The reason for the blue-black complex is due to the formation of polyiodide chains during the reaction between starch and iodine. The amylose in starch forms helices with which the iodine molecules align, causing a transfer of charge. This charge transfer corresponds to the absorption spectrum, in which the blue-black colour is the complementary colour.
The details of this reaction are not fully known yet. The strength and deepness of the colour is dependent on the amount of amylose present. 3The rate of the reaction can be explained by the collision theory, which shows that the rate can be altered by4: concentrations, pressure, temperature, intensity of radiation, particle size, surface area and a catalyst. In this experiment I will be looking at concentrations, temperatures and the affect of a catalyst.
The collision theory also states a reaction will only take place if threeconditions are met: 1. Reactant particles collide with each other
The reactant particles must collide with the correct orientation. 3. The collision must provide enough energy to overcome the activation energy. 2This is due to the fact, if particles collide but are not orientated correctly the molecules will just bounce of each other, this is often due to charge of the molecules which causes repulsion if the orientation is incorrect.
If particles collide with the correct orientation, they must collide with a minimum energy otherwise they will just bounce of each other. The activation energy is used to break some of the original bonds, which is essential for a reaction to occur. The activation energy is this minimum energy and can be shown on an energy profile. We can see from the below graphs how in an endothermic reaction the energy of the reactants is lower than the energy of the products, this is why it absorbs heat.
Whereas an exothermic reaction will give out heat as the reactants have a higher energy than the products. 2The activation energy can be marked on the Maxwell-Boltzmann distribution curve. The Maxwell-Boltzmann distribution refers directly to gases, however the principles can be taken and applied to liquid reactions also.
5The area underneath the curve represents the different particles and their level of energy. Only the particles that have energy higher than the activation energy will undergo a reaction. We can see that the kinetic energy of a molecule can never be zero, but theoretically can be ever increasing, as there is no defined maximum energy value. You can then change the shape of the curve or move the activation energy in order to increase the number of collisions that overcome the activation enthalpy. Changing certain variables can do this, which is what will be done in this experiment.
6 7 The first variable I will be varying is concentration of the different reactants; I will be using 5 different concentrations for each reactant. Increasing the concentration of a reactant means that there are a higher proportion of particles per unit volume. This then causes the rate of reaction to increase because particles have a higher chance of colliding with sufficient kinetic energy to cause a reaction. 8By varying the concentration of each reactant it allows me to calculate the order of the reaction, by investigating their effects. By finding the order I can then calculate the rate constant and rate equation.
As shown above the majority if the percentage errors are minimal, however we can see the largest error was that of the colourimeter which came to 7.14%. Unfortunately there is little that can be done to prevent this, as the reading is so small. I ensured the same colourimeter was used each time, to eliminate as much error as possible. In replacement of the thermometer, a digital thermometer could be used to gain a more accurate reading of the temperature, as the precision error for it is ±0.05°C. Other than this the equipment used was well chosen as it gave the smallest amount of error possible.
The investigation was accurate as most of the percentage errors are very low and mostly insignificant. Other errors are likely to be that the room temperature varied between 19 and 23 degrees C, throughout the experiment on a day-to-day basis, a factor out of my control, therefore this would have affected the rate constant.
To reduce this error and improve the experiment, each experiment could be done in a thermostatically controlled water bath. After looking at the published data of the activation enthalpy for a non catalysed, I can see my experiment is reliable as the value I got was only 5.794 KJ mol-1 off the actual activation enthalpy, however in order to be able to fully justify my findings I would need to repeat each experiment numerous times, a minimum of three, to then allow me to calculate an average and to disregard any anomalous data. As each experiment was only conducted once, it could be that all the data is anomalous.
By using two different methods at looking at how concentration affects the rate of reaction, my results are more reliable as both methods back each other up. To improve this aspect of the investigation I would use more and a wider variety of concentrations of each reactant, during the colourimetery, as only two concentrations of each were used. The main reason for this was the time allocated to this aspect of the investigation, as each run of colourimetery took around 25 minutes.
To improve and investigate further into this experiment I would create a calibrate curve through the colourimeter by making up solutions of the coloured substance of known concentration, then measuring the absorbance of each, ensuring to use the same conditions as the experiment will be done as. The graph of absorbance against concentration will give your calibration curve. This would then allow me to see how much iodine was produced at each stage of the reaction.
Allowing me to closely monitor the rate of reaction. A limitation of my experiment would be the catalysed experiment; due to the fact the reaction occurred so rapidly. The human reaction time is only accurate to 0.5 seconds, and in some cases the reaction took only 3 seconds, meaning the percentage error is 16.7%. To look into this particular catalyst further, I would dissolve the catalyst and dilute it to lower the concentration; this may give a longer time period before the blue-black complex forms. You would also be able to investigate if the concentration of catalyst changed the rate of reaction. Another potential way to improve this would be to investigate how different catalysts may affect the reaction and to what extent do they lower the activation enthalpy.
A catalyst that could be tried would be ammonium iron sulphate, using the iron (III) ions to catalyse the reaction. Another limitation of the experiment that would have reduced the accuracy would be the fact the blue-black complex forms gradually, therefore it subject able as to when to stop the stopwatch. In order to reduce this error a black cross was marked and the experiment was stopped once the black cross could no loner be seen.
However whilst conducting the temperature experiments the reaction was done in test tubes and no cross could be used,
to minimise the error the stop clock was stopped immediately at the first sign of the blue-black complex. The final main limitation would be that the conical flask was swirled in order to mix the two solutions.
Although best efforts were made to ensure the solutions were swirled evenly in each experiment, it is difficult to control. An improvement would be to use a magnetic stirrer, set at the same speed to ensure the same kinetic energy throughout the solutions, ensuring this did not affect the reaction. As found in the Nuffield Book of Data, I can see that the order of the reaction with respect to H+ ions is dependent on the concentration. To further my investigation it would be interesting to look further into how the concentration of sulphuric acid may cause the order to be either 1 or 0 with respect to the H+ ions. I could then look at the point at which it changed from being zero order to first order. It would then be interesting to see how this affected the activation enthalpy of the experiment.
I can conclude my results are accurate and reliable, due to the fact the equipment was chosen with low precision errors and any errors given were too small to have a large impact. This can be backed up by the published data found in the Nuffield Book of Data.
Courtney from Study Moose
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