- Repeat twice and take an average for each concentration
4. Apparatus
The apparatus used is listed below in detail:
- Measuring beaker
- Thermometer
- Marking pen
- Filter paper
- Large beaker (x3)
- Small beaker
- Conical flask
- Stopwatch
- Sodium thiosulphate (600cl)
- Hydrochloric acid (80cl)
- Water
- Tissues
The apparatus we will use for the following experiment is listed below:
- A burette (for measuring)
- Thermometer
- Marking pen
- Filter paper
- Large beaker (x3)
- Small beaker
- Conical flask
- Stopwatch (x2)
- Sodium thiosulphate (600 cl)
- Hydrochloric acid (80cl)
- Water
- Tissues
5. Fair Testing
To make this experiment fair, we will have to first look at the issues which occurred in the original experiment.
The following are issues which were discovered during the experiment.
- A lack of continuous temperature measurement
- Measuring of the reagents were not completely accurate (a burette could have been used)
- Lack of readings taken (more experiments could have been conducted)
- Lack of repeats undertaken (1 could be far off and this would remain unknown)
- A larger variety of concentrations could have been used
For issue 1, we will take a measurement during the experiment to make it a fair test. Temperature affects the rate of reaction in the way that the higher the temperature, the quicker the reaction. Temperature is a measure of the kinetic energy of particles in a reaction, so a higher temperature forces more collisions to occur, as the kinetic energy of the particles is increased due to heat. The ‘Collision Theory’ dictates that there must be a collision with sufficient energy to start a reaction (i.e. the activation energy). It also states the particles must have enough energy to react. Increasing the temperature makes more particles have this amount of energy, meaning the rate of reaction will increase.
For issue 2, we will use a burette to measure the amounts needed correctly. This will mean there will be no variance in the measures of substances used.
For issue 3, my partner and I will both use separate stopwatches and average all our results. This will eliminate the chances of error as if there is an error, the experiment will be repeated.
For issue 4, we will repeat the experiment 4 times per concentration to eliminate the chance of error and get results that concur.
For issue 5, we will use a larger variety of concentrations stretching from 16 g/l-40 g/l, increasing it by 4 g/l each time. This means the graph which we will use will be more accurate, having a more accurate line of best fit, more accurate results and the ability to show if one or two results are removed from the norm.
6. Prediction
My prediction for this experiment is that as the concentration of one of the reagents (in this case sodium thiosulphate) increases, as does the rate of reaction. This is proven in the graph taken of our set of results which shows a distinct increase of rate of reaction in proportion with the increase of concentration. This is because increasing the concentration of one of the reactants will increase the frequency of collisions between the two reactants. In solutions of a low concentration, the particles are separated further apart than in a high concentration. This means they will not collide with each other as much, as there are fewer particles to collide.
The Maxwell-Boltzmann theory indicates that in any particular mixture of moving molecules, the speed will vary a great deal, from very slow particles (low energy) to very fast particles (high energy). Most of the particles however will be moving at a speed very close to the average; however no particles would have zero energy. This is shown in the graph below.
From Yahoo Images
Using this graph, I can see that if I double the concentration (i.e. the number of particles that have sufficient energy to react) then I double the chances of a collision happening. This will therefore mean that the rate of reaction doubles.
My prediction claims that there will be a proportional increase in rate of reaction as the concentration increases. Using the Maxwell-Boltzmann theory however I predict this increase will be directly proportional to the concentration increase. This means that if the concentration doubles, the rate of reaction will double and if the concentration triples, as will the rate of reaction (and so forth).
7. Improved Table of Results
Safety Aspects
As we were handling chemicals, it was important to be careful. Goggles had to be worn, as did aprons and the laboratory safety rules had to be observed and carried out.
Reliability & Accuracy of Results
The results were fairly accurate. They supported the prediction in the respect that the time taken for the ‘x’ to go increased as the concentration of water increased. However during the experiment the temperature differed slightly, which could have had an impact on the results. Even a small difference in temperature (in this case 1◦C) can make a substantial difference to the results. This is a reason the results were not directly proportional as predicted.
Anomalies
There were no major anomalies in the experiment so the averages all came out the way it was mentioned in the prediction: As the concentration of the sodium thiosulphate decreased, the time taken increased. If anomalies were present, I would repeat that part of the experiment and record the retaken result, ignoring the original result. The point of repeats was to spot the anomalies and treat them accordingly. If they were included in the average, the whole average would become awry, possibly to the extent where it goes in opposition to the prediction.
An Experiment to Investigate the Effect of Concentration on the Rate of Reaction
Analysis
Concentration affecting time
As the concentration increases, the time taken for the reaction to complete and for the ‘X’ to disappear decreases. As the concentration increases there are more particles present and therefore more collisions occur. As the amount of collisions increase the time it takes for the reaction to complete decrease.
Concentration affecting Rate of Reaction
As the concentration increases, the rate of reaction also increases. As the concentration increases, more particles are present. All these particles increase the chances of a collision taking place, increasing the rate of reaction. The rate of reaction is how many particles collide in a second, and though it is mostly chance, the more particles in a solution the more the chances of a collision are.
Prediction and Quantitative Prediction Check
My prediction was only partially correct. Though I predicted and increase in rate of reaction as the concentration increased, I predicted it would be not only proportional but directly proportional. Theoretically, my prediction was correct however in practice the results show that it was not. The maximum concentration was 40 g/l of Na2S2O3 had a rate of reaction of 0.0526/s. By my theory a concentration of 20 g/l should have a rate of reaction of 0.0263/s; however my results show it had a rate of 0.0235/s. Also in a solution of 30 g/l of Na2S2O3 the rate of reaction is 0.0318/s. A concentration of 15 g/l should have a rate of reaction of 0.0159/s; however it actually had a rate of 0.0182/s.
Discrepancies
The discrepancies between the results and the quantitative prediction could have occurred for many reasons. These include:
- Inaccuracies of seeing the ‘X’ go
- The time it takes to press the stop watch
- Inaccuracies in measuring the solutions (burettes are only accurate to +/- 0.06ml)
- The collisions are purely chance, the collisions not occurring could have been pure bad luck
- Other substances affecting the experiment
- Temperature (increase or decrease)
With every set of eyes being different, watching the ‘X’ go by 2 sets became harder because of the differences in timing. If the concentration blurred out the ‘X’ and my partner could still see it, the timings would be off, even if it was minimally. A minimal difference, however, made quite a substantial difference to the results.
Though the stop watches were in working condition, the few milliseconds it took to press the stopwatch at the start and at the end affected the results as well. The time to press the stopwatch would have differed each time, the reaction time being near (but not completely) perfect.
Burettes are the most effective way to measure out the solutions, but even an inaccuracy of +/- 0.06ml can alter the results. Also the difference affected all the concentrations, so each experiment could have had an inaccuracy of up to +/- 0.12ml which comparatively is very big.
The collision theory states that a reaction depends on the collisions. These collisions are completely random but by increasing the amount of particles the chances of collisions increase. Though this is true, there is no guarantee that the collisions will take place.
The reaction was not closed off and, though other gases in the air won’t have affected the experiment too much there is still the possibility that it did. There is an abundance of oxygen and nitrogen in the air (99% of all the air around us) and these could have had a small impact on the experiment.
The temperature of the room the experiment was performed in affected the experiments rate of reaction. In an increased temperature, there would be more collisions and in a decreased temperature there would be fewer collisions. Unless this temperature was able to be kept constant, there would be more (or less) heat energy and more collisions or fewer collisions would occur respectively.
An Experiment to Investigate the Effect of Concentration on the Rate of Reaction
Evaluation
Success and Evidence
The evidence suggests that the concentration and rate of reaction is proportional. As one increases the other does as well. The experiment proved this theory well, as there were no anomalies supporting this theory. All increases in concentration increased the rate of reaction. The second part of the experiment, proving that it is directly proportional failed as the results show that even though there is an increase, this increase is not directly proportional. Theoretically, this is true but in practice the experiment shows it is does not hold.
Anomalies and Evidence
The experiment had no obvious anomalies but it failed to prove the second part of the aim. My theory, with the help of the Maxwell-Boltzmann theory, was that the rate of reaction should double as the concentration doubles. The evidence suggested otherwise, making all of my results theoretical anomalies. There were a few problems with the experiment, however if these were solved and the experiment was done in perfect conditions I believe my theory would stand true.
Problems with Experiment and Solutions
I mentioned before that there were some problems with the experiment. I have chosen two problems and thought about a way to solve them.
- Other substances affecting the experiment
- Temperature
For problem 1, I would close off the top of the beaker being used in the experiment. This would prevent excess gases entering the solution and therefore would not affect the experiment. I though about using a cork top, however realized that it would be difficult to see when the ‘X’ underneath goes from the top, and so would suggest using a clear plastic or even glass top.
For problem 2, because the temperature affects the experiment, I would put the beaker in a thermal insulating box or container and this would keep the temperature constant while not affecting the experiment. Temperature increase meant that the particles would collide more, therefore changing the results. We had no control over the temperature, only a way of measuring it.
Improvements to Experiment and Reliability of Results
After doing the experiment, I would alter the above 2 issues but also would do more repeats so there can be no issue over doing the experiment wrong. Also I would use a larger range of concentrations as there was only 1 set of concentrations to check the possibility of tripling the concentration to see if it tripled the rate of reaction.
The evidence is not reliable enough to support the conclusion as there have been a few problems mentioned already with the experiment. However this experiment, done in closed conditions, could yield the results mentioned in my prediction. To prove the point that concentration and rate of reaction are proportional, this experiment is very reliable as every concentration increase yielded a higher rate of reaction without any exceptions.
Bibliography
The following are the origins of assistance I got in my chemistry coursework. I used no books, only internet websites.
Yahoo Images
www.alevelchemistry.co.uk
BBC: GCSE ‘bitesize’-chemistry
www.avogadro.co.uk
