IS A REACTION BETWEEN A BASE AND AN ACID LIKELY TO BE DANGEROUS?
Miri Kesner and Nurit Arad, Science Teaching Department, The Weizmann Institute of Science
Aim of the experiment: In specific terms: to find the reason for the tragic accident that
took place at the Coca Cola plant. In general terms: to study about the importance of
safety rules for chemical reactions and the need to look for safe procedures and
processes.
Before the experiment, the students are shown the press clipping in which the accident
that took place at the Coca Cola plant is reported.1 An analysis of the press report can
either be done with the help of guiding questions as suggested in the article, or in general
terms by posing the question, “What, in your opinion, were the possible reasons for the
accident that occurred at the plant?
The students will discuss this in pairs or larger groups, and raise assumptions. These can
be collated in a list to which the students will come back after the laboratory work in order
to decide which of the assumptions is supported by the experiment.
The first part of the experiment is usually not new and appears in many text books in the
section on energy. The innovation is in the examination of different ways of neutralizing
acid with various alkalis, either solid or in solution form, and the possible consequences of
similar processes on an industrial scale.
Timing: The experiments can be integrated into different sections of the syllabus:
1.
The energy section – to emphasize the importance of testing energy changes that
accompany chemical processes.
2.
Acids and bases2 - to show the application of acid-alkali reactions and the energy
changes that accompany them.
3.
The chemical industry – to show the importance of energy considerations in
maintaining safety. To emphasize the importance of the choice of a suitable neutralizing
agent and the need to execute the process on a small scale before doing so on a large
scale.
Apparatus and materials (for each pair):
An insulated beaker, Thermometer, Spoon
3 graduated test tubes – 25 cc.
5 glass beakers – 50 cc capacity.
8 pellets of NaOH(s).
50 ml NaOH solution at 2M concentration.
50 ml HCl solution at 2M concentration.
50 ml HCl solution at 1M concentration.
A bottle of distilled water.
1.06 gm dry Na2CO3(s).
2.86 gm dry Na2CO3.10H2O(s).
1
2
The accident occurred at the Coca Cola plant in the industrial zone in Benei Berak, Israel, on
29.10.93. According to reports, a concentrated solution of hydrochloric acid leaked from a tank in
the sewage purification facility. To neutralize the acid leak, sodium chloride crystals were poured
onto it. An explosion occurred and a worker was killed.
This is recommended only if this subject is studied after the section on energy.
Caution: Do not touch the sodium hydroxide pellets! Use a spoon! Throughout all the
experiments protective glasses must be worn.
General instructions:
All the experiments will be conducted in a glass beaker that is inside the insulated beaker
which isolates the solution from the environment.
The temperature will be measured using the thermometer, which will also be used for
mixing the substances in the beaker (with great care!)
The average mass of 4 solid sodium hydroxide pellets, NaOH(s), is about 1.00 gr, so they
can be counted instead of weighed.
Section A:
Neutralizing the HCl solution with solid NaOH or an NaOH solution.
Work stages:
Experiment A:
1.
Pour 20 cc of the HCl 1M solution into the beaker and measure its temperature.
2.
Add 4 pellets of solid sodium hydroxide, NaOH(s), to the HCl solution. Stir gently
and measure the maximum temperature change until the substance is completely
dissolved.
Experiment B:
1.
Pour 20 cc of water into a glass beaker and measure the temperature.
2.
Add 4 pellets of solid sodium hydroxide, NaOH(s), to the water. Stir gently and
measure the maximum temperature change until the substance is completely dissolved.
Experiment C:
1.
Pour 10 cc of the HCl 2M solution into a glass beaker and measure the
temperature.
2.
To the acid solution, add 10 cc of the NaOH(s) 2M solution. Stir gently and measure
the maximum change of temperature.
Processing the results (Section A)
1.
Formulate the reactions that took place in each of the three experiments.
2.
Calculate the amount of energy released in each of the three experiments.
3.
Calculate the amount of energy that will be released when I kg of solid sodium
hydroxide is dissolved in 1 liter of water. What will the rise in the temperature of the
solution be?
4.
Calculate the amount of energy that will be released when I kg of solid sodium
hydroxide is dissolved in 1 liter of HCl 2M solution. What will the rise in the temperature
of the solution be?
5.
Calculate the amount of energy that will be released when 0.5 liter of HCl 2M
solution is mixed with 0.5 liter of NaOH 2M solution. . What will the rise in the
temperature of the solution be?
6.
In your opinion, what is the safest possible way of neutralizing an HCl solution?
Section B:
Can other substances be used for neutralizing an HCl solution?
Work stages:
Experiment A:
1.
Pour 20 cc of HCl 1M into a glass beaker and measure the temperature.
2.
Calculate how many grams of dry (anhydrous)3 Na2CO3(s) must be added in order to
neutralize the acid.
3.
Weigh out the required weight of this substance on weighing paper and add it
carefully to the acid solution. Mix gently while measuring the maximum change of
temperature.
Experiment B:
1.
Pour 20 cc of HCl 1M into a glass beaker and measure the temperature.
2.
Weigh out 2.86 gm of the hydrate Na2CO3.10H2O(s) and add it to the acid solution.
Mix gently and measure the maximum change of temperature.
Processing the results (Section B)
1.
Calculate the amount of energy that will be released when 1 liter of the HCl solution
is neutralized with a suitable amount of dry (anhydrous) sodium carbonate.
2.
In your opinion, what are the advantages or disadvantages of using dry sodium
carbonate compared with the use of sodium hydroxide in solid or solution form for
neutralizing the acid solution? Explain.
3.
What, in your pinion, are the advantages or disadvantages of using a sodium
carbonate hydrate compared with the use of the other two substances for neutralizing the
acid solution? Explain.
4.
What general conclusions can you draw from the experiments you have
conducted?
3
The hydrate can be completely dried in an oven and kept.