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275cm3 of Sodium Thiosulphate
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275cm3 of water
- 3 Beakers
- Measuring Cylinders
- Conical Flask
- Stopwatch
- Thermometer
- Black cross
Preliminary Work -
Prior to my experiment I found out which amounts of Hydrochloric Acid and sodium Thiosulphate I would need. I worked out measurements so that the reaction was not too slow and not too fast. I worked out that 20 cm3 of Hcl with 40cm3 of Sodium Thiosulphate and 10 cm3 of water would make a reaction time of 35 seconds. Working from this I made the other concentrations take longer than this but shorter than 5 minutes.
Prediction I predict that as the concentration of sodium thiosulphate increases the faster the reaction will take place. So the less water you add to the Sodium thiosulphate the faster it will react with the Hydrochloric acid to make sulphur and turn the solution cloudy. This is because as the Sodium Thiosulphate reacts with the hydrochloric acid, lots of particles are reacting with each other and have few water particles getting in the way of this reaction
fig 1a. In a weaker solution water particles get in the way of the reaction therefore slowing down the reaction. fig 1b . There are more sodium Thiosulphate and Hydrochloric acid In 1 ml of the solution in a high concentrate so more collisions occur per second, giving a faster reaction. Therefore there is a speeder reaction. When there are more water particles in 1 ml of the solution, there is less space for Sodium Thiosulphate so less collisions can take place in a second. Therefore you get a slower reaction.
If I double the amount of sodium thiosulphate There will be twice as many particles of sodium thiosulphate per 1ml and therefore there will be twice as many reactions per second so the reaction time will be halved.
If I half the amount of sodium thiosulphate There will be ½ as many particles of sodium thiosulphate per 1ml therefore there will be half as many collisions per second which would result in the reaction time being doubled.
Conclusion From my results and my best-fit graph you can see that as the concentration increased the rate of reaction increased. I found that this is because as the concentration decreases the more water particles there are per 1 ml. This means there is less room for the other chemicals so fewer reactions can take place in one second. If there is a high concentration there are less water particles per ml so there is more room for the Sodium Thiosulphate and Hydrochloric Acid to react. So more reactions can take place in one second. From my graph you can see that the line keeps on going up in a steady increase. This is because the more water particles there are in a solution the less volume of the reacting chemicals have. In a low concentrate there are any water particles which get in the way of the reaction between Sodium Thiosulphate and Hydrochloric Acid this means that the reaction cannot take place as fast hence it takes longer to produce sulphur and cover up the cross. In a high concentrate there are far fewer water particles to get in the way of the reaction between the Sodium Thiosulphate and Hydrochloric acid (Na2 s2 o3 2HCl SO2 S NaOH H2O) so more sulphur can be produced In a second so the cross can be covered up quicker and therefore it has a faster reaction time.
Evaluation I thought that this experiment went relatively well. I think I have conclusive evidence to show how concentration affects reaction rates. My graphs had no anomalous results and I have a nice best-fit line. In this experiment there could be a lot of human error e.g. looking through a solution and knowing when the solution is fully clouded could differ for each experiment. Measuring of solutions might not be very accurate, the time delay between the solution going cloudy and stopping the stopwatch could differ from differ.
I would use two different solutions such as using magnesium ribbon and different concentrates of sulphuric acid just to check if it’s not only that reaction that speeds up when there is a higher concentration.
