My aim is to see the effects of a change in concentration on the rate of reaction. The reaction I will use is:
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In this experiment I will vary the concentration of Sodium Thiosulphate while keeping everything else constant. As our sources are limited I am going to mix up my own concentrations of Sodium Thiosulphate by diluting the 1.0M available with water. The table below shows how much water is needed to make 50cm3 of Sodium Thiosulphate in the following concentrations:
I predict that as the concentration of the Sodium Thiosulphate increases the rate of reaction will also increase. This means that the graph I draw up in my analysis will have positive correlation, and probably be curved, as the increase in rate of reaction will not be exactly the same as the concentration is increased. This can be justified by relating to the collision theory. If solutions of reacting particles are made more concentrated there are more particles per unit of volume. Collisions between reacting particles are therefore more likely to occur.
Plan
To obtain my results and find out whether or not my prediction is correct I am going to the ‘Disappearing Cross’ experiment. This is the equipment I will need:
- Stopclock
- 25cm3 measuring cylinder
- 50cm3 measuring cylinder
- Conical Flask
- Paper with a cross drawn in the middle
- Water
- Glass Rod
- Safety Goggles
- 1.0M Sodium Thiosulphate
- Hydrochloric Acid
In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the Hydrochloric Acid and Sodium Thiosulphate will not be mixed up. The amount of Hydrochloric Acid will be 10cm3 each time and the amount of Sodium Thiosulphate will be fixed at 25cm3. Cleaning out the flask/measuring cylinders will ensure that no chemicals that aren’t suppose to at that time mix together and I will stir the Sodium Thiosulphate solution with a glass rod to be sure that it is mixed properly. As well as keeping an eye on the X at all times. All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus making the entire investigation more successful.
As well as making it a fair test I must also ensure that it is a safe one. A pair of goggles will be worn during the whole experiment to protect my eyes.
For this experiment I will have to measure the time the X takes to disappear- not literally but until the solution turns cloudy and the X is no longer visible. I will also have to measure accurately the amount of Sodium Thiosulphate/Hydrochloric Acid I want to use as well as using the same amount each time. My results will be recorded accurately in a table that is clear and easy to read, I will produce this on rough paper before I do the experiment to save time. To ensure my results are reliable, I am going to do each experiment twice and if the two results are not close together then I will be able to see that my experiment has gone wrong somewhere and is not accurate. Therefore, I will do it again until it looks reliable enough to continue.
Detailed Instructions for Investigation
- Collect all equipment and put on safety goggles.
- Set up equipment. (As shown below.)
- Ensure all equipment is clean and untampered with.
- Carefully measure 10cm3 of Hydrochloric Acid into a measuring cylinder.
- Draw a cross on a piece of paper.
- You now have to make the fist concentration of Sodium Thiosulphate, using the table on previous page to work out the ratio of Sodium Thiosulphate to water.
- E.g. To get 0.1M of Sodium Thiosulphate, carefully measure 5cm3 of Sodium Thiosulphate into a 50cm3 measuring cylinder and add 45cm3 of water to dilute it.
- Mix thoroughly with the glass rod.
- Accurately divide the 50cm3 of Sodium Thiosulphate into 2 separate 25cm3 cylinders for the convenience of doing the experiment twice.
- Pour the 10cm3 of Hydrochloric Acid into the conical flask and place on top of X.
- Pour in 25cm3 of Sodium Thiosulphate into the conical flask, as soon as it reaches the Hydrochloric Acid start the stopclock.
- Watch very carefully and stop the clock when you think the cross has disappeared.
- Record your results in a clear table.
- Repeat this for each concentration of Sodium Thiosulphate, using the table each time to measure the correct concentration.
- Once finished clear away all the equipment and wash everything thoroughly before you sit down and work out the averages of your results.
- REMEMBER: Wear safety goggles at all times right up until you have washed everything up!
Results
I have now conducted my experiment and am pleased with the results I achieved. There is an appropriate number and range of measurements to get a clear understanding on how concentration affects rate of reaction. I ensured that I used equipment safely for observations and measurements and recorded my results clearly in the table below.
After recording my results I worked out the average by adding the results for my 1st try and 2nd try together and dividing the sum by 2, because there are two sets of data. I did this for each set of results for each concentration.
Now I have obtained my results I will work out the rate of reaction for each concentration of Sodium Thiosulphate. The formula is:
Rate = Amount of Sulphur
Time
I cannot accurately measure the amount of sulphur produced so am going to substitute with 100, as this will make the maths easier. The results I obtain will not be the actual rate but will be directly related to it. So the equation will now be:
Rate = 100
Time
These are the results for the rates:
Analysis
In this experiment I have found that as the concentration is increased the time taken for the reaction to take place decreases. This means the rate of reaction increases as it takes less time for a reaction to take place, so more take place per second. This is basically what I predicted.
Using line graphs, with lines of best fit, I can draw a conclusion from my experiment. (Graphs are on the next page) Firstly, looking at the first graph, plotting concentration against time taken for the reaction to take place, I can see that there is negative correlation. This means that as the concentration is increasing the time is decreasing, thus showing that the rate of reaction is faster and my prediction was correct. The curve on this graph has a very steep slope to begin with, which then evens out as the concentration increases. The second graph shows concentration plotted against rate. This graph has positive correlation, which means that as the concentration increases the rate decreases. The opposite affect to time. The curve gets steeper as the concentration increases.
For this to fully make sense it is necessary to recap the collision theory briefly: For a reaction to occur particles have to collide with each other. Only a small percentage results in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy needed is called the activation energy and the size of this is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. An increase in the frequency of collisions can be achieved by increasing the concentration, which is what I have proved and shown in a graph.
Evaluation
Overall I am pleased with the way my investigation has turned out; I feel I have obtained some reasonable results, which can be used to prove that rate of reaction increases as concentration increases. The accuracy of the results cannot be perfect as I was relying on just my judgement to tell me when the X had disappeared, so the time it took for me to judge when the X had disappeared and then stop the clock will not be totally accurate. In my results table, the results for 0.1 M were not perfect, as there was a difference of 6 seconds between them. However I decided that this was close enough to carry on with the experiment and wouldn’t really interfere with my final conclusion. The experiment I conducted was suitable for its purpose as I was in a classroom and facilities are limited, although I would like to do the experiment properly in a laboratory and see how accurate my results actually were. If I were to do the experiment again I would probably measure the Sodium Thiosulphate into 2 25cm3 measuring cylinders rather than into a 50cm3 cylinder and dividing it into two. This is because it was quite hard to get 25cm3 exactly in each cylinder and took up a lot of time, even though I thought it would save time. All in all I believe that my results are reliable enough to support the conclusion that rate of reaction decreases as concentration increases and I have proven that.