Reaction Between Persulphate And Iodide Ions Biology Essay

Reaction Between Persulphate And Iodide Ions Biology Essay Persulphate (S2O82-) is reduced to sulphate (SO42-) in presence of iodide ions (I-) in aqueous solution. While persulphate is being reduced, iodide is being oxidised to iodine. The overall reaction equation being: S2O82- + 2I- à ¢Ã¢â‚¬  Ã¢â‚¬â„¢ 2SO42- + I2 On the other hand, iodine can be converted to iodide in presence of thiosulphate (S2O32-) as follows: I2 + 2S2O32- à ¢Ã¢â‚¬  Ã¢â‚¬â„¢ S4O62- + 2I- If iodine indicator is added to the solution a blue colour of the starch-based iodine indicator complex develops after all thiosulphate has been consumed. This is because the starch-based iodine indicator complex is very stable in high concentrations of iodine. The charges transfers and the energy level spacings in the resulting complex correspond to absorptions in the visible part of the spectrum blue. The strength of the resulting blue colour depends on the amount of iodine present. Therefore, the development of the blue colour is useful as it indicates a constant point in the progress of the reaction which can be taken as being representative of the end of the reaction (Practical Booklet p.37). Several kinetic parameters can be easily analysed and determined if those reactions are timed as they progress from start to end, hence the importance of the component time (t) in kinetic studies. By adding a known amount of thiosulphate and iodine indicator, the reaction rate can be determined as it obeys the following rate equation: AIMS This experiment was undertaken to determine the reaction rate, the order of reaction with respect to both iodide and persulphate, and the effect of variation in ionic strength on the rate constant of this reaction. EXPERIMENTAL Methods Procedures An exact copy of the methods provided in the practical booklet is attached to this document as no deviations were made from those given. RESULTS Reaction Time t (min:sec) Tim t (seconds) 1 19:32 1172 2 9:45 585 3 9:58 598 4 25:20 1520 5 33:30 2010 Table Timing of reaction from the exact time of mixing to the time of the blue colour appearance Treatment of results Reaction Solution A Solution B ( + 0.5g iodine indicator) [KI] (mol cm-3) [Na2S2O3] (mol cm-3) [KCl] (mol cm-3) [K2S2O8] (mol cm-3) H2O (cm3) 1 0.05 0.0005 0.100 0.004 25.0 2 0.10 0.0005 0.050 0.004 22.5 3 0.05 0.0005 0.088 0.008 15.6 4 0.05 0.0005 0.050 0.004 27.5 5 0.05 0.0005 0.000 0.004 30.0 Table Concentrations of all substances in the final reaction mixture and water volume to account for dilution to a 50cm-3 final volume. From the rate equation of this reaction: Interpreting the above results, doubling [I-] while keeping [S2O82-] constant doubles the rate of reaction halves t (time of appearance of the blue colour). Doubling [S2O82-] while keeping [I-] constant, also doubles the rate of reaction. The below equation gives the relationship of the variation of rate constant (k) with ionic strength (I): This equation is of type , where k0 is the limiting value of the rate constant at zero ionic strength. where zA and zB are the charges of the two ions involved in the rate-determining step at low ionic strengths. Since [I-] remains constant during each kinetic run, the integrated rate equation is: where a and (a-x) represent [S2O82-] at times t = 0 and t = t respectively. Making k the subject: Hence, for reactions 1, 4 and 5: Per 2 moles of S2O32- consumed 1 mole of S2O82- oxidises iodide because any iodine produced is converted back to iodide until all thiosulphate is consumed. Therefore, adding a known amount of thiosulphate allows to determine the concentration of persulphate at the end of the reaction, when t=t. From 0.0005 mol cm-3 of S2O32- consumed, 0.0005/2 = 0.00025 mol cm-3 of S2O82- were consumed. Therefore the remaining persulphate at the end of reaction = 0.0040 0.00025 = 0.00375 mol cm-3 For reaction 2: For reaction 3: At the start of this run [S2O82-] = 0.00800 mol cm-3 and [S2O32-] = 0.0005 mol cm-3. At the end of the reaction 0.0005/2 = 0.00025 mol cm-3 of S2O82- were consumed. Hence, from the initial concentration of S2O82-: 0.00800 0.00025 = 0.00775 mol cm-3 of S2O82- were present at t=t. Rate Constant Determination The value of k can now be calculated for each mixture. The values were calculated and recorded in the table below: Reaction Rate Constant (k) / mol cm-3s-1 1 2 3 4 5 Table Values of k constant for each reaction mixture Calculation sample: For reaction 1: t = 1172 s Ionic Strength Determination The contribution of each electrolyte to the total ionic strength is given by: Taking into account the dilution of the final volume to 50 cm3 of all electrolytes, and knowing that for 1:1 electrolytes, I=concentration, and for 2:1 electrolytes, I= 3 x concentration. Hence, for these mixtures: The ionic strength was calculated and recorded in the table below: Reaction Ionic Strength (I) / mol cm-3 1 0.164 2 0.164 3 0.164 4 0.114 5 0.0635 Table Ionic Strength per each reaction Sample calculation: Calculation of B and k0 In order to determine B and k0 a plot of v.s. logk was plotted. Reaction k(mol cm-3s-1) Logk(mol cm-3s-1) I(mol cm-3) 1 -2.96 0.164 0.288 2 -2.96 0.164 0.288 3 -2.97 0.164 0.288 4 -3.07 0.114 0.252 5 -3.19 0.0635 0.201 Table k constant and ionic strength for each reaction Graph Correlation between logk and ionic strength The correlation is given as being y = 2.6305x 3.7229. Hence: logK0= -3.72 mol cm-3 s-1 k0= mol cm-3 s-1 k0= mol dm-3 s-1 B = 2.63 à ¢Ã‹â€ Ã¢â‚¬  Hà ¢Ã¢â€š ¬Ã‚ ¡ à ¢Ã‹â€ Ã¢â‚¬  Sà ¢Ã¢â€š ¬Ã‚ ¡ Calculation DISCUSSION (Questions) As stated above: (..) doubling [I-] while keeping [S2O82-] constant doubles the rate of reaction halves t (time of appearance of the blue colour). Doubling [S2O82-] while keeping [I-] constant, also doubles the rate of reaction. Hence, in the rate equation: x= 1 and y=1. The reaction is of first order with respect to and with respect to . As a result, the overall rate of reaction is x+y=2. The overall rate of reaction is of second order in which the rate determining step is: S2O82- + I- à   IS2O83- This is consistent with the rate observed experimentally by comparing mixtures 1 to 2 and 1 to 3. The value of B (2.63) is rather high in comparison with the theoretical value but of same magnitude. As stated above, B is given by where zA and zB are the charges of the two ions involved in the rate-determining step. Hence, theoretically . The value of k0 determined experimentally is rather low in comparison to the published value. k0= mol dm-3 s-1 Published value: mol dm-3 s-1 Some source of error must have influenced this result. Perhaps wrong timing and contaminated glassware would allow a rather faster/slower reaction which results in a different rate constant (k). (Answer to Q1 above.) The ionic strength is the same for reactions 1, 2 and 3. Reaction 4 shows a characteristic ionic strength for the concentrations of ions present in the mixture (Table 2). Accordingly, reaction 5 with no KCl added shows a rather low ionic strength as expected. A variation of Ionic strength vs. Rate constant is notable. With reference to table 5, a decrease in ionic strength accompanies a decrease in the rate constant the lower the ionic strength of a reaction the lower its rate constant (k). It is important to mention that the ionic strength of each mixture remains constant up to time t (appearance of blue colour). This is perfectly understandable because the rate of production of anions is equal to the rate of their expenditure, hence manifesting a constant ionic strength. However, when all thiosulphate has been consumed the ionic strength decreases because an interruption in their equilibrium causes the consumption of iodine to stop. In other words: When the reactions reaches t: S2O82- + 2I- à ¢Ã¢â‚¬  Ã¢â‚¬â„¢ 2SO42- + I2 I2 + 2S2O32- à ¢Ã¢â‚¬  Ã¢â‚¬â„¢ S4O62- + 2I- Thus, at t there is an increase in the amount of I2 and a drastic decrease in the amount of I-. Therefore: At t, which results in a low ionic strength. (Answer to Q2 above) For this experiment, the value of calculated is rather low but positive. All systems tend to progress in a direction of increasing entropy and therefore an increase in the disorder of the system in question. For this bimolecular reaction in solution at 25 °C, the entropy of activation of the rate determining step is positive meaning the final starch-based iodine indicator complex is rather stable because increases in entropy correspond to irreversible changes in a system. This is because the amount of work the system can do is limited because most of the energy was wasted as heat; therefore this reaction is thermodynamically irreversible reaction. (Answer to Q3 above) A reaction which proceeds more slowly with increasing ionic strength is CONCLUSION Aims were accomplished. All kinetic parameters were determined although some major sources of error were present during this experiment. Possible sources of error may include difficult or inappropriate timing (non-digital watches were used). Difficulty to quickly detect the end point of the reaction (development of blue colour) as the rate of reaction to some of the mixtures was very slow; contamination of glassware could also contribute for such errors. Nevertheless, the order of reaction with respect to both iodide and persulphate was determined precisely; and the effect of decreasing ionic strength is known to decrease the rate of reaction.

Jobless

People work hard to get better lives and to accomplish what they want after graduating high school or college. However, they face some difficulties in finding a job. Unlikely, â€Å"Ben† (‘Young†) would not want to be an â€Å"independent adult† (A. 25). As the rate of youth unemployment is rising, many countries suffer from how to deal this unwelcome circumstance. According to statistic of Margaret’s article, â€Å"In Canada, the jobless rate for young adults is a relatively low 14 per cent. Across the European Union, the jobless rate is higher than 20 per cent.In the U. S. , the jobless rate for high-school-educated men between 20 and 24 – Ben's generation – has reached 22. 4 per cent† (A25). If the number of jobless people increases, many problems will arise in society. First, youth power could be wasted, and countries could lose human resources. Many jobs require capacity from men for some works that women could not handle. Ho wever, some people may argue that women could handle those jobs and there should be no gender discrimination.Although there is less sexual discrimination in work places, discrimination in hiring still exists. Jobs of women are â€Å"much more likely to be part-time, contract work, working through a temporary help agency, or working on their own† (301). Women also have to consider â€Å"child care† that could be â€Å"limits women’s choice of paid employment† (301). Thus, men would be widely used in many jobs for society. Generally, in terms of physical condition, young men are stronger than women so labor and some jobs require physical strength from men.For instance, young women work at building construction; they could have difficulties to carry heavy materials. Consequently, progress of constructing the building could be slower because of lack of strength. The slow progression could create that the time and money be wasted. Secondly, people without work could be effect on the society. Not working is associated in homelessness, criminal problem and social welfare. Unemployment could be homeless because they do not have enough money to buy or rent the house.Homeless people sometimes commit suicide because they cannot afford to buy basic needs such as food, clothing and shelter. It is hard to maintain those requisites if they do not work and there is no financial supports. Poverty from unemployment also could impact on their children. Children who grow upon poverty later suffer from more persistent and several health problems than children who grow up under better financial circumstances. Children raised in poverty also tend to miss school more often, so they could not have a high quality of education.When these children grow up, it is hard for them to find a better career and they could be jobless again as their parents who were unemployment as well. Moreover, people without work make the criminal rate increase because joblessness a nd homeless could be a major motivator in theft, burglary and violent crimes in order to get money easily. A result of criminal from unemployment may bring that our social security is jeopardized. Lastly, there may be less social welfare for children and disabled people because young men without work have not enough money to pay taxes.The result of fewer taxes could make government’s finance decrease and then government could not support the social welfare. Of course, people would not receive the better social welfare services. Finally, being without work could affect their individual life such as relationship, confidence and health. If they go out and talk with friends or other people who are working, they could not communicate with them because they do not have work experience and do not understand words that worker used.Thus, they will not be likely to go out and interact with other people, and prefer to stay alone at home. Not moving out could make many unemployment not w ant to get married because youth unemployment could be â€Å"trapped in a twilight world of failure to achieve adulthood† (â€Å"Young†). Indeed, they would have the lack of relationship with people. By not working, people also may begin to doubt their sense of self and abilities, be depressed and not have confidence. The result of emotional effects may give unemployment a huge stress.Suffering from stress is known as to have physical effects on a human body; therefore, they find doctor and do drugs more. Certainly, their health may be at risk. In conclusion, there is a problem that not only youth power but also time and money could be wasted if the unemployment rate keeps rising. In terms of society, people without work could be homeless and a cause of increasing criminal. They also may lead to lack of social welfare services. In terms of each unemployed individual, they may have a low interaction with other people and communication skills.By being jobless, people cou ld have not only emotional effects such as self-esteem, lack of confidence, depression and anxiety, but also physical effects from the stress. Thus, young unemployed people may be the problems to be ironed out in the world. Works – cited Townson, Monica. â€Å"Canadian women on Their Own Are the Poorest of the Poor. † Writing: A Journey. Ed. Lester Faigley and Melanie Rubers. Pearson, 2010. 300-301. Print. â€Å"Young men without work. †Ã‚  Globe ; Mail  [Toronto, Canada] 10 Nov. 2011: A25. Gale Opposing Viewpoints In Context. Web. 22 July 2012.

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