Chemical Reactions and Equations

In the electrolysis of water, hydrogen gas is produced at the cathode and oxygen gas at the anode. From \(2H_2O \rightarrow 2H_2 + O_2\), the volume ratio of hydrogen to oxygen is \(2:1\). Therefore, the greater volume of gas, hydrogen, is produced at the cathode. Oxygen is produced at the anode, but its volume is half that of hydrogen.

Pure water contains very few ions and is therefore a poor conductor of electricity. Dilute sulphuric acid supplies mobile ions, allowing current to pass through the solution and enabling electrolysis of water. Therefore, option A is correct. Adding an acid does not make the solution neutral; it makes it acidic.

The reaction is \(2H_2O \rightarrow 2H_2 + O_2\). At the same temperature and pressure, the gas volumes are in the ratio of their coefficients, so \(H_2:O_2 = 2:1\). Therefore, for 8 mL of oxygen, the volume of hydrogen is \(2 \times 8 = 16\) mL. The 8 mL option is incorrect because the volumes of the two gases are not equal.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The displaced copper deposits on the iron nail as a reddish-brown coating. The reaction is \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Iron oxide in option C is associated with rusting, whereas the coating in this reaction is copper.

The blue colour of aqueous copper sulphate solution is mainly due to hydrated copper(II) ions, \(\mathrm{Cu^{2+}}\). When the concentration of these ions decreases, the blue colour becomes less intense and the solution appears paler. Sulphate ions, \(\mathrm{SO_4^{2-}}\), do not cause the blue colour, so option B is not the correct ionic reason. Exam tip: For solution colour changes, first identify the ion responsible for the colour.

In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. For example, zinc displaces copper from copper(II) sulfate solution: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). Therefore, zinc is more reactive than copper. Option B is incorrect because copper cannot displace zinc from a zinc salt solution.

A more reactive metal displaces a less reactive metal from a solution of its salt. Since A displaces B, A is more reactive than B. Since B displaces C, B is more reactive than C. Thus, the reactivity order is A > B > C, so metal A is the most reactive. Option B is more reactive than C, but not than A.

Mixing BaCl₂(aq) and Na₂SO₄(aq) causes exchange of ions: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl. Barium sulphate is insoluble in water, so it separates as a white precipitate. Sodium chloride remains dissolved, while barium oxide and sodium oxide are not formed because no oxide ions are present. Hence option A is correct.

In \(\mathrm{BaCl_2 + Na_2SO_4 \rightarrow BaSO_4\downarrow + 2NaCl}\), the ions of the two compounds exchange partners. \(\mathrm{Ba^{2+}}\) combines with \(\mathrm{SO_4^{2-}}\) to form an insoluble precipitate of \(\mathrm{BaSO_4}\), while \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) form \(\mathrm{NaCl}\). Therefore, this is a double displacement reaction. Option A describes single displacement, in which a free element replaces an element in a compound. Exam tip: The general pattern \(\mathrm{AB + CD \rightarrow AD + CB}\) indicates a double displacement reaction.

In a precipitation reaction, ions from two aqueous solutions combine to form a solid that is insoluble in water. For example,
\(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\).
\(\mathrm{BaSO_4}\) separates from the solution as an insoluble white solid, so it is a precipitate. In contrast, the soluble substance described in option B would remain in a clear solution rather than form a precipitate.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Oxygen is removed from \(\mathrm{CuO}\) and combines with hydrogen to form water. Removal of oxygen is reduction, so copper(II) oxide undergoes reduction. In contrast, hydrogen gains oxygen and is oxidised.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Hydrogen gains oxygen from copper oxide to form water. As gain of oxygen is oxidation, hydrogen is oxidised. Copper oxide, on the other hand, loses oxygen to form copper and is therefore reduced. Exam tip: In oxygen-based redox questions, the substance that gains oxygen is oxidised.

In the oxygen-based definition of redox, removal of oxygen from a substance is reduction, whereas addition of oxygen is oxidation. Since A loses oxygen, A is reduced. Since B gains that oxygen, B is oxidised. These processes occur together as a redox change. Option B reverses the definitions, and options C and D incorrectly assign the same process to both substances.

Based on oxygen, oxidation is the addition of oxygen to a substance, whereas reduction is the removal of oxygen from a substance. For example, when copper gains oxygen, copper oxide forms; when oxygen is removed from copper oxide, copper is obtained. Therefore, option A is correct. Option B interchanges the definitions of oxidation and reduction.

In the hydrogen-based description of redox reactions, removal of hydrogen is oxidation, while addition of hydrogen is reduction. Therefore, the two processes in the stated order are oxidation followed by reduction. This is consistent with the broader electron-transfer idea: oxidation involves loss of electrons and reduction involves gain. Option B reverses the sequence; C and D name unrelated reaction types.

Rusting is a slow oxidation reaction in which iron reacts with oxygen in the presence of water or moisture to form hydrated iron(III) oxide, commonly called rust. Both oxygen and moisture are essential conditions. Salt or acids may speed up corrosion by increasing conductivity, but they are not substitutes for oxygen and water. Nitrogen, light, hydrogen, and sound are not the required pair. Thus option A is correct.

Rusting requires iron to be in contact with oxygen and water or moisture. Paint and oil form a protective coating on the iron surface, so air and moisture cannot reach the metal. Therefore, option A is correct. Raising the temperature does not create a barrier between iron and oxygen or moisture.

Zinc is more reactive than iron. When a small scratch occurs and moisture is present, zinc acts as the anode and oxidises preferentially. This sacrificial action keeps the exposed iron cathodic and protects it from rusting. Option C reverses these roles: in galvanisation, zinc is the anode, not iron. Iron can rust after the available zinc has been substantially consumed and it is exposed to air and moisture.

Oils and fats, especially unsaturated fats, react with oxygen in air and undergo oxidation. This oxidation produces foul-smelling substances and makes the oils or fats rancid; the process is called rancidity. Reduction is the opposite of oxidation, so it does not cause rancidity.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution:
\(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\).
The \(\mathrm{FeSO_4}\) solution is green, and the reddish-brown coating is copper. This is a displacement reaction because a more reactive metal removes a less reactive metal from its salt solution; it is not a combination reaction, in which reactants form a single product.