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Rates of Reaction

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Science Course Work - How does the dilution of Hydrochloric acid affect the rate of reaction between itself and Sodium Thiosulphate? Aim: To see how changing the concentration of Hydrochloric Acid affects the rate of reaction. Prediction: In this experiment I predict that the more diluted the acid is, the longer it takes for it to react. For example, If 2 molar HCL added to X molar Na2S2O3 took 40sec to react, 1 molar HCL added to X molar Na2S2O3 will take 80sec to react. This is my prediction of the results table (not necessary the times, but the pattern of the times and rate) HCL(cm3) H20(cm3) Na2S2O3(cm3) Time (sec) Rate (sec) (1/X) (2mol/dm3) 25(from bottle A) 0 25 40 0.025 (1mol/dm3) 25(from bottle B) 0 25 80 0.0125 (0.5mol/dm3) 12.5(from bottle B) 12.5 25 160 0.00625 (0.25mol/dm3) 6.25(from bottle B) 18.75 25 320 0.003125 (0.125mol/dm3) 3.125(from bottle B) 21.875 25 640 0.0015625 I believe this will happen because each time the acid becomes more diluted, the amount of acid particles is halved. Therefore if there are half of the acid particles to react with, the time must be doubled because it will take twice as long for the Sodium Thiosulphate particles to collide with the Hydrochloric acid particles. ...read more.


100 0 1(from bottle B) 100 0 0.5 (from bottle B) 50 50 0.25(from bottle B) 25 75 0.125(from bottle B) 12.5 87.5 3. After making all of the dilutions, wash equipment with distilled water; place the black cross underneath a clean beaker. 4. Add both liquids (following the measurement table below) to the beaker and time how long it takes until the cross cannot be seen. Make dilutions for 1.75, 1.25 0.75, this will allow for results to be even more reliable so we can check in between the acid dilutions as I original predicted. (Xmol/dm3) HCL(cm3) +_ H20(cm3) Na2S2O3(cm3) (2mol/dm3) 25 25 (1.75mol/dm3) 25 25 (1.5mol/dm3) 25 25 (1mol/dm3) 25 25 (0.75mol/dm3) 25 25 (0.5mol/dm3) 25 25 (0.25mol/dm3) 25 25 (0.125mol/dm3) 25 25 5. Repeat each dilution three times to get an average. Fair Test: * Na2S2O3 25cm3 each time. * Acid strength varied but volume stayed the same. * Cleaned all the equipment after use, to avoid contamination. * Same temperature each time. * No catalyst used in any reaction. Results: HCL(cm3) H20(cm3) Na2S2O3(cm3) Time (sec) ...read more.


* Contaminated glass wear * Wrong dilutions * No successful collisions with the particles for a long time. I didn't really get any odd results on my experiments, only on my averages. Now moving on to the rates, and how the concentration affected them... The pink line shows my prediction, seeing as my prediction was based on maths and halving the rate each time, it will appear to be straight. But the direction of the lines is both sloping downwards. Although they don't have the same gradient they both share the same principal...The more diluted the acid is the slower the reaction, the lower the rate. Evaluation: Over all I think the experiment went quite well, although I would improve it in a few ways... * Instead of using the "watching the cross" disappear method, use a light meter, that way its more accurate as to when an amount of a reaction has taken place. * Instead of just washing the equipment out with distilled water each time, get a new clean beaker for the reaction to take place in, this will avoid any left over from the last experiment. * Instead of using a measuring cylinder use a burette or a pipette to get really accurate measurements. ...read more.

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This is a straight forward practical task but does require good precision particularly when making stock acid dilutions. The equipment selected is basic and only collects a limited amount of data. There is a lack of preliminary work used to adapt the apparatus to ensure the data is of a high quality. Improvement to specific parts of the report have been highlighted throughout.

Marked by teacher Cornelia Bruce 18/03/2013

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