The reason for this occurrence is because a precipitate of sulphur is formed as the substances react. The stop clock is terminated immediately when the cross is no longer perceptible. The time will then be recorded into a table, along with the results of the other experiment. In this reaction sodium thiosulphate reacts with hydrochloric acid forming salt, sulphur water and sulphur dioxide gas.
Sodium thiosulphate + hydrochloric acid → sodium chloride + sulphur + water + sulphur dioxide
Na2S2O3 + 2HCl → 2NaCl + S + H2O + SO2
Fair test
To make this experiment accurate and reliable, I will repeat it twice so that an average can be initiated. It provides me with an opportunity to distinguish any outcomes which look irregular and rectify them. The measurements have to be precise so there are no more particles than there should be, the stop clock must be started at exactly the same time the sodium thiosulphate is poured in to have an exact time. As I am only testing one variable, I will maintain the amount of sodium thiosulphate all the way through the experiment. The experiments will all be conducted in the same area at room temperature and will not be shaken so that substance particles do not get different amounts of kinetic energy and therefore be more or less active. I will use different pieces of paper and draw a cross on each one to prevent the fading of ink if the liquid smudges it. If the ink fades then an experiment does not need to react as much for the solution to conceal the cross. I have to make sure the liquid solution becomes completely opaque and then stop the timer. When I am transferring liquids I do not want to spill it.
Hypothesis
I think that when the concentration of the hydrochloric acid is higher, the reaction will be faster. The rate is proportional to the concentration; therefore it will be doubled if the concentration of hydrochloric acid is also doubled. This can be justified with the collision theory. Only a small percent of collisions result in a reaction. If there are more particles per unit volume, there is a larger chance of sodium thiosulphate and hydrochloric acid particles colliding as they are closer together. With more particles, this is more likely to happen. A larger concentration increases the frequency of collisions between the two reactants. For a reaction to occur, molecules must collide not only with sufficient energy but with the proper orientation.
Concentrated Dilute
Because energy is spread between particles, only a certain number will have the extra amount of energy above the activation level needed to start a reaction. The more concentrated the solution, the more particles there will be with energy levels with this extra energy, therefore more particles will react with each other, causing the rate of reaction to increase.
Results Table
Graph
To plot my graph of averaged results, I first have to find an average of my times in order to gain a more accurate and reliable data set. To do this I use the following formula;
Time (1) + Time (2) + Time (3)
__________________________
3
I also have to calculate the rate of each reaction. To do this I use the formula;
1
_______________________________
Time taken for reaction to occur
Then I have to calculate the concentration of the hydrochloric acid which goes on my other axis. To do this I take the original 2 Molar concentration and convert that to 40g/dm3. I then have to rearrange numbers to find every concentration
40g/dm3 = 50cm3 hydrochloric acid
The formula is as follows;
Concentration of previous experiment
_____________________________________ * Next volume of acid
Volume of Hydrochloric acid
Example;
40g/dm3 / 50cm3 = 0.8 0.8 * 40cm3 = 32 g/dm3
Hence:
A Graph Showing the Correlation between Concentration and the Rate of Reaction
Analysis
From my graph I can see a positive correlation showing that when the concentration increases, the rate of reaction subsequently enlarges as well. This means that because the rate of reaction increases, it takes a reduced amount of time for a reaction to occur, so more happens in an amount of time. The general pattern is that as the concentration decreases, the rate of concentration decreases as well, showing that the time taken for the reaction to occur is inversely proportional to the concentration of hydrochloric acid. However, it is not directly proportional due to
When solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur. The axis for concentration show that they were quite high, and that the increase in rate of reaction was relatively small.
The correlation between concentration and rate of reaction is reasonably strong. I can work out the gradient of my graph and of the correlation by using the following formula with two points of my best fit line.
Gradient = difference in y / difference in x (3sf) (8, 161) (32, 201)
Rate of reaction in the y-axis
Concentration is the x-axis
Gradient = 40 / 24
Gradient = 1.67 = +1.67 * 10-4
My results tend to agree with my earlier prediction as I hypothesized that if the concentration is higher, the rate of reaction will be higher as well. The collision theory is right in the sense that with a higher concentration of acid, there are more particles per unit volume to collide with particles of sodium thiosulphate, making the rate faster. We know that this reaction has been a success because although collisions often do not make a product, they were strong and orientated enough to produce a precipitate of sulphur and turn the solution into a milky yellow colour. This is what happened during my experiments; however, the statement I made was that the rate of reaction “will be doubled if the concentration of hydrochloric acid is also doubled”. This is not correct as I discovered from my graph.
This graph is an example of what the correlation between concentration and the rate of reaction looks like;
Conclusion
From this investigation I can say that the rate of reaction does increase as the concentration of hydrochloric acid increases, but the data is inversely proportional to each other. For this to happen, collisions occurred between acid and sodium thiosulphate particles and the amount of energy reached surpassed the activation level. The collisions were fast enough with the correct orientations, therefore reactions occurred.
Although I have found out that the higher the concentration, the faster the rate of reaction, this will only work up to a certain point. The law of limiting factors states that during each experiment, there will come a point when a restricting factor does not allow the general trend of the data to continue. For this experiment, I can conclude that this factor is the amount of sodium thiosulphate. Because the line of best fit tended to curve upwards towards the higher concentrations, there was no sodium thiosulphate left to react with the hydrochloric acid as it had already been used up in previous collisions and had complete converting into the precipitate of sulphur.
Evaluation
In total, there is one anomalous point on my graph. Although there was another point not directly on the line of best fit, it was so close that it does not count. Reasons for the anomalous point are that I was rushing so they were the outcome of human error; they may not have been measured correctly. However, the timer was not accurate enough as it only measured to 1/10th of a second so because I had 2 rates of reaction at 0.0200 (experiments 3 and 4), I cannot differentiate between them, even if one may have been rounded up and the other rounded down. This detracts from the accuracy of my investigation. The timer was not started and stopped fast enough, allowing extra time on some results and taking time off on others. This inaccuracy prevents the results from being very reliable. Another problem that may have occurred is that I may not have cleaned the measuring cylinder and conical flask out properly after using them, causing a contamination. The conclusion that I have formed based on this evidence is perhaps not as strong as I had originally thought. After considering all the possible errors I have decided that my conclusion is correct but there could be more accurate evidence which I can collect to support it.
The method I used was simple to follow and the results seem fairly accurate and precise. I feel that it was an average method as it didn’t allow for too many errors to occur. However to improve on this method and ensure that no mistakes happen, instead of using hydrochloric acid as a variable in a following experiment, I can make it constant and dilute the sodium thiosulphate instead to see if there is any change in the results. I would be able to analyse differences and compare the two experiments. This would then bring more evidence that the higher the concentration, the faster the rate of reaction. If that wasn’t enough, I could go on to record the results for an experiment where instead of having sodium thiosulphate, I would use magnesium. The evidence I have is enough to draw up a suitable conclusion although there are still improvements to be made.
For a better idea of how concentration affects the rate of a reaction, I could use a wider range of concentrations so that there is more evidence to analyse and conclude.
Next time, I will take more time with obtaining he evidence instead of rushing the work to complete it. That lack of haste was probably the cause of mistakes. I only managed to do three lots of experiments which are still not enough to get a reliable set of results; more would be needed to be satisfactory. This is the reason why there was an inconstancy within my results. I will have to measure the level of precipitate produced during the experiments as in my investigation, I did not measure the level of cloudiness of the solution. This would make the experiment fairer.
I think that if better equipment such as a more precise measuring cylinder the flaws would start to decrease. A more accurate timer is needed. We would have more truthful measurements to work. Pipettes of a better standard could be used to make the amounts of solution even more reliable.
Jingyi Zhang 10TF