Surface Area: - if a substance had a large surface area, another substance reacting with it would have more reactant atoms to collide with at any one time, increasing the rate of reaction.
Concentration: - If a solution is more concentrated, then there are more particles of reactant between fewer water particles, resulting in more collisions between reactant particles and thus an increased rate of reaction.
Catalyst: - if a catalyst is introduced, then the reactant particles would stick to its surface and be bombarded by incoming particles, increasing the number of collisions and rate of reaction.
I know that the collision theory works because of an earlier experiment I completed which involved hydrochloric acid and the surface area of marble chips. My observations from this experiment support the collision theory.
Reactions occur over very different periods of time, ranging from the oxidation of iron to an explosion. One takes years, while the other takes milliseconds. The rate of reaction depends on the aforementioned variables. It can be measured by the presence of the following physical changes: -
✓ Precipitation – this is when a solid product of the reaction clouds the solution. To measure the rate of reaction, the solution may be placed between the observer and a marker. The time it takes for the precipitate to form and completely obscure the marker is noted. I will use this method, as it is simple and requires no special equipment, only things I carry around with me all the time, paper and pen. I have done a preliminary experiment to find out how temperature affects the rate of reaction using this method, and I found it an easy way to find out good results. It is for that reason as well that I plan to use this method.
✓ Change in mass – any reaction which gives off a gas can be carried out on a mass balance, and the mass lost can be easily found. The results can then be plotted on a graph; the steeper the line, the faster the rate of reaction.
✓ The volume of gas given off – in a similar experiment, the amount of gas exhausted by a reaction can be measured, and plotted on a graph. Again, the steeper the line, the faster the reaction.
The times recorded are not the rate of reaction. This number must then be multiplied to the minus one to find the actual rate of reaction. I will use the precipitation method to find out how concentration affects the rate of reaction.
This information was obtained from sources noted at the end of this report.
Prediction
I think that an increased concentration of acid will result in an increased rate of reaction, as there is more probability of a collision happening between the reactants, and less probability of water particles getting in the way, therefore increasing the speed at which the product is formed. I think 40g/dm3 will take much longer than 8g/dm3, as there will be more interference from the water particles, and so collisions between the reactants won’t happen as often. I also predict that the rate of reaction for a concentration of 20g/dm3 of HCl will be half that of 40g/dm3.
Variables
As discussed in my preliminary research, a number of factors may affect the rate of reaction. These are temperature, surface area, and concentration. In this experiment, I only want to test the effects of concentration on the rate of reaction, and so I need to limit the effects of the other variables. I cannot control temperature, but I will be conducting the experiment at room temperature, which doesn’t change dramatically in the short space of time needed to complete the experiment. Surface area does not apply because I am conducting the experiment using aqueous substances.
Equipment
To complete the experiment and to ensure my safety, I will need to use the following: -
✓ Conical Flask
✓ A piece of paper marked with a black cross
✓ Thermometer
✓ Sodium thiosulphate
✓ Hydrochloric acid
✓ Goggles
✓ Timer
✓ Measuring cylinder
Diagram
Method
First I will need to set up the equipment up as in the diagram shown above. I will then don the appropriate safety equipment as discussed on the Equipment list. I will then add the two reactants together, at the following concentrations: 40g/dm3 (50cm3 of HCl), 36g/cm3 (40cm3 of HCl to 10ml of water), 24g/dm3 (30cm3 of HCl to 20ml of water), 20g/dm3 (25cm3 of HCl to 30ml of water), 16g/dm3 (20cm3 of HCl to 40ml of water) and 8g/dm3 (10cm3 of HCl to 10ml of water). Then I will begin timing. Once the precipitate has formed and I am no longer able to see the cross marked on the paper, as shown in Fig.2 below, I will then cease timing and begin the experiment again with a different concentration of HCl.
At the beginning of the experiment, the solution is clear, and the cross can be easily seen (1). Mid-way, the precipitate will begin to form, and the cross will look hazy (2). At the culmination of the reaction, the cross will have disappeared from view, as it has been covered by the precipitate (3).
Results
Graph
Conclusion
Although none of my results are perfectly on the line of best fit, they show a strong positive correlation, which provides me with the answer to the question ‘How does Concentration Affect the Rate of Reaction?’. This answer is that the more concentrated something is, the faster it will react with something else. This agrees with my prediction; the less water particles to get in the way of the actual reactant particles, the more chance that they will react and the faster the rate of reaction. Furthermore, as I predicted, a concentration of 20g/dm3 has half the rate of reaction of an experiment with 40g/dm3 (double) that concentration. Therefore, I can say that rate of reaction is proportional to concentration to a ratio of 1:1, and double the concentration equals double the rate of reaction.
My prediction is correct because for the rate of reaction to get faster, the reactant particles need to collide more often. The factor which may limit this is other particles which do not react; they just get in the way. The more concentrated the solution, the less non-reactant particles to get in the way, and the higher ratio of collisions will be between reactant particles, causing the reaction to occur at a faster rate.
Evaluation
Although I achieved my aim with the results, I feel that the final answer would have been more comprehensive had I done the experiment with smaller gaps between each test. This would have allowed me to calculate if the rate of reaction is proportional to concentration or not, and if not, the experiment may have provided clues. I also think that the timer was started too early, before all of the sodium thiosulphate had been added, so maybe the results were a little too slow, but that has not affected my actual goal, of proving that the more concentrated a chemical is, the faster it’s rate of reaction. My results are not very accurate, but in my view it is only the correlation between concentration and rate of reaction that mattered in this experiment, although I would have been able to get more information on the proportion with more accurate results. I feel that the method was a good one, because it was clear and easy to follow. There was, however, one clearly anomalous result. This was probably caused by human error.
If I was to try this experiment again, I would place a light sensor in the place of the cross, and if I could, link it to the timer. I would start the timer, and when the sensor detects an absence of light, it would
automatically stop the timer. I would also measure the change in mass instead of the formation of precipitate, as it would be more accurate. If I was to try a different experiment to find out more about the effect of
concentration on the rate of a reaction, I could react solid magnesium with dilute hydrochloric acid, and then use a gas syringe to measure the amount of hydrogen gas given off over a specified period of time.
Sources
☑ www.bbc.co.uk/schools/gcsebitesize/chemistry
☑ CGP GCSE Chemistry Revision Guide Higher Tier
Author: Richard Parsons Published in: 2002
