1 Cm 3 Starch Solution example essay topic

PLANNING Investigating the Kinetics of the reaction between Iodide ions and Peroxodisulphate (VI) ions By the use of an Iodine clock reaction I hope to obtain the length of time taken for Iodine ions (in potassium iodide) to react fully with Peroxodisulphate ions (in potassium Peroxodisulphate). I will do three sets of experiments changing first the concentration of iodide ions, then the concentration of Peroxodisulphate ions and finally the temperature of the solution in which the reaction is taking place. From these results, I hope to draw conclusions as to the effects of these changes to the environment of the reaction on the rate and also determine the order of the reaction and the activation enthalpy. Background information The rate of a reaction is determined by a number of factors. These include: pressure, temperature, concentration of reactants, surface area of reactants, presence of a catalyst and radiation. The effect of these factors can be explained using collision theory.

Reactions occur when the reactant particles collide, provided the colliding particles have enough energy for the reaction to take place. As the molecules approach their electron clouds repel. This requires energy - the minimum amount of which is called the 'activation enthalpy' - and comes from translational, vibrational, and rotational energy of each molecule. If there is enough energy available, this repulsion is overcome and the molecules get close enough for attractions between the molecules to cause a rearrangement of bonds and therefore an 'effective' reaction has taken place.

The more collisions of particles with kinetic energy over the activation enthalpy that occur, the faster the overall reaction. During this investigation I am focusing on the effect of temperature and concentration while aiming to maintain other rate determining factors at a constant level in order to ensure reliable results. Effect of concentration Taking the collision theory into account the effect of concentration is simple in that the more particles of the reactants there are in the same area of space the more likely the collisions and therefore the faster the overall reaction. The following equation has been determined through experimentation showing that the rate of a reaction depends on concentration of reactants A: Rate [A] nWhere n is a constant called the order of the reaction. This tells us the exact dependence of a rate of a reaction on the concentration. From this relationship the rate equation is obtained: Rate = k [A] nWhere k is a constant of proportionality called the rate constant.

Effect of Temperature basic law of physical chemistry is that an increase in temperature causes an increase in the rate of any reaction. As the collision theory states, for a reaction to take place the particles need to collide. If the temperature is increased, each particle has greater kinetic energy transferred from the heat energy, and therefore is moving faster (the average speed of molecules is proportional to the square root of the absolute temperature.) The faster the particles are moving, the more likely they are to collide and therefore the faster the reaction. Also, the more energy transferred to each particle due to increased temperature the more likely it is to surmount the activation enthalpy and again the higher the number of effective collisions. As a general rule, the rate of a reaction doubles for every increase of 10 K in temperature. The diagram below demonstrates the effect of temperature on the rate of a reaction.

Despite the initial increase in the energy of particles of a lower temperature, one can see that those at a higher temperature eventually surpass and lead to an overall higher amount of particles with energy higher than the activation enthalpy and therefore a greater number of effective collisions. The exact relationship between temperature and rate of reaction was first proposed by a Swedish chemist called Arrhenius in 1889, most famous for his self-named equation that stated: k = Ae -Ea / RTw here k is the rate constant for the reaction; A is a constant for the reaction; Ea is the activation enthalpy; R is the gas constant and T is the temperature in kelvin. In terms of log to the base 10 this is: log k = log A -Ea/ 2.303 RT Reaction between Iodine ions and ions 2 O 82- (aq) + 2 I- (aq) 2 SO 42- (aq) + I 2 (aq) In order to make the reaction clearer, during my experiment I will add starch and a small known amount of sodium (to act as a que ching agent). The ions turn iodine back to iodine ions: 2 S 2 O 32- (aq) + I 2 (aq) S 4 O 62- (aq) + 2 I- (aq) Which means that no starch-iodine colour will appear until all the has been used up. The amount of time taken for this occur (and the reaction to suddenly turn blue) is the same amount of time for the reaction to produce the amount of Iodine. Apparatus (For making up solutions) weighing boatsscalesBeaker (150 cm 3) 3 Volumetric flasks (250 cm 3) Distilled water Glass rod (for concentration and temperature change experiments) 4 thermometers (0-110 oC) A large number of boiling tubes (roughly 50 depending on repeats) 5 Burettes with funnels for filling 5 Clamp stands (for burettes) Stopwatch (for temperature change only) Two large beakers (400 cm 3) Chemicals Freshly made starch solution Pottasium Iodide (made to solution with conc.

1.00 mol dm-3) Pottasium (made to solution with conc. 0.0400 mol dm-3) Sodium Thiosulphate (made to solution with conc. 0.0100 mol dm-3) Each Solution made up in a 250 cm 3 volumetric flask (see method below) with the exception of the starch solutions which will be made each day of the experiment by the technician as the time I have to complete this project does not allow me to do this myself. Method Making up standard solutions 1.

Weigh out appropriate amount of chemical in weighing boat and record mass. 2. Tip contents into beaker and record weighing boat mass. Subtract this from original mass to give the actual mass of substance in solution 3. Add distilled water and stir using a glass rod until substance has fully dissolved.

4. Pour solution into volumetric flask through funnel and using distilled water rinse beaker, glass rod and funnel into the volumetric flask so that no solution remains on any apparatus used. 5. Add distilled water up to the mark It is important to ensure that all apparatus is rinsed otherwise the mass recorded of the amount of substance in the solution is inaccurate and the concentration is unknown.

From the mass recorded of each substance one is able to work out the exact concentration of the solution using the following equations: Mass (g) Moles. RMM = Moles Volume of solution (dm 3) = Concentration Rate of reaction experiment (concentration) 1. Fill three burettes using a funnel with each of the solutions and in the remaining two: water and freshly made starch solution. Ensure each funnel is removed and the burette reading can be taken easily at eye level. 2. Measure out mixture 1 (see table 1 below) into a boiling tube expect for the unvaried reactant (in the first set of experiments this is the K 2 S 2 O 8 (aq), in the second set its Na 2 S 2 O 3 (aq) ) which should be measured into a separate boiling tube.

3. Pour the unvaried reactant into the mixture and immediately start timing. Shake the mixture once vigorously to ensure thou rough mixing and record time taken for the solution to turn blue. 4. Repeat for each different mixture a minimum of three times, until results are within of 8% of each other. For example, if my results for a mixture are around 40 seconds, each repeat must be within 3 seconds of the others (3/40 = 7.5%) 5.

Record the temperature for each experiment Set Mixture Volume of KI (aq) (cm 3) Volume of Water (cm 3) Volume of Na 2 S 2 O 3 (aq) (cm 3) Volume of Starch sol. (cm 3) Volume of K 2 S 2 O 8 (aq) (cm 3) 10 2 4 2 1 1 Table 1 Rate of reaction experiment (temperature) 1. Fill a large beaker with hot water, adjusting the temperature by adding cold water if required (this is your water bath) 2. Using the volumes of Mixture 3 again measure out all the solutions and distilled water (expect K 2 S 2 O 8 (aq) ) into a boiling tube. 3. Measure out the K 2 S 2 O 8 (aq) into a separate boiling tube. Place both boiling tubes in the beaker.

4. Put one thermometer in each boiling tube and wait until both thermometries read the same temperature. Record this temperature. 5.

As in the previous set of experiments, quickly add the K 2 S 2 O 8 (aq) to the rest of the mixture and start the stopwatch. One vigorous shake should be enough to mix the reactants fully. 6. Record the time taken for the solution to turn blue. 7.

Repeat with varying temperatures of the water bath. And at least three repeats at each. Keep the thermo metre in the mixture throughout the reaction and note down any temperature changes (an average of which can be taken at the end). 8. (The remaining equipment can be used to set up another experiment during the lower temperatures so that one is able to have two experiment on the go at the same time (or at least warm up the solutions for the next set) in order to save time) JustificationQuantitiesThe concentrations of the solutions were suggested in a similar experiment found on a website (listed in references) and when considering the moles involved in the reactions the concentrations are appropriate. I have chosen to make up 250 cm 3 of each solution because this amount is manageable whilst allowing me numerous repeats for each experiment (which will be until I obtain results within 8%) and also allows for trials, errors and mistakes where I may have to start again.

Through calculations I have determined that when making 250 cm 3 of solution the smallest amount of solid to be added is 0.62 g of sodium. The scales used will allow me to measure this amount accurately so I do not feel the need to make up a larger volume of solution. The amount of each solution used in each set of concentration-focused experiments was determined through trial and error (although again I had come across suggestions from similar experiments on websites and in books). The chosen quantities allow use of a small amount of solution each time (to save wasting) yet still maintain a high level of accuracy. 1 cm 3 starch solution gives a very obvious dark blue whilst not effecting the occuring reactions. In a preliminary experiment I found that 1-5 cm 3 of the reactant I was varying (the remaining amount made up with water so that there was always the same volume in each individual experiment) meant that at lowest concentration the length of time taken for the solution to change colour was not too long to effect what I am able to achieve in the time allocated for my project (roughly 500 seconds for I- ions and 350 seconds for S 2 O 82-ions), and at highest concentration the colour change took long enough to record (roughly 40 seconds for I- ions and 60 seconds for S 2 O 82- ions).

The choice of using mixture 3 in the set of temperature-focused experiments was decided upon after the concentration experiments had been done. Form my results, I was able to see that a mixture of 3 cm 3 KI, 2 cm 3 distilled water (and the rest as normal) took on average 82.3 seconds to change colour at around 20^0 C. For every 10 K increase I expected the rate to half, and so at 50^0 C I expected a result of just over 10 seconds which is long enough to record accurately. Methods will use burettes to measure out the as this means the qualities are accurate +/-0.05 cm 3 compared to a measuring cylinder which can only measure quantities +/-0.1 cm 3. I have chosen to record three results for each different concentration / temperature and repeat until these three results are within 8% of each other. This means that at my highest concentration and therefore rate, there will be no more than 3 seconds between results, and at my lowest concentration there will be no more than 25 seconds between results. Although this seems like a large variant through preliminary experiments I have discovered that it is extremely difficult to obtain results that are within 20 seconds at the lower concentrations, and the amount of time I have would not allow me to continue repeating until this occurs.

A margin of 8%, therefore, I do not feel is inaccurate enough to supply me with unreliable results. The turning point (at which I stop the stopwatch) will be at the very first hint of blue colour. Through practice I could see that if not mixed fully the blue colour begins to form each at the surface or at the bottom, yet this is not the correct turning point as if shaken the colour disappears. Therefore, it is important to shake vigorously at the beginning and in longer experiments again at around 2 minute intervals so that the recants do not settle. Although text books have informed me that the reactions are only very very slightly exothermic and the overall of the solution will not change during the experiments, it is important to record the temperatures of every experiment and take into consideration how this may affect the results. Also, it is important to record the start and end of the solution during the temperature-focused experiments because higher temperature lose their heat more quickly and if the temperature drops during the time of the reactions this information has to be involved in finding the average temperature.

Predication sI predict that the higher the concentration of the reactants the higher the rate of the reaction. I predict that the higher the temperature of the reactants the higher the rate of the reaction. I predict that the order of the reaction with respect to I- ions will be 1 and the order of the reaction with respect to S 2 O 82-ions will also be 1 giving a overall order for the reaction of 2. This is information obtained from data books and backed up by information from the internet. (see references) I predict that the activation energy of the reaction will be 52.9 kJ mol-1. This information is taken from the ILIAC Resource Pack for Advanced Practical Chemistry and is for the reaction between iodide ions and ions.