5. Pressure (in gases) - The speed of the reaction increases when the pressure is increased. The reason is that increasing the pressure decreases the volume of a certain mass of gas and so pushes the gas molecules closer together. Therefore there is a greater likelihood of collision and a faster rate of reaction at higher pressures.
The variable I will change will be temperature, as I am investigating the rate of reaction I therefore will need to measure the time taken until the reaction reaches a certain stage.
A stopwatch is the best way to measure time elapsed, as I will be measuring time to an accuracy of one second. I will only be able to record the result at the end of the experiment for that is the only time there is a visible signal that something has happened – there has been a chemical change. However, I will repeat each experiment three times and take the mean, in order to gain the best average obtainable in the time available to me. When we increase the temperature, we give the particles more energy, which has two effects; it makes them move faster, which, in turn, makes them collide with each other more often, and it increases the average amount of energy each particle has, so more particles will have the “activation energy”. Both of these changes make the rate of reaction go up so we see a decrease in the time taken for the reaction; an increase in time taken reflects the rate of reaction.
I think that because temperature has an effect on both the speeds at which the particles react and the activation energy they have, there will be a greater effect on the rate of reaction than other variables would have.
The collision theory
Reactions usually require collisions between reactant molecules or atoms. The formation of bonds requires atoms to come close to one another. New bonds can form only if the atoms are close enough together to share electrons. Some collisions are not successful. These are called ineffective collisions. The particles simply hit and then rebound.
Collisions that lead to products are called effective collisions. An effective collision must happen with a great enough speed, energy and force to break bonds in the colliding molecules – it must have enough ACTIVATION ENERGY.
Collisions between molecules will be more violent at higher temperatures. The higher temperatures mean higher velocities. This means there will be less time between collisions. The frequency of collisions will increase. The increased number of collisions and the greater violence of collisions result in more effective collisions. The rate for the reaction increases. Reaction rates are roughly doubled when the temperature increases by 10 degrees.
It should be clear that if you can increase reaction rates by increasing temperature you could decrease reaction rates by lowering the temperature. You do this every time you put something in the refrigerator. If you want to see the effect of high temperatures increased reaction rates you can leave some dairy product out of the refrigerator for a few days and compare its condition with the same age dairy product that was kept cold.
The Iodine clock reaction
This reaction has been used repeatedly down the years in the study of reaction rates. Most famously by Harcourt and Esson in the late 18th – early 19th century.
Prediction
I predict that as the temperature is increased the time taken for the reaction will decrease. Therefore the rate of reaction will increase. This means that the graph drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the temperature is increased. This can be justified by relating to the collision theory. When the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully. All this can be understood better with full understanding of the collision theory itself:
For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.
Range
The temperature range I will use for the water will be from 20 to 80oc at 10oC intervals. I have chosen to start at 20 because room temperature will be around 21 degrees and I think it is much more convenient to start at a higher temperature rather than a lower one because then I will not have to worry about cooling the water down. From my own scientific knowledge I think that if the temperature is increased much higher than 80 the reaction will occur too rapidly and so the time will be difficult to measure because of human error. I will repeat the experiment 3 times at each of the 7 different temperatures.
Repeat results and averages will be taken to improve the credibility of the findings and present solid grounding for the final conclusion. The repeat results will help iron out any anomalies and the average will give a good summary of the results of the experiment. However if one set of results is entirely different to the other, a fourth experiment will be produced to replace the anomalous set of results.
Two graphs will then be drawn so that my findings are clear; they will show temperature against average time for reaction and temperature against rate of reaction. (This is proportional to 1÷the time taken for the reaction to take place).
Safety
Standard laboratory safety regulations will be followed. A pair of goggles/safety glasses will be worn this is particularly important during the heating part of the experiment in order to protect the eyes. I will also wear an apron so that chemicals do not get spilt over school uniform. I will not handle hot beakers instead I will leave them on the tripod and turn off the Bunsen burner. A gauze and heatproof mat will be used while heating to avoid any damage to the equipment. Bags and blazers will be kept under benches so that they cannot be tripped over. Long hair will be tied back so that it does not become dangerous. I will also wash my hands after each practical lesson so that any spills of the chemicals will be washed off.
Fair Test
In order for my findings to be valid the experiment must be a fair one. I will make sure that the measuring cylinders for the potassium iodate and sodium disulphate will not be mixed up. I will also keep the concentration of the solution the same each time:
- The amount of potassium iodate used will be 5cm³ each time
- The amount of sodium disulphate used will be 5cm³ each time
- The amount of distilled water used will be 30cm³ each time
I chose these volumes because in a previous experiment I carried out to find out the effect of concentration on the rate of reaction I used the same solutions and found that at room temperature (approx 21ºc) the concentrations above took (on average) 117 seconds to turn a blue/black colour. This is almost two minutes, it is important that the timing is not too short because then human error would play a bigger part. I felt that this concentration would give me sensible values for the rate of reaction at each temperature without getting too small too quickly.
During the heating stage of the experiment, a blue flame will be used throughout. Also the same Bunsen burner and gas tap will be used to maintain continuity. All of these precautions will make my final results more reliable and keep anomalies at a minimum and thus make the entire investigation more successful.
Pilot Test
I will carry out a pilot test to ensure that the way I have planned my experiment will actually work in practice and give me the range of results I require. I will do this by carrying out one test run for each extreme of temperature i.e. 10°C and 80°C and checking the time taken for the solution to turn a blue/black colour.
If the amount of reactant used up is the same each time, as in our case, then the only thing that changes is the time taken, so the reaction rate is inversely proportional to the time taken.
For particles to react:
a) They have to collide with each other.
b) They need a certain amount of energy to break down the bonds of the particles and form new ones, the “Activation Energy”.
I will also use the same type of container for each test, a 250ml conical flask, and rinse it thoroughly before each test. I will use the same stop clock each time, and ensure the tube is in plenty of light at all times, so the colour, when it changes more slowly, can be clearly seen. I will always try to take the reading from the same colour point, although this is unreliable because of human judgement. I will try to stir continually, and will only stir, not shake.
When I investigate temperature’s effects, I will ensure that all three repeats are at the same temperature, heating the water bath if necessary. I will place the thermometer in the chemicals, not the beaker; in order to gain the temperature the reaction will take place at as opposed to the surroundings.
Method
- Fill a 250ml beaker with water and heat it using a Bunsen burner to a few degrees above the desired temperature so as to account for any heat lost. This will act as a water bath.
- Using two 10ml measuring cylinders and a 50ml measuring cylinder measure out 5cm³ of potassium iodate, 5cm³ of sodium disulphate and 30cm³ of distilled water.
- Transfer each of these into 3 different test tubes which should be labelled with what they contain so there is less room for errors in adding the wrong chemicals. And put the three test tubes into the water bath.
- Use a thermometer to check for each of these to reach the required temperature, the water because there is more of it will probably take longest to warm up.
- It is important to put the thermometer in the chemicals (rinsing between each one) instead of the water bath because it is the temperature of the chemicals and not their surroundings that is important.
- When all the reactants are at the correct temperature pour the water and potassium iodate into the conical flask and then start the stop clock at the same time as the sodium disulphate goes in.
- Stir continuously using a glass rod.
- As soon as the mixture changes from clear to dark blue, stop the stopclock and record the time shown.
- Repeat the experiment three times and take the average.
Results of preliminary experiment
To back up my evidence I carried out a preliminary test which resulted in gaining another set of results. These results agree with what I have said in my prediction; that as the temperature increases the time taken for the reaction to happen decreases; so I feel that my prediction will be correct.
Results of actual experiment
The experiment was carried out safely. It was important that the solution did not become too hot because this would make the experiment unsafe. It was also important to make sure that the concentrations of chemicals A and B and water were the same each time ensuring that it was a fair test. It was very important to use the equipment safely so as to obtain accurate results. For each different temperature, I measured the time taken for the reaction 3 times as I felt this would give me a more reliable average. My data also has quite a large range, going from 10°c to 80°c. This makes the evidence much more reliable because it shows that there is a general relationship between the temperature and the time taken for the reaction to happen. We carried out the amount of tests and got the amount of results we were planning to get and we carried them out in a systematic and sensible way. We took the results using the same equipment and in the same period of time so all experiments were subjected to the same factors.
All the results have been recorded in a clear, neat table that shows all measured factors clearly and is easy to read and extract information from. We used all our equipment as accurately as we could, making sure we did each experiment in the same conditions as the previous ones. This minimalised the risk of there being any major differences between experiments and so gave us much more accurate results. We kept to our original guidelines and we made sure all the other factors remained constant.
My first graph clearly shows that as the temperature increases, the reaction time decreases and so the rate of reaction increases. This can easily be seen by the line of best fit which I have drawn in. It proves that my prediction is correct. It also provides sufficient evidence to back up my statement that temperature affects the rate of reaction.
In this experiment I have found that as the temperature of the solution is increased the time taken for the reaction to take place decreases. This means the rate of reaction increases because it takes less time for a reaction to take place, so more take place per second.
Using the graphs, with lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that with the “time” graph (that plots temperature against time taken for the reaction to take place) the graph has negative correlation, meaning that as the temperature increased the time taken for the reaction to take place decreases. Secondly with the rate of reaction graph (that plots rate of reaction against temperature) it has a positive correlation, meaning that as the temperature is increased so is the rate of reaction,
This is because when the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully.
My results are plotted on two graphs, which are shown on the next page.
The first graph shows the speed of reaction and the graph has a line of best fit.
In my graphs I have used the average results, so that my graph is a representation of the whole experiment.
My second graph shows the rate of the reaction vs. the temperature of solution.
In conclusion: from this experiment I have found that as the concentration is increased the time taken for the reaction to take place, decreases. This means the rate of reaction increases, as it takes less time for a reaction to take place, so more take place per second
Using the graphs, which have lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that the graph that plots concentration over time has negative correlation, meaning that as the concentration increased, the time taken for the reaction to take place decreases.
Obviously, the above means that the graph-plotting rate against concentration has positive correlation – as the concentration is increased so does the rate of reaction. This is because when the temperature is increased, reactant particles are moving faster with greater average kinetic energy and therefore a greater proportion of them will collide with enough energy to be converted from reactants to products. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully, and when solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur.
From my graph that shows the rate of reaction I can see that when the temperature is low, the rate of reaction is lower than when the temperature is higher.
For this to fully make sense it is necessary to recap the collision theory briefly:
For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same.
Evaluation
Whilst carrying out my experiment I encountered no abnormal results, all complying with one another and with my prediction. However my average result for 30°c doesn’t quite fit with my line of best fit however it is not far from it and so I think that maybe the temperature was not at exactly 30°c for one of the experiments and it distorted the average.
I think that if I were to carry out my experiment again I wouldn’t change the way I actually carried out the experiment because I found that the way I did it anyway was quite successful, however, like all experiments there is always room for improvement and this one is no exception. My main limitation was time. To improve the reliability of the results more tests could have been done but there was limited time so this became a problem. I would also try to make my experiment even more accurate. I would do this by monitoring all the factors much more closely such as trying to make sure that the temperature of the solution remained the same throughout the time it took to react. I think it would be interesting to use a temperature probe connected to a data logger so that I could see (as a graph) if the temperature fluctuated at all during the time when the chemicals were reacting.
Overall, I would state the experiment as a success since my predictions were supported by my results. This is important in reflecting success only if my prediction was sensible and logical. Just as important is where the experiment was not a success and why.
I think that even though we didn’t have any equipment to notice slight changes in temperature, that overall our experiments were accurate enough to make an certain conclusion, as all our experiments gave very similar results as you can see in the results table and there weren’t any glaringly obvious errors or differences between them. I think that with the equipment we had we managed to carry out a very accurate experiment and produce an accurate conclusion and evaluation. We could do more work to take the investigation further, such as increasing the range of temperatures investigated by going below 20°c the intervals between recordings could also be altered, such as taking results every 5°c instead of every 10°c.
This, as well as adding “extra-sensitive equipment” would all add to making the results more accurate and we would be able to give a definite, firm conclusion due to the sheer volume of results we would have and how accurate all these would be.
