All the above-mentioned factors will make sure that the test is fair.
Sodium Thiosulphate + Hydrochloric Acid
Na²S²O³ (aq) + 2HCl (aq) > 2NaCL + S + H²o + SO²
Prediction:
Before starting my experiment I can predict that the following things will happen. I think that as the concentration of acid is increased, the rate of reaction will also increase. Having said this I could also say that if we double the concentration, we double the number of particles. I looked in my chemistry revision book, which was called GCSE Chemistry Success Guide and was published by ‘Letts’; it states that more collisions increase the rate of reaction. I can therefore conclude that the rate of reaction is directly proportional to the concentration.
From this knowledge, I predict that as the concentration of thiosulphate is increased the rate of reaction will become faster. For example a solution with 50cm3 thiosulphate will react quicker than a solution with 0cm³ of thiosulphate. This will mean that both graphs drawn up in my analysis will have positive correlation, and will 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.
Planning
Collision theory
When the concentration of acid is increased there are more particles Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully. If solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur. All this can be understood better with full understanding of the collision theory itself:
If the concentration of a solution is increased there are more reactant particles per unit volume. This increases the probability of reactant particles colliding with each other.
Pressure - If the pressure is increased the particles in the gas are pushed closer. This increases the concentration and thus the rate of reaction.
Surface Area – If a solid is powdered then there is a greater surface area available for a reaction, compared to the same mass of un-powdered solid. Only particles on the surface of the solid will be able to undergo collisions with the particles in a solution or gas.
The rate of the reaction will slow down during the chemical reaction thus the rate curve is usually steepest at the beginning of the reaction. As the reaction progresses the concentration of reactants decreases and the concentration of products increases. When the rate curve is horizontal the reaction is complete.
The initial rate of reaction depends upon the initial rate of concentration of at least one of the reactants. The higher the initial concentration the higher the initial rate.
Planning
Prediction
I can predict that my graph will be curved and steeper at the beginning as this is when the rate of reaction will be at the fastest.
I can also predict that the fastest experiment should take around 30 seconds with a high concentration of acid; the longest should take around 3 minutes with a low concentration of acid. From this prediction I know that I will not have to alter my plan as the time taken for the reaction is not to long.
Aim: To find out if the rate of reaction is effected by the concentration of an acid.
To find the rate of reaction of thiosulphate solution with hydrochloric acid that will vary in concentration.
Apparatus: list of the apparatus I will use for the experiment.
The apparatus used for the following experiment is shown below:
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150cm³ conical flask
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10 cm³ conical flask
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50cm³ conical flask
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10cm³ measuring cylinder
- Test-tube
- Stop clock
- Test tube rack
- White tile
- Marker pen
Chemicals:
The chemicals I will use for this experiment.
- Sodium thiosulphate
- Hydrochloric acid
The measuring cylinder is easy to use and it is easy to read from, the marker pen will be used to mark a cross on the white tile. The stop clock is also easy to use and the test tube rack holds the solutions, which is very helpful when carrying out an experiment.
Planning
Safety
Before starting there are a few safety aspects, which we have to be careful of. Firstly, safety glasses should be worn at all times, and secondly, we have to be careful because the acid is highly corrosive to our bodies and should be kept away from skin and from our clothes.
Method
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I put 10cm³ of thiosulphate solution into the conical flask.
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I added 40cm³ of water.
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I measured 10cm³ of hydrochloric using the 10cm³ cylinder and poured it into the test tube.
- I drew a small cross on the tile and put the flask on top.
- I poured the acid into the flask and started the clock.
- I swirled the flask.
- I timed how long it took for the cross to disappear.
- I washed out the flask thoroughly. I repeated the method using the next volumes of thiosulphate and water as the table shows.
The apparatus will be set up as in the diagram shown below:
Obtaining Evidence
Table of results
I repeated the experiment three times.
Room temperature was 22 degrees.
My experiment was done almost exactly as I had stated in my plan. The only difference that I can think of is that I started by using 10cm³ of thiosulphate instead of 50cm³. However this will not affect my results.
Observations
I observed that when I shook the solution it started to become slightly cloudy. This meant that I helped speed up the reaction by mixing it. As time went by the solution became more and more cloudy until I could no longer see the cross.
Analysis
Analysis of my results:
My results are plotted on two graphs, which are shown on the next page.
The first graph shows the concentration over time and the graph has a line of best fit.
In my graph I have used the averages, this will make it easier to read.
I have also drawn a graph, which displays concentration over time, times 100. I did this to make it easier to read.
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
Using the graphs, which have lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that the graph that plots concentration over time has negative correlation, meaning that as the concentration increased, the time taken for the reaction to take place decreases.
Naturally, the above means that the graph-plotting rate against concentration has positive correlation – as the concentration is increased so does the rate of reaction. This is because when the concentration is increased the number of particles increases. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully, and when solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur.
Analysis
From my graph that shows the rate of reaction I can see that when the concentration is low, the rate of reaction is not as high as when the concentration is large.
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 percent result 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 that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area
Below is a diagram of the energy barrier; before the reaction can take place the particles have to overcome it.
Evaluation
The graph shows, along with my tabulated results, that my prediction is right. My prediction of the concentration being directly proportional to the rate of reaction is also correct. This is highlighted by my graph. My prediction of the collision theory is also correct. I have learnt that the rate of reaction is low when the concentration is lower than that of a higher concentration.
Conclusion:
Before I did the experiments I made some predictions they were that the more concentrated the acid was the quicker the reaction would be thus the cloudier the solution would become in a shorter time. My prediction was correct as what I predicted happened.
Evaluation and Improvements:
The main drawback with my experiment was the accuracy. To get the best range of results possible I needed to be as accurate as possible. With my equipment I think that I achieved this, but I could have used more complex measuring devices.
I had no anomalies in my results as they all fitted into my pattern on my graph.
If I were to do the experiment again I could improve it by using a wider range of concentrations, which would lead to a more accurate line of best fit. I would also have done each reading more than three times and taken a larger average, which would have increased the accuracy. Possible problems with my coursework could have been caused by human error. Firstly, there could have been error in stopping the stopwatch, although this would have been minimal. Also, some of the solution could have escaped
If I were to change a variable, I would use the temperature. I think that this would be interesting to see if the rate of reaction was directly proportional.
Overall, however, I think that this experiment has been successful because my prediction has been accurate which shows that the results are accurate. I have also made sure that the test was done fairly.