decay of a substance and is used to detect and measure radioactive isotopes.
It assists in estimating the age of rocks, fossils, and archaeological finds using carbon dating.
Therefore, it is mainly used to detect or measure the presence and quantity of a radioactive element.

The element X combines with chlorine in different proportions by mass to form several compounds such as XCl₂, XCl₃, and XCl₄.
This shows that the same elements can combine in more than one ratio to form different compounds, illustrating the law of multiple proportions.

Given [H⁺] = 2.0 × 10^-11, then pH = -log [H⁺] = -log(2.0 × 10^-11) = 11 - 0.3010 = 10.70
Since pH + pOH = 14, then pOH = 14 - 10.70 = 3.30
Therefore, the pOH of the orange juice sample is 3.30.

The reaction is exothermic (ΔH negative), meaning heat is released during the formation of SO₃.
According to Le Chatelier’s principle, increasing temperature favours the endothermic direction, i.e. the reverse reaction.
Thus, the equilibrium shifts to the left, and the equilibrium constant decreases because the forward reaction becomes less favourable at higher temperatures.

Candle wax is mainly composed of hydrocarbons such as paraffin. When burned completely in air, it reacts with oxygen to produce carbon dioxide and water vapour.
Incomplete combustion would produce carbon monoxide, but under normal burning conditions, the major gaseous product is carbon (IV) oxide.
Therefore, the main products are water and carbon (IV) oxide.

The collision theory states that for a reaction to occur, the reacting particles must collide with sufficient energy (activation energy) and proper orientation. The rate of reaction depends on the number of effective collisions per unit time. Hence, reaction rate increases with the frequency of collisions between reactant molecules, provided they have enough energy to overcome the activation barrier.