Method
- I will set up the apparatus as shown below.
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The volume of acid will be 100cm3 and the length of the magnesium will be 3cm.
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I will be using different concentrations of hydrochloric acid. They will be 0.5M, 1.0M, 1.5M, 2.0M, 3.0M and 4.0M. I will not use a concentration lower than 0.5M because I think it will take too long to produce 20cm3 of hydrogen. I will not use more than 4.0M because I think the reaction will be too quick for me to stop the stopwatch in time.
- I will put the bung on as soon as I drop the magnesium.
- I will start timing the reaction as soon as the magnesium hits the hydrochloric acid.
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When the amount of hydrogen reaches 20cm3, I will stop the stopwatch and record the time.
Diagram
Prediction
I think that as the concentration of hydrochloric acid increases, the rate of reaction will be quicker. This can be explained by the collision theory. The theory states that for a reaction to happen, the particles in the reaction need to collide with each other. Some of these collisions break bonds and form new bonds. These collisions are called successful collisions. However, for successful collisions to take place, the particles need energy. This is called the activation energy. Therefore, I think that as the number of hydrochloric acid molecules increase, there will be a greater chance of successful collisions with the magnesium atoms. However, from my second preliminary experiment, I found out that during my experiment, the temperature of the reaction will increase. This is because the reaction is exothermic (it gives out heat). This heat will mean that the particles in the reaction will have more activation energy, so there will be more collisions, which will mean that the rate of reaction will increase as the concentration increases.
Results
Conclusion
Graph 1 shows that as the concentration of the hydrochloric acid increases, the time it takes for the amount of hydrogen produced to reach 20cm3 decreases. This is because there are more successful collisions. This happens because there are more hydrochloric acid molecules for the magnesium atoms to collide with. This increases the chance of successful collisions.
Graph 2 shows that as the concentration increases, the rate of the reaction is quicker. This is because the particles in the reaction collide but only some of these are successful. For a successful collision to happen, the particles need activation energy. When they have enough activation energy, they are able to break existing bonds and make new ones, which in this case is hydrogen. When I increased the concentration of the hydrochloric acid, the particles in the reaction were closer together (because the volume of acid stayed the same). This increased the likelihood of collision, which therefore increased the chance of successful collisions. My proof of this was that it took less time for 20cm3 of hydrogen to be produced as the concentration increased.
However, my second graph clearly shows that the line is curving upwards. This means that as the concentration increases, the rate of reaction increases as well, but at a greater rate each time. This can be explained by a preliminary experiment that I did. In that experiment, I found out that from the start of the reaction to the end of the reaction, the temperature of the reaction increased by 1oC. This is because the reaction is exothermic. It becomes exothermic because each time the hydrochloric acid molecules and the magnesium atoms collide, some of the kinetic energy that they have is converted to heat energy when they collide due to the friction, and this heat causes the temperature of the reaction to increase. This means that the particles have more energy, which means that they have more kinetic energy which means they move faster which increases the chances of collisions, which therefore increases the amount of successful collisions, which makes the rate of reaction quicker. This meant that my line of best fit curved upwards, because as the concentrations of the reactions increased, the reactions with more concentration of hydrochloric acid had a higher increase in temperature, due to the fact that there were more particles in the reaction for the temperature to increase. This meant that the rate of reaction increased at an increasing rate as the concentration increased.
Also, from my graph, I can predict that if I use a concentration of 3.5M of hydrochloric acid, the rate of reaction will be 0.425s-1.
In my prediction, I stated that I thought that as the concentration of hydrochloric acid increased, the rate of reaction would be quicker. I now know that this is true because of my graph. The graph clearly shows that as the concentration increases, the rate of reaction increases with it and the line of best fit is curving upwards.
Evaluation
Overall, I thought that my experiment went well and I was able to make some clear conclusions which were supported by the graphs. However, there were many things which may have affected my results and some things which I could have improved on to make the experiment better.
However, I do not think that there were any anomalies in my results. I concluded this because the biggest difference in my results was just 1 second either side of the average time. I do not think that this affected my graph in any way because the actual differences would have moved the points such a short distance, that they would not have affected the shape of my line of best fit in any way.
However, there may have been other factors that affected my graph:
- Firstly, there may have been a delay in me putting the bung on the conical flask and when the reaction started. This delay may have let some of the hydrogen produced to escape into the air. This would have meant that the time that I recorded would include the amount of hydrogen in the gas syringe but also the hydrogen which escaped, which may have increased the recorded times.
- Another problem was that I measured the magnesium with a ruler, and then cut it but I may have measured it wrongly. This could have increased or decreased the amount of time it took for the reaction to take place depending on how wrong my measurement was. This could have affected my recorded times and ultimately, my rate of reaction measurement.
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Also, I may not have had the same volume of acid in the conical flask each time I did the experiment. If there was more than 100cm3, then there would have been more collisions and if there was less, then there would have been fewer collisions. This would have affected my time measurements.
- Lastly, I could not control the temperature of the reaction. If the temperature increased, the rate of reaction would have been quicker. If it decreased, the rate would have been lower. However, I do not think there would be a realistic way of avoiding this problem.
A way I could have improved my experiment would have been to use the same volume of hydrochloric acid and the same length of magnesium each time I did the experiment. I could make the volume more accurate by using a more accurate measuring cylinder. A way to improve the accuracy of the magnesium would be to have someone hold one end of the magnesium and then I would cut it 3cm in. This would improve the accuracy because the first time I did it, the magnesium may have moved while I was cutting it and I may have cut the wrong length.
Another problem with my experiment was that I could not keep the temperature constant. A way to see how temperature changes during an experiment would be to investigate how temperature affects the rate of reaction. I could do the same experiment but I would use a water bath to control the temperature. This is what my results table would look like:
