2πr2 + 2rl (+2πr2) (+2πr2)….
I will assess my results by using a mathematical method called Product moment correlation coefficient (PMCC). This will show how closely correlated my results are and therefore show how reliable they are. The result of the PMCC equation will be between 1 and –1. The closer to one the closer the results are correlated i.e. how closely they follow a pattern. Below 0.7 would show a weak correlation.
This is the formula:
Σ×y-ΣxΣy
n
(Σx2 – (Σx) 2) (Σy2 - (Σx) 2)
n n
The values for x will be the times that I took readings and the values of y will be the volume of gas produced at the time. Each set of readings for each surface area are imputed into the calculator to produce values that are put into the formula above then I use these to work out the correlation.
I will also calculate the rate of each reaction. This is done by dividing the amount of gas used by the amount of time it took to get that gas. This should show simply how the volume of gas increases as the surface area increases.
I conducted preliminary work to devise an experiment with the best set up that would give accurate and reliable results. For the first trial the set up was basic, using a gas syringe, using mashed potato and a conical flask. No measurements were taken however I learnt that mashed potato was impossible to measure the surface area accurately however it did show dramatic results compared to a plain chip. The gas syringe was also in accurate, it could easily tampered with and sometimes it stuck and hence stopping any results being read. For the second set up I used a glass cylinder filled with water turned upside down with a bowl. The chips were place in a conical flask and hydrogen peroxide was poured into the flask. I measure the volume of oxygen produced every 10 seconds. The following were the results:
From this test I learnt that the chip was not completely submerged and that it would float horizontally therefore not the whole surface area would be exposed to the hydrogen peroxide. Less would be unexposed in a boiling tube as they are thinner so the chip shouldn’t be able to lie horizontally. Measuring at 10-second intervals gave results that did not change very regularly so unnecessary measurements were recorded, the results did not show any indicate of a curve of enzyme activity. The results changed about every thirty seconds. I believed that pouring the hydrogen peroxide in just before placing the bung in the flask was inaccurate as it could easily change throughout the experiment as oxygen would be able to escape. Therefore using a syringe would be more accurate as the reaction would take place in a sealed environment.
Fair testing: I will need to keep the temperature the same because as temperature increases, molecules move faster. In an enzyme catalyzed reaction, this increases the rate at which the enzyme and substrate molecules meet and therefore the rate at which the products are formed. The results would not provide accurate comparisons if the temperatures change. Therefore I will be using tongs to handle the apparatus so that the heat from my hands will not affect the Catalase and measure the temperature of the hydrogen peroxide before I fill the syringe each time. I will also keep the amount and concentration of hydrogen peroxide the same so the chip is as submerged as possible. The concentration will start at 20 vol. for each surface area, if the hydrogen peroxide was too strong it would break the bonds of the enzyme therefore the tertiary structure of the active site would change causing it to be denatured. This is due to the hydrogen ions in the acid interacting with the r-groups of the enzyme form the tertiary structure, too many hydrogen ions would be effecting the r-groups. One factor I will need to take into consideration is the fact that hydrogen peroxide has an acidic pH which will break down into water which has a neutral pH. This may effect the enzyme action as many enzymes work faster in a neutral environment, the only control over this is the fact that it will happen in each test and to consider its effect at the end of the experiment. I will need to use the same potato and cut a fresh chip at each stage of the experiment as the catalase may become effected by exposure to the air. This may effect results so I will keep the chips as similar as possible. The apparatus should have been washed with distilled water and I will use a different conical flask for each test so that any reactants left in a flask wont affect further tests. The procedure will stay the same so that the results can be compared.
Apparatus list and setup:
- Potato
- Potato borer
- Ruler
- Scalpel
- 6 conical flasks
- 20ml syringe (to hold enough hydrogen peroxide to cover the chip in the boiling tube)
- Double holed bung
- Retort stand
- Clamp
- Tongs
- Goggles
- 50ml glass measuring cylinder (to measure gas produced accurately and allow space for the displaced gas. As the cylinder is larger than necessary I can take the next reading directly after the first without having to fill the cylinder as much)
- Bowl of water
- Hydrogen peroxide 20vol
- Splint
- Timer
- Delivery tube
- Tile
diagram
Method
- Set up apparatus including filling glass cylinder with water and placing it upside down in water, measure width of potato borer and apply this to surface area formula
- Read the level the glass measuring cylinder shows
- Cut a piece of potato 4cm and calculate the surface area. (take care when using the scalpel)
- Put the chip in the first conical flask
- Measure 20ml of hydrogen peroxide into the syringe. Care must be taken when handling the hydrogen peroxide as the acid will be an irritant to broken skin. Glasses should be worn throughout the procedure so that acid cannot contact the eyes.
- push hydrogen peroxide and start the timer immediately
- Read the gas measurement at 30 seconds, 60 seconds, 90 seconds, 120 seconds, 150 seconds and 210 seconds. Find the difference between the starting amount of gas and the finishing amount. The displaced gas from the hydrogen peroxide must be discounted.
- use a new conical flask and repeat steps 3-7, cutting the chip once more than previously
- repeat step 8 until 6 different surface areas are used
- the experiment should then be done two more times so average reading can be made adding to the level of accuracy
Surface area calculation
Each rounded to a whole number
r = 5mm
d = 10mm
l = 40mm
π = 3.14…
New surface area exposed by each cut = 157.1mm2
2πr2 + 2rl (+2πr2) (+2πr2)….
1st chip formula = 2πr2 + 2rl = 2π25+400 = 1414mm2
2nd chip formula = 1st chip surface area + 2πr2 = 1414mm2 + 157.1= 1571 mm2
3rd chip formula = 2nd chip surface area + 2πr2 = 1571+ 157.1 = 1728 mm2
4th chip formula = 3rd chip surface area + 2πr2 = 1728 + 157.1 = 1885 mm2
5th chip formula = 4th chip surface area + 2πr2 = 1885 + 157.1 = 2042 mm2
6th chip formula = 5th chip surface area + 2πr2 = 2042 + 157.1 = 2199 mm2
Results
The averages have been rounded to the nearest 0.5ml decimal places as reading values smaller than 0.5ml would be inaccurate as the glass cylinder reads in increments of 1ml.
Evaluation
The experimental procedure went well and the results are evidence of this. The results follow a regular or expectable pattern. However there are some factors that may have affected the results. I realised that the potato chip often floated in the boiling tube and the top circle of the chip would not be exposed, this did would has less effect as the surface area amount went up. The chip did not always float so it was not a constant variable, this would make the results slightly less accurate. However it may account for the anomalous result and the end of the readings for 2042. The last two reading seems to jump at a higher gradient to usual. As the area of potato unexposed to the hydrogen peroxide goes down in proportion to the surface area the effect of the unexposed area would become less and less. Therefore at the point of my anomalous result the chip section may have all been submerged, therefore making a higher reading of gas. This aspect combined with the evidence for the enzyme preferring a neutral environment my have made the anomalous results higher than expected. A factor that could have affected accuracy may have been syringeing in the hydrogen peroxide. I tried to keep the action as similar as possible however there is no firm control of this. This is why I took an average of three readings of gas at each time. The average would minimise the effect that inaccurate reading has.
The method proved my prediction as the rate of reaction clearly increased as the surface area was increased. However if I wanted to see the full rate of reaction of the enzyme action I should have recorded the reading until no further gas is produce and until the solution is water. This should a cumulative curve, all surface area readings would eventually finish at the same point. Lengthening the experiment would make good extension work.
The results are relatively accurate and support a firm conclusion even when considering the variables that could not be controlled (chips floating occasionally and acidic environment turning neutral). If I repeated the experiment using a buiret and a gas syringe instead of glass measuring cylinder and the results were similar to my previous ones, the reliability of the results would become even stronger. Using a buiret would be more accurate as they are clearer to read.
