Conclusion:
I predicted that the higher the concentration, the faster the reaction would be. This prediction was based on the collision theory’s affecting factor, concentration. If the concentration increased, the number of reacting particles would increase. This means that successful collisions were more likely so the rate of reaction increased. I also predicted that although the concentration halves each time, the average time will not exactly half because as the concentration is weaker, it takes a longer time for all the particles to collide. I guessed that maybe all the hydrogen ribbon may not disappear, and if it does, there may be errors to the results, as the human eye cannot detect the exact second to which all the ribbon has disappeared in each experiment.
My prediction was right as I can tell from the results table. The higher the concentration, the faster the rate of reaction was. My timing accusation was also proved right by the experiment. Looking at the table, you can see that the magnesium ribbon took approximately three times more in the 1M concentrated solution of hydrochloric acid then the 2M solution. You can also see that it took over six times the time in the 0.5M solution than the 2M solution.
My results support the scientific knowledge as it shows the higher the concentration, the faster the rate of reaction. The graph also supports my prediction, as the jump between 1M and 0.5M is a lot higher than 2M and 1M. This proves that the weaker the concentration, the slower the particles react, or stop reacting.
Evaluation:
I understand that my results may not be extremely reliable because as we know temperature is an affecting factor for a rate of reaction experiment. We know that the particles at higher temperature move faster so they are more likely to collide with more energy increasing the chance of a successful reaction, therefore increasing the rate of reaction. The temperature of the acid may not have been exact for all the experiments as people touching them could have changed the temperature, which would have made certain reactions faster then they should be and other reactions slower then they should be. To be more accurate, we should have used a thermometer to check the temperature of all the acids before we used them.
The surface area of the magnesium ribbon may not have been the same or weight. This could have made our experiment unfair because if there is less magnesium ribbon, there are less magnesium particles that need to react with the hydrochloric acid, making the rate of reaction faster. To avoid this, we should have weighed the magnesium ribbon all to the same weight.
Another reason that the results may not be accurate is because we used our human judgement to detect when the ribbon had totally disappeared, this timing could have been taken too early or too late in comparison with the other experiments. This would have obviously affected the results and the graph, which again could make our experiment unfair. However, overall the results were not totally anomalous but enough to prove my prediction and the collision theory correct.
Aim:
To find out how concentration of sodium thiosulphate will affect the rate of a chemical reaction.
Method:
-
Measure out 20cm3 of hydrochloric acid in a measuring cylinder, and 20cm3 of sodium thiosulphate at a concentration of 0.1M in another cylinder.
- Pour the contents of both the measuring cylinders into a conical flask on top of a paper marked with an “X”, (HCL first).
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of hydrochloric acid, and 20cm3 of sodium thiosulphate at a concentration of 0.09M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of hydrochloric acid, and 20cm3 of sodium thiosulphate at a concentration of 0.08M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of hydrochloric acid, and 20cm3 of sodium thiosulphate at a concentration of 0.07M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of hydrochloric acid, and 20cm3 of sodium thiosulphate at a concentration of 0.06M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of hydrochloric acid, and 20cm3 of sodium thiosulphate at a concentration of 0.05M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of hydrochloric acid, and 20cm3 of sodium thiosulphate at a concentration of 0.04M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
- Now repeat the entire experiment two more times to get three sets of answers to calculate an average.
Note:
To get a solution of x concentration, you calculate (1000x)% of 20, and use that amount of sodium thiosulphate. You then add the remaining amount to add up to 20ml of water.
This is a diagram to show the layout of our experiment.
Below is a table showing the amounts that will be used in the experiment:
Apparatus:
Conical flask
Hydrochloric acid
Sodium thiosulphate
Water
Measuring cylinder
Paper with an “x” on it
Pipettes
Timer
Safety:
To conduct this experiment I will need to follow rules that apply to being in a laboratory among others. They are:
- Wear goggles during the experiment.
- Use the correct equipment to ensure that I don’t endanger myself when using acid.
- Wear gloves when handling the acid, and make sure none gets onto my skin or into my eyes, as it is an irritant.
Fair Test:
To make this experiment a fair test, I must ensure I am using the correct amount of hydrochloric acid and sodium thiosulphate each time. I must have allocated pipettes for the water, hydrochloric acid and sodium thiosulphate because if one chemical gets into another chemical beaker, then a chemical reaction will start to take place. It will then not be accurate when we dilute our sodium thiosulphate concentration, and when we do each experiment. I must also use a timer to get as accurate timings as possible.
Prediction:
I predict that the higher the concentration, the faster the reaction will be. This prediction is based on the collision theory’s affecting factor, concentration. If the concentration increases, the number of reacting particles increases. This means that successful collisions are more likely so the rate of reaction increases. I predict that although the concentration halves each time, the average time will not exactly half because as the concentration is weaker, it takes a longer time for all the particles to collide. Looking back at the difference between the timings for the preliminary work using the magnesium ribbon, I think that the difference between these timings will also be quite a lot, meaning not a difference of a few seconds, but a difference of a minute or more each time. I also think that the difference will gradually get more as the concentration decreases, this is probably because there may not even be enough reacting particles to have a full successful reaction.
Results:
Using my scientific knowledge from the beginning of the coursework, I know that:
Rate of reaction = 1 / time
Here is a table to show the rate of reaction for my average time for each concentration.
Conclusion and Evaluation:
I predicted that the higher the concentration, the faster the reaction would be. If the concentration increased, the number of reacting particles would increase. This meant that successful collisions were more likely to happen so the rate of reaction increased. This was a correct prediction as you can see from the table above that the rate of reaction decreased as the concentration got weaker. The range of the rate of reaction is 0.002799286.
I also predicted that although the concentration halves each time, the average time will not exactly half because as the concentration is weaker, it takes a longer time for all the particles to collide. However, theoretically the results should have a doubling affect.
For example, imagine that you use:
Concentration = 0.04M
Time = 6 minutes
Rate of Reaction = 0.002777778
Then you use:
Concentration = 0.08M, theoretically you should also get:
Time = 6 x 2 = 12 minutes
Rate of Reaction = 0.00277778 x 2 = 0.005555556
This obviously did not happen, but we did get results near half way.
Example 1:
Concentration = 0.04M
Rate of Reaction = 0.002582845s-1
Now use:
Concentration = double 0.04M = 0.08M
Theoretically, you should get:
Rate of reaction = double 0.002582845s-1 = 0.00516569s-1
This has a difference of 0.000972994s-1 in comparison with the real result for 0.08M, which was 0.004192696s-1.
Example 2:
Concentration = 0.05M
Rate of Reaction = 0.002779708s-1
Now use:
Concentration = double 0.05M = 0.10M
Theoretically, you should get:
Rate of reaction = double 0.002779708s-1 = 0.005559416s-1
This has a difference of 0.000177285s-1 in comparison with the real result for 0.10M, which was 0.005382131s-1.
I have to take into consideration that there is other affecting factors, as the experiments were not done with total fairness, some of which may be beyond our classroom control, such as the temperature in the atmosphere may have changed as bodies moved around the room. It may have other substances that could have been released in the air affecting the timings. Another reason how the temperature may have affected the experiments are because the chemical reaction between hydrochloric acid and sodium thiosulphate is called an ‘Exothermic Reaction.’ ‘Exo’ means ‘to exit’ or ‘leave’, and ‘thermic’ means ‘heat’.
What is an exothermic reaction?
An exothermic reaction involves the release of energy. It is a chemical reaction or a physical change that produces heat. An example of this would be the thermal packets that keep your hands warm when you break them. All chemical reactions require a small amount of energy before they will start. This energy is called ‘activation energy’. The activation energy is used to break the bands in the reacting chemicals. Some particles will have enough energy for the reaction to take place. You can lower the amount of activation needed by using a catalyst. A catalyst is a chemical that speeds up a chemical reaction without being used up. All enzymes are catalysts as well as transition metals.
Diagram of an exothermic reaction:
Activation Energy
Reactants Energy (heat)
Released
Energy
In
Chemical
Products
Time
Breaking bonds require energy, and making bonds releases energy. If more energy is released from new bonds being formed then is needed to break bonds, then the reaction is exothermic. If more energy is needed to break bonds then is made from new bonds, then it is endothermic. So for my experiment, the energy released as heat would have affected the results. We know from previously, that the particles at higher temperature move faster so they are more likely to collide with more energy increasing the chance of a successful reaction. Therefore the more concentrated my Sodium thiosulphate was, the more energy the chemical had in from the beginning of the reaction. As I decreased the concentration, more activation energy was needed disallowing as much (heat) energy to be released causing the more dilute mixtures to take longer to react. I noticed that by 0.04M, the cross on the paper did not completely disappear. I think this may have happened because so much of the energy was used up as activation energy that no (heat) energy was released to catalyse the reaction between the hydrochloric acid and the sodium thiosulphate. They also simply do not have enough reacting particles to collide and allow a “perfect reaction.”
A also drew two graphs using the horizontal axes for ‘concentration’ and the vertical axis for ‘rate of reaction (s-1)’ in my first graph and ‘time taken for cross to disappear (secs)’ on my second graph. In theory, the lines should be a straight line if you go by the doubling affect already mentioned.
Example (this data is not correct)
They both look like I have produced fairly realistic results as they both show a line of roughly the same steepness. In my first graph, (this shows negative correlation) the line of best-fit shows that as the concentration increases, so does the rate of reaction, as does the second graph, (this shows positive correlation). This also proves that my calculations to convert seconds into rate of reaction was correct as the plots are both in a very similar shape:
One anomaly I found on both graphs was the time taken and the rate of reaction for 0.07M. This is quite far away from the line of best bit in comparison to the other plots. So in theory, the plot for 0.07M in relation to the lines should be little higher to be nearer to the lien of best fit. This means the time taken for the cross to disappear should have been longer then what is plotted on the graph. There could be two reasonable explanations of this anomaly and they are:
- An affecting factor interrupted more severely in the experiment for 0.07M then in any other experiment catalysing the reaction.
- Anomalous result blamed on human errors. It is difficult to detect the exact second that the mixture becomes opaque.
Also, in comparison to the shape of the plots on the Time/Concentration graph, to the shape of the plots on the Rate of Reaction/Concentration graph, 0.05M is anomalous result. To resemble the shape of the Time/Concentration graph, the Rate of Reaction for it was plotted a bit lower than it should be. It should be plotted at about 0.00285 to resemble the Time/Concentration graph, however it is plotted at approximately 0.00277, which is a difference of 0.00008.
Anomalies were avoided by repeating the experiments for each concentration three times to get as accurate results as possible. But obviously when calculating an average, a single anomalous result could result in a less extreme anomaly.
For further investigation, I could use another method. Instead of changing the concentration of the Sodium thiosulphate, I could alter the concentration of the Hydrochloric acid. This in theory has the same number of reacting particles; therefore I should end up with the same results as to changing the concentration of sodium thiosulphate. I think that it is obvious that the results would not be the same as my experiment unless I had the exact same working area and affecters, which is impossible to recreate. This would assure that any variables that affected my experiment previously, would do exactly the same to the new method that guaranties me the same results. However, this would not happen, but chances are that my results should be roughly the same. Here is a plan of a plan for possible further investigation:
Aim:
To find out how concentration of sodium thiosulphate will affect the rate of a chemical reaction.
Method:
-
Measure out 20cm3 of sodium thiosulphate in a measuring cylinder, and 20cm3 of hydrochloric acid at a concentration of 0.1M in another cylinder.
- Pour the contents of both the measuring cylinders into a conical flask on top of a paper marked with an “X”, (sodium thiosulphate first).
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of sodium thiosulphate, and 20cm3 of hydrochloric acid at a concentration of 0.09M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of sodium thiosulphate, and 20cm3 of sodium thiosulphate at a concentration of 0.08M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of sodium thiosulphate, and 20cm3 of hydrochloric acid at a concentration of 0.07M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of sodium thiosulphate, and 20cm3 of hydrochloric acid at a concentration of 0.06M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of sodium thiosulphate, and 20cm3 of sodium thiosulphate at a concentration of 0.05M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
-
Then fill another flask of 20cm3 of sodium thiosulphate, and 20cm3 of hydrochloric acid at a concentration of 0.04M.
- Using a stopwatch, record how long the reaction takes until the cross disappears.
- Now repeat the entire experiment two more times to get three sets of answers to calculate an average.
Note:
To get a solution of x concentration, you calculate (1000x)% of 20, and use that amount of sodium thiosulphate. You then add the remaining amount to add up to 20ml of water.
Below is a table showing the amounts that will be used in the experiment:
Apparatus:
Conical flask
Hydrochloric acid
Sodium thiosulphate
Water
Measuring cylinder
Paper with an “x” on it
Pipettes
Timer
