peroxide
Method
In the preliminary work I tested yeast, potato, carrot and celery in a 100% concentration of hydrogen peroxide solution. I collected the results by measuring the amount of oxygen given off in a measuring cylinder after one minute, two minutes and three minutes.
These preliminary results will help me to decide which substance to use. My group and I have chosen to use yeast. The reason behind this choice is that it gives off the most oxygen within three minutes.
This is how my group will carry out the main experiment:
- A rough amount of hydrogen peroxide will be measured out into a spare measuring cylinder. This is so that it will be easier to measure out the exact amount of hydrogen peroxide.
- We will measure out the exact amount of hydrogen peroxide from the measuring cylinder into a smaller measuring cylinder (so that the amount will be more accurate).
- Then, I shall measure out exactly 0.2g yeast into a small plastic container.
- I will then add the yeast to the measuring cylinder (with the hydrogen peroxide in).
- As soon as I have added the yeast to the hydrogen peroxide, I will attach the bung to the conical flask. When I attach the bung, I will make sure that another member of my group immediately starts the stop clock.
- I will get a member of my group to tell me when one minute, two minutes and three minutes of the reaction have passed. At these points, I will take a measurement, from the burette, of how much oxygen has been given off and record the amount of oxygen given off in the back of my exercise book.
I will repeat this process three times for all of the hydrogen peroxide concentrations (10%, 25%, 50%, 75% and 100%).
Here is diagram of the apparatus we used for the experiment:
I will make sure that my results are reliable by taking a number of precautions; firstly, I shall make sure that I use the same equipment for all of the individual experiments so that there is no bias for any of the recordings. Naturally, I will try to make all recordings as accurate as possible. When I measure the volume of hydrogen peroxide, I will take the measurement from the bottom of the meniscus consistently, in order to prevent any inaccuracies.
I will also perform each experiment three times, so that I can take an average of the three for all concentrations of hydrogen peroxide. This shall make the results more accurate than if we only took the measurements for one experiment. If any recordings seem incorrect or mistakes are made during that experiment, my group shall perform the experiments for that concentration again.
Safety is an important factor in a science lab. Hydrogen peroxide is toxic and so it is important that my group and I are cautious when using the hydrogen peroxide. In order to ensure safety during the experiment, I, along with the rest of my group, shall be wearing safety goggles to protect my eyes from the reaction. I shall also wash my hands thoroughly if I happen to spill hydrogen peroxide on them and after each experiment in case I have spilt hydrogen peroxide on them.
Results
Graph
Conclusion
From my preliminary work I found out that yeast gives off more oxygen than potato, carrot or celery when reacted with a 100% concentration of hydrogen peroxide solution.
From the main experiment, however, I found out that my prediction was correct. I saw that the reaction for all the concentrations began more vigorously than when it was coming towards the end of the three minutes, as all the reactions slow down. The general trend for the reaction of yeast with a solution of hydrogen peroxide is that as the concentration of hydrogen peroxide increases, the amount of oxygen given off within three minutes also increases. This proves that the higher the concentration of a substrate
Looking at my results and graph, I noticed that the higher concentrations of hydrogen peroxide tended to begin very vigorously and then slow down a lot, whereas the rates of reaction with the lower concentrations tended to stay similar throughout the three minutes. I expected this difference in the rates of reaction.
Enzymes are biological catalysts. Substrates (the substances that an enzyme acts upon) fit into the ‘active site’ (a special slot in an enzyme molecule). A chemical reaction takes place at the active site and products (oxygen in this case) are released in a different structure. Enzymes are usually effective for one substrate because each needs a different active site. This is called the ‘lock-and-key’ theory. This diagram shows that each enzyme has a unique slot for a substrate to fit into.
As there were more hydrogen peroxide molecules present in the higher concentrations, there were more reactions taking place within three minutes. This is because there is a higher chance of the hydrogen peroxide molecules coming into contact with the yeast molecules. If there are more reactions taking place, this increases the rate of reaction. As more reactions are taking place, more oxygen is produced.
The reason for the reactions slowing down towards the end of the experiments is that there are few atoms that have not reacted. At the beginning of each reaction, none of the atoms have reacted. Therefore, the rate of reaction at the beginning of the reaction is rapid (particularly in higher concentrations of hydrogen peroxide). As the reaction goes on, more and more atoms are reacting, and so the number of atoms that have not reacted is gradually decreasing. This explains why there are fewer reactions towards the end of the experiment than at the beginning and why less oxygen is given off towards the end of an experiment. This also explains the curve on the graph; the number of reactions gradually decreases throughout the experiment.
Evaluation
I felt that the experiment for my group went fairly well. The results we gained from the investigation supported the prediction made before the experiment.
However, that is not to say that our experiment went perfectly; there were still inaccuracies.
For example, after the yeast was added to the hydrogen peroxide solution, there was a moment before the bung could be attached to the measuring cylinder. To make the experiment more accurate, I would have needed to find a way to attach the bung immediately after the yeast was added to the solution, or use a different technique altogether; such as using apparatus with a pre-fixed bung, so that it would not have to be attached after the yeast was added to the hydrogen peroxide.
It could be argued that more than three experiments for each concentration would make the investigation more accurate. More experiments in this case, however, would be impossible, as we did not have enough time to perform more than three separate experiments for each concentration of hydrogen peroxide.
Another inaccuracy was that the measuring cylinders used by my group were only accurate to 0.5cm³. This problem could have been solved by using more accurate measuring cylinders (for example, accurate to 0.1cm³).
The experiments were also flawed in the sense that when the measuring cylinders were cleaned between experiments, they could have still contained traces of yeast, hydrogen peroxide or water. This would have made the investigation inaccurate as the measurements would be added to these traces; there could have been too much of one substance over the others. A way of combating this problem would be to use different equipment each time, but there would not be enough equipment to go round and it may take a lot of time to change all the equipment around after each experiment. Using different equipment may also make the investigation inaccurate. However, this problem would have made little difference to the results of the experiment anyway, so it would not be worth these suggestions.
There were no anomalous results in my group’s experiment, proving that it was carried out accurately. However, if there were any results that I found to be inaccurate or anomalous, I would simply discard those results from my results table, and perform that particular experiment again.
I think that the evidence that I have gained from this investigation is reliable as there were no anomalous results included and all of the experiments were carried out carefully and accurately. The evidence that I have gained helped me come to a firm conclusion that a high concentration of hydrogen peroxide gives off more oxygen than a low concentration. The graph clearly shows this trend of that the higher the concentration of hydrogen peroxide, the more oxygen is given off.
To clarify this pattern further, I would need to carry out more experiments for different hydrogen peroxide concentrations. For example, I may need to carry out experiments for concentrations of 5%, 15%, 35%, 60% and 85% in addition to the concentrations I used in the investigation (10%, 25%, 50%, 75% and 100%). For the experiment, I could have carried out an experiment for a 0% concentration of hydrogen peroxide, but as there would be no hydrogen peroxide to react with the yeast, there would be no reaction, and therefore no oxygen given off.
