Investigate the effect of concentration on the rate of reaction.

A reaction will only start if the particles involved have enough combined energy, to get over the energy barrier between reactants and products. This 'minimum' amount of energy, which is needed, is called the 'activation energy'.

Rate

The rate of any given reaction is the speed at which this reaction takes place. The speed of the reaction can vary enormously. From my prior knowledge, I know that the amount of chemical reaction taking place depends on the amount of chemicals being used within that reaction. The speed of a reaction can be seen either by how quickly the reactants are used up or how quickly the products are forming. I also know from previous knowledge that there are three ways to measure the speed of a reaction:

1. Precipitation
2. Change in mass
3. The volume of gas given off

Factors which affect rate of a reaction

The rate at which a reaction takes place depends on a number of factors. These factors can either speed up or slow down the rate of a chemical reaction.

1. Temperature
2. Concentration
3. Catalyst
4. Surface area

Hypothesis:

I predict that as the concentration of the sodium thiosulphate increases, the rate of reaction will increase.

I have predicted this due to my knowledge on the collision theory. For a successful collision, or reaction, to take place, the molecules must collide with enough energy and with the correct orientation. If these are not correct then the molecules will bounce off each other and not successfully collide. The minimum energy needed for a reaction to successfully occur is called the Activation Energy, EA, which I have previously mentioned.

If you increase the concentration, you increase the total number of molecules and therefore also the number with the required activation energy (but not the proportion of the molecules with more than the activation energy). This increases the number of successful collisions and hence the rate of reaction increases. (See diagrams below).

This means that the graphs drawn up in my results section will have a 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.

I also predict that the higher the volume of sodium thiosulphate, the less time it will take for ‘x’ to disappear. Therefore, the graphs that will be drawn up in my results section showing how quick it takes for the cross to disappear will have a high, negative correlation.

Variables

        In order for my results 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 HCl and thiosulphate will not be mixed up. I will also make sure that the amount of HCl will be the same each time.

All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus make the entire investigation more successful.

Method

For this experiment I took one conical flask and filled it with dilute hydrochloric acid. I placed this flask on top of a piece of paper, which I put in a plastic wallet. On this piece of paper I drew an ‘x’ shape. I positioned this x shape in the centre of the bottom of the flask. In doing this I was able to see the shape through the dilute hydrochloric acid and the bottom of the flask. I then added the sodium thiosuphate in the concentration of 40g/l. As soon as all the solution was added, I started the clock. I then watched as the mixture started to turn cloudy and when the mixture reached a point whereby it had become so cloudy that I could no longer see the ‘x’ at the bottom of the flask, I stopped the clock. This was the time I recorded for the cross to disappear. After doing this, I then rinsed the flask out and repeated the process, only this time I varied the concentration of sodium thiosulphate, and the volume of water. However, I made sure that the concentration of Hydrochloric acid remained constant throughout to ensure the accuracy of my results and to ensure a fair test. I then reduced the concentration of the sodium thiosulphate by taking out 5g/l each test I did. I repeated the same experiment three times in order to collect enough results to get a meaningful average.

Risk assessment

1M hydrochloric acid is an irritant.  Wear eye protection.  Some sulphur dioxide can be given off.  Ensure laboratory is well ventilated.  Pupils should be told to take care not to inhale sulphur dioxide and pour away solutions as quickly as possible, preferably down a fume-cupboard sink. Also, sulphur is given off during experiment. It is irritant, corrosive and irritates the throat. If a group member is asthmatic, move them away from the sulphur, which is given off from the conical flask when experiment is taking place.

Apparatus:

Below is a list of apparatus I will use during my preliminary series of experiments. If everything goes accurately and correctly then I will use this list of apparatus for my real experiment.

• Sodium thiosulphate solution (starting with 40 g/l and decreasing by 5 each time)

• Hydrochloric acid (acid concentration will be fixed) 

• 3 measuring cylinders

• Conical flask
• Piece of paper (to draw a cross on)
• Stop watch

• Eye protection-Goggles

• Water (starting with a volume of 0 and then increasing by 5 each time)

• Apron

Diagram

        Below is a diagram of how I will set up my equipment for my experiment at the moment. However, due to my preliminary series of experiments I will confirm which apparatus and what measurements to use later on.

During my preliminary experiments, everything worked out perfectly so therefore, below is a list of apparatus, which I will use for the real experiment:  

• Sodium thiosulphate solution (starting with 40 g/l and decreasing by 5 each time)

• Hydrochloric acid (acid concentration will be fixed) 

• 3 measuring cylinders

• 1 conical flask
• 1 piece of paper (to draw a cross on)
• 1 stop watch

• Eye protection-1 pair of goggles

• Water (starting with a volume of 0 and then increasing by 5 each time)

• 1 apron

The diagram I provided earlier for my provisional experiments is the exact one I will use for the real experiment.

Sources used

        To aid me with this plan I used the following sources:

• School Key Science GCSE textbook by Eileen Ramsden
• CGP Chemistry Revision Guide by Richard Parsons

Obtaining, analysing and considering Evidence

        I have concluded my experiment and everything worked out to plan, and now that I have finished, below, is a list of my results, which I have gathered from completing it. I have adjusted the results of the time in the second furthest column to the nearest second and the rate of reaction to four decimal places. In order to calculate the rate of reaction, I completed the following sum:        1  

                                                          time

 

Now, I have created a table below, showing the results with the average time and the average rate of reaction over the three sets of results. I have gathered and rounded the average time to the nearest second, and the rate of reaction to four decimal places.

        Overleaf, I have used various graphs to display my results in the tables above. By using these graphs, I have been able to identify any anomalous results, and therefore I have highlighted them in bold red ink above.

        On the page after the graphs, I will re-write the same set of tables as above, but without the anomalous results. Due to this, there will be some gaps in the table. Because the anomalous results have been deleted, this will effect the overall average time and rate of reaction time, and then I will be able to see how my results have changed.

Now, I will create ‘the average graph of how long it took for the cross to disappear’ again but without the anomalous result to see if there is any difference compared to when the anomalous result was calculated into the average. (Figure 6).

On each graph, I have plotted a curve of best fit, and below, are some points to note about the curves:

• There are no particles with zero energy.

• The curve does not touch the x-axis at the higher end, because there will always be some particles with very high energies.

• The area under the curve is equal to the total number of particles in the system.

• The peak of the curve indicates the most probable energy.

Analysis and conclusion:

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 reactions take place per second.

Using my two final graphs (Figure 5 and figure 6), with the lines of best fit, I can draw a conclusion from my experiment.

Firstly, I can see that with figure 6 (that plot the concentration of sodium thiosulphate against time taken for the reaction to take place), the graphs have a negative correlation, meaning that as the concentration increased, the time taken for the reaction to take place decreases.

This was because the hydrochloric acid and sodium thiosulphate molecules collided with enough energy and the correct orientation. As the concentration was increased, the total number of molecules increased and therefore also the number with the required activation energy. This increased the number of successful collisions and hence the rate of reaction increases.

For most of the graphs, there was a reasonably steep curve meaning that the decrease in time taken for the reaction was far more rapid.

For the final average graph, plotting the concentration against the reaction time (figure 5), there is a positive correlation. This is because 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. There was also a slight curve on the line of best fit. This was because as the concentration was increased, the increase in the rate of reaction will not be exactly the same.

Evaluation

Overall, I think that the results were very reliable as they turned out to be just what had been expected (figure 5 and 6). I had collected plenty of evidence and it was as accurate as it could have been. I didn’t have to repeat any results from the experiment, however, there was one anomalous result. It came from the first attempt (figure 1), which can be seen in the results section. This result had a reasonable difference compared to the other results. Nevertheless, there could be some perfectly logical explanations such as the following:

• There were two other people in my group who could have measured the amount of sodium thiosulphate and hydrochloric inaccurately.

• There was not enough time for the whole set of results to be collected in just one lesson. Therefore, the equipment had to be left for a few days until our next lesson. When the equipment was used again, another member of the group may not have washed the conical flask out after and before the next set of results were taken. Therefore, there may have still been some remains of sulphur from the previous set of results.

• The amount of water in the test tube and beaker might have differed.

• A new conical flask was used half way through the first set of results because the first one smashed.

As a whole, I think that the experiment worked out extremely well. However, one way which I could have used to improve my investigation is to have recorded even more results to get a meaningful average and also to repeated the anomalous result so that I had a full set of results for each attempt. Another technique I could have used to improve my investigation would be data logging.

        I would have loved to extend this investigation in many ways. Firstly, I could have kept the concentration of sodium thiosulphate the same and changed the amount of hydrochloric acid to see what the difference would be compared to my results.

This would have been very enjoyable. However, I did not have enough time and there was also a lack of equipment which I would have needed.