Investigation to find out how the rate of reaction between sodium thiosulphate and hydrochloric acid is affected by changing the concentration
Background
We place the reaction mixture on a paper with a black cross-drawn on it. When the cross is completely obscured, the reaction will have finished. The time taken for this to happen is the measure of the rate of reaction. We must do this several times, and change the concentration of sodium thiosulphate.
The rate of reaction is a measure of the change that happens during a reaction in a single unit of time. The things that affect the rate of reaction are as follows:
· Surface area of the reactants
· Concentration of the reactants
· The temperature at which the reaction is carried out
· Light
· Use of a catalyst
Rate of reaction = I/T
The factor, which we are using, is Concentration of the reactants.
Temperature
Increasing the temperature of a reaction increases the rate of a reaction. At the higher temperatures reactant particles move faster and collide more often and more violently. Increasing the temperature will cause the particles to move faster with more energy. They will therefore collide. If they have more energy they will collide with grater force this means there is more successful collision per second and therefore a faster rate of reaction.
Surface Area
Breaking up solids into smaller pieces increases reaction rate because this increases the total surface area. Increasing the surface area must increase the number of particles of the solid in contact with the other reactant in solution.
Catalyst
Using a catalyst increases the rate of a reaction. Catalysts are not used up in a reaction. A catalyst is a substance that increases a reaction rate without being used up by the reaction. It does this by lowering the activation energy for the reaction, thus allowing the reaction to proceed at a lower temperature. If a solid reactant or a solid catalyst is broken down into smaller pieces the rate of reaction increases. The speed increase happens because smaller pieces of the same mass of solid have a greater surface area compared to larger pieces of the solid. Therefore, there is more chance that a reactant particle will hit the solid surface and react. Catalysts increase the rate of a reaction by helping break chemical bonds in reactant molecules. This effectively means the Activation Energy is reduced (see diagram below). Therefore at the same temperature, more reactant molecules have enough kinetic energy to react compared to the uncatalysed situation. Although a true catalyst does take part in the reaction, it does not get used up and can be reused with more reactants. A solid catalyst might change physically by becoming more finely divided, especially if the reaction is exothermic.
Concentration
Increasing the concentration of reactants increases the rate of a reaction. This is because there are more particles in the same volume so more collisions are possible every second. If there is more of a substance in a system there is a higher chance that molecules will collide and speed up the rate of the reaction. If there is less of something there will be fewer collisions and the reaction will probably happen at a slower speed. If the concentration of any reactant in a solution is increased, the rate of reaction is increased. Increasing the concentration increases the probability of a collision between reactant particles because there are more of them in the same volume. Examples...
Activation energy
For a chemical reaction to take place, some bonds in the reactants must be broken. The colliding particles must have enough energy to break these bonds. This minimum amount of energy is called the activation energy. Only the very fastest moving particles have enough energy to break bonds.
In gases, liquids and in solution, the particles move at a range of speeds. Some are moving very slowly and others are moving very fast. To react, particles must collide with enough energy and in the correct orientation for bonds to be broken.
Aim
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Activation energy
For a chemical reaction to take place, some bonds in the reactants must be broken. The colliding particles must have enough energy to break these bonds. This minimum amount of energy is called the activation energy. Only the very fastest moving particles have enough energy to break bonds.
In gases, liquids and in solution, the particles move at a range of speeds. Some are moving very slowly and others are moving very fast. To react, particles must collide with enough energy and in the correct orientation for bonds to be broken.
Aim
Investigation to find out how the rate of reaction between sodium thiosulphate and hydrochloric acid is affected by changing the concentration. Also we may try and find out what variable effect the rate of reaction.
Prediction
I predict as we reduce the concentration the slower the rate of reaction will be think that the higher the concentration we use of sodium thiosulphate, the faster the reaction will happen.
Hypothesis
The reason I think that this will happen is because, in a reaction particles collide to give the end results of the reaction. So if there is a higher concentration of particles on a solution, then there will be a high number of collisions. If there are a high number of particles colliding, then there is a higher chance of them reacting. So as we decrease the concentration the rate of reaction will be slower I have found this out from investigating the collision theory
Variables
There could be many things that we could change to alter the rate of reaction but we had decided only to change the concentration. We are going to change the concentration of the sodium thiosulphate. I am going to keep some things the same to make it a fair test we are going to keep the amount of hydrochloric acid the same this will our constant. I am going to change the concentration of water. We are going to use the same equipment and also the same paper with the cross on it.
Safety
There are a lot of safety issues we must abide by in this experiment also we must remember that the substances which we use in this experiment can be very harmful if used the wrong way.
When we do this experiment, it may be necessary to wear safety goggles, as things are very unpredictable, and even though it is very unlikely that the solution would come out of the conical flask during the experiment, one must still be cautious of spills.
We must make sure that coats and bags are all out of the way while doing the experiment. Ties and hair should be tucked out of the way, so they do not make contact with any of the chemicals. It would also be preferable to wear a scientific apron, however this is not essential and we have decided not to wear an apron.
We should also try our best not to spill any chemicals, and we must not eat or drink in the lab while dealing with these harmful chemicals, as they can get on to our hands.
Fair Test
To make this experiment a fair test, we need to make sure we do a number of things. In this experiment we are trying to find the rate of reaction using concentration as a factor, so there is a number of things we need to make sure we do to keep it a fair test. Firstly we need to keep a chemical at a constant concentration. So, in this experiment we have chose to keep hydrochloric acid at a constant concentration. We could have, however, used sodium thiosulphate as a constant, but we had chose to use hydrochloric acid. Next we must make sure that the solution is kept at a constant volume throughout the experiment. If the volume is different, then it could give different results to if it was at a constant volume.
We must also make sure that we add both the water and the sodium thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could effect the results of the experiment.
We must start the stopwatch at the exact time as we put the water and the sodium thiosulphate into the conical flask. To do this it is a lot easier if there are two or more people doing the experiment, so one person can put the two substances in the HCL, and one person can start the stop watch.
Another thing we must do is to make sure that the conical flask is completely clean and free of any water or any other substance before we attempt to start the next experiment. We must also make sure that the measurements are as accurate as we can make them. Also we will have the same person deciding when the cross cant be seen this will illuminate any augment's about when the cross can be seen. To make it a fair test we will make sure that the temperature is the same when the experiment is taking place. To make the results true we will try and make sure that there are no catalysts in the solutions. Also we will try and keep the surface area the same.
Apparatus
* Conical flask
* Sodium thiosulphate (different concentrations)
* Hydrochloric acid (same concentration each time)
* Water (different concentrations)
* Stop watch
* Paper with black cross on it
* Large measuring beaker
* Wash bottle of distilled water
* Small measuring beaker
Method
. Firstly, we drew a black cross on a white piece of paper.
Then, we put 10 cm³ of hydrochloric acid in a conical flask (measuring the HCL with a small measuring beaker)
2. Then put the conical flask on the black cross on the paper. We then added 14 cm³ of sodium thiosulphate to the hydrochloric acid.
3. At this time we do not need to add any water. Immediately.
4. We started the stopwatch.
5. We then waited for the black cross to be completely obscured by the mixture.
6. When this happened we stopped the stopwatch.
7. By this time the mixture was completely yellow.
8. Then we recorded what the time was.
9. We then emptied out the contents of the conical flask, and cleaned it out, and dried it.
0. Next, we added 12 cm³ of sodium thiosulphate; we kept the HCL acid constantly at 10 cm³.
1. Now, because we have to keep the volume the same for it to be a fair test.
2. We added 2 cm³ of water. (We had to add the sodium thiosulphate and the water at the same time for this to be a fair test also).
3. We then recorded the time for the cross to become completely obscured.
4. We repeated this many times, adding different amounts of sodium thiosulphate and water each time
5. Recording the time taken each time.
6. We also work out the averages to make the results more accurate.
Diagram
Analysis
By doing a calculation similar to underneath all the results from the same concentration were added together and divided by three (the number of experiments).
To find the mean this calculation was used
first result + second result +third result / 3
= Average result
The mean results are
From these results we can conclude that by increasing the concentration of Sodium thiosulphate the reaction rate is increased. This can be seen as the highest concentration had the fastest rate reaction and reached 260 cm3,where as the lowest concentration only reached a maximum of 29 cm3 at the end of the two minutes. The increase does seem to be proportional, as the gradient of the line seems to decrease by roughly the same amount each time. This is shown underneath.
To calculate the gradient the method below was used.
=Gradient
Point on Y-axis
Point on X axis
From this the following Gradients were found
Concentration molars Gradient
2.0= 2.3
.6= 1.9
.2= 1.5
0.8= 0.9
0.4= 0.2
The results that were found were what was expected to a certain degree. The reaction rate did not seem to show any signs of slowing down though which is what was expected. This was probably due to the fact that the experiment was not times for long enough though.
The fact that the reaction rate was highest at the highest concentrations shows that the higher the concentration the more particles that are present per set volume which shows increases the rate of reaction. This was not exactly followed in the experiment, which was probably due to a margin of error in the results, but the pattern was still showed. This also proves tat activation energy does not play a part because if this did the gradients of the lines would not increase by the amounts they did. This is due to the fact more/less energy would be needed to start the reaction and there would be a higher or lower gradient of the line so the gradients would not go up /down by the same amounts.
Evaluation
There are anomalous results that I have recorded. The result may have turned out anomalous because of basic human error, or maybe because we measured the substances wrong. It may have even been because we did not clean the apparatus properly.
Apart from this, the accuracy of my experiment has been more or less accurate. Although there are a number of ways in which we could have made the results more reliable. For instance, we could have used better measuring equipment, because the apparatus we used was mainly basic equipment.
Another thing we could have done to bring more evidence is to have tried to use the hydrochloric acid as the variable substance, and used the sodium thiosulphate as the constant substance. This would have brought more evidence to support the idea that the higher concentration of a substance, the faster it will react.
I think that the evidence, which I have received, is enough to reach a suitable conclusion, but there are a few flaws to the experiment (which I have mentioned). Apart from them, the experiment is fine.
The results that were derived from the experiments showed a pattern and showed my prediction was completely supported because the reaction rate was faster when the concentration was higher. There were a few anomalous results, however I think if this experiment was repeated it would give results that matched the pattern. It is not certain why this result was found but it was probably the solutions measured slightly inaccurately because we measured the solutions to the nearest whole number this would account for a slight margin of error.
The measurements were accurate to about cm³ because this how often the cylinders were marked.
The method did show the relationship between the concentration and the rate of reaction but there was a slight margin of error because when putting the solutions into the beakers maybe some acid was lost. To solve this problem we could use a gas syringe, but the problem with this is we could not test the higher volumes of solutions because they wouldn't all fit into a syringe, also the reaction will start happening before the beaker is put over the cross. Which would mean the range of results would be lower which would mean the pattern we got would be less obvious.
The experiments were fair tests to a certain extent but there was a certain margin of error. This was because the results recorded were only to the nearest cm³ and the values of the controlled variables were not exact amounts so the may have been slightly differently each time which would have affected slightly the reliability of the results. This would have made little difference to the results though as the volume of solutions were only measured to the nearest cm³.
To find if the prediction was supported or to find if the reaction rate was faster when the concentration increased we could have changed the constant and seen how the concentration of the new acid affected the rate of reaction. If we was to do the experiment again we would have like to extend the experiment and seen what other variables affect the rate of reaction for example we would have like to investigated if changing the temperature would effect the rate of reaction. From looking at our background work I can clearly see that the higher the temperature there faster the rate of reaction. Also another thing which I would have like to investigate if the experiment was done again is how catalyst effect the rate of reaction I think that this would have been more interesting and more challenging for me and my group.
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