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Introduction

The rate of the reaction between calcium carbonate and dilute hydrochloric acid can be judged by measuring the volume of carbon dioxide evolved with time. In this simulation you will discover the effect on the rate of reaction of altering the surface area of a solid reactant, and explain this effect in terms of kinetic particle theory.

Task 1: Effect on rate of varying surface area


Simulation for measuring the effect on rate of changing surface area

  1. Open Yenka file Model 1.
  2. Select 10 g of coarse calcium carbonate powder and place it in the conical flask. Pause the simulation. Reload the graph using the button on its toolbar, pour the acid into the flask and then unpause the simulation. When 10 cm3 of gas has been collected in the syringe pause the simulation again. Record the time taken.
    Answer
    E.g. 4.8 seconds.
  3. The rate of production of gas can be calculated by dividing 10 cm3 by the time taken. Do this for your experiment, showing your working and giving units for the final answer.
    Answer
    E.g. 10 / 4.8 = 2.08 cm3/s
  4. Reload the simulation (F5) and repeat the experiment using 10 g of medium calcium carbonate powder. You will need to change the vertical scale to a maximum of 40 cm3. This time pause the simulation when the volume reaches 40 cm3. Write down the time taken.
    Answer
    E.g. 7.6 seconds.
  5. To calculate the rate of the reaction, divide 40 cm3 by the time taken. Show your working and give units for the final answer.
    Answer
    R.g. 40 / 7.6 = 5.3 cm3/s
  6. Now repeat the experiment using 10 g fine calcium carbonate powder. This time pause the simulation after 100 cm3 of gas has been produced and write down the time taken.
    Answer
    E.g. 8.5 seconds.
  7. To calculate the rate of the reaction, divide 100 cm3 by the time taken. Show your working and give units for the final answer.
    Answer
    e.g. 100 / 8.5 = 11.8 cm3/s
  8. All factors other than the size of the calcium carbonate pieces were kept constant in this series of experiments. List the factors, and explain the importance of keeping them constant.
    Answer
    Volume and concentration of acid.
    Mass of calcium carbonate.
    Temperature.
    If these are not kept constant, it will not be a fair test.
  9. From the diagram below, it is apparent that when a cube of a substance is cut in two, two new faces are exposed, yet the mass and volume of the substance have not changed.


    Increasing surface area by cutting up a sample.

    If this is repeated again and again, for example by grinding up the substance, the surface area exposed increases dramatically. Write down the samples of calcium carbonate you used in increasing order of surface area.

    Answer
    Coarse, medium, fine.
  10. What happens to the rate of the reaction as the surface area increases?
    Answer
    It increases.
  11. Explain your conclusion in terms of collisions occurring between particles. You must mention activation energy in your answer.
    Answer
    Particles can only react if they collide with an energy greater than the activation energy. Increasing the surface area of the calcium carbonate increases the number of collisions per second therefore increases the number of reactant collisions per second with energy greater than the activation energy. This increases the rate.
  12. Is it possible to measure the surface area of a powder sample, and thus find a mathematical relationship between surface area and rate? If not, suggest a practical method of finding this relationship.
    Answer
    Cannot measure surface area of a powder. However, if you cut 1x1x1 cm cubes of calcium carbonate, and perform experiment, then repeat with one cut in half etc., it should be possible to find the relationship.
  13. In industry it is important to make reaction go as fast as possible in order to save money. Reactants are often ground up and mixed together (e.g. sintering of coke and iron ore in the blast furnace) in order to increase the surface area. Catalysts are often in the form of fine mesh in order to maximise contact with the reactants. Catalytic converters in car exhausts contain expensive platinum and rhodium sprayed thinly on to a honeycomb structure made of a ceramic material. Suggest two reasons why this arrangement is used.
    Answer
    High surface area, allows gases to pass through, inert support material is cheap, thin coating to save money.

Summary

Increasing the surface area by finely crushing solid reactants increases the rate dramatically as a result of more successful reactant collisions per second. This technique has many practical applications where a fast rate of reaction is essential.

Teacher Summary

  • The exercise measures the rate of reaction very simply, and complex maths is not required.
  • It can be used to reinforce the idea that a quantitative relationship between rate and surface area is difficult to deduce.