I need to find out what the areas of error fall into. Firstly, the measuring cylinder is only accurate to within +/- 1cm3. Also, the volume readings will be measured at the bottom of the meniscus at eye level.
Background Knowledge
As a general rule, the rates of chemical reactions increase with increasing concentration. This is due to a number of reasons, namely because there are more molecules in a higher concentrated solution, meaning that there is an increased kinetic energy. This in turn means that there will be more collisions between molecules. When there is a reduction in the concentration, there is a diluting effect on the solution, so there are fewer molecules of enzyme and substrate in the solution. This means that there is less chance of the particles colliding.
Hydrogen Peroxide (H202)
Hydrogen peroxide is a molecule with the above formula. It is a compound of hydrogen and oxygen, and is clear and toxic. Hydrogen peroxide has a metallic taste and is corrosive when in contact with the skin. When concentrated, this is an unstable liquid, which will explode when heated to over 100°C. Hydrogen peroxide can decompose for any number of reasons, including exposure to sunlight, heat, chemical catalysts, and rust. When decomposed, the byproducts of water and oxygen are stable and harmless. In order to prevent natural decomposition, it is stored at low temperatures and in the dark.
Hydrogen peroxide is manufactured in massive quantities for use in industry. It is produced by electrolyzing aqueous sulphuric acid, ammonium bisulphate, or potassium bisulphate. Germicides and antiseptics utilize hydrogen peroxide at concentrations of 3 to 5%. It is also used as a bleaching agent for textiles and paper. It acts as both a reducing and an oxidizing agent as the oxygen released on decomposition readily joins with other molecules and can be used as part of the fuel for rockets.
Enzymes
Many chemical reactions are helped along and sped up with the use of catalysts, which alter the rate of reaction, without being altered themselves. Catalysts are found in all organisms and control the rate of reaction for most chemical reactions in the human body. Catalysts found in living organisms are called enzymes. Enzymes are globular proteins, which provide a large surface area for chemical reactions to take place on. They have active sites which have a lock and key function. Reactant molecules which fit the lock and key shape of that enzyme attach to the enzyme and so are in closer proximity to one another to collide. This causes more collisions, which means an increased rate of reaction.
Enzymes are secreted in many parts of the body, importantly though, the alimentary canal sees the secretion of different enzymes in many areas of the digestive tract. These are called digestive enzymes and they speed up the breakdown of large molecules that we ingest
Kinetic Theory
This is the study of reaction rates according to the energy of the molecules in it. Three conditions need to be adhered to if a chemical reaction will be successful; the molecules must collide, they must be located so that the reacting products are combined in a transitionary state between reactants and products, and the collision must have enough energy to convert the reactants into byproducts.
Collision Theory
Particles of an acid which are suspended in a solution are in a constant state of rapid movement. As a result, many molecules will collide and bump into each other, which will cause an increased rate of reaction. For a chemical reaction to take place there must be collisions occurring between the initial reactants. The higher the concentration of the reactants, the quicker the rate of reaction, and the greater the rate of reaction as there are more reactants in the solution. Collisions must also be effective in order for these assumptions to be correct.
Effective collisions result in a successful reaction only of they have reached the activation energy level. This is the energy that is required to cause a successful reaction.
Changing the rate of reaction
For a molecular collision to be effective, the following criteria must be met;
- The collision must have sufficient impact energy to overcome the activation energy. The impact energy should be enough to break chemical bonds and to create new bonds.
- The molecules must be in the correct position to ensure successful collision.
Changing reaction times
There are 5 main factors influencing the reaction time of particles. These are temperature, concentration, pressure (for gases), surface area, and use of catalysts.
Temperature
As the temperature of a liquid or gas increases, so does the kinetic energy of the molecules in that substance. Therefore, the particles will move faster and the probability of them bumping into each other increases dramatically. The increased energy of particles also means that they are likely to have increased impact energy for chemical reactions to successfully take place. At lower temperatures, the particles have less kinetic energy and will move slower, resulting in less collisions at the right energy level, and so a reduced rate of reaction.
Also, enzymes work best at an optimal temperature of 35°C, because this best matches the temperature of the human body. At higher temperatures, enzymes become denatured. This is where their chemical bonds break and the activation sites change shape. This means that the lock and key function is not in place so the chemical reaction is much slower.
Diagram 1, below shows particles at lower temperatures, where reactions are reduced. Diagram 2 shows particles at higher temperatures where there is an increase in the number of collisions
Concentration
Increasing the concentration means increasing the number of molecules in a particular volume. This means that there will be more particles or molecules that can collide. A reduced concentration means that water has been used to dilute the solution, which means that the reactant particles are further apart, so less collisions. If there is a high concentration of hydrogen peroxide, this will mean that it will react more with the yeast, and there will be a greater volume of the byproducts in a given time.
Pressure
Increasing the pressure of a gas plays the same role as increasing the concentration of a solution. Increasing the pressure means that particles are compressed into a smaller area, so they will collide more. If the same number of particles is exposed to lower pressures, they will collide less as the area they are in will increase.
Surface Area
Smaller particles will have a greater reaction rate than larger particles. This is because smaller particles have a larger surface area: volume ratio. Therefore, those with a larger surface area will have a larger area on which the reaction can take place. Thus, they will have in increased rate of reaction compared with particles of large surface area.
Catalysts
Catalysts are substances that speed up a chemical reaction without changing themselves. The catalyst supplies a surface on which the reaction can take place. The catalyst is substance specific and enzymes are the biological catalysts found in living organisms. Without these, chemical reactions in the human body would take forever to take place.
Enzymes have a lock and key structure, the activation site on the globular protein where the reaction takes place. Yeast is a product that naturally contains the enzyme catalase, which is used to hydrolyze starch. It can also be used, in conjunction with washing up liquid to breakdown hydrogen peroxide into water and oxygen.
Plan
Apparatus
- Glass test tubes x 18
- Retort stand
- Three clamps
- Conical flask (with tube and bung)
- Graduated pipette
- Burette
- Stop watch
- Yeast – dried and in granules
- water bath, at 35°C
- Hydrogen peroxide at concentrations of 100%, 80%, 60%, 40%, 20%
- Distilled water
- Washing up liquid
Safety: Goggles, gloves, and protective clothing must be worn at all times. Hydrogen peroxide is corrosive and is harmful if ingested or gets into eyes. Spillages should be wiped up immediately and any spillages on skin should be reported.
Instructions:
- Prepare the glass test tubes first. There should be three test tubes for each concentration of the hydrogen peroxide so that repeated measurements can be taken.
- Place the test tubes containing the hydrogen peroxide into the water bath for 5 minutes so that they can acclimatize to the temperature.
- In the mean time, measure out 3 lots of 2ml of yeast. Place these aside and wait for the five minutes to acclimatize to be up.
- When the hydrogen peroxide has been in the water bath for 5 minutes, take a pipette and measure 1 ml of the washing up liquid. Place this into the test tube containing 100% hydrogen peroxide. Immediately after this, place the 2ml of yeast into the same test tube and start the stop watch.
- Measure the volume of gas collected every 5 seconds from 0 second until 45 seconds. Measure the volume of the solution at the meniscus at eye level to ensure that the volume is correctly measured.
- The tube that carries oxygen to the measuring cylinder must be put into the water in such a way that no air is trapped underwater that could rise into the measuring cylinder to ruin the results.
- I decided to use 2ml of hydrogen peroxide because preliminary tests showed that oxygen was too quickly released with larger volumes of hydrogen peroxide and they could not be easily measured.
- Repeat this for the two other test tubes containing 100% hydrogen peroxide. Then repeat three times for each of the different concentrations of hydrogen peroxide.
Factors to think about
In order to reduce human error and experimental error, the following need to be considered and accounted for;
- Use a measuring cylinder and measure to the meniscus at eye level
- Take three readings for each concentration of hydrogen peroxide to obtain a mean result
- Maintain the temperature of the reactants in order to control for the effects of temperature on the rates of reaction
- Ensure that the stop watch is used accurately so that all recordings are taken from the same time scale
Results
The volume of yeast was consistent at 2 ml for each test tube and the volume of washing up liquid in each test tube also remained the same at 1 ml.
I then drew a graph that represented the volume/time graph for the 6 concentrations of hydrogen peroxide (graph 1). From this, I was able to work out the initial rate by working out the gradient of the line. I then plotted a graph to show initial rate/ concentration graph to show how the concentration of the hydrogen peroxide was linked to the rate of reaction (graph 2)
The graph above shows the time vs. volume of oxygen collected for the 6 concentrations of hydrogen peroxide.
- Series 1 – 100% hydrogen peroxide
- Series 2 – 80% Hydrogen Peroxide
- Series 3 – 60% Hydrogen peroxide
- Series 4 – 40% hydrogen peroxide
- Series 5 – 20% hydrogen peroxide
- Series 6 – 0% hydrogen peroxide
I then needed to work out the gradient of each line for the 6 concentrations using the straightest part of each line graph.
- 100% Hydrogen peroxide solution:
(50 – 20) / (14 – 6) = 30 / 8
= 3.75 mls-1
- 80% Hydrogen peroxide solution
(50 - 20)/ (22 – 7) = 30 / 15
= 2 mls-1
- (60 – 50) / (36 – 30) = 10 / 6
= 1.67 mls-1
- (20 – 10 ) / (17 – 8) = 10 / 9
1.11 mls-1
5. (20 – 12) / (19 -10) = 8 / 9
0.89 mls-1
Then I drew a graph showing the initial rates of reaction which was calculated above, against the concentration of Hydrogen peroxide to show how they are linked.
Conclusion
From my results and graphs, I found that the trend supported my predictions and that as concentration of the hydrogen peroxide increases, so the volume of oxygen created does. This is also similar for the initial rate/concentration graph, where increasing concentration of hydrogen peroxide solution resulted in an increased initial rate of reaction. This proves that my predictions were correct and support my background theory. I believe that the reasons for the changing initial rate of reaction as the concentration of the hydrogen increased was due to successful collisions taking place. This would have been due to increased kinetic energy of a greater concentration of hydrogen peroxide molecules, which in turn would mean that the increased thermal energy would cause more collisions, therefore and increased rate of reaction. For the lower concentrations of hydrogen peroxide, the initial rates of reactions were low, being directly proportional to the concentration of the hydrogen peroxide in the solution. This corresponds with the theory that the further away the particles are from each other, the less the rate of reaction. Being diluted in a water solution, the lower concentrated solutions showed the predicted decreased initial rate of reaction. The frequency of the successful collisions would have decreased as all the substrate was broken down to its respective products, and so the reaction slowed down and began to plateau. This can be seen in the first graph.
Limitations
There are a number of limitations to this investigation and they may have inadvertently affected the results. A fluctuation of any of the measuring would have adversely affected the results, resulting in biased, anomalous, or incorrect readings.
The major factors that could have influenced my results were:
- Fluctuations in the setting up of the experiment. If some air had become trapped in the apparatus, this would have resulted in a reading far greater than the actual results
- Accuracy of diluting of hydrogen peroxide. If this was not done accurately, this would have affected the reaction rate
- Gradual breakdown of the hydrogen peroxide due to exposure to sunlight means that the results were not 100% accurate. Unfortunately, it was not possible to do this experiment in the dark. Nor was it possible to conduct this experiment at low temperatures as the enzyme needed to be as close to its optimum temperature as possible. Unfortunately, increased temperatures also facilitated the breakdown of hydrogen peroxide
- Accuracy of measuring the volume of gas. The volume of solution would have had to be measured at the meniscus to ascertain what volume of gas was in the cylinder
Evaluation
Although my results did support my predictions, I do not believe that there were a significant number of results to determine an accurate initial rate of reaction for each concentration of hydrogen peroxide. Furthermore, perhaps I should have measured the volume of gas in the tube for more than 45 seconds. There didn’t appear to be any real anomalies in my results, but again, it would have been wise to conduct more experiments on the varying concentrations of hydrogen peroxide. Unfortunately, due to time constraints, there was little time to undertake an initial study to determine how to collect my results. I think that if I could have conducted this again, I would have counted the number of bubbles emitted as a result of hydrogen peroxide breaking down to water and oxygen. The theories behind this experiment appear to be sound and the collision theory supports my predictions.
