Diagram: The diagram below shows the equipment we will use for the experiment.
Magnetic stirrer Water
Method: First we will cut a piece of magnesium 1cm in length. Then we will add 40ml of Hydrochloric acid to a beaker (these measurements we agreed were the best in our preliminary work and will use throughout the experiment), and heat it to the appropriate temperature using a Bunsen burner (all equipment shown in the diagram above). We will then place the beaker of acid on a magnetic stirrer, which causes a vortex in the acid, into which we will put the magnesium and simultaneously start the stopwatch, and time how long it takes for the magnesium to react away. For each temperature we use, we will experiment three times, and will use 6 different temperatures, ranging from 20ºc – 70ºc, going up in 10ºc steps. We will record our results in a table, and from this, we will be able to work out the rate of reaction for each temperature.
Prediction: I predict that as the temperature increases so will the rate of the reaction.
I predict that for every 10ºc rise in temperature, the rate of the reaction will double.
Explanation: To back up my prediction, I have researched into the collision theory and reaction rates etc.The diagram below shows the Maxwell-Boltzmann distribution curve. It shows the distribution of energy within a reaction. In a reaction, only a small amount of particles have enough energy to collide and react. The diagram shows the line of activation energy, meaning any molecules with greater energy than this value are the only ones, which have sufficient energy for effective collisions. As the temperature rises, the number of molecules with greater energy than the activation energy value increases, causing more, greater collisions and therefore a faster reaction rate. There is a disproportionally large increase in the number of high-energy collisions. It is only these collisions (possessing at least the activation energy) which result in any action. This is the Maxwell-Boltzmann theory, which also tells us that for every 10ºc rise, the rate of the reaction doubles.
Collisions in all but the very simplest of chemical reactions also require to have correct orientation for effective collisions, meaning that the reactive atoms in each of the two molecules have to be the parts to collide with each other in order for the reaction to take place. The angle is important, as you can see in the diagrams below. If the particles collide head on (a direct collision) then the impact is greater so the collision is more effective. However if the molecules collide at a different angle (an indirect collision), they may simply bounce off each other so the collision is not effective.
The Maxwell-Boltzmann Distribution curve and line of activation energy
Number of
Particles
Energy
Observation
During the experiment we took our results (times) down in order to be able to see what had happened, and to draw a conclusion from the results.
Results. I have recorded my results in a table, and calculated the rates of reaction. To calculate the rate of a reaction you divide 1 by the average time and multiply by 1000.
Table to show the results of the experiment to find out how temperature affects the rate of reaction between Magnesium and Hydrochloric acid
Analysis
From this table I can graph the results by plotting temperature against the rate so that any patterns stand out and the results become clearer. I can also identify and explain any anomalous results for my analysis. I have drawn the graph on graph paper. (See attached).
The graph shows my results and proves that my prediction was correct, that as temperature rises, so does the rate. However, the graph does not show that the rate of reaction has doubled. For example, the rate of the reaction for 20ºc is 20, and 30ºc (10ºc higher, so it should be double) is only 25. I backed up my prediction with scientific knowledge and research into A-level chemistry. The information I discovered could have been wrong in the book, and therefore the prediction would be inaccurate. I think that we have anomalous results due to a problem or possibly human error during the experiment. We can at least say that the rate of reaction rose, even though in our case it did not double.
I have redrawn the Maxwell-Boltzmann distribution curve showing how much more energy has been gained, and worked out on the table below, by how much the rate of reaction has increased by for each temperature. Again you can see the anomalous result which I have highlighted on the table.
This shows that, from our results, the rate of reaction does not double for every 10ºc rise, in fact the rate of reaction decreased between 40 and 50ºc, suggesting our results were incorrect, which I will later find reasons for, and explain this in my evaluation.
Maxwell Boltzmann distribution curve showing the change in energy.
Number of
Particles
Energy
Conclusion
The rate of reaction rose and should have doubled, because when temperature increases, more particles gain enough energy to make effective collisions (see above). (For every 10º rise in temperature, the reaction rate should double). Because of this particles collide more frequently, and because they are travelling faster due to their gain of energy from the rise in temperature there is more chance of direct collisions occurring, as opposed to indirect collisions. As I have explained before a direct collision is one that happens at a speed and angle by which the particles do not bounce apart, and the impact is great enough for a reaction to take place. An indirect collision is one that happens at an angle and speed that causes the particles to bounce apart and the impact is not great enough for the reaction to occur.
More particles possess the activation energy or more when they increase in temperature because the initial bonds are broken, releasing more energy. The Maxwell-Boltzmann distribution theory tells us that for every 10ºc rise in temperature the reaction rate doubles, meaning double the amount of particles have the energy to perform effective collisions.
Evaluation
Anomalous results: From looking at the graph I can see that there are three points which do not fit in with the others, they are not on the line of best fit. They are at 20º where the rate is 20, 40º where the rate is 48, and 50º where the rate is 37. On the results table in my observation, the only apparent anomalous result is at 50º where the rate decreases from the previous temperature where it should have increased. Thee could have been a number of reasons for this, as explained below.
Problems and solutions: We had some problems during the experiment procedure. It was difficult to see when the magnesium had reacted away, so some of the reactions may have been mis-timed. Also when we heated up the acid, it was very hard to get the exact temperature, as when the required temperature was reached, we took the beaker of acid away from the Bunsen burner and the temperature continued to increase for a further 3-4 degrees. This means we could have had the wrong temperature for some of the reactions.
For future experiments, solutions to these problems need to be found so that the experiment will run accurately.
- To resolve the problem that we can’t see when the magnesium reacts away we could speed up or slow down the magnetic stirrer. Unfortunately, this does not help, as when we slowed down the magnetic stirrer, although we could see when the magnesium reacted away, the magnesium floated, so not all of its surface was in contact with the acid, a problem we were presented with in our preliminary work. When we speeded up the magnetic stirrer, the acid covered the magnesium, but again, we were unable to see when the magnesium reacted away.
- To resolve the problem that the temperature was too high, we could remove the beaker from the heat when it reached 3 or 4 degrees below the required temperature, so the required temperature is reached and maintained for the reaction.
Other causes for anomalous results could include:
- Wrong measurement of acid or magnesium
- Wrong times recorded
- Mis-calculations or errors on graph
These problems are of human error, so during experiments we must concentrate, be precise and fair at all times.
Further work: We could take this experiment further in a number of ways.
- We could experiment using a wider range of temperatures
- We could try a different method of following the reaction to see how results differ, eg gas collection as explained in my preliminary work.
Dissociation: We could also experiment to find out how the dissociation of different acids affects the rate of reaction. Dissociation is used to measure the strength of an acid. It depends on the equilibrium of an acid. The greater the extent of dissociation, the greater the amount of hydrogen they contain, making them stronger acids. We used hydrochloric acid which goes 100% in a reaction. A strong acid we could use is sulphuric acid, which reacts 90%, releasing 2 hydrogen atoms for each time it reacts, making it stronger than hydrochloric acid which releases 1 hydrogen atom for each time it reacts.
Distribution curve showing the amount of particles which gain enough energy for effective collisions after a rise in temperature.
This diagram shows particles colliding with the correct orientation for an effective collision. The two reactive atoms in each molecule have to collide in order for the collision to be effective.
