The calcium carbonate (CaCO₃) in antacid tablets reacts with dilute hydrochloric acid (HCl) according to the following balanced equation: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l) ΔH < 0 5.1 Is the above reaction EXOTHERMIC or ENDOTHERMIC? Give a reason for the answer - NSC Physical Sciences - Question 5 - 2019 - Paper 2

To calculate the average rate of reaction, we use the formula:

$ext{Rate} = -\frac{\Delta m}{\Delta t}$

Here, ( \Delta m ) is the change in mass of the tablet, which is ( 2 g - 0.25 g = 1.75 g ), and ( \Delta t ) is the time taken, which is 30 seconds.

Thus,

$ext{Rate} = -\frac{1.75 g}{30 s} = 0.0583 g\cdot s^{-1}$

Therefore, the average rate of the reaction is approximately 0.058 g·s⁻¹.

First, we find the moles of calcium carbonate in the antacid tablet:

Since the tablet weighs 2 g and contains 40% calcium carbonate:

$m(CaCO_3) = 2 g \times 0.4 = 0.8 g$

Now, convert grams to moles using the molar mass of CaCO₃ (100 g/mol):

$n(CaCO_3) = \frac{0.8 g}{100 g/mol} = 0.008 mol$

From the balanced equation, 1 mole of CaCO₃ produces 1 mole of CO₂. Therefore, the moles of CO₂ produced:

$n(CO_2) = n(CaCO_3) = 0.008 mol$

At STP, 1 mole of gas occupies 22.4 L. Thus, the volume of CO₂ produced:

$V(CO_2) = n(CO_2) \times 22.4 L/mol = 0.008 mol \times 22.4 L/mol = 0.1792 L$

Therefore, the volume of carbon dioxide collected at STP is approximately 0.18 L.

One controlled variable could be the concentration of the hydrochloric acid (HCl) used in the experiment.

The graph indicates that as the temperature increases, the reaction rate (1/time) also increases. This suggests a direct proportionality between temperature and reaction rate.

According to collision theory, the rate of a chemical reaction depends on the frequency of collisions between reactant particles. As temperature increases:

1. The average kinetic energy of the molecules increases, resulting in more frequent and energetic collisions.
2. More molecules achieve the activation energy needed for reacting effectively.

This ultimately leads to a higher reaction rate, explaining the observed increase in rate as temperature rises.