Rates of reactionIB MYP Sciences: Subtopic test

1. 1
   A student in Auckland adds magnesium ribbon to dilute hydrochloric acid in a flask and collects the hydrogen gas in a gas syringe, recording the volume every 10 seconds. She plots the volume of gas against time. The line is steepest at the start and becomes horizontal after about 120 seconds.

   (a)

   Which measurement best shows the rate of this reaction?

   [1 mark]

   • AThe mass of magnesium at the start
   • BThe volume of hydrogen collected per second
   • CThe temperature of the acid at the start
   • DThe total volume of acid used

   (b)

   What does the horizontal part of the line show?

   [1 mark]

   • AThe reaction is at its fastest rate
   • BThe reaction is going at a constant fast rate
   • CThe reaction has stopped because a reactant has been used up
   • DThe rate of reaction is increasing

   (c)

   Explain why the reaction is fastest at the start.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Dublin drops one effervescent vitamin tablet into 200 cm³ of water at 10 °C and times how long it takes for the fizzing to stop. It takes 120 s. She repeats the experiment with water at 40 °C and it takes 35 s. The tablets are the same size.

   (a)

   What do the times show about the reaction?

   [1 mark]

   • AThe reaction is faster at 40 °C than at 10 °C
   • BThe reaction is faster at 10 °C than at 40 °C
   • CTemperature has no effect on the rate
   • DMore gas is made at 40 °C than at 10 °C

   (b)

   Which statement explains, using collision theory, why a higher temperature increases the rate?

   [1 mark]

   • AThe particles get bigger, so they collide more often
   • BThe higher temperature acts as a catalyst
   • CThere are more particles in the water at a higher temperature
   • DThe particles move faster, so they collide more often and more collisions have enough energy to react

   (c)

   The student repeats the experiment at 10 °C using a tablet that has been crushed into a powder. Predict how the time for the fizzing to stop will change and explain your prediction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student in Cape Town investigates how the concentration of sodium thiosulfate solution affects its reaction with hydrochloric acid, which makes a cloudy yellow precipitate of sulfur. She stands the flask on a cross drawn on paper and times how long it takes for the cross to disappear when she looks down through the liquid. For sodium thiosulfate solutions of concentration 0.10, 0.05 and 0.025 mol dm⁻³, the times are 40 s, 80 s and 160 s.

   (a)

   Describe the pattern in the results and explain it using collision theory.

   [3 marks]

   (b)

   Evaluate the investigation. Identify the independent variable and the dependent variable, name one variable that must be controlled and suggest one limitation of timing until the cross disappears.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Catalytic converters in cars in cities such as Delhi contain the metals platinum, palladium and rhodium. They speed up reactions that change poisonous carbon monoxide and nitrogen oxides in the exhaust gases into carbon dioxide and nitrogen. The converters work best when they are hot, and they work poorly for the first few minutes after a cold engine is started. Platinum group metals are rare and expensive to mine.

   (a)

   Explain, using collision theory, how the catalyst and the high temperature help the converter to speed up the reactions, and why the converter works poorly just after a cold engine starts.

   [6 marks]

   (b)

   Discuss and evaluate the rule that all new cars must have a catalytic converter.

   [6 marks]

   Total for question 4: 12 marks

End of questions