INVESTIGATING THE DIFFERENT RELATIONSHIPS THAT CONCENTRATION AND TEMPERATURE HAVE ON THE RATE OF REACTION Aim To study the effect that temperature and concentration of jodide ion solution have on the rate of iodide ion l-oxidation by peroxodisulphate ion S208, creating an iodine clock reaction. Introduction I decided to choose as the topic for my investigation the rate of reaction for its vital importance in the human body. Indeed lam really interested in Biology and especially physiology and I would like to study medicine.
The rate at which different reactions happens in the human body is very important for it to correctly work. Reactions in the body are affected by the concentration of the reactants, but also by the temperature: fever speeds up the chemical reactions of the immune system and inhibits the growth of some microorganisms. According to the Collision theory, the concentration of reactants and the temperature should both affect the rate of reaction, so I tried to investigate how much they do affect it. During a iodine clock reaction, two colourless solution, in this case potassium iodide, Kl and sodium peroxodisulphate, Na2S2O3, would be added together. Even if at the beginning nothing would seem to be happening, after a certain amount of time the observer should be able to observe a sudden change in the colour of solution: from colourless to blue-black.
Measuring this time it is possible to find the rate of reaction, reciprocal of the time. The chemical reaction taking place is a redox reaction where iodide ion is oxidized and peroxodisulphate ion is reduced. This is the full ionic equation: 21-(aq) + S2082-(aq) 12(aq) + 25042-(aq) (All potassium and sodium ions are spectator ions) This reaction happens instantaneously. For this reason during the experiment a delaying mechanism has been used, in order to delay the formation of the blue-black starch complex. Doing so it has been made possible to measure the time taken by the reaction and so the rate of reaction.
The delaying substance used was sodium thiosulphate . Indeed sodium thiousulphate was added, in additional to potassium iodide and sodium peroxodisulphate, to reconvert iodine in iodide, as in the following ionic equation: 12(aq) + 252032-(aq) T-(aq) + 25042-(aq) When all thiosulphate will be exhausted, iodine will then be free to form the blue-black complex with the starch added at the beginning Hypothesis According to the Collision theory , the reacting particles involved need three things to react: They must collide, there must be physical contact They must have the correct mutual orientation The reacting particles must have sufficient kinetic energy; their energy must overcome the activation energy Reducing or increasing the presence of a reagent we can therefore affect the effective collisions number, and so the rate of reaction.
On the other hand, changing temperature we should be able to see a change in the time taken for the reaction, and so in the rate of the same reaction. In this experiment, the effects of concentration and temperature are studied singularly, or rather changing only one time by time and analysing the different effects they have. Specifically for the iodine clock reaction, the concentration of iodide ions should, following this theory, affect the rate of conversion of iodide to jodine: indeed, as the concentration decrease the number of particle per volume decreases, and so the possibility of collision. The other variables will be kept constant, so to be able to isolate the effect only due to the change in the concentration of iodide ions.
Reducing or increasing its concentration would increase or decrease the probability of collision between iodide and peroxodisulphate, and by extension also the number of effective collisions. As the number of effective collision decreases, the time taken by the reaction would increase, and so the rate of reaction would decrease. On the other hand, changing temperature the number of collisions would change, but also the effectiveness of them would be affected. Indeed, increasing temperature the number of collisions would increase, because the particles are more excited. Like what happened changing concentration, increasing temperature the rate of reaction should increase.
However, changing temperature we are also affecting the kinetic energy of each particle: increasing temperature more particles will have enough energy to start the reaction and so more collisions will be effective. Changing temperature would affect more the reaction than changing the concentration of iodide ions, because it would affect the number of collision, but also the effectiveness of them. Maxwell-Boltzmann energy distribution and temperature VARIABLES Independent variables : Concentration of Kl solution used, temperature Dependent variables : Time taken for formation of blue starch solution, rate of reaction Fixed variables : Volume of sodium peroxodisulphate Na2S203 solution used, volume of starch solution used, volume of iodidel- used CONTROL OF VARIABLES Concentration of iodide solution Potassium ions are spectator ions in this reaction, so the concentration of iodide solution is equivalent to the concentration of potassium iodide Kl.
From a standard Kl 1.0 mol dm-3 solution used as stock solution, a diluition will be made to obtain solutuions with different concentrations (0.8, 0.4, 0.2 and 0.1 mol dm-3) Time taken for blue-black solution to form The time for the reaction to happen, for the formation of the blue-back solution, will be taken using a digital stop watch, so to reduce uncertainty. It is the dependent variable. For each concentration the experiment will be repeated three time, reducing random errors Temperature Thanks to a water bath, the temperature at which the reaction happens will be changed. The experiment will be repeated for 6 different temperatures, 20, 30, 35, 40, 45 and 50 °C.
With concentration, this is the other independent variable. Those two will be changed one by time, not together Rate of reaction It is the reciprocal of time taken for formation of blue-black solution. Given that there are three different times for any concentration and any temperature, it will be the reciprocal of the average time taken. Volume of sodium peroxodisulphate Na2S208 solution used It will be kept fixed at 5 cm3 throughout the experiment. Volume of sodium thiosulphate Na2S203 solution used It will be kept fixed at 2 cm3 throughout the experiment. Volume of starch solution used It will be kept fixed at 5 drops throughout the experiment. Volume of iodide |- solution used It will be kept fixed at 10cm3 throughout the experiment. Volume and concentration are related by the equation n=MV/ 100 where Mis the concentration, V the volume and n is amount of solute. Given that the varying variable is the concentration is the iodide solution, the volume of iodide solution must be kept constant.
Apparatus/ Materials Pipette (25.00+0.03 cm3), pipette (10.00+0.02 cm3), pipette (5.00+0.001 cm3), burette (50.00+0.05 cm3), micropipette (1.00+0.003 cm3), digital stop watch (+0.01s ???), test tubes, beakers, test tube rack, volumetric flask, , retort stand with clamp, distilled water, pipette filler, spatula, dropper, water bath, thermometer, standard potassium iodide Kl 1.0 mol dm-3 solution, sodium thiosulphate Na2S203 0.05 mol dm-3 solution, sodium peroxodisulphate Na2S208 0.04 mol dm-3 solution and starch solution Experimental procedures Preparation of chemical solutions n(A)=(m( A))/(M(A)) , therefore m(A) To prepare 500 cm3 of standard potassium iodide Kl 1.0 mol dm-3, 83.000g is needed. The mass of KI was weighted and then diluited with distilled water to produce 500 cm3 of standard KI 1.0 mol dm-3 solution A dilution was then made, with Kl solution as the stock solution. For concentration 0.1 mol dm-3, 10 cm3 of stock solution was transferred in a 100 cm3 beaker using a burette. With a different burette, 90 cm3ofdistilled water was added. For concentration 0.2, 0.4 and 0.8 mol dm-3, the stock to distilled water ratios are 2:8, 4:6, and 8:2 respectively.
To prepare Na2S203 0.05 mol dm-3 solution, 1.977 g of Na2S203 was needed. The mass of Na2S203 was weighted on the balance and then transferred and diluted in a 250 cm3 volumetric flask. Distilled water was added drop-wise till reaching the 250 cm3 mark To prepare Na2S208 0.04 mol dm-3 solution, 2.381g of Na2S208 was needed. The mass of Na2S208 was weighted on the balance and then transferred and diluted in a 250 cm3 volumetric flask. Distilled water was added drop-wise till reaching the 250 cm3 mark. The lodine Clock Reaction Changing the concentration of iodide ions. Three test tubes were labelled 1, 2, and 3 respectively.
With a pipette (10.00+0.02 cm3), 10 cm3 of standard potassium iodide Kl 1.0 mol dm-3 solution was moved into test tube 1. Using a pipette (5.00+0.01 cm3), 5 cm3 of sodium peroxodisulphate Na2S208 0.04 mol dm-3 solution was moved into test tube 2. Using a micropipette (1.00+0.003 cm3), 1 cm3 of sodium thiosulphate Na2S203 0.05 mol dm-3 was moved into test tube 3. This should act as a delaying mechanism for the reaction. Solutions from test tube 1 and 3 were mixed together in a conical flask, and 5 drops of starch were added too. This solution was added to allow the formation of blue-black complex when iodine would have been added. The mixture in the flask was shuffled to create a homogeneous mixture. The solution from test 2 was transferred into the flask and the digital stop watch was started.
The mixture was again swirled for almost 10 seconds. Once the blue-solution was formed, the stop watch was stopped and the time taken was recorded in a table. The experiment was then repeated two more time, so to obtain three readings Step 1 till 7 were repeated using different concentrations of iodine solution in the order of 0.8, 0.4, 0.2 and 0.1 mol dm-3 Changing the temperature Step 1 till 8 were repeated using always potassium iodide Kl 0.4 mol dm-3 solution, because the most suitable concentration. However this time the reactants were mixed in a boiling tube, and this one was immersed in a water bath at the temperature of 30°C. Once the blue-black solution was formed and the
