Preliminary
Before I can start my main experiment into the rates of reaction I must first find out which metal and which concentration of acid will best suit my purpose. I will do this by carrying out a few simple experiments.
For the main experiment I will need a metal which will show me what happens when acid rain reacts with it, this metal has to have a fast rate of reaction as time is limited for the main experiment. For the preliminary experiment I added several metals to a beaker of 25cm3 of sulphuric acid with a concentration of 1 M. The metals I used are iron, zinc, magnesium, lead, and copper. These are the results for the experiment.
Once I completed this preliminary experiment I found that magnesium would be the best metal as the reaction has a clearly defined start and stop time, and has the fastest reaction. This means I will be able to complete my experiment within the time I have been set.
I will then have to test if the concentrations I have chosen will work sufficiently. I am going to use 25cm3 of sulphuric acid for all of my experiment, as it is an easy amount of acid to work with.
After considering these results I came to the conclusion that the concentrations of acid I had chosen were sufficient for my experiment. Also during this preliminary experiment I have chosen to stir the solution as the reaction takes place, this is because I believe that when the reaction is proceeding there will be a build up of magnesium sulphate this is a salt. This salt will then build up around the magnesium creating a barrier; this barrier will impede the rate of reaction. Therefore stirring the solution will disperse the salt away from the magnesium.
Now that I have finished all of the preliminary experiments I can start the main experiment. Also for the experiments I have used a concentration of 0 M because I wish to see if the reaction caused by acid rain isn’t just by water so this measurement is being used as a control experiment.
Plan for Main Experiment
Diagrams
Apparatus
- Burette
- Beaker
- Stand
- Clamp
- Sulphuric acid
- Glass rod
- Thermometer
- Distilled water
- Stop Watch
- Magnesium ribbon
Method
To ensure my experiment was a safe one I will wear safety goggles all the time while around the sulphuric acid. Also to make the experiment fair we will use the same person stirring and timing the experiment.
- Step 1- First set up a burette and carefully measure out the required concentrations of acid into beakers, for each concentration we will have 3 beakers. This will give us an average and it makes the test even fairer because if any anomalous results were gained then we could fall back on the rest of the results.
- Step 2- Next we will add the magnesium ribbon to the acid in the beaker. We will time each reaction and record how long each reaction will take from start to finish. We will do this for each of the 3 beakers and then using maths get an average of the 3 readings for all of the concentrations.
- Also during the experiment we will stir the solution and keep a thermometer in with the reaction to measure if an exothermic reaction is taking place.
Results
Analysis
Graph 1 shows the relationship between concentration of the acid and the time it takes for the magnesium to dissolve. The graph has a negative correlation and is a curve. The fastest reaction was with the concentration of 1 M, this concentration had an average time of 18 seconds. I cannot take a gradient as it is a curved graph, this curve means that the higher the concentration the faster the reaction. In the graph there were no anomalous results.
When compared with my prediction, graph 1 does prove some of the theory. I predicted that as the concentration increased the time of reaction would get faster and that is exactly what this graph is showing. Yet in order for me to discover if the rate of reaction doubles with the concentration I must create another graph.
Graph 2 shows the rate of reaction compared to the concentration of acid. I worked out the rate of reaction by using this formula:
Rate of reaction = length of Mg / time of reaction.
The graphs line of best fit does pass through the origin, which gives me the conclusion that the graph is proportional. This means that as the concentration increases so does the rate of reaction.
My graph does not show a directly proportional relationship between the rate of reaction and the concentration of sulphuric acid, because of the many anomalies caused from the variables that I could not control during the experiment. So my results are not directly linked but they are near enough to show a relationship.
These variables maybe to do with temperature, and these variables may add to the problem of the relationship, which may have already been disturbed due to the time period of the experiment. The experiment was completed over several days meaning the temperature on certain days may have been higher or lower than on other days having an adverse affect on the experiment. As if the day was hotter then the particles would have more energy than normal therefore the rate of reaction would be quicker and the opposite can be said for colder days.
The graph has a positive correlation and is curved up wards. The highest rate of reaction is 10.6 while the lowest is 0.442 with a concentration of 0.2 M. This graph does have an anomalous result; I believe it is due to the above explanation.
The reason the rate of reaction was highest with the highest concentration of acid was because a higher concentration means that there are more particles in the same volume this then means that there is a higher chance of collisions between the acid particles and the metal particles, as there is a decreased volume for the particles to move about in so more and more collide. So because the concentration was higher it means that more of the particles collided with the magnesium causing a reaction and corroding the metal faster.
Using these 2 graphs I am able to test my prediction of:
Double concentration of acid = double rate of reaction = ½ of reaction time.
I will use a table to show the test.
As you can see the time taken for the reactions are usually only off the predicted mark by 30 seconds once doubled, yet the rate of reactions are not near there predicted outcome.
This leads me to believe that my prediction of doubling the concentration will double the rate of reaction and half the time taken for the reaction is only partly right. As when you double the reaction the time taken for the reaction will come close to the expected amount of time.
Also during the experiment I found that the reaction taking place is that of an exothermic reaction as heat is given off in the process. This means that more energy is given out when making bonds than is taken in when bonds are broken. Energy is needed to break the bonds between the molecules and if the reaction is exothermic it means that the product produced will have a lower energy than the reactants.
Evaluation
I believe that the reliability of my results is the best they could have been under the circumstances yet they could still have been improved with effort and more attention.
All the points on my graphs fit the curves that they produce meaning that none of them are anomalous and that I have completed the experiment properly and efficiently.
The good things that made my results reliable up to a point were the repeated tests on the concentrations giving me a more accurate average reading, also using a burette instead of just a measuring cylinder has also increased the reliability of my experiment by giving me a more accurate measurement of concentrations.
Yet the variables that could affect the experiment were not properly dealt with leaving a small problem with the results. This was because the time to do the experiment in was short meaning the experiment was set over different days so the temperature could’ve changed increasing or decreasing the rate of reaction according to the weather also the rate at which we stirred the solution was different each day as we could not keep a steady pace all the time. This may have altered the rate of reaction causing the anomalous result in my second graph.
There were other problems like the magnesium sticking to the sides of the beaker or floating to the surface, this means that the whole of the surface area of the magnesium would not have been in contact with the acid throughout the experiment.
Some ways to ensure a fair experiment would be to keep the experiment in a room where the heating can be regulated on hot or cold days to a satisfactory level, or an insulated container to maintain a constant temperature throughout the experiment, also we could’ve used an electronic stirring machine so that the constant stirring may be kept at a good pace without stopping. To make the results even more accurate I could’ve done more tests to use for a more accurate average reading i.e. use 5 readings per concentration instead of 3.
To further the experiment in order to fulfil the brief better I would have to use the average concentration of acid rain instead of using concentrations which are not related to the real world, also I should have been able to use actual building materials with which buildings are made of to get a more true to life experiment. I believe I could’ve done more with time, as I would’ve been able to accomplish much more with further experiments and time to think about the processes that occur during the reactions of acid rain.
