John's Science Activity of the Month

September 1996

Rate of Reaction: Alka-Seltza races

Activities

Activity 1. Surface area & rate of reaction

Activity 2. Effect of Temperature on rate

Activity 3. Make sherbet

General teacher notes


Activity 1: How does size affect the speed of dissolving?

Aim:

To demonstrate how increasing the surface area of a chemical increases it's reaction rate.

Materials:

Method

  1. Put 100 ml of tap-water into the beaker.
  2. Drop a whole tablet of Alka-Seltza into the beaker. Time how long it takes to dissolve completely. Record this into the data table.
  3. Empty the water and rinse out the beaker.
  4. Again put 100 ml of water into the beaker...make sure it is about the same temperature as before.
  5. Cut a tablet into two. Drop this into the beaker and time how long it takes to dissolve. Record this into the data table.
  6. Repeat steps 3-5 for a tablet broken into quarters, and a tablet crushed. To crush the tablet simply wrap in paper and press firmly with a wooden block or hit with a hammer.

Results:

Size of Tablet Whole tablet 1/2 tablet1/4 tablet Crushed tablet
Time to dissolve sec sec sec sec

Q1. How did breaking the tablet into smaller pieces change how fast it reacted? Write this as a rule.
Q2. What do you think will cook first... cup-cakes or a large cake? Why?
Q3. Will life-savers last longer in your mouth if they are whole or in pieces?
Q4. Why is saw-dust easier to ignite than a block of wood? Explain.
Q5. Why was it important to keep the temperature and the volume of water the same in each of the experiments?

Teacher Notes for Activity 1


Activity 2. How does temperature
affect the speed of dissolving?

Aim:

To demonstrate how increasing the temperature of the reaction system generally increases the rate of reaction.

Materials:

Method:

  1. Put 100 ml of tap-water into the beaker. Measure the temperature.
  2. Drop a whole tablet of Alka-Seltza into the beaker. Time how long it takes to dissolve completely. Record this into the data table.
  3. Empty the water and rinse out the beaker.
  4. Repeat for the cold, warm and hot water.

Results:

Type of water Cold water Tap water Warm water Hot water
Temperature oC oC oC
Time to dissolve sec sec sec sec

Q1. Display your results as a line graph.
Q2. How did changing the temperature change the speed of the reaction. Write this as a rule
Q3. Would ice melt faster on a cold day or a hot day? Explain your answer.

Teacher Notes: Activity 2

Extension: Activity 3. Making sherbet

Aim:

To demonstrate the acid/base nature of effervescent tablets/mixtures. Applications of neutralisation reactions.
Find out how to make sherbet (you may need to look up some cooking books). Try to make some. How is sherbet similar to Alka-Seltza?

TEACHER NOTES:

This series of activities relates to the level 5 Reaction & Change substrand and can form the basis of the SCNPM54 task.

Concepts:

Background:

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Teacher Notes - Activity 1: How does size affect the speed of dissolving?

Alka-seltzer tablets contain sodium bicarbonate (base) and citric acid (acid). Both of these are solids, so no reaction will occur until the tablet gets wet. Once wet, protons transfer from the acid to the base according to the neutralisation reaction:

3 NaHCO3 + C6O7H6 ---- > Na3C6O7H + 3 CO2 + 3 H2O
Sodium + Citric Sodium + Carbon + Water
Bicarbonate Acid Citrate Dioxide

As a consequence of this reaction, bubbles of carbon dioxide are produced (the fizz) and a basic solution (due an the excess of sodium bicarbonate as well as the sodium citrate in solution) which will neutralise excess stomach acid.

Clearly, the speed of reaction depends on how quickly water can get to the two (solid) chemicals. So the smaller the tablet is broken up, the faster it will dissolve.

This activity will work for any effervescent tablets (eg soluble aspirin etc). All effervescent tablets and powders work in the same manner as Alka-Seltza... a mixture of sodium bicarbonate or sodium carbonate as the base with either Citric acid (C6O7H6) or tartaric acid (C4O6H6) as the acid, with acid & base both being solid so the neutralisation reaction does not occur until the solid is placed in water.

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Teacher Notes: Activity 2.

In general, the warmer the water, the faster the rate of bubbling by the Seltza tablets. The speed of reaction is not so much determined by the transfer of protons (which is very rapid under the temperatures being studied), rather it has to do with the speed of dissolving of the solids.

In general, the warmer the water, the faster a solid will dissolve. The increased rate of bubbling highlights this. This situation mimics many chemical reactions: they react faster if warm.

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Created: 28/8/96
Last Modified: 31/8/96
Author : John Werry (werry@netspace.net.au)