Chemistry 2022 Jamb Examination

According to the electrochemical series, ions lower in the series have a greater tendency to be discharged at the cathode because they require less energy to gain electrons.
Copper(II) ions \( \text{Cu}^{2+} \) are lower in the electrochemical series compared to \( \text{Sn}^{2+} \), \( \text{Fe}^{2+} \), and \( \text{Zn}^{2+} \).
Hence, \( \text{Cu}^{2+} \) will be preferentially discharged at the cathode to form copper metal.

Activated charcoal is a good adsorbent. It is added to sand filters during water treatment to adsorb organic impurities and unpleasant odours.
It does not kill germs or prevent diseases but rather improves the taste and smell of the treated water by removing offensive odours and colours.

Metals that react with steam are those above hydrogen in the reactivity series but not so reactive as to react with cold water.
Iron reacts with steam to form iron(II,III) oxide and hydrogen gas according to:
\( 3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2 \).
Metals like copper, tin, and lead are less reactive and do not liberate hydrogen from steam.

Organic compounds with fishy odours are typically amines.
Amines have the general formula \( RNH_2 \) for primary amines.
The odour is characteristic of volatile amines such as methylamine or ethylamine, which emit a distinct fish-like smell.
Other groups like amides \( RCONH_2 \) and ketones \( RCOR_1 \) do not exhibit this odour.

The reaction is exothermic (ΔH = negative), meaning heat is released.
According to Le Chatelier’s principle, lowering the temperature favours the exothermic direction (formation of \( N_2O_4 \)).
Also, fewer moles of gas are produced on the side of \( N_2O_4 \) (1 mole) compared to \( 2NO_2 \) (2 moles), so increasing pressure shifts equilibrium towards \( N_2O_4 \).
Therefore, low temperature and high pressure favour maximum yield of \( N_2O_4 \).

Zinc loses electrons to form \( Zn^{2+} \) ions in the reaction:
\( Zn \rightarrow Zn^{2+} + 2e^- \).
The loss of electrons indicates oxidation.
Therefore, zinc is oxidised, while hydrogen ions are reduced to form hydrogen gas.

Ammoniacal silver nitrate is used in the Tollens’ test to detect terminal alkynes and aldehydes.
Terminal alkynes, containing a hydrogen atom attached to the carbon–carbon triple bond (–C≡CH), reduce the silver ion \( Ag^+ \) to metallic silver, causing decolourisation.
Alkenes, alkanes, and alkanols do not react in this way.

Chlorine gas is greenish-yellow and highly toxic.
It reacts with moisture in the respiratory tract to form hydrochloric and hypochlorous acids, which can severely damage the mucous membranes of the lungs.
Carbon(II) oxide is colourless, while hydrogen sulphide and sulphur(IV) oxide are less corrosive to lung tissue compared to chlorine.

During silver electroplating, the object to be plated (spoon) is made the cathode so that silver ions are deposited on it.
The silver rod serves as the anode, which dissolves to replace the silver ions in the electrolyte solution.
The electrolyte is silver trioxonitrate(V) solution, providing \( Ag^+ \) ions for deposition.
Therefore, the anode is a silver rod and the cathode is the spoon.

Crude petroleum is a mixture of hydrocarbons with varying boiling points.
Fractional distillation separates these components based on their differences in boiling points.
Lighter fractions such as petrol and gas rise to the top of the distillation column, while heavier ones like diesel and bitumen remain at the bottom.
Thus, fractional distillation is the industrial method for refining crude petroleum into useful products.