Learners use the reaction of MgCO₃(s) with EXCESS dilute HCl(aq) to investigate the relationship between temperature and the rate of a chemical reaction - NSC Physical Sciences - Question 5 - 2022 - Paper 2

1. The concentration of hydrochloric acid (HCl) must remain constant to ensure that variations in the reaction rate are solely due to temperature changes.
2. The surface area of the solid MgCO₃ must be kept constant to prevent changes in the rate of reaction due to different particle sizes.

The collision theory states that for a reaction to occur, reactant particles must collide with sufficient energy and proper orientation. As temperature increases, the kinetic energy of the reacting molecules also increases, resulting in more frequent and effective collisions. This leads to an increase in the rate of reaction. The graph indicates that as temperature rises, there is a corresponding increase in the average rate of CO₂ production, demonstrating the direct relationship between temperature and reaction rate.

To determine the time taken for the reaction to run to completion, we apply the formula:

$t = \frac{n}{R}$

where $n$ is the number of moles of CO₂ produced and $R$ is the average rate of production. Given the mass of MgCO₃ is 5 g, the molar mass of MgCO₃ is 84 g/mol, so:

$n = \frac{5 g}{84 g/mol} = 0.05952 mol$

Let’s assume, from data, the average rate of production of CO₂ is 0.0114 mol/min. Thus, the time taken for completion is:

$t = \frac{0.05952 mol}{0.0114 mol/min} \approx 5.22 min$.

Therefore, the time taken is approximately 5.22 minutes.

Using the ideal gas law and knowing that at 40 °C, the molar gas volume is approximately 25 dm³/mol, we can find the mols of CO₂ collected:

$n(CO₂) = \frac{V}{V_{m}} = \frac{1.5 dm³}{25 dm³/mol} = 0.06 mol.$

Thus, the molar gas volume at 40 °C based on the collected CO₂ is 25 dm³.

In the response document, the existing curve representing CO₂ production should be carefully redrawn and labeled 'A'.

At 20 °C, the average rate of CO₂ production will be lower than at 40 °C due to decreased kinetic energy and fewer effective collisions. The sketched curve 'B' should lie below curve 'A', indicating a slower reaction at this lower temperature.