- This process of heating the water, dropping the alka seltzer tablet into it, timing how long it took to dissolve, cleaning out the beaker and re-filling it was repeated for each temperature twice.
- Because of the beakers high temperature at some parts of the experiment I had to handle it using a clamp.
Here are the results that I obtained from this preliminary experiment:
Things I shall change for my final experiment
- Instead of just measuring the water in the beaker, I shall measure water in a measuring cylinder to increase accuracy. I did this because I realized that the beaker only measured approximate values.
Relevant Background Knowledge:
1. Temperature:
As the temperature increases so does the rate of reaction.
EA (activation energy) is the minimum energy particles must have on collisions for them to react.
By increasing the temperature, particles move faster, i.e. they have more energy.
So, they collide with more energy and more particles have the required EA (and also they collide more frequently.
The dissociation constant of acids and bases increases:
This is affected by the increase of temperature: As temperature increases, the dissociation of the acids and bases into their ions increases. Therefore the release of the ions is quicker and so is the rate of the reaction.
Citric acid (aq):
C6H8O7 (aq) H+ (aq) + C6H7O7-(aq)
Sodium Hydrogen Carbonate (aq):
NaHCO3(aq) Na+(aq) + HCO3-(aq)
So, ions will be freely available to collide and form neutralization reaction and when H+(aq) ions collide with HCO3 (aq) ions, CO2(g) and H2O(l) will be formed.
The Q10 theory:
According to this theory: With every 10°C rise in temperature, the rates of reaction double.
pH of water changes slightly as temperature increases:
Normally, pH of pure water at 25°C is 7. When temperature is increased, pH decreases.
At 50°C, the pH of water is 6.6 . This is due to dissociation of H2O into its ions and the availability of H+ in the water. This will increase the rate of reaction.
Not only the pH changes at higher temperatures, but also hardness of water is broken which might affect the trend of the reaction.
Hardness of water:
The water I used was tap water. It is hard: It contains calcium hydrogen carbonate Ca(HCO3)2 and magnesium hydrogen carbonate Mg(HCO3)2 When heated to above 60°C , this metal hydrogen carbonate breaks down to form a solid calcium carbonate and carbon dioxide and water.
Ca (HCO3)2 (aq) CaCO3 (s) + CO2(g) + H2O(l)
Mg (HCO3)2 (aq) MgCO3 (s) + CO2(g) + H2O(l)
This process lowers the amount of hydrogen carbonate and increases the three products. So the water composition which I have started with initially will change with higher temperature. I would therefore expect a change in the trend of my results but still I expect the rate of reaction to increase.
2. Surface Area:
If the surface area is increased (more powdered) the rate of reaction also increases. This means that there are more collisions between the solid and liquid.
The smaller the particles involved the greater the surface area where the reaction can take place and the faster is the reaction.
3. Concentration:
If the concentration is increased, the rate of reaction increases as well. There will be more collisions because there are a greater number of particles in the same volume. I think this applies until the solution is saturated, so beyond saturation point there will be no effect on increasing the rate reaction.
4. Pressure (for reactions between gases):
If the pressure is increased, the rate increases. There are more collisions because there are a greater number of particles in the same volume.
5. Catalysts:
If a catalyst is added the rate increases. The EA is lowered. These change the rate by providing an alternative reaction pathway along which the reaction can occur.
A catalyst works by giving the reacting particles a surface to stick to where they can bump into each other. This increases the number of collisions
6. Activation Energy:
Before a reaction takes place it has to break through an energy barrier by going past the activation energy. A rise of temperature lowers the activation energy which makes the reaction quicker.
7. Collision Theory:
Collision theory describes the way temperature, concentration and surface area of the solid reactant affects the rate of reaction, by affecting collisions between particles. Particles react when they collide with sufficient energy.
At a higher temperature collisions are more frequent, and they also have more energy, both because particles are moving faster.
At a higher concentration collisions are again more frequent, as there are more reactant particles in the liquid. So, more tablets, means more collisions.
Increased surface area of the solid again increases the frequency of collision between reacting particles, as the liquid reactant has greater contact with the solid one.
Rate of reaction can also be increased by adding a catalyst, but there is not one for this reaction.
If we use an acidic solvent instead of water, example vinegar, the collision increases than that of water because base particles in the tablets will have more acid particles to collide with. I.e. the ones in the tablets and the ones in the solvent. Hence higher rate of reaction.
8. Knowledge about tablets
Alka-Seltzer tablets contain sodium hydrogen carbonate (NaHCO3) and citric acid (C6H8O7). Both are solids, so no reaction will occur between the base(s) and the acid(s) until the tablet dissolve in water or get wet. Both have to be in aqueous form to react. When this happens, hydrogen ions (protons) transfer from the acid to the base according to the neutralisation reaction.
In my experiment, the base is a metal hydrogen carbonate (NaHCO3)
So:
Acid + metal hydrogen carbonate Metal salt + Carbon dioxide + water
Citric acid + Sodium hydrogen carbonate sodium Citrate + carbon dioxide + water
C6O7H8 (aq) + NaHCO3 (aq) Na (C6H7O7) (salt) + CO2 (g) +H2O (l)
As a consequence of this reaction, bubbles of carbon dioxide are produced (the fizz) and a basic solution (due to the excess of sodium hydrogen carbonate as well as the sodium citrate in solution).
As long as carbon dioxide is produced, I believe that the reaction is still going on.
Note: Formula of citric acid: HOOCCH2C (OH) (COOH) CH2COOH
If we take tablets as medicine, the reaction above will neutralise excess stomach acid. Alka-Seltzer tablets also contain aspirin (acetyl salicylic acid which is covalent).
Alka-Seltzer tablets dissolve in warm water, so therefore the warmer the water the quicker the reaction will be.
Bibliography:
Classroom notes.
Chemistry in context, Graham Hill and John Holman, Fifth edition, 2000.
Complete Chemistry, Rose Marie Gallagher and Paul Ingram, 2000.
Hypothesis:
As the temperature increases, so will the rate of reaction. Therefore the Alka-Seltzer tablets dropped into the hottest water will dissolve the fastest.
Prediction:
As the temperature increases, so will the rate of reaction.
Therefore the Alka-Seltzer tablets dropped into the hottest water will dissolve the fastest.
This is due to the fact that the particles (reactants) of something must physically touch each other to produce a chemical reaction. Increasing the temperature of the reactants can increase the rate of chemical reaction.
As the temperature increases by 10°C the rate of reaction would approximately double and the time taken for the tablet to dissolve is less. (My prediction is based on the Q10 theory)
I expect a linear graph where a negative correlation between time taken for the tablet to dissolve and the increase in temperature.
The graph I expected would look like:
Planned Method:
- The variable that I will change will be the temperature of the water that the Alka Seltzer is put in.
For my results I will measure the time taken for the Alka seltzer tablet to completely dissolve into a solution with the water.
- I am going to measure the time taken for the Alka Seltzer tablet to dissolve by using a stopwatch. (see method)
- I am going to measure it by using a stop clock and setting it to count up. At the same time that the pill will be dropped into the water the timer will be started.
- I will have seven temperatures (ranging from 20°C to 80°C with intervals of 10°C each time) which I will test each of which will be repeated three times. I believe that these results will be more than enough to plot a decent graph.
Apparatus list:
. 150ml measuring cylinder
· 150 ml beaker
· Timer (stopwatch)
· Tap Water
· Alka-Seltzer Tablets
· Thermometer
· Bunsen Burner
· Tripod
· Heat mat
. Gauze
. Goggles
. Clamp to handle the beaker when hot
Method:
- A 250 ml measuring cylinder will be filled with 150ml of tap water and this will be transferred to the 300 ml beaker which is to be placed over the Bunsen burner.
- A thermometer will be placed in the beaker of water to measure the temperature (°C) of the water at intervals until the required temperature is reached. I made sure not to place it in the beaker while being heated.
- The experiment will be carried out at different temperatures. A Bunsen on a roaring flame will be placed directly underneath the beaker to heat it with it being supported by a tripod and gauze.
- One Alka-Seltzer tablet will be dropped into the beaker at the require temperature. The time taken for the tablets to dissolve will be timed and recorded. This is done as follows: The beaker will be removed from the Bunsen burner at a certain temperature and then an alka seltzer tablet would be dropped slowly in the water in one hand and the stopwatch is set on in the other at exactly the same time. Time taken for it to dissolve (No more traces of tablet was seen at the bottom) would be recorded. After doing so the beaker filled with alka seltzer solution would be emptied and cleaned out thoroughly to make sure that it is not contaminated from previous attempt.
- Proper care was taken to ensure that setting the stopwatch was done at exactly the same time as the Alka-Seltzer tablet touched the water.
- This process of heating the water, dropping the alka seltzer tablet into the water, timing how long it takes to dissolve, cleaning out the beaker and re-filling it will be repeated for each temperature three times.
- Because of the beakers high temperature at some parts (>40°C) of the experiment I had to handle it using a clamp.
The temperatures to be investigated are:
20°C
30°C
40°C
50°C
60°C
70°C
80° C
Skill O - Obtaining Evidence
Results:
- I decided to perform two experiments with two runs each because I saw that there were enough results which showed a noticeable relation.
- The second experiment is a replicate of the first.
Safety Precautions:
- Throughout the experiment, I made sure to implement the following:
- Goggles were on all the time.
- Tie was tucked inside the shirt.
- I walked all the time with caution while handling the apparatus.
- A clamp was used to carry beaker when it was at high temperatures.
- The flame was put on roaring (blue) while heating the water. It was turned on safety (yellow) on other times when no water was heated.
Conclusion:
From the results obtained it has been seen that the rate of reaction increases as the temperature increases, supporting my hypothesis in which I stated that:
“As the temperature increases, the rate of reaction increases. The Alka-Seltzer tablets dropped into the hottest water 80°C will dissolve the fastest.’’
The Alka Seltzer tablets dissolved quicker at higher temperatures because of the collision theory which states that as the particles are heated up they move quicker and therefore collide with each other more often and with more force.
By looking at my graphs, It is apparent that as the temperature increases, the time taken for the Alka Seltzer tablets to dissolve decreases. This shows that the rate of reaction increases with the temperature. But it doesn’t follow exactly what the Q10 theory states because the time between each consecutive temperature did not halve each time. Initially, in experiment (1), at 20°C, the average time was 42.5s and at 30°C the average time was 31s and this was the biggest difference among my data between two consecutive readings. The rate change was 1.37. So Q10 theory was not followed in my results.
There is a curve that at first goes down steeply (looks like a sloping down line), but then after 60°C flattens off horizontally. The link breaks up, and this can be explained by the fact that the Ca2+ and Mg2+ ions are removed from the metal hydrogen carbonate due to their decomposition. As I stated in my scientific background, when water is heated above 60°C, the metal hydrogen carbonate breaks down to form a solid carbonate and carbon dioxide and water which would change the composition of water that I initially started off with.
So as my graphs show, the trend that the reaction was following at 20°C, 30°C, 40°C, 50°C and 60°C is not evident any more (not been followed) at 70°C and at 80°C. But I will not consider these two results anomalies because they are the outcome of dissociation of calcium and magnesium ions.
Evaluation:
My results were very reliable, because by looking at my graphs I can see a trend which relates to my scientific background. It is also evident that I have obtained a reliable set of results when I see that the replicate shows the same trend as the first experiment.
I used a measuring cylinder and a beaker to measure out the amounts of water; however these did not seem to affect the quality of my results. To increase the accuracy of my results I could have perhaps used a burette.
Even though I did the best I could to keep the experiment accurate, at some places there were mistakes that unintentionally occurred. Firstly, when testing temperatures at 30°C and 40°C, the water was sometimes heated more than needed, so I had to wait until it cooled down to the required temperature. To avoid this happening, a thermostatic water bath could have been used, because I could set it to my required temperature. Also it would mean that the solution inside it would reach the exact temperature or close to It., and not increase or decrease once the water inside it has reached equilibration.
It was also very difficult for me to observe exactly when the Alka-Seltzer tablets had totally dissolved and consistently decide this for every result obtained. This was probably the main factor that would have decreased the accuracy of my readings. To avoid this happening I would have needed to use specialized piece of equipment which would tell me exactly when the tablet was dissolved
Overall, I can come to firm conclusion from my results that support my prediction and background knowledge that: As the temperature increases, less time will be required for a reaction to take place i.e. the rate of reaction increases.
To improve the experiment overall I could use distilled water to avoid inaccuracies related to the hardness of tap water. I believe I should get a straight line curve.
Another improvement I could have made is to increase the range of the temperatures investigated. (10°C to 100°C).