Measuring rates of reactionIB MYP Chemistry: Subtopic test

1. 1
   A student in Cape Town reacts magnesium ribbon with excess dilute hydrochloric acid in a conical flask connected to a gas syringe. She records the volume of hydrogen collected at regular intervals and plots volume against time.

   (a)

   Which quantity is used to measure the rate of this reaction?

   [1 mark]

   • AThe total volume of acid used
   • BThe pH of the acid at the start
   • CThe mass of magnesium ribbon at the start
   • DThe volume of hydrogen made per second

   (b)

   The line on her graph is steepest at the start and becomes horizontal after 90 seconds. What does the horizontal line show?

   [1 mark]

   • AThe reaction has not yet started
   • BOne reactant has been used up and the reaction has stopped
   • CThe reaction is at its fastest rate
   • DThe temperature of the flask has fallen to zero

   (c)

   Between 0 and 20 s she collects 30 cm³ of gas, and between 60 s and 80 s she collects 6 cm³. Calculate the mean rate of reaction, in cm³ s⁻¹, for each period.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician in Manchester places a conical flask containing marble chips and dilute hydrochloric acid on a top-pan balance. A loose plug of cotton wool is fitted in the neck of the flask, and the balance reading is recorded every 30 seconds as the reaction proceeds.

   (a)

   Why does the balance reading decrease as the reaction proceeds?

   [1 mark]

   • ACarbon dioxide gas escapes from the flask
   • BThe marble chips dissolve and disappear
   • CThe acid evaporates from the flask
   • DThe cotton wool absorbs the acid

   (b)

   Why is a loose plug of cotton wool used rather than a tight bung?

   [1 mark]

   • ATo stop all of the gas leaving so that it can be measured
   • BTo keep the temperature of the mixture constant
   • CTo let the gas escape while stopping acid spray leaving the flask
   • DTo increase the surface area of the marble chips

   (c)

   The first reading is 152.40 g and the reading after 60 seconds is 151.92 g. Calculate the mean rate of mass loss, in g s⁻¹, over the first 60 seconds.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a school in Mumbai, a student mixes sodium thiosulfate solution with dilute hydrochloric acid in a flask standing on a cross drawn on paper. A yellow solid forms and makes the mixture cloudy. The student times how long it takes until the cross can no longer be seen when looking down through the mixture.

   (a)

   Explain how the time taken for the cross to disappear can be used to compare the rates of reaction in different experiments.

   [3 marks]

   (b)

   At 20 °C the cross disappears after 80 s and at 30 °C it disappears after 40 s. Calculate the relative rate (1 ÷ time) at each temperature, state how many times faster the reaction is at 30 °C, and suggest one reason why this method is not very precise.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A manufacturer of indigestion tablets in São Paulo is comparing two brands of antacid tablet that both contain calcium carbonate. The tablets react with the dilute hydrochloric acid in the stomach and release carbon dioxide gas. The company wants to know which brand reacts with acid faster, and a laboratory technician is asked to find out.

   (a)

   Plan an investigation, using a gas syringe, to compare how fast the two brands react with dilute hydrochloric acid. Include a testable hypothesis, the variables you would control and one precaution.

   [6 marks]

   (b)

   The company could make and sell a faster-acting tablet. Discuss the benefits and drawbacks of faster-acting tablets for patients and for society, and evaluate whether the company should sell them.

   [6 marks]

   Total for question 4: 12 marks

End of questions