A thermometer: to measure the temperature at the start and end of the reaction.
A conical flask: for the reaction to actually take place in.
2 measuring cylinders: One of them will measure the 5cm of hydrochloric acid each time, so will be smaller, and one needs to go up to at least 50cm to measure out the Sodium Thiosulphate and water each time.
A stopwatch: to measure the reaction time.
2 measuring pipettes: To alter the amounts when measuring them out. Using them makes it more accurate. Two are used so that the chemicals do not mix and start reacting in one pipette while we are measuring them out.
Pair of Goggles: To protect the eyes from substances entering them whilst doing the experiment.
Piece of paper with “X” on: To measure the reaction rate. This will be laminated with some plastic to stop the reactants and products dripping onto it and smudging the cross.
Prediction
I predict that the higher the concentration of Sodium Thiosulphate, the faster the reaction will happen. This prediction can be justified by referring to the collision theory: The higher the concentration of the substance, the more particles there are to react. This means there are more reactions (collisions between the Sodium Thiosulphate and the Hydrochloric Acid), so more of the product (which is a gas) will be produced, and the cross will be clouded over by the gas quicker. Even though when you increase the concentration, there is the same proportion (percent) of particles with the needed activation energy, there are more of the particles, and so more gas is produced. For example; 0.1 molar Sodium Thiosulphate has 1 in 5 of the particles with more than the needed activation energy. In 0.2 molar, which is more concentrated, there are 2 in 10 of the particles with more than the needed activation energy. This means that even though they both have the same proportion of particles with the activation energy, 0.2 molar has more particles in total, so more collisions happen.
The particles are all moving, and after colliding, some gain speed, some lose speed, and some stay more or less the same. There are some particles with faster speeds, some with slower speeds, but most of the particles speeds are in the middle. This theory is called the “Boltzmann distribution” theory. There is a Boltzmann distribution curve to go with this theory:
You will notice that there are no particles without any energy
When I add water instead of Sodium Thiosulphate, the concentration will go down. If I half the concentration of Thiosulphate, the time that it takes to see the cross disappear should be double that of the full concentration before halving.
This is the table of results I will collect the information in:
To get the overall concentration (Measured in Molar), the equation is:
Concentration of Volume of Thiosulphate (cm3)
Hydrochloric acid X Total volume (Cm3)
(Molar)
I will take three sets of results for each experiment so that if I have taken two sets, and they differ, the third one will show which result is right, and if non of them are close, then I may repeat the experiment again if I have the time.
I will make a rough graph of my results, and I will circle any anomalous results, try to find the cause of them being anomalous, and if they are far wrong, I will not include them in my averaging.
Preliminary Experiment
I carried out a preliminary experiment to see which concentration of Hydrochloric acid we would use, as this concentration is staying the same throughout the experiment. We used 0.2 molar, 0.1 molar and 2 molar concentrations of Hydrochloric acid. The concentration of Sodium Thiosulphate we used throughout the preliminary and the rest of the experiments were 0.2 Molar.
After this set of results, we decided to use the 0.2 Molar concentration. This is because, with the 0.1 Molar concentration, it would take too long, and we would not finish the experiments in the time allotted. With the 2 Molar concentrations, the reaction happened very quickly, and, because there is a human error in timing anyway, the error would be bigger if the time is smaller, as it is a bigger error proportionally. For example, if my human error were 0.5 seconds every time, then for the 0.2 Molar experiments, the error would be:
0.5 / 25 = 2 / 100
This is a 2% error.
For the 2 Molar concentration, if the error is still the same:
0.5 / 7 = 14 / 100
This is a 14% error. This is 7 times more than the error with the 0.2 Molar concentration, so I t is more accurate to use the 0.2 Molar, so we will use the 0.2 Molar Hydrochloric Acid.
Results
I believe an appropriate degree of accuracy for averaging the results is to the nearest second. This is because, when measuring it, I was never accurate to within about half a second, and this could be rounded to the nearest second, so if I rounded to the nearest tenth of a second, this would not be accurate because my human error was larger than that, so it would be irrelevant if I rounded it to that degree of accuracy. However, on the single results, I have done them to the nearest tenth of a second because, if I did them to the nearest second, when I averaged them, there would be a rounding error.
Analysis
My results show that when you halve the concentration of Sodium Thiosulphate, it takes a lot more time to react. This is because there are a lot less particles of Sodium Thiosulphate, so there are fewer reactions; therefore it takes more time for the product (a gas) to be produced, so it takes longer for the cross to be clouded over by the gas. In this experiment, the concentration halves twice; from 0.2 Molar to 0.1 Molar, and from 0.16 Molar to 0.08 Molar. This is so that I can justify my prediction. If it was only doubled once, it could be a fluke if what I predicted was right. There is a pattern that the lower the concentration of Sodium Thiosulphate is, the longer the reaction time is. This shows that concentration affects reaction rate in an inversely proportional way; the lower the concentration goes, the higher the reaction time gets.
Conclusion
From all my work, I have found that firstly, concentration is inversely proportional to rate of reaction, and that if you halve the concentration, the rate is doubly slower (It takes double the time to react)
This is because, if there are a certain number of particles in a concentration (for example 0.2 Molar), then when you halve the concentration, the proportion of the particles with enough energy (activation energy) to react and break down the chemical bonds is half as much, so it will take longer for the same amount of reactions to happen.
Ludwig Boltzmann devised a theory, wherein he explains that even though a substance has equal energy throughout, the particles are going at different speeds. This is because when they collide, some travel off fast, and some go slower. Some gain energy from reactions, and some lose it. In a substance, some particles are always faster, some are always slower, but most of the particles are in-between.
The three particles have all collided. One goes away fast, one slow, and one in-between. This is a Boltzmann distribution curve:
I predicted that when I halved the concentration, the reaction time would double. This is shown in the line of best fit I drew on my rough graph. I was quite accurate, but obviously my prediction was not perfect. So, seen as I predicted that the reaction time would be double for half the original concentration, the 0.2 Molar concentration experiments took an average of 25 seconds, which should double to 50 in the 0.1 Molar experiments. The 0.1 Molar experiments were an average of 47 seconds long, so this is 3 seconds out.
3 / 50 = 6%
This is a 6% error. This may sound like a large number, but the actual error was only 3 seconds, and this can be accounted for by human error, and other uncontrollable factors, like temperature, and accidental mixing of chemicals. I am happy with this degree of accuracy for my prediction. Seen as the concentration doubled twice, I will see if I can justify my prediction further by working out the percentage error for the 0.16 Molar to 0.08 Molar concentration halving:
The average time it took the 0.16 Molar concentration experiments to react was 28 seconds, this doubled is 56, and so the 0.08 Molar experiments should be 56 seconds. It turns out that the 0.08 Molar experiments average was 56 seconds, so my prediction is correct for this concentration halving. There was a rounding error with all of the experiments, but the truthful error, if the time had not been measured, would probably be less than 2%, so my prediction was very near to the results.
Some of the error is not rounding though; it is from measuring the chemicals out. If I measured out 1cm3 wrong each time I measured, then (for example) in a 5cm3 measurement, I would have been 20% out. For measuring 50cm3 though, I could still make the same error (1cm3) but this would only be a 2% error. This means that more of the error that occurs is from human error again, this time in the measuring of the reactants.
A graph cannot prove that reaction rate and concentration are inversely proportional, because, even though the graph may exactly follow the pattern,
INVERSELY PROPORTIONAL GRAPH
there are so many mistakes that the graph is never completely accurate. Because the concentration is inversely proportional to rate of reaction, this means that concentration is directly proportional to 1
Time
This is because 1 is the inverse of time, and so, the
Time
inverse, inverse is the direct. A directly proportional graph goes through the origin, and looks like this:
DIRECTLY PROPORTIONAL GRAPH
I made my own directly proportional graph to try and justify my prediction further, in more ways, and to see how accurate my prediction was. I worked out the results in 1 format, so I could see how large the axes on my
Time graph have to be:
The graph shows the average result for 0.16 Molar to be anomalous. This is accounted for by the human error factor, probably the timing. I probably started the stopwatch too late, and this would explain why the time is less tan it technically should be.
I have made a graph of the results in my main results table, just to see if I could spot any anomalies.
On my graph I have highlighted a result in red, which is an anomaly that was far away from the rest of the results for the other experiments of that concentration. I have not included this in the averaging of this concentration’s results, as it would throw the average away from what it should be, and that would give an inaccurate depiction of this experiment.
The only result that I spotted to be far anomalous was the 1st experiment of the 0.14 Molar series. There is no obvious reason for this to happen, the probable reason is that it was human error. We repeated the experiment 3 times again shortly afterwards so that the conditions did not alter much, and got these results:
This shows that the result circled was an anomaly, and was probably human error, and we now have clear, reliable results for the 0.14 Molar concentration experiments.
Evaluation
It was quite hard to tell when the cross had disappeared, because when it had disappeared was always a point of view, not a fact. While one person may think the cross has disappeared, another may think it is still visible. The averaging of the results also causes a rounding error, and this accumulates as more and more things are rounded, and added to each other. To get the average of 2 numbers, you add them together, and then divide them by 2, for example:
1. 50 + 30 = 80. 80 / 2 = 40
2. 39 + 41 = 80. 80 / 2 = 40
In number 1, the results are 20 apart, but still have the same average as the two results that are only 2 apart. They are exactly the same average, but number 2 is much more accurate because the two numbers are only 1 away from the average, whereas in number 1, they are 10 away.
If I relate this theory to my results, I find that fortunately, it is not applied, so I do not have to investigate further into those results. It is not applied because there is not much spread of results in my table, there are all relatively close to the average.
Some of the other problems with the experiment were; the temperature changing from time to time. This means that the reaction gets quicker or slower if it is higher or lower temperature respectively. This is because temperature gives particles more energy to break bonds with more speed and force. It was also difficult to measure out the substances accurately, because measuring is another matter of opinion. We tried to combat the problem of opinion by having the same person measuring the reactants and temperature, and the same person watching the cross disappear.
Matter of opinion is probably the chief cause of unreliability in this experiment. Other factors including temperatures and other human errors (like spilling reactants) are also causes of unreliable results, but opinion is the biggest I believe.
I still believe that my evidence is reliable enough to support my conclusion though. This is because all the sources I used for my prediction told me that rate of reaction is inversely proportional to concentration, in the way that my results show. So, because my conclusion supports my prediction, I believe my results are sufficient enough to back up the conclusion.
I believe the anomaly I circled on my graph is justified by the fact that human error comes into it. I believe that I made an error by starting the stopwatch too late, and this means that, because the reaction is still happening, the cross will cloud over in the same time, but we will interpret this as slower, because the stopwatch was started too late.
To make the experiment more accurate, there are many things we could change;
We could do the whole experiment in a controlled temperature water bath, so that temperature doesn’t affect the rate of reaction like it has in parts of this experiment.
We could use equipment to measure when the cross has disappeared. We would use a light gate to measure how much light is passed through the conical flask. We would set the light gate to stop the timer at a certain light intensity, and the results we wrote down would be the time it took for the light to reach the given intensity with every different concentration. The computer measures it, so it automatically stops. This will help collect much more reliable evidence to support my conclusion.
DIAGRAM OF LIGHT GATE
We could use more accurate measuring instruments. The pipettes can be a lot more accurate, by using a longer, thinner pipette. We could also use longer, thinner measuring cylinder to measure out the water, Hydrochloric Acid and Sodium Thiosulphate in more accurate, larger quantities.
To get more evidence for my conclusion, we could use a completely different reaction to see if my theory worked for that reaction. We could use:
Mg + 2HCl MgCl2 + H2
From this reaction we could measure the amount of hydrogen released by the reaction, because in this reaction, the more hydrogen that is released, the faster the rate of reaction.
To get more reliable evidence for my conclusion, we could use a data logger, which will measure the reaction time automatically, and gives no error in the place human error would occur. We could do the experiment for a lot longer or a lot shorter amount of time. We could use 10 Molar concentrations, and the reaction would be over in seconds, but the reading would be exact. We could also read 0.05 Molar concentrations, and the experiment would take hours, but the data logger could be left overnight and would log the time when it finished, whereas a human may miss it because of being tired or not being there. This means that wider gaps can be measured to make the evaluation and overall results more accurate, and will give a clearer pattern.