Low Concentration High Concentration
From the diagrams, we can see that it would be very hard to collide with a particle in the first diagram because there are not many to do it with. In the second diagram we can see that it is harder not to collide with particles because there are so many (highly concentrated). This shows that it would be quicker to react in a substance that has a high concentration like the second diagram than the first diagram.
The following is a step-by-step guide towards the procedure used
- Fill the plastic container around a quarter of the way with water.
- Measure 1.5g of calcium carbonate. All of the marble chips should be small in size.
-
Measure 25cm3 of hydrochloric acid.
- Place a burette of water upside down in the plastic container.
- Place marble chips inside a conical flask with a thistle tube and small funnel.
- Connect this to the plastic container using a delivery tube.
-
Pour in the 25cm3 of acid inside the conical flask and start the timer.
- Observe the time it takes for the burette of water to become empty.
- Record the time in minutes and seconds.
- Repeat same experiment two more times.
- Use a different concentration for the hydrochloric acid.
The marble chips should be small in size for every single experiment because it gave the most reliable results throughout the preliminary tests. Large chips took a lot longer to break down because the particles have a smaller area to work on. The surface area for smaller chips is greater so there is an increase in the rate of reaction. By using small marble chips throughout, it will be a fair test.
Repeating the same experiment with the same concentration of acid makes the results more reliable than just doing it once. Each experiment will be done three times, but an average time will be worked out for the results. To make the results precise, I will be calculating and recording the times to the nearest second and plotting a graph showing the time in seconds.
This reaction involves a carbonate and an acid, producing a salt, water and carbon dioxide. The carbon dioxide flows through the delivery tube and into the water in the plastic container. Because it is less dense than water it will rise to the top of the burette taking over the water. The time the carbon dioxide takes to fill the burette tube is the time it takes for the reaction, i.e. the reaction rate.
I predict that as the concentration of acid increases, the rate of reaction will decrease. This is because there will be more particles for the reactant between the water molecules. This will mean that the chance of collisions occurring is more likely. Therefore a higher concentration will result in a faster rate of reaction.
Analysis & Conclusion
All the experiments have now been completed and the results table below shows what evidence has been obtained to prove that a higher concentration results in a faster reaction rate.
The table above gives the rate of reaction time in minutes and seconds for all attempts for all the different molars. For every attempt, looking down the column we can see that the rate of reaction decreases because the concentration increases. This is a trend present throughout all the attempts. The average time has been calculated in the last column. This gives the results in a more reliable way. The results can be relied upon, but there is an instance where the reaction rate increases instead of decreasing in the average section. The time has gone up by 2 seconds for 2.5 moles concentration. The following graph shows the affect of concentration on the rate of reaction.
From the graph I have come to the conclusion that the more concentrated a substance is, the quicker the reaction will occur. So the rate of reaction will be increased if the substance is more concentrated. The graph shows a decreasing time. There is a reasonable negative correlation here. This is because all the points aren’t so close to a straight line but this is probably due to some minor flaws in the procedure, which will be analysed in the evaluation.
The only anomaly here is the reaction using a 2.5 mole concentration. The reaction rate increases by 2 seconds instead of decreasing. The difference between a 2 molar and a 2.5 molar concentrated acid is not very big at all. They are similar to one another and therefore during the three attempts one of the attempts for 2.5 moles resulted in a slower reaction. This gave a lower average time. This does not affect the overall claim because the difference between 2 molar and 2.5 molar concentrations is so small that it is likely that it could have happened anyway.
The results gathered supports my prediction that as the concentration of acid increases, the rate of reaction will decrease. This occurred because there were more particles in a given volume of 25cm3. These particles were more ‘squashed up’ in the more highly concentrated acid. This meant that there was not a lot of space to move around, so the chance of colliding with the calcium carbonate particles was greater. This increased the rate of reaction because there were more collisions and they happened quickly.
The shape of the graph shows that as the concentration increases the rate of reaction. A low concentration decreases the rate of reaction. Therefore we can say that the rate of reaction is proportional to 1/time. The curve of the graph shows that the rate of reaction is proportional to 1 divided by the time. The shape of the graph represents an inversely proportional shaped graph. The graph shows a decreasing time, so the rate of reaction is proportional to 1/time or it is inversely proportional. With this statement I can now prove that concentration is inversely proportional to time. The following graph shows this:
The straight line of this graph shows that the concentration is proportional to 1/time or inversely proportional to time, therefore the concentration is proportional to rate of reaction. The conclusion supports the predication made because I stated that the rate of reaction would decrease and that is what happened. The conclusion has explained this by showing how this is possible. By looking at the results in some cases I could also say that as the number of moles for the concentration doubled, the reaction rate halved. A perfect example of this is when we increase the concentration from 0.25 to 0.5, an increase of 100%. The average rate of reaction time halves from exactly 15 minutes (900 seconds) to 7 and a half minutes (450 seconds). This theory is not so evident throughout all of the results. This does occur but the results are not exactly half, but are close to it.
Evaluation
The results from the many experiments performed showed a clear pattern with one anomaly. The pattern was that as the concentration increased the rate of reaction decreased. The evidence obtained was very reliable. This was due to the repetition of the same experiment two more times. I took the average of the three results and used that to produce the graph. I think that my results were quite accurate and precise because they were recorded to the nearest second. However, there was one anomaly and this was a result of performing three attempts and taking the average. Somewhere in the three attempts, the results were lower than expected so I obtained an anomaly. The results that I obtained were a good set of results due to repetition and precise recording of the results. However, I think I could have improved the results by performing 5 attempts instead of 3, giving even more reliable results that were very trustworthy. This would have also prevented any anomalous results from occurring.
There was one major problem in the procedure that I used. I found out that by covering the top of the small funnel, the rate of reaction increased. This was shown by the number of bubbles being given off and appearing at a faster rate in the burette. Covering the small funnel kept it air-tight and therefore the reaction was quicker. I found this out during the middle of the 1.5 mole experiment, but I quickly changed back so it did not affect any of the results. This was a flaw in the design of the experiment. Keeping air out of the system would have made the results more efficient because the presence of air could not be controlled. The effect of the air could have made the results unreliable, however, because each experiment was exposed to the same amount, this should account for a fair test. I think that this could be solved by securing the top making it air-tight and not allowing any air to get inside the system. It could be sealed using a cork-screw that is the same size as the flask top. This could be the improvement to the current method.
I think the evidence obtained is enough to support the conclusion, but I don’t think that it proves collision theory, just supports it. The evidence does support the theory of collision but with these results and this experiment, you cannot prove that this theory actually exists.
The evidence that I have obtained here is quite reliable, but further work could be done to make the results more reliable and prove other things. Further work could include measuring the rate of reaction. There are 2 ways in which this could be done:
-
This reaction produces a gas (CO2). If we place the flask on top of a mass balance then we can observe the change in mass and see how long it takes for it to disappear.
- Another method is to use a gas syringe and observe the volume of gas that is given off when there is a reaction. Note the change in volume and how long it has taken.
For both of these methods you would need to vary the concentration each time and then see what the effect is on the rate of reaction.