Concentration - 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.
Method
Before I did the final experiment I carried out a preliminary experiment. To do this, we mixed solutions of water and Sodium Thiosulphate to make concentrations of (1, ½, ¼, ¾) m/cm . Then we added 5ml of Hydrochloric acid and timed the reaction until we could see the “X” no more. See below for diagram of apparatus:
Safety
A pair of goggles will be worn during the experiment in order to protect the eyes.
Fair Test
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 HCl and thiosulphate will not be mixed up. The amount of HCl will be 5ml each time, and the amount of thiosulphate solution will be fixed at 40ml . 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.
Here are the results that I obtained from my preliminary experiment:
After doing the preliminary experiment I concluded that the concentrations we used were to strong, too fast respectively in reacting to be worth testing and including in my final results. So when doing the actual, final experiment we will dilute the concentration of the solutions to 1/10 strength.
When recording the results I will note down the “concentration (m/cm )” and “time taken for X to disappear” in a table like the one below:
Once I have obtained my results I will plot a graph of rate over concentration to find out the relationship between concentration and rate of reaction. To find the rate of reaction I will do the calculation 1/time but then I will multiply this by 1000 to make the figures easier to plot on a graph.
I know from being taught that, as we increase the concentration of the Sodium Thiosulphate the time taken for the “X” to disappear will decrease and that the rate of reaction will increase.
E.g. If I double the strength of the concentration the rate of the reaction will double too!
I would expect my graph to look something like this:
Obtaining Evidence
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.
Using the graphs, with lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that with the scatter graphs (that plot concentration against time taken for the reaction to take place) the graph has negative correlation, meaning that as the concentration increased the time taken for the reaction to take place decreases.
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 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.
The graph for concentration against rate shows that when the concentrations were relatively low (0.02, 0.04, 0.06m/m ), the increase of rate x1000 was also fairly small (increasing from 4.00 to 5.70 to 9.46). For this to full make sense i need 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 because the particles must over come the energy barrier. The minimum energy needed to overcome this is known as the activation energy. If the frequency of collisions is increased the rate of reaction will increase. But the percent of reacting collisions stays the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.
Evaluation
The method used was good and produced good results but it could have been improved. The results are mainly good, there are no odd results and everything came out as expected. This could mean that the experiment was done perfectly but it does not.
Improvements that could be made if the experiment was repeated:
When doing the results that took less time, it would have been more accurate to have two people so one person could put the substances together while the other person started timing. Obviously, it would have been good to do more repeats. Three tests were managed each time but if one had been wrong, this could have dramatically changed the average time and therefore rate of reaction.
However, thankfully I used the most accurate measure equipment available to me, a glass burette for measuring concentrations Sodium Thiosulphate and a pipette for measuring the Hydrochloric Acid. Distilled water was used as it is pure and has no impurities because the impurities in the water could have affected my results. I believe that my results are reliable enough to make a valid conclusion as there were no anomalous results and they all stuck close to the lines on best fit on the graphs.
Overall, I think this was a good experiment and the best that could have been done with the time and resources available. The results supported my predictions and they seem to be reliable results.
If I was to investigate the rate of reaction between Sodium Thiosulphate and Hydrochloric Acid further, then I could investigate another variable. E.g.
Temperature change – in accordance with the collision theory. When the temperature is increased, the particles will have more energy and move faster. 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 react successfully.
- Or Even –
Use of a Catalyst - A catalyst works by lowering the activation energy need to overcome the energy barrier. A catalyst would not affect the distribution of the results. Because a catalyst allows the reaction to take place with a lower activation energy, however, a greater proportion of particles will have energy greater than the activation energy.
