This question is about hydrogen chloride and hydrochloric acid - AQA - GCSE Chemistry Combined Science - Question 6 - 2022 - Paper 1

A strong acid is one that completely ionizes in aqueous solution. This means that when the acid dissolves in water, it releases all its hydrogen ions (H⁺) into the solution, resulting in a high concentration of H⁺ ions and a low pH.

When magnesium is added to a strong acid, it reacts vigorously and fizzes, producing hydrogen gas quickly. In contrast, when added to a weak acid of the same concentration, the reaction occurs at a much slower rate and may produce less fizzing, indicating a weaker acid.

The change in pH can be calculated by the formula: $ext{pH} = - ext{log}_{10}[ ext{H}^+]$. When the concentration of hydrochloric acid is increased by a factor of 100, the resulting concentration of H⁺ ions also increases by 100 times. Hence, the change in pH is a decrease of 2 units, as each tenfold increase corresponds to a decrease of 1 in pH.

To calculate the bond energy X, we can apply the following steps:

1. Calculate the total bond energy of the bonds broken:

   • Bonds broken: $4 \times 413 + C=C + 431 = 2083 + C=C$

2. Calculate the total bond energy of the bonds formed:

   • Bonds formed: $346 + 339 + 5 \times 413 = 2750$

3. The energy released in the reaction is given by $\text{energy released} = \text{bonds broken} - \text{bonds formed}$ Thus, $56 = 2750 - 2083$ which leads to $C=C = 611 \text{ kJ/mol}$. Therefore, bond energy X is 611 kJ/mol.