2024 - JAMB Chemistry Past Questions and Answers - page 9

Here's how to solve this problem:

1. Write the balanced chemical equation:
   HCl + NaOH → NaCl + H₂O
   This equation shows that hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) in a 1:1 molar ratio.
2. Calculate the moles of NaOH:
   Moles of NaOH = Concentration × Volume
   Moles of NaOH = 0.20 mol/dm³ × 20 cm³
   Convert cm³ to dm³: 20 cm³ = 20 / 1000 = 0.02 dm³
   Moles of NaOH = 0.20 × 0.02 = 0.004 moles
3. Determine the moles of HCl required:
   Since the reaction is 1:1, 0.004 moles of HCl are required to neutralize 0.004 moles of NaOH.
4. Calculate the volume of HCl:
   Volume of HCl = Moles / Concentration
   Volume of HCl = 0.004 / 0.12 = 0.0333 dm³
5. Convert the volume to cm³:
   0.0333 dm³ × 1000 = 33.3 cm³

Therefore, 33.33 cm³ of 0.12 mol·dm^-3 HCl is required to neutralize 20 cm³ of 0.20 mol·dm^-3 NaOH.

When a metal reacts with an acid, the products formed are a salt and hydrogen gas. The salt is a compound formed by the metal replacing the hydrogen in the acid.

Hydrochloric acid (HCl) is considered a strong acid because it ionizes completely in water. This means that when HCl is dissolved in water, it fully dissociates into its ions, H⁺ (hydrogen ions) and Cl^- (chloride ions). This complete ionization results in a high concentration of H⁺ ions, which is the characteristic feature of a strong acid. The other options are not the primary reason why HCl is considered a strong acid.

When sulfur(IV) oxide (SO₂) is passed into an acidified solution of tetraoxomanganate(VII) (permanganate, MnO₄⁻), a redox reaction occurs. The permanganate ion acts as an oxidizing agent and is reduced, while sulfur dioxide (SO₂) is oxidized. The characteristic purple color of the permanganate ion fades as it is reduced to colorless manganese(II) ions (Mn²⁺).

The rate of a chemical reaction is affected by several factors, including:

• Surface area: Increasing the surface area of reactants (especially solids) increases the reaction rate.
• Temperature: Increasing the temperature generally increases the reaction rate.
• Catalyst: A catalyst speeds up a reaction without being consumed itself.
• Volume: While the concentration of reactants (which can be affected by volume changes) impacts the reaction rate, the volume itself is not a direct factor that influences the rate.

Potassium hexacyanoferrate(II) is a coordination compound. Here's how to determine the correct formula:

1. Hexacyanoferrate(II): This indicates a complex ion containing iron (Fe) and cyanide ligands (CN). The Roman numeral (II) signifies that iron has a +2 oxidation state. The hexacyano part means there are six cyanide ligands.
2. Complex Ion: The complex ion is \[Fe(CN)_6\]^{4-}. Each CN ligand carries a -1 charge, and there are six of them, resulting in a total negative charge of -6. Since the iron has a +2 charge, the overall charge of the complex ion becomes +2 + (-6) = -4.
3. Potassium: Potassium (K) is the counter-ion and has a +1 charge.
4. Formula: To balance the -4 charge of the complex ion, we need four potassium ions (4 × +1 = +4). Therefore, the correct formula is K_4[Fe(CN)_6].

The correct answer is noble metals. Here's why:

• Noble metals: Metals that are resistant to corrosion and oxidation. This is why they are found in their elemental form (uncombined) in the earth's crust. Silver and Gold are classic examples.
• Corrosive metals: Undergo corrosion (breakdown) and do not exist in their free state.
• Reactive metals: Readily react with other substances and are not found in their free state.
• Natural metals: Not a scientific term.

Here's how to determine the difference in molecular mass:

1. General Formulas:
   Alkene: C_nH_2n
   Alkyne: C_nH_2n-2
2. Given Information: Both the alkene and alkyne have six carbon atoms (n=6).
3. Calculate the molecular formulas:
   Alkene (C₆H₁₂): 6(12) + 12(1) = 84 g/mol
   Alkyne (C₆H₁₀): 6(12) + 10(1) = 82 g/mol
4. Find the difference: 84 - 82 = 2 g/mol

A spontaneous reaction occurs naturally without any external influence. The spontaneity is determined by the Gibbs free energy change (ΔG), combining enthalpy change (ΔH) and entropy change (ΔS) as follows:

A reaction is spontaneous if ΔG is negative. While both ΔH and ΔS contribute to ΔG, the most direct indicator of spontaneity is a negative free energy change. For this question, the most appropriate answer is:

• Enthalpy change is negative: This refers to an exothermic reaction, which can be spontaneous.

Calcium ethynide, also known as calcium carbide (CaC₂), reacts with water in a hydrolysis reaction. This reaction produces ethyne (C₂H₂) gas and calcium hydroxide (Ca(OH)₂). The balanced chemical equation is: