Hydrogen peroxide is widely used in industry. It is manufactured in huge quantities by the electrolysis of aqueous solutions of sulfuric acid or of potassium bisulfate or ammonium bisulphate. Solutions containing 3 to 6 per cent hydrogen peroxide are used as antiseptics and germicides and as a skin cleanser. Higher concentrations are used in the manufacture of many chemical compounds. They also serve as bleaching agents for textiles and paper pulp, and as rocket propellants.
Hydrogen peroxide acts as both an oxidizing and reducing agent, because the oxygen freed in decomposition readily combines with other substances. Its oxidizing properties are used in bleaching thing such as hair, feathers, ivory and delicate fabrics e.g. wool, silk, nylon etc. that would be destroyed by other agents. In a different form, it can be used as an antiseptic or a throatwash. It restores the original colour in paintings, by oxidizing the black lead sulfide to white lead sulphate. It can also be used as a source of oxygen in the fuel mixture for many rockets and torpedoes. So on the whole, Hydrogen peroxide is a very useful substance.
Enzymes
Scientists have proved that if you make a solution of starch and water, and kept it at your normal body temperature, it would take quite a long time before any of the starch molecules transform into glucose molecules. However, inside your alimentary canal, this reaction happens much quicker.
Many chemical reactions can be speeded up by components called Catalysts. A Catalyst alters the rate of reaction, without being converted itself.
Catalysts control almost all the chemical reactions that occur inside the human body. The Catalysts which are found in living organisms are known as Enzymes.
Enzymes are protein molecules that control most of the chemical reactions inside the human body. How they work can be understood easily. The enzyme provides a surface for the reaction to take place on. On the surface of the enzyme molecule, there is a cavity called the active site. Reactant molecules become trapped in this site and so collide more often. This results in more effective collisions, which will increase the rate of reaction.
Enzymes have many different uses. One enzyme, called Lipases, speeds up the ripening of food, such as cheese. Proteases are found in washing powders. They are good for removing stains of clothes, like blood.
In several parts of the alimentary canal, digestive juices are secreted. In these digestive juices lie enzymes, which can speed up the breakdown of large molecules to small ones, by the use of surface area. They are called Digestive enzymes. This breakdown is known as hydrolysis because water is also involved (hydro means ‘water’, lysis means ‘splitting’).
Molecularity
The number of collisions taking place depends on the following factors:
- The concentration of the colliding species
- The molecularity of the process
The steps in a reaction are classified in terms of molecularity, which describes the number of molecules consumed. When a single molecule is consumed, the step is called unimolecular. When two molecules are consumed, it is bimolecular.
Because the overall rate of a step wise reaction depends on the rate determining step (slowest), the way that the reaction rate changes with concentration alteration will depend on the molecularity of the slowest step.
Chemical Kinetics
Chemical kinetics, the study of reaction rates, shows that three conditions must be met at the molecular level if a reaction is to occur: The molecules must collide; they must be positioned so that the reacting groups are together in a transition state between reactants and products; and the collision must have enough energy to form the transition state and convert it into products.
The Collision Theory
The collision theory states that the particles of acid in a solution are in continual rapid motion. This results in many particles bumping into each other, which will cause a quicker reaction.
This then makes the suggestion that for a reaction to take place there must be collisions between the reacting species. This means that the rate of reaction depends on 2 main factors; the number of collisions per unit per time, and the fraction of these collisions that are successful. There is a relationship between the concentrations of the particles and the number of collisions. The higher the concentration, the higher the amount of possible collisions. This means the rate of reaction is directly proportional to the concentrations.
According to the collisional theory of reaction rates, products can only be formed when there are effective collisions between the reactant molecules in the rate determining step of the process. Having reactant molecules colliding is not adequate in itself. The collisions have to be effective.
Effective collisions are successful in causing a reaction. There are very few of these in a chemical reaction. Colliding particles only bring out a reaction when they have a certain amount of energy. This energy is called the activation energy.
Altering a rate of reaction
In order for a molecular collision to have an effect it must meet the following 2 conditions:
-
The collision must have enough impact energy to overcome the amount of activation energy. The activation energy is the minimum required amount of energy that is needed for product to form. The impact energy must be sufficient so that the bonds can be broken in the reactant molecules. After this, new bonds are formed to produce the new substance (after the reaction has taken place).
- The molecules must be in the right place (position) for effective collisions to take place.
The collision theory explains how certain factors affect the rate of a reaction. There are 6 main factors that can alter reaction time. They are listed below:
Temperature,
Surface Area,
Concentration,
Pressure (for gases),
The use of Catalysts,
And Light.
Temperature - this is arguably one of the best ways to alter the rate of a chemical reaction. The hottest solution will cause a reaction first. Increasing the temperature increases the reaction rate. The hotter the particles, the more kinetic energy the particles have, which makes the probability of them bumping into each other higher. When the particles have more energy, they are more likely to react when they bump into each other. If the temperature is decreased, the particles slow down. This will make the reaction slower, as they will have less kinetic energy. This is also the reason why food is kept in a fridge. The cold temperature stops the chemical reactions that make the food go bad.
Example
Particles at a higher temperature have more energy. Particles with more energy are likely to react when they collide.
Surface Area – If we break up the particle, we are increasing the total surface area where collisions can occur. This will have a greater effection the rate of product formation. Gas particles have a higher amount of kinetic energy. Therefore, the quantity of impact energy will be greater than the activation energy, which will cause an effective reaction. A good example of this is a potato. A potato takes about half an hour to cook in the oven, but chips take less than half the time. This is because they have a larger surface area.
Concentration – If we increase the concentration, we are increasing the total number of collisions. When the concentration is low, it means that water has been added. This will space out the acid particles, which will make the probability of them colliding lower. There will be less Hydrogen Peroxide particles to react with the potato. If there are more acid particles within the same volume, the reaction will be quicker due to more collisions. The highest concentration will react quickest.
Pressure – Changing the pressure on a gas is similar to altering the concentration of a solution. Increasing the pressure on a gas quickens the reaction. In a low pressure, particles are in a big area. The likeliness of them meeting is low. In a high pressure, particles will be compressed together into a smaller area, which will make them more likely to collide and react.
Catalysts – A Catalyst is a substance that will make a reaction happen faster. Hydrogen Peroxide decomposes to give off water and oxygen. At room temperature, this can take a long time. Dropping some black manganese dioxide powder (a catalyst) will speed up the reaction time.
What the catalyst actually does is supply a surface whereby the molecules might position themselves to give them a better chance of reacting. In this example, only a little amount of the catalyst is needed. However, the catalyst does not get used up itself, as it has the same mass at the end of a reaction that it had at the beginning.
A potato is a plant that contains a catalyst. Also, Blood in animals contain a catalyst. Every living thing contains biological catalysts – also known as Enzymes.
1.The effects of pressure 2.The effects of concentration 3.The effects of surface area
Low Pressure
Particles are far apart(1&2) and have a small area(3) and aren’t likely to meet & react.
High pressure
The same number of particles are in a higher pressure(1), There are more particles(2) and there is a larger surface area(3). The particles are now more likely to collide and react.
Light – Increasing the amount of light will boost the rate of various reactions. Unlike the other factors, except Temperature, light increases the amount of energy in the particles, not the number of collisions. Temperature does both of these.
In Photography, the photographic film is covered with chemicals that react when they are in contact with light. Silver Nitrate, like Hydrogen peroxide, is stored in a brown glass bottle to diminish the effect of light.
Extra Information
- Hydrogen Peroxide is also prepared by the action of acid on other peroxide, such as those of sodium and barium.
- Inside cells, enzymes work best at the neutral pH of 7. On the other hand, enzymes in the stomach work best at an acidic pH of 2.
- If you increase the temperature of water in an experiment by 10ºC, you are in essence doubling the speed of the reaction.
- Using a catalyst means chemicals can be produced large scale cheaply and quickly.
- In Los Angeles, Catalysts were used to control pollution given off from cars. A special catalyst chamber had to be fitted onto the exhaust pipe of every registered car in the city. It converted the harmful pollution – such as carbon monoxide and unburned oil – into harmless gases such as Nitrogen and oxygen.
- As the factor (e.g. Temperature) drops, the number of collisions will drop, which means the rate of reaction will drop.
Equipment
- Hydrogen Peroxide
- A potato
- Water
- 2 Test tubes
- Stopwatch
- A Bung
- A Test tube rack
Method
Once we had set up our equipment, as shown in the plan, we poured the hydrogen Peroxide and water into the two test tubes. We dropped the potato in the hydrogen peroxide, put one end of the bung into that test tube and the other end into the other test tube, then started the clock. We then looked in the test tube of water and counted the amount of bubbles being released into the water. This was done over a time period of 3 minutes. We then recorded our results.
Now we changed the concentration of the hydrogen peroxide as shown in the plan. Every time we changed the water and hydrogen peroxide. We then recorded all of our results and drew a graph of our results.
Fair test
The whole of the experiment was virtually fair apart from the fact that we done the experiment over two days. We might not have had exactly the same equipment and it might not of been of the same quality. The tube we used the second time might have been thicker or thinner than the previous tube. This would affect the speed that the bubble was released. Also, although the mass of the pieces of potato were all the same, the shape of them were all different. This might have made an alteration to my results.
Results
Evaluation
My results seemed a little odd. The results seemed to go up in between 10 to 20 after each experiment. But then there was this massive leap where it jumped from 40 to 95. Apart from this fact, the results were quite accurate, but because of it, they are not that reliable.
The method was suitable although there might have been a few things that I had not noticed which could affect the results.
The size and mass of the potato in each experiment was different. This could have affected my results.
Conclusion
I found out that the higher the concentration, the quicker the reaction time. My prediction was correct, and the collision theory was correct. The particles moved faster when they had more kinetic energy. This is what the increase in the concentration gave them. There didn’t seem to be a pattern in my graph, but if I changed the difference in concentration or added more results, a pattern could appear in the results.
Improvements
To improve this experiment I could have done the experiment in one day. My results would then be more accurate. In future experiments, I could also use Temperature and Pressure or the use of Catalysts and surface area to see if the results differ. Additionally, I could make the size and mass of the potato exactly the same.