# Question 5

The reaction of 15 g of an IMPURE sample of calcium carbonate, CaCO3, with EXCESS hydrochloric acid, HCℓ, of concentration 1,0 mol∙dm-3 , is used to investigate the rate of a reaction. The balanced equation for the reaction is:
CaCO3(s) + 2HCℓ(aq) → CaCℓ2(aq) + H2O(ℓ) + CO2(g)
The volume of CO2(g) produced is measured at regular intervals. A sketch graph representing the total volume of carbon dioxide gas produced as a function of time is shown below.

5.1 Define the term reaction rate. (2)

5.2 Give a reason why the gradient of the graph decreases between t2 and t3. (1)

5.3 Changes in the graph between t1 and t2 are due to temperature changes within the reaction mixture.
5.3.1 Is the reaction EXOTHERMIC or ENDOTHERMIC? (1)

5.3.2 Explain the answer by referring to the graph. (3)

5.4 The percentage purity of the sample is 82,5%.
Calculate the value of X on the graph assuming that the gas is collected at 25 °C. Take the molar gas volume at 25 °C as 24 000 cm3 . (5)

5.5 How will the reaction rate change if 15 g of a PURE sample of CaCO3 reacts with the same HCℓ solution?
Choose from INCREASES, DECREASES or REMAINS THE SAME. (1)

5.6 Use the collision theory to explain the answer to QUESTION 5.5. (2)
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