The rate at which Alka-Seltzer tablets react with water.

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Preliminary work

When reacting the Alka-Seltzer tablets in different conditions, I found out many things that I was not originally going to do. In the actual experiment I was going to use a water bath. This was to prevent the temperature of the water changing too much during the experiment. However, I found out that it took long to set up the water bath, and it was difficult to place the beaker inside the water bath and also take it out. I repeated this without using a water bath and I found it more efficient and easier, especially due to the time span, because when using a water bath less replicates were done whereas doing the experiment without the water bath gave me more time to do replicates. Also, when I did not use a water bath the temperature of the water only changed by 2oC or so and these could later be averaged out.

 In the preliminary I also found out that I should use temperatures from 20oC – 90oC. This is because anything below 20oC would need freezing off the water as it is below room temperature and anything above 90oC would cause the water to start evaporating. The evaporation would change the amount of water level.  

  To find out how much water I should use, I tested different volumes of water with an Alka-Seltzer tablet, and timed how long the Alka-Seltzer tablet took to dissolve. The volumes of water I used were 25ml, 50ml, 75ml, and 100ml. I found out that using 25ml and 50ml of water produced a quite a slow reaction whereas a volume of 100ml produced a reaction rate nearly the same as using 75ml of water. In the end I found out that 75ml of water was the right volume of water to use, and produced a steady reaction.

  To find out what temperatures to use, I experimented with temperature from 20-800C. From 600C onwards up to 800C the rate of reaction was the same. For this reason in the actual practical I will only use temperatures ranging from 20-700C.

 

Below are basic results, which I achieved in finding out the results for the preliminary

Results comparing different volumes of water:

Results comparing stirring to not stirring:

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Pilot Experiment

Method

Below is a basic outline of what I did:

• The apparatus was set up as in the diagram
• A measuring cylinder was filled up with 75ml of water.
• This water was then poured into a beaker.
• This beaker was placed on top of a blue flame Bunsen burner with the air hole fully opened.
• A thermometer is used to measure the water inside the beaker.  
• Once the water in the beaker reached the adequate temperature, the Alka-Seltzer tablet is inserted into the beaker and simultaneously the stop clock is started.
• The beaker is then taken of the gauze and laid on the table.
• Once the Alka-Seltzer tablet had dissolved, the stop clock is stopped and the final temperature of the water inside the beaker is measured.
• This is repeated 3 more times.

(Note: when doing the experiment at room temperature, the water in the beaker is not heated, and so the Alka-Seltzer tablet is placed straight into the water.)

Results

From the results obtained, and from the graphs, the experiment went quite well. At 60oC onwards the graph tended to curve, and this was due to the fact of the hardness in water being dissolved. The reaction also seemed to get slower and slower.

  The 1/time graph was also good, and showed that 1/time was inversely proportional to time.

                                                        

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Background Information

The Alka-Seltzer tablet dissolves in solution. The higher the temperature the quicker the tablet dissolves in water. This is because as the temperature increases the kinetic energy of the molecules increases. This causes them to move around faster. As they move around faster they are more likely to collide with each other with more force that is more likely to produce a reaction.

They have to collide with more than a particular amount of force to break the activation energy barrier. Different reactions have different activation energies. A reaction rate will only take place if the colliding particles have more than a minimum amount of energy. This minimum amount of energy is known as the activation energy. If the activation energy is very large, only a small amount of molecules have enough energy to react. Therefore the reaction is very slow.

  However if the activation energy is very low, most molecules have enough energy to react and the reaction proceeds fast. An increase in temperature gives molecules more energy and the reaction also proceeds fast.

 When the Alka-Seltzer tablet is reacted with water, the reaction produces a fizzing sound, releasing a gas. The gas released is carbon dioxide.  The Alka-Seltzer tablet consists of several chemicals. These include sodium bicarbonate also known as

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baking powder, and citric acid. When the tablet is dropped in water, the citric acid and the sodium carbonate react producing a fizz and bubbles. This fizz or bubbles shows that carbon dioxide is being produced. And a chemical reaction takes place.

Below is the chemical equation for the reaction:

 3NaHCO3 + C6O7H6 → Na3C6O7H + 3CO2 + H2O

Below is the word equation for the reaction:

Sodium bicarbonate + Citric acid → Sodium citrate + Carbon dioxide + water

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Hypothesis

As the temperature is raised, the rate of the reaction will increase. However, as the temperature increases the general trend of the graph would be broken. This is because the calcium hydrogen carbonates which causes hardness in water is dissolving and making the water softer. I would also expect the experiment to proceed fastest at the highest temperature (in this experiment- 70oC) and slowest at the lowest temperature (in this experiment- room temperature).

 

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Predictions

As the temperature increases the movement of molecules also increases as they gain more energy. This theory is based upon the kinetic theory.  This also means that the rate at which the particles collide also increase. As more particles are colliding in a given time and also as the particles have more energy they collide with more force producing a reaction breaking the activation energy level. Some of the particles colliding will not break this activation energy barrier and so will not produce a reaction.

  The higher the temperature is the higher the rate of reaction.  I expect the reaction to occur fastest at around 60oC or 70oC.  However, after this increase in temperature I do not expect there to be an increase in the rate of reaction. This is because around this time the molecules would be reacting as fast as they can already.

 I would expect the graph for Time against temperature to like this:

The reason why the graph curves at the end is due to the fact about hardness in water. As the temperature is increased I expect the calcium hydrogen carbonate in water to dissolve. This is what causes the hardness in water. As it dissolves, it will be making the water softer.

I would expect the graph for 1/time against temperature to like the one below:

The reason why the graph levels off is again because of the hardness in water as mentioned above. The reason why the graph is a straight line is because 1/time is inversely proportional to time.  

In the reaction, I would also expect carbon dioxide to produce. This is because in the preliminary experiment done, the reaction produced bubbles and also fizzed. This also showed that a chemical reaction took place.

                                                        

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Safety Precautions

In order to ensure safety the following must considered;

• Protective eyewear should be worn e.g. goggles to ensure that no Alka-Seltzer tablets in solution get into your eyes as it can cause allegories.
• An apron should be worn to ensure that no Alka-Seltzer tablets in solution get onto your clothes. This is because it can cause stains.
• Caution should be taken when handling hot beakers and other equipment so that there is no damage of equipment.
• Care must be taken when using the Bunsen burners.

Fair Test

In order to ensure a fair test the following must be considered;

• The same sized apparatus is used. This is to ensure a fair test and different sized equipments can lead to unfair results.
• The same sized tablet produced by the company must be used to ensure more accuracy over the experiment and so that there is no higher amount of the reactant.
• The same volume of water must be used to ensure more accuracy over the experiment and so that there is not more concentration of water.
• The same method is used for each experiment, and the temperature is only altered.

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Planned method

 Apparatus:

100ml Measuring Cylinders

100ml Beaker

Thermometer

Bunsen Burner

Tripod

Gauze

White Mat

Stop Clock

Apron

Safety Specs

Water supply

Diagram:

Method:

For this experiment I am experimenting on how the change in temperature would affect the rate at which an Alka-Seltzer tablet dissolves in water.  I will measure how long it takes for the tablet to dissolve at different temperatures.

All the apparatus is set up as shown in the above diagram. 75ml of water is measured in a 100ml measuring cylinder. This water is then poured into a 100ml beaker. The beaker is then placed on top of a gauze lying on the tripod. After turning open the gas

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tap, using a lighted splint the Bunsen burner is then lit to a blue flame with the air hole opened completely. Once this is done, a thermometer and stop-clock are then taken. The thermometer is then placed into the beaker, and the temperature of the water inside the beaker is measured, reading the temperature on the thermometer at eye level. When the temperature is correct, one Alka-Seltzer tablet (The tablet is the same sized tablet supplied by the company) is placed into the beaker and simultaneously the stop watch is started. The Alka-Seltzer tablet must not have been in contact with water before.  Also at this time the beaker is removed from the gauze and placed on the table. Once the tablet has dissolved the stop-watch is immediately stopped, and the final temperature inside the beaker is measured.

  The average temperature is then found out using the equation:

         

Average temperature =     The Initial temperature- The final temperature

                                           2

This method is used to test the following temperatures:

Room

30oC

40oC

50oC

60oC

70oC

Each temperature is replicated 3 more times and the whole experiment is then repeated one more time.

(Note that for the room temperature the water inside the beaker is not heated. The Alka-Seltzer tablet is just inserted into the beaker).

Justifications For Method

• A 100ml-measuring cylinder is used to measure 75ml of water as it is the most accurate. A 75ml-measuring cylinder was not available by the school and so using the 100ml-measuring cylinder proved to be the most accurate. A 250ml-measuring cylinder would have been far too large and therefore not accurate as the space between the numberings was not very clear. Only large amount of water was used would the 250ml-measuring cylinder prove successful.
• The water is not measured using into the beaker, because the beaker is not a reliable piece of equipment for measuring amounts, as there are no clear numberings. Using a measuring cylinder would therefore ensure more accuracy.
• The air hole is opened completely so that the water inside the beaker, which the flame from the Bunsen burner is heating, will heat more quickly, than a half opened air hole.
• The Alka-Seltzer tablet must not be in contact with any water before the temperature. This is because it would react with the water, and therefore it would disrupt the final result.

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• The temperature of the thermometer is measured at eye level. This is because it is directly in the line of sight, so no misreading can occur.
• The experiment is replicated, so that if there was anything wrong with the final result, you would know from the replicate.
• The temperatures range from room temperature to 70oC, because after 60oC the rate of reaction does not change, as found out from the preliminary. The temperatures start of from room, because anything below that needs freezing. Normally, for a good graph 6 points are needed to plot a good graph, and that is why 6 temperatures are taken.

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Safety Precautions

In order to ensure safety the following must considered;

• Protective eye wear should be worn e.g. goggles to ensure that no Alka-Seltzer tablets in solution get into your eyes as it can cause allegories.
• An apron should be worn to ensure that no Alka-Seltzer tablets in solution get onto your clothes. This is because it can cause stains.
• Caution should be taken when handling hot beakers and other equipment so that there is no damage of equipment.
• Protective eyewear should be worn e.g. goggles to ensure that no Alka-Seltzer tablets in solution get into your eyes as it can cause allegories.

Fair Test

In order to ensure a fair test the following must be considered;

• The same sized apparatus is used. This is to ensure a fair test and different sized equipments can lead to unfair results.
• The same sized tablet produced by the company must be used to ensure more accuracy over the experiment and so that there is no higher amount of the reactant.
• The same volume of water must be used to ensure more accuracy over the experiment and so that there is not more concentration of water.
• The same method is used for each experiment, and the temperature is only altered.

Method Update

Nothing was changed from the last method written.

Conclusion

The results achieved in the practical supports the hypothesis stated. As the temperature is increased the rate of reaction also increases. This is seen in the 1/time graph, where we can see that the graph is getting steeper.

On the 1/time graph at 60oC as you can see from the graph, the rate of reaction stays the same, and does not increase. This is not an anomalous result but merely because, as is stated in the predictions due to the fact of the hardness in water. Hardness in water is caused by calcium hydrogencarbonate. When the solution is boiled the calcium hydrogencarbonate breaks down and turns to insoluble calcium carbonate. Therefore the calcium ions are removed from the water.

Here is the equation:

Calcium hydrogencarbonate → Calcium carbonate + carbon dioxide + water

Ca(HCO3)2                           →  CaCO3 + CO2 + H2O

As the hardness in water is slowly removed, the reaction would have proceeded faster, and this therefore caused an unfair test.

  For the graph for time against temperature the graph is curved at the beginning but becomes straight as you go down it. This is because again due to the fact of hardness in water.

 In my predictions I stated that I would expect a big change in the trend of the graph at around 60 oC, and this was right.

 A number of judgements

 

 Evaluation

Generally the experiment went quite well on the whole. However there were many things that could be improved. One was the equipment. Burettes could be used instead of measuring cylinders, as they are more accurate. They measure to one decimal where as measuring cylinders only measure to a whole number. This would therefore ensure more accuracy as a whole.

  Also a thermostatic water bath would also ensure more accuracy over a heated beaker. This is because the water inside the water bath can be set to a particular temperature, and the water would stay to that temperature. When using a beaker to heat the water, the temperature could always change in that it could increase in temperature if heated more, or decrease in temperature, if not heated.

As you can see form the results and the graph there was an anomalous result. This could have been caused for many reasons the main being human error.

 Below are points of what could have one wrong:

• If the amount of water was not measured correctly it would change the overall result. This is because too much water would cause an increase in the rate of reaction, and too less water would cause as slower reaction. To ensure more accuracy over this a burette could be used as mentioned above.
• If the Alka-Seltzer tablet was not put into the beaker at the correct temperature there would be inaccuracies in the overall result, and if the final temperature at the end of the experiment was not measured properly, it would have caused even more inaccuracies. This is because at a particular temperature the rate of reaction would be different from what it should be. To solve this problem a thermostatic water bath could be used as stated above.
• If the stop watch was stopped to early or late, again the overall reading would not be as accurate as it could have been.
• It was almost impossible to tell when the Alka-Seltzer tablet had dissolved, each time the experiment was done. This was a huge problem for the experiment as this could have totally caused problems to the experiment. A special type of detector apparatus, which bleeped when the correct amount of Alka-Seltzer tablet dissolved, could improve this, each time the experiment was done. This would give us an extra measure of accuracy each time.

Another way to improve the experiment and to produce consistent readings was to used distilled water. This is because the distilled water contains no impurities and therefore no hardness in water.