In liquids the particles are constantly moving and so constantly colliding with each other. They will normally always react when they collide with enough energy. The greater the number of reactant particles means the collisions between these and the particles of hydrochloric acid would be more frequent. If there were less than there would be more of the non- reactant particles (water molecules) colliding and therefore not reacting. The more of these reactant particles there are, the more space they take up, so the chance of the hydrochloric acid particles colliding and reacting with them, and not the non- reactant particles, becomes higher.
I am also predicting that as I double the concentration, the rate of reaction will double. If I double the concentration, than the number of reactant particles in the solution would double, so therefore the chances of collisions between these reactant particles and the hydrochloric acid particles would double, and so the reaction rate should double. If the concentration of sodium thiosulphate is 0 than the rate of reaction has to be 0 because there are no reactant particles, this means that in my graph the best fit line will have to pass through the origin. If my prediction that as the concentration doubles, the rate of reaction doubles is correct and the line also passes through the origin then the rate of reaction is directly proportional to the concentration.
My prediction basically is that as the concentration increases, the reaction rate increases. However more precisely, I am predicting that doubling the concentration of the reactant will result in the rate of reaction doubling.
RESULTS:
CONCLUSION: My suggested values are suitable because none of the reactions were too quick to measure, nor were they too slow. When I do my main experiment I know I can pick concentrations within the range above (10 – 50), because none of the reactions between this range will be too fast or too slow.
The selected apparatus for the preliminary experiment was appropriate. They allowed me to do the experiment quite quickly yet at a fairly accurate standard. Measuring cylinders are not the most accurate form of measuring; however they were accurate enough for this experiment. The main experiment does have to be much more accurate. That is why I will be using burettes instead of measuring cylinder to measure my solutions. They will allow me to be very accurate and hopefully obtain more accurate results. In the preliminary the results didn’t have to be so accurate, because their aim was only to tell me about how long the reactions will take. Also burettes take longer to use and in the preliminary, I wanted to work very quickly so the measuring cylinders were more appropriate.
Another change I am going to make is that I am going to obviously take readings for more concentrations of sodium thiosulphate, but I am also going to repeat each reading three times. This will allow me to then take an average reading, which will make my results even more accurate.
Main Experiment (variable 1)
APPARATUS:
- Safety Glasses
- Conical Flask
- Burette
- Stopwatch
- Piece of Paper (for measuring when reaction is completed)
- Dilute Hydrochloric Acid
- Solution of Sodium Thiosulphate
- Retort Stand and Clamp
- Water
Prediction (variable 2)
I believe that as I increase the temperature of sodium thiosulphate and hydrochloric acid, the rate of reaction will increase. In liquids the particles are constantly moving and so constantly colliding with each other. They will normally always react when they collide with enough energy. The minimum amount of energy needed in a collision for the particles involved to react is called the Activation Energy. It is needed to break the bonds. If the particles collide with insufficient energy, the particles simply bounce off each other. If every collision occurred with energy, at least that of the activation energy, then the reaction would be extremely fast.
As you increase the temperature, the reactant particles gain more kinetic energy and start to move faster. This means that there is a greater chance of collisions between the particles, since they are moving about quicker in the same volume of solution. Also a greater proportion of the particles acquire the activation energy as you increase the temperature, so more particles are able to collide and definitely react. This will surely cause the rate of reaction to increase. On the whole there will be more collisions at a quicker speed which will increase the reaction rate.
I am predicting that as you increase the temperature, the rate of reaction will increase which indicates that they both are directly proportional to each other.
Analysing Evidence
Simple Conclusion
I noticed that the concentration is directly proportional to the reaction rate, because as I increased the concentration of sodium thiosulphate, the reaction rate increased, and when I decreased the concentration of sodium thiosulphate, the reaction rate decreased.
Identification of Trends
My graph shows that as the concentration increases, the rate of reaction increases. The line of best fit on the graph shows that the results go up steadily. It also passes through the origin, which indicates that the rate of reaction has to be directly proportional to the concentration. The graph shows that as the concentration doubles, the rate approximately doubles. For example, as the concentration doubles from 0.03 to 0.06, the rate according to the graph more or less doubles from 0.0055 to 0.011, which happens to be exactly double.
Explanation and Comparison with Prediction
I believe the rate increased as the concentration increased because there were more reactant particles in the same volume of solution and therefore there was a greater chance of collisions with these reactant particles. There were more of the reactant particles so they took up more space, so the chance of the hydrochloric acid particles colliding and reacting with them increased, leading to more collisions and a quicker rate of reaction.
I believe the rate doubled as the concentration doubled because the rate of reaction is proportional to the concentration of sodium thiosulphate. When the concentration doubled, the number of reactant particles in the solution doubled, so therefore the chances of collisions between the reactant particles and the hydrochloric acid particles doubled, and so the reaction rate doubled.
The results agreed with my predictions, which is why I got a straight line through the origin. Apart from one result, the results did give me a clear pattern which supports my prediction. The one anomalous result which does not support my prediction is circled in orange on my graph. This result is for when the concentration of sodium thiosulphate was 0.15 mol dm3, and it is anomalous because it is just about above experimental error.
Evaluating Evidence
Simple Evaluation
My method did work because I did collect a good set of results. Apart from one result, all my results are very close to the best fit line. This shows that they were pretty consistent and therefore I collected the results accurately. Therefore my method was good because following its instructions I managed to collect accurate results which gave a clear pattern that agreed with my prediction.
Anomalous Results
I have only one result which was just about outside experimental error. On the graph it is circled in orange. It was for when the concentration of sodium thiosulphate was 0.15 mol dm3, and the rate of reaction for it was 0.0294 s-1. The line of best fit suggests the rate of reaction should have been about 0.0275 s-1. This means it was only 0.019 s-1 above what it ‘should’ have been.
The result could have been anomalous because of a few reasons. One reason for the error could be that I didn’t measure the concentrations accurately enough. It could be that I stopped the stopwatch too soon because I decided that I couldn’t see the cross too early. It could be that only one result was wrong which affected my average. I believe it was just a combination of these reasons which caused the anomalous result.
Accuracy and Reliability
Like I have said, there was only one anomalous result, and generally my results were reliable enough to support a conclusion. They were accurate because the rest of the results were very close to the best fit line.
I took a lot of measurements, six different concentrations and three readings for each. If I had taken results for three concentrations then the line of best fit would have to pass through three points, and so could have done so without being extremely accurate. But if the line of best fit manages to pass though many points then it means the results must be very accurate. My results for five out of the six concentrations were very close to the best fit line, so they must be accurate. It is very unlikely that it is just coincidence because I did get quite a lot of accurate results.
By doing two repetitions I managed to get an average and reduce the impact of an anomalous result. My repetitions were very similar to each other (when the concentration was 0.075 mol dm3 the reaction times were 72s, 73s, and 73s), which is again proof that my results are accurate and reliable.
Improvements
There are improvements I could make to my experiment which would give even more accurate results. I could:
- Take more results by doing more concentrations and more repetitions. The more I do, the more accurate my results become, but I wouldn’t be able to do lots more because there is a time limit to this experiment.
- Take the results to more decimal places, since this would make my graph more accurate. However, if I do lots of decimal places it would be difficult to plot these on a graph of reasonable size.
- Put the conical flask in a water bath, to keep the temperature constant. The one other variable apart from concentration which can affect the rate of reaction of solutions is temperature. I did the experiment at room temperature which is about constant. However, to make the experiment even more accurate I could put each conical flask in a water bath before I add the two solutions to each other. I could put the conical flask in a water bath at around 20°C, and I would do this before each reaction to make sure it stays constant. It must be 20°C every time so that the temperature is constant, and I know that temperature is not affecting the rate of reaction.
