I also think that increasing the H2O2 will slow down the reaction because there is more substrate to be broken down into the active sites.
The factor I will be changing in my experiment is the amount of catalase; I will do this by using different lengths of potato.
Firstly I will do a pilot experiment to discover the best lengths of potato to use.
I will keep my evidence as reliable as possible by using a range of potato lengths and repeating the experiment. I will take all readings to 1 decimal point, and have the same person take the readings each time to keep the results as accurate as possible.
RESEARCH
Lock-and-key mechanism
A mechanism proposed in 1890 by Emil Fischer (1852-1919) to explain binding between the active site of an enzyme and a substrate molecule. The active site was thought to have a fixed structure (the lock), which exactly matched the structure of a specific substrate (the key). Thus the enzyme and substrate interact to form an . The substrate is converted to products that no longer fit the active site and are therefore released, liberating the enzyme. Observations made by X-ray diffraction studies have shown that the active site of an enzyme is more flexible than the lock-and-key theory would suggest.
A Dictionary of Science, Oxford University Press, © Market House Books Ltd 1999
From .com
In my investigation the lock is the enzyme catalase and the key is the substrate Hydrogen peroxide.
Collision theory
What constitutes an effective collision?
In order for a molecular collision to be effective it must meet two conditions:
- The collision must have sufficient enough impact energy to overcome the Activation Energy. The Activation energy is the minimum energy necessary for product to form. This impact energy must be sufficient so that bonds can be broken within the reactant molecules and new bonds formed to produce the products.
- The molecules must have a proper positioning for effective collisions to occur.
If we examine the four factors that influence the rate of a reaction we can see how each of these postulates result in these factors influencing the rate.
- Nature of the reactants. If we crush the reactants, we are in essence increasing the total surface area that collisions can take place. This will have an enhanced effect on rate of product formation. Gaseous reactants have a higher Kinetic energy ,and therefore the impact energy will be greater resulting in a higher rate of product formation
- Concentrations of the reactants in the rate determining step. If we increase the concentration, we are, in essence, increasing the total number of collisions. This ,in turn, will increase the number of effective collisions, and the rate will increase.
- Temperature- Increasing the temperature increases the average Kinetic Energy of the molecules. This will increase the impact energy enough to overcome the Energy of Activation.
- Catalyst- Catalyst provides a surface whereby the reacting molecules might position themselves more favorably for collision.
The Collisional Theory of Reaction Rates explains how each of the above factors affects the rate of the reaction.
From: http://members.aol.com/logan20/ratetheo.html
- reactions are the result of collisions between reactant particles
- not all collisions are successful
- sufficient kinetic energy (KE) and favourable geometry are required
- to increase the rate of a reaction one must increase the frequency of successful collision
- energy changes are involved in reactions as bonds are broken and formed
From: http://www.bced.gov.bc.ca/irp/chem1112/ch122.htm
Hydrogen Peroxide particles need kinetic energy to have successful collisions with the catalase particles, the reaction will stop once the active sites have all been occupied, at this point there are no other active sites for the enzyme to occupy.
PILOT EXPERIMENT RESULTS
WHAT I HAVE LEARNED FROM MY PILOT EXPERIMENT
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The 50mm potato in 5cm³ H2O2 experiment- the catalase didn’t get a chance to fully react with the H2O2 because all of the potato wasn’t covered by the H2O2.
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The 10mm potato in 5cm³ H2O2 experiment- The height of the froth was very hard to measure because only a small amount is produced.
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The 10mm potato in 10cm³ H2O2 experiment- Again the amount froth produced is very little so it is hard to measure.
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The 50mm potato in 10cm³ H2O2 experiment- This was the best experiment done, it was done twice and the results were within 10%.
TIME: From our graphs, the line is straightest at 1 minute i.e. this is when the rate of reaction is fastest, therefore this is the time we will be using.
AMOUNT OF H2O2 (SUBSTRATE): We will use 10cm³ of H2O2 because the potato (enzyme) may not be fully covered with 5cm³ H2O2.
LENGTH OF POTATO(ENZYME): We will start at 10mm and go up in 5mm’s until we reach 50mm. we’ve chosen this range because we will have more points for a graph, therefore hopefully making our results more accurate.
TABLES OF RESULTS
EXPERIMENT 1
REPEAT EXPERIMENT
AVERAGE OF EXPERIMENT 1 AND 2
ANALYSIS
The line of best fit on my graph is straight but after 50mm of enzyme was used it seems to be levelling off.
I predicted that increasing the length of potato will speed up the reaction, and as you can see from my graph, this is true, to a point. The reaction is constantly occurring faster, until 50mm of catalase is used, here all of the active sites have been occupied, this is when the enzymes work at its best, and is the v max of the graph.
EVALUATION
I think the practical went reasonably well, because we managed to get two full sets of results within 10%, although towards the end we were rushed a little for time.
To me, all my results look accurate, they were within 10% of each other and there do not seem to be any anomalous result, the only problem I can see is that the line is mainly straight and does not fully reach the v max, so I think we should have chosen longer lengths of potato for the practical.
In my opinion the method was not carried out as accurately as possible, although we were able to carry out a fair test, we were not able to control the part of the potato the cylinder was cut from, the technicians did that, therefore we do not know that all the cylinders that we used came from the same area of the potato, e.g. through the centre of the potato. Different areas of a potato might contain different levels of catalase, this would make the test unfair.
I do not think enough measurements were made; we could have carried the experiment on with longer lengths of potato. We did not need to repeat any of the experiments because all the results were within 100%.
We could have used a different method such as the displacement method or the gas syringe method, where the volume of oxygen produced is collected in a syringe, rather than measuring the height of froth produced. Both seem more reliable methods.
I think the evidence is good enough to support a conclusion, because the experiment was done twice and the results were within 10%, and the results show that increasing the length of potato will speed up the reaction, to a point.
I think the plan could have been improved by using a more reliable form of measuring, instead of a ruler and the height of froth, and had longer to do the practical work.
