CIE iGCSE Co-Ordinated Science C7.3 Preparation of salts Exam Style Questions Paper 4
Question
Describe how the rate of reaction changes during the experiment.
Explain your answer using ideas about collisions between particles.
This time they use warm dilute hydrochloric acid instead of cold dilute hydrochloric acid. The reaction is much faster.
Explain why the reaction is much faster by using ideas about collisions between particles.
Acid rain reacts very slowly with marble buildings.
Suggest why the reaction is so slow.
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):
• Topic C6.2 — Rate of reaction (Part (a), (b)(i)–(ii))
• Topic C10.1 — Water (Part (c)(i))
• Topic C10.2 — Air quality and climate (Part (c)(ii))
• Topic C7.3 — Preparation of salts (Part (d))
▶️ Answer/Explanation
(a) 100 \( \text{cm}^3 \)
From Table 3.1, the volume of gas remains constant at \( 100 \, \text{cm}^3 \) from 35 s onwards.
This constant reading shows that the reaction has stopped producing gas.
(b)(i) Rate decreases as the reaction proceeds
As the reaction proceeds, the concentration of hydrochloric acid decreases, so there are fewer acid particles in the same volume.
This means the particles are less crowded, so there are fewer collisions per second between reacting particles.
Since rate depends on the frequency of collisions, the rate of reaction decreases over time.
(b)(ii) Warm acid increases particle energy and collision frequency
In warm acid, particles move faster and have more kinetic energy than in cold acid.
This results in more frequent collisions between particles.
A greater proportion of these collisions have energy greater than or equal to the activation energy, giving more successful collisions per second.
(c)(i) Acid rain is very dilute / marble has a small surface area
Acid rain is a very dilute acid, meaning it contains a low concentration of \( \text{H}^+ \) ions.
Marble buildings also present a relatively small surface area compared to their volume.
Both factors reduce the frequency of collisions between acid particles and the marble surface, slowing the reaction.
(c)(ii) Combustion of fossil fuels
Sulfur dioxide is released mainly from the combustion of fossil fuels that contain sulfur compounds, such as coal and some petroleum products.
This sulfur dioxide dissolves in atmospheric water to form acid rain.
(d) \( \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2 \)
The equation must balance for calcium, chlorine, hydrogen, carbon, and oxygen atoms.
Two moles of \( \text{HCl} \) are needed to supply the two chloride ions in \( \text{CaCl}_2 \) and balance the hydrogens across water.
The carbon dioxide gas produced is what is collected in the gas syringe in Fig. 3.1.