- The temperature of the hydrogen peroxide
- The volume of celery and water it is blended with
- The time the celery is blended for, as this might affect the amount of enzyme. For example if one batch of celery is blended more than others, then the enzymes may work better to render the hydrogen peroxide harmless, giving inaccurate results
- The volume of hydrogen and peroxide, in whatever ratios.
Measuring the rate of reaction
The variable to be changed is the concentration of hydrogen peroxide, but how can the reaction rate be measured? I know that the reaction is catabolic; therefore it will produce two substances. These are water and oxygen. Measuring the volume of oxygen can be done in various ways, counting the bubbles, collecting the gas and measuring its volume or collecting the oxygen over water. As I am able to use gas burettes, I will measure the gas over water. This is the most accurate way, as counting bubbles is very susceptible to human error.
Diagram
Preliminary Experiments
To determine the amounts of celery and hydrogen peroxide to be used I must carry out preliminary experiments. This will enable me to work out what quantities will give a good set of results. As a starting point, 35 grams of celery was blended, for one minute, with 20 cm3 of water. This gave a fairly mushy substance, with no large lumps of celery.
All of the preliminary experiments will be carried out with full concentration of hydrogen peroxide, which from the bottle is 6%. This is the largest concentration of H202 that will be used. It was also decided that the experiment would be left for 30 seconds each time, and then the volume of gas collected would be read off of the burette. 10 cm3 of non- dilute hydrogen peroxide was measured out, as well as 20cm3 of the celery mush. The experiment was set up as shown in the diagram. The amount of gas given off was far too much, as it displaced the water in the burette far too quickly, and was not able to be measured. It was assumed that the amount of hydrogen peroxide was too much, so the experiment was repeated with 5cm3 of hydrogen peroxide and 20cm3 of celery. This again gave off to much gas, and we were unable to take a result as it displaced the water so quickly. We could not start the experiment at a more dilute concentration as we were to half the concentration each time, and we needed various concentrations. Starting at a lower concentration would mean having to go into fiddly, hard to measure out decimal numbers, to get a good set of different concentrations.
Next we tried reducing the celery concentration. The experiment was repeated, in the same way but using 10cm3 of hydrogen peroxide and 5cm3 of water. This time the gas displaced hardly any water, and more dilute concentrations would displace even less, giving a poor set of results. It was decided to use 10cm3 of hydrogen peroxide and 10cm3 of celery. When it was tested it gave off a large volume of gas, but still stayed on the scale of the burette and was able to be read off.
Problems to overcome
The setting up of the experiment will be very tricky, as the stop clock has to be started and then as quickly as possible, the H202 added, and the delivery tube put up the gas burette as not to loose any of the oxygen in the first few seconds. To make this easier the gas burette will be clamped in place, so it doesn’t have to be held.
Safety
- Eye protection must be worn as H202 is being used, and can cause serious damage to the eye.
- Care must be taken when using knives to cut up the celery.
- Bags must be tucked under the benches as they may cause an accident.
Method
- The apparatus was set up as shown in the diagram
- 35grams of celery was cut up and mixed with 20cm3 of water, which was blended for 1 minute.
- 10cm3 of the celery mush was then placed in a conical flask
- The gas burette was filled with water and clamped in place over a bath of water
- 10cm3 of the hydrogen peroxide (in whatever concentration) was added to the celery
- The stop clock was started
- As quickly as possible, the bung was placed in the top of the conical flask and the delivery tube angled so that the gas would be directed up the burette.
- After 30 seconds the stop clock was stopped
- The amount of gas collected was noted, by reading the scale on the gas burette.
- Each concentration was repeated three times to confirm the validity of the data and so an average can be made.
Results
Red numbers = anomalous results
Analysis
From my results and graph it can clearly be seen that they agree with my prediction, “ diluting the H202 will decrease the rate of decomposition of the H202, therefore producing less gas.” The results table shows that at a concentration of 10:0 there was an average of 14.70cm3 of gas collected and this steadily decreases as the concentration of H202 becomes weaker.
In both graphs, all readings and averages, there is a positive correlation. As the concentration of hydrogen peroxide increases, so does the amount of gas collected. The line of best fit is fairly straight but seems to increase more rapidly at the start and then smooth off towards the end. This can also be seen in the rate of reaction graph. There is a negative correlation and the lower the concentration, the faster the rate of reaction. My results also back up the collision theory. As there are more H202 particles present, there is more chance of collision with the enzyme and so the overall reaction will occur much faster, in this case the production of water and oxygen. Also as I predicted the reaction was faster with a higher concentration, as there was more hydrogen peroxide for the catalase to render harmless, so more oxygen and water were produced.
I can conclude from my results, which adhere to scientific knowledge and my prediction that the higher the concentration of hydrogen peroxide, the more gas given off. I can conclude this for my experiment, as the highest concentration of hydrogen peroxide used was 6%. In my prediction I stated “ an increase in the substrate concentration will speed up the reaction if there are enough enzyme molecules to cope with the additional substrate.” In my experiment there were enough enzyme molecules to cope, but the H202 wasn’t that strong. There must be a point at which the concentration of H202 is too large for the enzyme to render harmless and not produce any more oxygen and water. This might also be a contributory factor to the rate of reaction. The lower the concentrations of hydrogen peroxide the faster the reaction. This maybe because when there is a small concentration of hydrogen peroxide all the enzymes can work quickly to render it harmless, and turn it into oxygen and water. When the concentration is higher there is more to render harmless, so there will still be more water and oxygen given off as there is more substrate in the first place, but there is more for the enzymes to work with and it takes longer. Therefore the rate of reaction is slower when the concentration is higher.
Evaluation
The data that I collected was fairly reliable, but there was one anomalous result. This is shown on each of my graphs. This anomalous result was the first result collected when doing my experiment. This means that as it was the first try, using the equipment, I was maybe slower at starting the clock, or putting the delivery tube into the burette, as I had had no practise. This anomalous result can be put down to human error.
Looking back on my experiment there are many ways I think I can improve it. I could use more concentrations of hydrogen peroxide to get a more accurate graph, and a better line of best fit. I also believe that the results could have been more accurate if we had got all of the results in one lesson, using one batch of celery. This would mean that the celery would be more likely to have the same number of enzymes. It would also make the experiment more reliable if there were more people doing each experiment. In our case there were only two of us. This meant that my partner had to pour the H202 into the celery, while I started the stop clock, and then she put the delivery tube into the gas burette. This wasted valuable time and gas as she had to do two things simultaneously, which was impossible. If we had another person putting the delivery tube into the burette, then less time and gas would be wasted, and the results would be more accurate.
To extend the experiment further I could keep concentration of H202 constant and change another variable. If I were to carry out the experiment with the hydrogen peroxide at different temperatures, my prediction would be that the higher the temperature, the more gas would be given off. This is because, to make reactions work between the enzyme and H202 there must be a certain amount of force to combine them together, they have to collide to set the reaction off. The total kinetic energy of the reactant molecules must be at least as big as the activation energy. So if the H202 was heated up, the kinetic energy would be larger, so there will be more chance of collisions, so the overall reaction would occur faster. This is true up until a certain point. As enzymes are specific, they work best between a small range of temperatures. If the temperature is too high, then the enzymes are denatured, they change shape and they can no longer combine with the substances.
I could also extend the concentration experiment further by looking at the quote “ an increase in the substrate concentration will speed up the reaction if there are enough enzyme molecules to cope with the additional substrate.” As I put in my analysis, in my experiment there were enough enzyme molecules, but I could do an experiment to find out the specific concentration where the concentration is too high for the enzyme molecules to cope with.
