- I think this method can be modified to produce good results. Also in the preliminary experiments I worked out a good volume of gas to collect (see page 7). It does rely on human judgement (in starting and stopping the stopclock) but all other methods involving timing do also.
Within this, there are two different ways that the gas given off can be measured:
- I can measure how long it takes for the reaction to give off a fixed volume of gas.
- I can see how much gas is given off in a certain time.
I am going to use the first method, as I think that the stopclock will be more accurate than the gas syringe as I only want to measure the time to the nearest second. Also the gas syringe only has markings every for every 5cm³ of gas given off, and therefore it would be easier to measure to a marking on the syringe.
Prediction and Hypothesis
I predict that as the temperature increases, the rate of reaction will increase also. This is because at a higher temperature there is more heat energy to be converted into kinetic energy, making the particles move around faster. Faster moving particles have more chance of colliding with each other and the greater quantity of heat and energy means more effective collisions are created. As I wrote earlier (see page 2), a graph of temperature/rate should look like this:
- Because the two wouldn’t mix at 0ºC or less I have to start at a temperature above that. Therefore the graph won’t go through the origin.
- To begin with the rate of reaction rises slowly.
-
It increases faster as the temperature increases, giving a line which gets rapidly steeper.
Plan
NB. Although this is the final plan for the actual experiments, a lot of the volumes and other measurements weren’t worked out until the preliminary experiments were done (see pages 6, 7 and 8). In the preliminary experiment section I have specified the areas that I was investigating.
Get out equipment as specified.
Measure out 50cm³ of hydrochloric acid (concentration 4mol/dm³) into a conical flask using a burette.
- Measure the temperature of the solution and adjust it until it is the required temperature (by adding more cold/hot water into the beaker it’s in).
- Make sure the gas syringe is back to 0.
- Add two whole tablets to the solution, put the bung in and start the stopclock (or get someone else to start the stopclock).
- Swirl the solution gently to aid the reaction.
- When 50cm ³ of gas has been collected stop the stopclock
- Remove the bung from the conical flask to avoid too much gas being collected.
- Note the temperature and the time taken.
- Repeat the experiment, and then do the same for six other temperatures.
To ensure that this is a fair test, there should only be one variable, temperature. Therefore the temperature of the acid is the only thing that I should change. I should try to keep the volume of acid the same (by using a burette), keep the concentration of acid the same, only use whole tablets (to keep the surface area the same) and try to swirl the acid in the same way each time.
Range and Readings
Before the preliminary experiments I had decided to take six readings, all with repeats. These readings would have been either five or 10°C apart and would have ranged from approximately 10°C to 50°C. During the preliminary experiments I investigated this further (see page 8), and revised this original method. However, I did realise beforehand that there were several different ways I could get temperatures of 50°C. I could use a Bunsen burner to heat the acid or I could stand the acid in a beaker full of boiling or hot water. The second method seemed the best because it was easily controllable, the temperature wouldn’t rise too quickly, and you could also stand the acid in ice to cool it to 10°C. I didn’t change this part of the method, as it seemed to work effectively.
Diagram of Equipment
Safety
Certain safety precautions must be taken when using acids and other potentially harmful substances:
- Goggles must be worn to protect eyes. This is especially important as I am heating acid (although gently) and hot acid may spit before or during the reaction.
- If there are spills with corrosive acids they must be cleaned up immediately and thoroughly to prevent accidents (for example slipping on acid), and any skin that comes into contact with the acid must be thoroughly washed. Hands should be washed anyway after using acid.
- After my experiment I must make sure that everything is cleared away effectively. I must not leave any acid or other substances out to prevent it becoming a safety hazard.
- As I am using hot (even boiling) water I must be careful not to leave it out on the bench where it could be knocked over onto someone and cause burns or become a safety hazard in other ways.
Preliminary Experiments
I did five different preliminary experiments to help me decide on my final method.
- One experiment at room temperature using one tablet and 20cm³ hydrochloric acid (4 mols/dm³) taking the temperature before and after to determine whether the reaction was exothermic. If proved true this would have had a profound effect on my experiments, as I needed to try and keep the temperature constant throughout the reaction. Alternatively I could have taken ‘the temperature’ as the final temperature after the reaction for all of the experiments.
Fortunately, the temperature remained the same to the nearest degree before and after the experiment, proving that the reaction was not exothermic, or at least not to an extent worth noting or altering my method for.
- To decide which overall method to use (gas syringe or weighing to find total loss of mass) I did one of each, both at room temperature.
For the mass changing experiment, I measured the mass loss of one tablet in 50mols/dm³ acid at concentration 4 mols/dm³. I measured it at 30-second intervals to see how long the reaction took. My results were:
I felt there was not a sufficient difference in mass (only 0.21g) to make a difference when I did the same at different temperatures. Different tablets may have slightly different masses, and the original mass of the acid and tablet on the top-pan balance kept changing before we even mixed them, suggesting that maybe it wasn’t a very accurate method of measurement. Also, it still relies on human judgement, as the mass has to be measured at different points in the experiment.
For the same experiment with a gas syringe, I decided against taking one measurement every few seconds as the reaction happened too quickly. I also decided against noting the time every 10cm³ gas collected as the reaction was too fast, the gas syringes are not accurate enough and it would be difficult trying to get an accurate time without stopping the stopclock, as I would have to look at both the syringe and the stopclock at the same time.
However, when I measured how long it took to collect 25cm³ of gas from one tablet in 50cm³ of acid it gave me a simple reading of 23 seconds. I decided that this would be a good and clear method provided enough readings were taken to give an accurate representation of any trend.
- I tried both crushing the tablets and keeping them whole.
Crushing the tablets made the reactions happen so fast (due to an increase in surface area) that changing the temperature made little/no difference to the times I got. However, keeping the tablets whole produced a wide range of clear results.
- I tried both different concentrations of acid (4 mols/dm³ and 2 mols/dm³).
Acid at concentration 2 mols/dm³ took a long time to react with one tablet. Increasing the concentration increases the rate of reaction. Therefore, I decided to use acid at 4mols/dm³, as it would react more quickly. I also realised that in both concentrations of acid one tablet took too long (especially at a low temperature) to give adequate results within the time-span I had.
- I then found the maximum and minimum temperatures I could reach using my method of heating, and from there determined what would be a good volume of gas to collect (not too quick at the highest temperature, not too slow at the lowest temperature).
The minimum temperature I could reach using ice was approximately 10°C, as I had very limited time. The maximum temperature I could definitely reach using boiling water worked out as approximately 50°C. I decided that collecting 50cm³ of gas gave a wide range of results. The results I had at that point were:
This showed me that most of the reaction took place between 10 and 20°C, and therefore, as the difference in time between 20 and 50°C was only 12 seconds, I didn’t see any point in going as high as 50°C again. I altered the range of my experiment to be between 10 and 40°C. This would allow me time to take more readings in areas that needed to be looked at closer. However, I shall leave the 50°C reading in the final table and graph.
Revised Range and Readings
I am going to take seven readings, all with repeats. These will be at: 10, 15, 20, 25, 30, 35 and 40°C. Repeats enable me to take an average, and help me ensure that my results are accurate and not just due to circumstance or errors in measurement. As it is not possible for me to complete all of my investigation on one occasion (and therefore ensure that all the equipment used is the same) I will take my repeats on different occasions wherever possible, to ensure that my readings were not due to faulty equipment.
I have already done one reading at 10°C and one at 20°C, and I will keep my original reading of 50°C. I will not worry too much about the exact temperatures of the original readings, but I will try to ensure that the temperatures of the repeats are the same as the original temperatures, to the nearest °C. Should I have an anomalous point I will repeat the experiment at that temperature until I get a more likely answer. Also, if there happens to be a big jump in results between two points, if I have time at the end I will try and get results for a point in between. I will record my times to the nearest second, and the volume of gas collected as closely as possible to 50cm³.
Results
A table to show the length of time taken to collect 50cm³ of carbon dioxide gas when two indigestion tablets are added to 50cm³ of hydrochloric acid, including repeats
After obtaining these results, I decided that I should take another reading at 18°C, between my results of 41 and 24 seconds, to enable me to draw a clearer and more accurate graph, and to further support my prediction and predicted graph. My repeats all seemed close to the original experiment, and none of my results were anomalous enough at this stage to warrant taking further repeats. The only slightly odd value is my repeat reading for 15°C, which is the same as my original reading for 10°C. I can only assume that this is due to the fact that to begin with the acceleration is quite gradual, and a small error in timing or measurement, or slightly faulty equipment may have altered the results a little.
I also worked out the averages of my results, and I decided to display all this in another table.
A revised table to show the length of time taken to collect 50cm³ of carbon dioxide gas when two indigestion tablets are added to 50cm³ of hydrochloric acid including repeats and averages
I am going to first display my results as a graph of temperature against time taken, and then as a graph of temperature against rate, similar to the one I drew while making my predictions. This will enable me to comment on my results easily.
A graph showing temperature against time taken to collect 50cm³ of carbon dioxide gas when 50cm³ of Hydrochloric acid is added to two indigestion tablets
As this graph and these tables show, all my results are very close together, and on my graph, the average is barely distinguishable from the other two lines. Therefore, when plotting a graph of rate against temperature, I shall use the average result. I will do this by using the equation
Rate equals:
A change in something (eg. loss of mass/mass of gas given off)
Divided by
The time it takes to change/disappear
(see page 1)
Therefore, here, rate equals 50 divided by the average time it took to give off 50cm³ gas (in seconds). For example, at 10°C I would do:
50
= 1.1 (to one decimal place)
44
A table showing the rate of the reactions at the different temperatures
A graph showing temperature against rate of reaction while collecting 50cm³ of carbon dioxide gas when 50cm³ of Hydrochloric acid is added to two indigestion tablets
Analysis of my results
My results when plotted on a graph, and the best-fit line accompanying them, show a strong positive trend, as I had predicted. They do not fall along a straight line, but I didn’t expect them to do so (see original graph, page 2). They do not strictly follow the pattern I originally expected them to do (start off accelerating slowly, then speed up), indeed, they start off accelerating slowly, speed up, slow down, speed up again and then slow back down.
Conclusion
My conclusion fully supports my previous hypothesis and predictions that ‘as the temperature increases, the calculated rate of reaction increases.’ This is shown in my final table of results and graph. However, it does not support my predicted results exactly, as the pattern of the graph is not as I had expected. This can be explained if necessary by inaccuracy of equipment, error in timing or simply human reaction times and judgement and therefore is less important than the accuracy of the predicted fact that the temperature and the rate of reaction are proportional.
From my earlier notes I can see that this is because as the temperature increases there is more heat energy. This can be converted into kinetic energy, which makes the particles move around faster, and therefore have more chance of colliding with each other. Also, larger volumes of heat and energy means more effective collisions are created.
I therefore conclude that the relationship between temperature and rate of reaction is indirectly proportional, in that as the temperature increases, the rate of the reactions taking place increases also.
Evaluation
I feel that both my method and equipment were fairly reliable, as my results fit fairly closely with how I imagined they would. I have taken enough results and repeats, and these are close enough to my prediction to create and support a firm conclusion. However, a few of my results were slightly anomalous, and the graph did not end up in the exact shape I had predicted, and therefore there must be ways to make my experiment more accurate. There are several things I have noticed which could have adversely affected any results I got, and I have tried to think of ways in which I could alter them if I was to re-do the experiments now.
It is important to remember that the line of best fit on my graph is a straight line, showing only a basic upward trend, not a graph which accelerates as I predicted.
When, towards the end of the given time for practicals, I was looking to re-do any points, I only looked at my first graph of time taken against temperature (see page 10), not at the graph of rate against temperature. Therefore, instead of trying to repeat points which fall a little away from my best-fit line, such as that at 24°C or that at 50°C, I added another point at an important stage in my original graph, at 18°C. This seemed a valuable point, but looking at my rate graph it lies exactly on my best-fit line. So while it does serve to highlight the accuracy of my results and best-fit line, it would probably have been more valuable to repeat the points at 24 and 50°C. Because of reasons stated (on page 8), the point at 50°C didn’t have a repeat to begin with, and therefore ought to be looked at as less valid than the other points anyway.
Most of the potential inaccuracies in my experiment were caused by my equipment, rather than the method, as I felt that my method was reliable. I especially liked my system for heating, as it almost invariably kept the acid and tablets at a constant temperature and it produced accurate results.
There are a few reasons that I can identify why the rate may have been above my best-fit line (such as at 24°C). If this is the case, it means that the product of 50cm³ carbon dioxide was produced within a shorter time than expected. This could be due to the temperature being higher than I had believed (for example if it rose during the reaction), or the gas syringe not being back to exactly zero before starting the experiment (sometimes I found it got knocked so it was already at approximately one or two cm³). Therefore, in theory, checking and making sure of both these things before the start of each experiment could have improved my method, and if I was to repeat the experiment I would ensure I was doing these things.
There are also reasons why the rate may have been below my best-fit line (such as at 50°C). This means that the product (50cm³ of carbon dioxide) took a longer time to be produced than expected, or that it took a longer time than average to show in the gas syringe. This could be due to the temperature being lower than I thought (for example having dropped during the reaction). In future, this should be checked before the experiment is started.
It could also be due to faulty equipment, like the gas syringe getting stuck or the rubber joining the tube not being on properly, or being on different amounts so that the gas took longer to travel up the tube. If the experiment was repeated, the gas syringe should be chosen carefully as one which is not either too loose or too stiff, and the joining tube should be put on properly, so that no gas is allowed to escape when the pressure builds up inside the conical flask.
Also, a certain degree of error must be allowed for in that a second person is necessary when starting an experiment. This is because you need one hand to add the tablets, one hand to put the bung in and a third hand to start the stopclock. This increases the chances of the experiment being inaccurate, as two different people are very unlikely to be completely synchronised. Therefore the gas has time to escape unnoticed at the start of each reaction before the bung is put in. I can see little which could be done about this in future experiments.
In both cases (above or below the best-fit line) the experiment would have benefited from a greater number of results being taken at smaller intervals. This would probably have further supported my conclusion and may also have more accurately displayed a less obvious trend such as the one I originally predicted. Taking results in a wider range, (for example from 5°C to 70°C) with repeats to verify accuracy, would have strengthened my evidence. It would only have done this if, as I imagine, a continuing trend showed in the extra results.
In all of the experiments, a degree of error in continuity must be taken into account. It is extremely unlikely that the tablets all had the same mass, or that there was the same volume of hydrochloric acid down to the last drop. I tried to make this as accurate as possible, using a burette instead of a measuring cylinder, and only getting the tablets out at the last minute so that they were as whole and un-tampered-with as they could be. If the experiment was to be repeated on a larger timescale, I could measure the mass of the tablets to increase accuracy. It is also impossible to swirl the beaker at exactly the same vigorousness every time, although I tried to keep it as constant as I could.
If I was looking to test these results on a much wider scale, I could use other common antacid tablets with similar active ingredients, keeping the type and volume of acid constant. This would prove that the tablets that I used were regular, and didn’t just coincidentally produce typical results.
