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Concentration and Reaction Rate.

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Introduction

Concentration and Reaction Rate The aim of this investigation is to see how the concentration of a reactant in ratio to the reactor affects the rate of a reaction. When hydrochloric acid and Thiosulphate react together sulphur is liberated this means that as the reaction goes on the solution will become yellow and will change from being transparent to translucent to opaque. The sulphur is formed as a solid but not in the usual precipitation way. Na2 + S2 + 2HCL 2NaCl+SO2 + S2 +H2O To time the reaction I will draw a black cross on a piece of plain paper on which the beaker of reactants will be placed (HCL and Thiosulphate). When the chemicals come into contact with each other I will start timing with a stopwatch and will stop timing when the cross is longer visible through the beaker from above. A reaction: A chemical reaction between to chemicals can only happen if their molecules can collide into each other. Out of many collisions there will be a few successful collisions, which means that the two molecules will exchange electrons and that means that they have reacted. These molecules have to hit each other in the right direction and at the right speed; in short the rules for a "successful collision" are specific and complex. But if the number of collisions per second increase so will the number of successful collisions increase. This means that the rate of the reaction has increased. For a reaction to occur you also need the required activation energy which means that if there isn't enough the reaction won't take place although catalysts can lower this. Input Variables: Catalyst Concentration of acid or thiosulphate Temperature Light Temperature: If you increase the amount of energy in a group of molecules the reaction rate will increase. When you give energy to molecules they tend to move about a bit more. ...read more.

Middle

I think it would be suitable to make5 observations with the following constituencies of HCL and thiosulphate. HCL volume in cm cubed Volume of H20 in cm cubed Thiosulphate volume in cm cubed 10 0 50 10 5 45 10 20 30 10 30 20 10 40 10 Precision and accuracy: Unreliable evidence is evidence that is faulty or was obtained by a faulty method. There are various ways to prevent this: Do not look at the meniscus sides when measuring a liquid, look at the plain in the middle. Do check readings so any anomalous results will become obvious. Refrain from contaminating either chemical with another. Keep head always at same height just before deciding when to stop timing, and always use the same pair of eyes, and keeping on any glasses. Fair test. I will use separate measuring cylinders for both chemicals so I don't contaminate either of them until I'm ready to time the reaction. Because I' am (always) measuring 10ml of HCL I will use a small measuring cylinder, i.e. 100ml so I can accurately identify where the 10ml mark is. I will always use my eyes to tell me when to stop timing because the quality of eyes varies from person to person. For my big measurements of liquid I will use a large measuring cylinder. I will not introduce a catalyst to the experiment neither will I increase the temperature of the liquids by holding their containers too long, and I will keep the area's temperature in which I'm working in constant. To be on the safe side I will try to keep the light intensity the same throughout the entire experiment by working in the shade. Prediction: I predict that as concentration of thiosulphate increases so will reaction rate. Concentration of a reactant affects reaction rate because there are more frequent successful collisions. Assume a person has two chemicals x and y, which react when put together. ...read more.

Conclusion

When looking at my graph I don't see any anomalous results because they are all very close to my line of best fit, and my method is quite reliable as shall be explained later. I think the method could have been changed to a far more suitable one as seen below: During this set-up the room would be kept dark so as not to interfere with the LDR. The light would shine through the reactants onto the LDR. As the test goes on less light would fall on the LDR, decreasing the resistance through the component. This should be quite noticeable on the voltmeter which would be set on an appropriate level found whilst doing preliminary, at a chosen resistance the person may stop timing. This method would be a lot more accurate than the as it takes out human error by itself judging when you should timing. I think I read my measurements to a good degree of accuracy because I made sure I used the right sized measuring cylinder for each liquid, so I had a small scale for small amounts etc. etc. I also made sure I was level with meniscus whilst measuring amounts of liquid. I don't think there was a large enough range of results taken, it would have been useful (when plotted) to test reaction rate whilst varying concentration of HCL. It would be interesting to see if it showed the same pattern of results as my thiosulphate did. If there was a larger range of results then I could have thoroughly observed and different patterns to ones I got. Perhaps I should have got a meter rule and made sure my head was always at the same height, this is because as your height increases away from the beaker the cross becomes less visible, which means you stop timing a little earlier. Chemistry Coursework - 10IW Robert Smith ...read more.

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