Investigate the effect of the concentration of the enzyme catalase on the decomposition of hydrogen peroxide.

In this investigation I could use two variables to see if the concentration of the enzyme catalase will have an effect on the decomposition of hydrogen peroxide:

• I could measure the amount of oxygen released after a set amount of time.
• I could measure the amount of time taken for a set amount of oxygen to be released.

I will find the amount of oxygen produced after 3 minutes by measuring the volume of gas produced using downward displacement of water.

Method

1. I will set up my apparatus as shown below.
2. I will find out how much hydrogen peroxide I will need to cover the surface of the potato using a measuring cylinder and measure it out into the conical flask.
3. I will then place the potato into the conical flask and start the stopwatch.
4. I will record the amount of oxygen present in the graduated test tube after 3 minutes.
5. I will repeat the experiment with 6 different pieces of potato with different surface areas. The surface areas I will use are 24cm2, 32cm2, 40cm2, 48cm2, 56cm2 and 64cm2. I will not do repeat readings for each piece of potato.

Apparatus

Equipment list

• Hydrogen peroxide
• 2cm3 potato cubes
• 250cm3 Conical flask
• Delivery tube and bung
• Graduated test tube
• Tub
• Water
• Tile
• Scalpel
• Stopwatch
• Thermometer
• Safety goggles
• Access to a tap

Preliminary Results

                                                 

                

Preliminary Conclusion

From my preliminary I found that my prediction was correct and as the surface area increased thereby increasing the concentration of catalase the rate of reaction increased and oxygen was produced more quickly.

        I will use the same method in my investigation because I got the results I expected. I also found that to cover the potato fully in hydrogen peroxide I needed to use 60cm3. I found the best way of cutting up the potato was using a scalpel on a tile. I found that if I recorded the amount of oxygen after 3 minutes the reaction was working well and I got large amounts of oxygen produced so I could accurately prove my prediction.

        In my preliminary experiment I didn’t take repeat readings so my results may not be very accurate. In my investigation I will take 3 repeat readings and then find an average to make the results more precise.

Experiment plan

Safety precautions

Hydrogen peroxide is a powerful bleach and is an irritant to the skin so I must take the following safety precautions when performing my experiment.

• Wear safety goggles to prevent contact with eyes,
• Wear gloves and if my skin comes into contact with hydrogen peroxide I will wash it off with plenty of water

Factors I need to control

The independent variable which I will not control will be:

• Rate of reaction

The variables that I will need to keep the same will be:

• Temperature - I will need to keep the temperature the same because the collision theory states that as the temperature increases the average kinetic speed of the particles increases, so more pairs of particles will have more energy between them and so there will be more successful collisions.

• Concentration of Hydrogen peroxide - I will need to keep the concentration of hydrogen peroxide the same because the collision theory states the higher the concentration the faster the rate of reaction because there is a larger number of particles in the same volume so there will be more collisions. This means there will be more successful collisions, which increases the rate of reaction.

• Volume of hydrogen peroxide - I will need to keep the volume of hydrogen peroxide the same because if I increase the volume of hydrogen peroxide I will increase the amount of particles of hydrogen peroxide, which will increase the amount of collisions between the hydrogen peroxide and the catalase which will increase the amount of successful collisions therefore increasing the rate of reaction and the amount of oxygen produced.
• I will not shake or stir the reaction because this would give the particles more kinetic energy and so there would be more collisions and so there would be more successful collisions and so the rate of reaction would increase.
• I must use the same type of potato - I must keep the type of potato the same because different potatoes may have different amounts of catalase in them which would change the amount of oxygen being released and the rate of reaction.

Equipment I will need to perform my investigation

Method

When I am measuring the hydrogen peroxide I will make sure the measuring cylinder is on a flat level surface and measure to the bottom of the meniscus.

        The different surface areas I will be investigating will be 24cm2, 32cm2, 40cm2, 48cm2, 56cm2 and 64cm2. I have chosen to use 6 different concentrations of the enzyme catalase because I think that I will gain a range of results so that I can make a reliable conclusion.

1. I will ensure that all the apparatus is airtight and then I will set up the apparatus as shown below.

2. I will measure out 60cm3 of hydrogen peroxide into the conical flask.
3. I will fill the graduated test tube with water then turn it upside down with my thumb placed over the entrance whilst it is still emerged in water thus ensuring that no air bubbles are let into the graduated test tube.
4. I will place a 2cm3 potato cube into the conical flask. I will immediately push the bung into the conical flask and simultaneously start the stopwatch. I will make sure that the delivery tube is pointing into the graduated test tube so no oxygen is lost.  
5. After 3 minutes I will record the amount of oxygen produced.
6. I will repeat each surface area 3 times so that I can find an average.
7. I will then repeat the experiment with the same amount of hydrogen peroxide and the potatoes with different surface areas.

Observations

Table to show results for amount of oxygen produced when a piece of potato was added to hydrogen peroxide.

Highlighted = anomalous result

Conclusion

I predicted that as the surface area of the potato increased thereby increasing the concentration of catalase, oxygen would be released more quickly as the rate of reaction increased. My results showed that there is a definite relationship between the variables I investigated as a piece of potato with a surface area of 10cm2 produced 3.5cm3 of oxygen and a piece of potato with a surface area of 20cm2 produced 7cm3 of oxygen. This is an increase of 3.5cm3 of oxygen. This happened because as the concentration of catalase is increased the rate of reaction is increased because the collision theory states that in a higher concentration there is a larger number of particles in the same volume. For the particles to react with each other they need to collide. So in a higher concentration there will be more collisions, therefore there will be more successful collisions, which increases the rate of reaction.

I also predicted that as the surface area doubled the amount of oxygen produced would also double. My results also proved this. A piece of potato with a surface area of 30cm2 produced 11cm3 of oxygen and a piece of potato with a surface area of 60cm2 produced 21.5cm3 of oxygen. This is very nearly double the amount of oxygen produced but this could have occurred through experimental error. Another example is that a piece of potato with a surface area of 20cm2 produced 7cm3 of oxygen and a piece of potato with a surface area of 40cm2 produced 14cm3 of oxygen. This happened because there was double the amount of particles in a given volume so there was double the amount of successful collisions and therefore the rate of reaction doubled and double the amount of oxygen was produced.

My predicted graph and my actual graph were quite similar. I think my actual graph would have looked very similar to my predicted graph if I had carried on my investigation for a longer period of time. This was because I didn’t use enough higher concentrations to reach the concentration at which level all the hydrogen peroxide has been decomposed and the reaction had stopped after 3 minutes.

                                

Evaluation

        I think that my investigation was quite accurate and I gained enough results to make a clear conclusion but improvements could be made to make my results more precise and reliable as I do have some anomalous results. My first results for a piece of potato with a surface area of 24cm2 was my only anomalous result because it was much lower than the rest of my repeats for that surface area.

My anomalous results may have occurred through many problems with my method:

• My potato cubes that were cut for me were not very accurate and the size differed. I could use a chipper to make the potato chunks identical or I could cut them myself with a ruler and marker to make them more accurate. I could have also weighed each cube to make them very exact. Also because my potato cubes were given to me pre-cut, the cubes I used could have been from different potatoes. This could mean that the amount of catalase was different in each potato and this would have affected the rate of reaction and the amount of oxygen released.
• A more accurate way of measuring the amount of oxygen released during the experiment would be using a gas syringe. The gas produced during the experiment pushes the plunger in the syringe up so you can read off the amount of gas produced on the syringe. A gas syringe would correct the problem of the oxygen that is produced being dissolved into the water when it is collected. I also found it quite difficult to hold the graduated test tube upright so I could read off the results accurately, this problem would be solved if I used a gas syringe.
• The rate of reaction can be increased by increasing the temperature because the collision theory states that as the temperature increases the average kinetic speed of the particles increases, so more pairs of particles will have more energy between them and so there will be more successful collisions. In my investigation I didn’t control the temperature this could have altered the amount of oxygen released. To improve the experiment I could perform it in a water bath with a thermostatic control at a constant temperature of 25oC.
• Other problems I had with my experiment were that at the beginning of the experiment after I had placed the potato into the hydrogen peroxide I had to push the bung into the conical flask to prevent any oxygen being released but if I pushed the bung down too hard air was displaced causing inaccurate results. So I will have to ensure that I push down the bung with the same pressure.
• I also had a problem with the slices of potato sticking together in the hydrogen peroxide. This made the investigation unfair because this reduced the surface area causing the results to be inaccurate. To prevent this I could use a wider conical flask so I could spread the potato out more so the pieces wouldn’t stick together.
• It was also difficult to cut a 2cm3 cube of potato 5 times so if I was to redo the investigation I would start with a larger cube maybe a 4cm cube.
• One of the variables that affect enzymes is pH. Every enzyme has an optimum pH level at which it can bond best with its substrate. Any change in the pH level will denature the enzyme and change the shape of the enzyme’s active site so it cannot bond as well with the substrate, which in this case is the hydrogen peroxide. A change in the pH could have reduced the rate of reaction and make my results unfair. I could improve the investigation by maintaining a constant pH using a pH buffer of pH 7. I would use a pH 7 buffer because this is equal to the natural environment of the enzyme in the potato tissue.

I could extend my investigation by using higher concentrations of the enzyme catalase by increasing the surface area of the potato even more. If I did this I could find out at what concentration the rate of reaction is quick enough to decompose all the hydrogen peroxide. If I did this I could draw a graph more like my predicted graph. I could also extend my investigation by using a larger number of concentrations between those I have already recorded so I could draw a more accurate line of best fit.

I used a potato as my source of catalase. The concentration of catalase could have changed in the different potatoes which would have made my results unfair. To make my investigation a lot more accurate I could use a 1 molar solution of the enzyme catalase instead of the potato cubes. I could have diluted the enzyme to create different concentrations. This way I could have measured the concentrations far more accurately and greatly reduce the chances of error in my investigation.