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# Concentration and rates of reaction - &quot;Disappearing cross&quot; experiment.

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Introduction

Concentration and rates of reaction - "Disappearing cross" experiment Aim I am investigating the effect of the concentration of Hydrochloric Acid on the speed of the reaction between Sodium Thiosulphate and Hydrochloric Acid. When these react together, sulphur is formed. A balanced chemical equation for the reaction is... Na2S2O3 + 2HCl --> S + 2NaCl + SO2 + H20 Apparatus * Total of 10cm3 of distilled H20 * Total of 15cm3 of HCl * Total of 125cm3 of Na2S2O3 * Paper with cross - to see how long it takes for cross to disappear under flask * Conical flask - to hold solutions needed for experiment * 2 measuring cylinders - to measure out correct amounts of solutions * 2 beakers and 2 pipettes - to contain solutions before going into the conical flask. The pipettes are for accurate measuring of the solutions. Method 1. I will put a big black cross on a piece of paper, the size of the bottom of the conical flask. 2. The conical flask will be put on the paper. 3. 25cm3 of Sodium thiosulphate will be added to the conical flask. 4. Added to the Na2S2O3 (Sodium Thiosulphate) ...read more.

Middle

This would be unfair to the experiment. * I am changing the concentration, which means the temperature will have to stay the same. If the temperature was to change, the experiment would be unfair. With colder water, the particles move slowly and do not collide very often. If you increased the temperature, the particles would begin to move faster with greater energy. They will collide more often with many more collisions being successful than before. This is because temperature affects the amount of kinetic energy the particles have and therefore the reaction time. Results 0.05M of Sodium thiosulphate 2M of Hydrochloric acid 60cm3 5cm3 Volume of Thiosulphate - cm3 Volume of water - cm3 Time taken for X to disappear - seconds Average time taken - seconds 1st test Repeat 1 Repeat 2 60 0 132.6 130.8 129.6 131 50 10 153 153.6 151.8 153 30 30 262.2 259.8 252.6 258 20 40 480.6 440.4 441.6 454 15 45 633 622.2 610.8 622 10 50 1147.2 1092 1269 1169 All averages are rounded to nearest whole number The averages are plotted on graph (1) overleaf Concentration/M Rate/ S-1 x 1000 0.05 7.6 0.04 6.5 0.025 3.8 0.017 2.2 0.0125 1.6 0.008 0.8 These results are plotted on graph (2) ...read more.

Conclusion

Evaluation My method worked well because I was able to get a set of results at the end. However, there were a few problems. On graph (1) there was an anomalous result, and because we did the experiment over two days, we could not use the same apparatus as the day before. The stopwatch however was the same, so we were able to get as accurate results as the previous day. Graph (2) does not show any anomalies. Graph (1) shows a point that doesn't fit the pattern, as does the second point of graph (2). Even though the graph does join up fairly smoothly, the point is slightly out. This point has been circled on the graph. To make sure there are no anomalous results next time, you will need to follow the method and equipment list fully, and make sure the flask and beakers are cleaned out and dried before a new experiment is taken out. If done again, I would change the temperature rather than the acid concentration. When sodium thiosulphate and Hydrochloric acid are reacted together, Sulphur is formed. The temperature increase would make the Sulphur form faster. The ions would have more kinetic energy so would collide more often and more vigorously and so there is a greater chance that they will react. ...read more.

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