This table shows all the results that I have collected during the entire experiment. There are five sets of Ph, and three readings for each Ph. At the end of the graph there is a column showing the average of the results collecting from each reading. The average was calculated by:

Average = 1^st Reading+2^nd Reading+3^rd Reading

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The average gives me more clear and concise results from my readings and helps to determine the patterns that can be seen from the results. The average results were used to plot the points on the graphs that I drew.

Graphs & Calculations

I sketched various different graphs to represent my data in a clear manner. My graphs show the patterns and trends that were recorded from the experiment. A few calculations were needed to sketch some of my graphs, such as finding the averages of the recorded results so that I could plot the average points onto the graphs.

Also, to sketch the graph that shows the rate of reaction, I had to first work out the gradient of the lines. To do this I had to divide any point on the y- axis by any point in the x-axis. For example, 6/266= 0.023

By dividing any point on the y-axis by the x-axis will always give me a similar result.

As well as this, to plot the error bars on my graphs, I had to do quite a lot of calculations that involves statistics. First of all I had to work out the mean for the three readings that I’m going to use to plot the error bars. I will work out the mean by adding the three readings and dividing by three. After working out the mean I had to work out the standard deviation of the three reading. Instead of using complicated equations to work out the standard deviation, I used Microsoft excel to work it out. I used Microsoft excel because it saves a lot of time since I only have to type in the equation and it automatically works out the standard deviation for the readings. Then after working out the standard deviation for the three readings, I added the standard deviation figure to the mean and took away the standard deviation number from the mean and plotted these two plots down. The range between these two plots is the error bar.

This is the equation to work out the standard deviation.

 

This box is the averages of the three readings stated above.

 

Trends and patterns

From analysing my results, I can see that the time it took to produce 10cm³ of oxygen gas is considerably different as the Ph of the solution increases.

The rate of reaction was much slower for Ph 3 and 5, but for Ph 7, 8 and 11 it was reasonably fast. The rate of reaction was highest for Ph 11, while Ph 8 was slightly slower. The rate of reaction was slowest for Ph 3, but slightly faster for Ph 5.

The graphs that I have drawn of my results show a definite trend. I can see that as the Ph increases, the gradient of the line in the graph becomes steeper. This shows that the rate of reaction increases as Ph increases. The steepest line was for Ph 11, that had a gradient of 0.071 and the line which wasn’t the steepest was for Ph 3 that had a gradient of 0.023.

The results show that Hydrogen peroxide and the potato enzymes can react in any Ph Condition whether the solution is acidic or alkaline. However the rate of reaction seems to be faster at more alkaline solutions such as Ph 8 and 11 as 10ml of oxygen gas was produced much faster.

Scientific analysis

In the experiment, the enzyme is the hydrogen peroxidase and the substrate is the hydrogen peroxide. The enzyme has a complementary shape to the shape of the substrate so that the substrate can fit in to the enzymes active site. This theory is known as the lock and key theory, the active site of the enzyme being the lock and the substrate being the key. When the hydrogen peroxide binds to the enzymes active site, it forms an enzyme-substrate complex. The enzyme then breaks the hydrogen peroxide into water and oxygen gas. This is a catabolic reaction.

The equation for the breakdown of Hydrogen peroxide into water and oxygen is:

2H₂O₂                         2H₂O + O₂

The Hydrogen peroxide was broken down at a slower rate in the Ph 3 and Ph 5 solutions because these solutions contain more H+ ions due to the fact that they are acidic. This means that the H+ ions are attracted to the charges on the enzyme, which will disrupt the bonds that are essential in maintaining the molecules three dimensional shape. This can mean that the active site of the enzyme can be destroyed and the enzyme can become denatured and that the original structure of the enzyme is lost, so therefore there is less enzyme-substrate complex present. This means that it would take a longer time for oxygen gas to be produced.

This theory of ions disrupting the bonds in the enzymes structure also applies for alkaline solutions as alkaline solutions contain OH- ions.  However, my results show that the Ph 11 solution has the fastest rate of reaction. This is because a buffer solution usually contains a weak acid. So when a strong base (conjugate base) is added to the solution, the hydroxide ions are consumed by the weak acid in the buffer solution, which then forms water. The amount of weak acid decreases while the amount of conjugate base increases. If the buffer was not present, the Ph would rise rapidly and the rate of reaction for Ph 11 would not be as high as the active site of the enzyme would be denatured.

This theory explains why Ph 11 has the fastest rate of reaction even though it seems logical to think that Ph 7 would have the fastest rate of reaction.

Evaluation

My results are extremely reliable as the gaps between the error bars are not too wide. However, for Ph 5 and Ph 11, some of the error bars do stretch out further than the other error bars in the other Ph graphs. But overall, there is very little difference between each reading for each separate Ph. The precision of this experiment is decent and firm conclusions can be made from it. As well as this, there were no anomalies in my results, since evidence is illustrated to prove it.

Limitations

In the experiment, there were a few factors affecting the accuracy of the results. For example, the potato pieces were kept in a bag bundled together, which could have meant that some of the enzymes from the potato had rubbed off onto the surroundings. It would have been better to have kept each potato piece on a tooth pick so that the pieces would have the same amount of enzymes.

Also another limitation would be the accuracy of the equipment that was used. For example, we measured 10ml of hydrogen peroxide using a 50ml burette. Instead I could have measured a slightly different measurement such as 10.1 ml of hydrogen peroxide due to the meniscus of the hydrogen peroxide being slightly off. It would have been better to have also weighed the hydrogen peroxide so that the measurements would have been more accurate and therefore the results could have been more accurate.

Another limitation would be the fact that you have to judge when the red liquid in the u-tube becomes level with each other. This can result in a slight error in recording the results as the red liquid may not be totally level, but the results are recorded anyway. It would have maybe been better to use a different apparatus to record the volume of gas given off, such as a gas syringe.

Also, another limitation would be the size of the flask used. During the experiment, a conical flask was used to react the potato pieces with the hydrogen peroxide. However the conical flask that was used was too wide, resulting in some sides of the potato pieces not having total contact with the hydrogen peroxide. This could have reduced the amount of oxygen and water given off. It would have been better to maybe use a smaller conical flask or even a test tube so that every side of the potato pieces could have contact with the hydrogen peroxide. This would have maybe increased the output of oxygen.

The potato pieces being kept on tooth picks would have been more accurate but I don’t think this made much difference in the results as only a small amount of enzymes from the potato would have been lost to the surroundings.

Also, judging when the red liquid becomes level with each other in the u- tube and the meniscus of the hydrogen peroxide in the burette being slightly off is also another factor that isn’t really significant. This is due to the fact that the measurements are still accurate enough as these factors are very minor differences.

However, the main limitation would be the conical flask that was used in the experiment as it was too wide; meaning that at times, 1/6^th of each individual potato piece was not exposed to hydrogen peroxide. As 5 pieces of potato were used, this would account for 6 out of 30 sides not being exposed, which means 1/5^th.

Data obtained

From carrying out this experiment I have proved that hydrogen peroxide is broken down the fastest by hydrogen peroxidase in Ph 11 solution. I have also confirmed the rate of reaction is very slow in acidic solutions such as in Ph 3.

To further extend my coursework, I could have tested out more Ph’s so that I could see the rate of reaction for each of them. This would tell me if Ph 11 solution has the fastest rate of reaction or if another Ph does. By doing this it would probably make the experiment a lot more accurate as a more conclusive pattern could maybe be seen.

I could have also used different kinds of potato to see if all potatoes contain hydrogen peroxidase and if some hydrogen peroxidase is better than others. Also, I could see if different hydrogen peroxidase in different potatoes affects the rate of reaction in the different Ph conditions.

Sources

http://en.wikipedia.org/wiki/Buffer_solution

http://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Buffers.htm#Bufferaddbase