The diagram below helps to see this. You can see that when the particles are smaller the acid molecules will more easily be able to attack the marble chips.
Figure 1.
Temperature- If the temperature is increased particles move quicker as they take in the heat energy and covert it into kinetic energy. With this added kinetic energy the molecules move faster and when they collide with more force with more collisions the rate of reaction is increased. Collisions aren’t always successful but the more of them obviously the higher the chance of successful collisions and also if the molecules is moving faster than it is more likely to be successful. Sometimes when molecules have low amount of energy they bounce apart and do not react collide more often and with more force so there are more successful collisions and the rate of reaction increases. When molecules have a low amount of energy they bounce apart and do not react.
Stirring- If I stirred the reacting products it would slightly affect the rate of reaction as by stirring heat is created and again the molecules have more energy. Also energy will be pushing the particles round and round and they will be travelling faster and so again it is more likely that a reaction will be more successful. Also by stirring you break up the molecules and the marble chips when you knock against them which creates a larger surface area and this leads to a faster reaction.
Concentration of hydrochloric acid- this is the proportion of hydrochloric acid in the solution and this is the one factor I will change. In my prediction I have stated why this factor effects the rate of a reaction.
Presence of light- Increasing Ultra-violet light helps bonds of existing molecules to break in some reactions. This factor is hard to control but all my experiments should take place in a sunlit lab and therefore the variation of light intensity should be quite limited.
Safety
To make sure we are safe in the lab whilst doing this experiment here are some safety precautions I will follow.
- Wear safety goggles because we are dealing with an acid that could be very harmful to eyes and corrosive if spilt on skin.
- Wear an apron. This is because the acid could react with our clothes and cause damage.
- Never run in the lab so things are less likely to be knocked over.
- Stand up when carrying out experiments so I can move away if I am in danger.
- Tuck stools in to make sure no-one falls in the lab
- Put all bags in the shelves so people do no fall over which could lead to things being knocked over and could harm the person.
Prediction
I predict that the higher the concentration the faster the reaction will take place this is because in higher concentrations of acid there are more hydrochloric acid molecules per set volume. This means that there will be a higher chance of the calcium carbonate molecules colliding with the hydrochloric acid molecules and reacting.
The increase in the concentration should be directly proportional to the increase of the reaction rate at a given time. This is because by doubling the number of hydrochloric acid molecules present the chance of a collision should be doubled, as there is now twice the possibility of a collision-taking place.
Activation energy should not be changed in this experiment though as the same amount energy will be required to break the bonds. This is because each particle has the same amount of energy the only reason the reaction is faster is because there are more particles.
I predict that there will be an optimum weight loss and then no more CO2 will be produced. I think this because the reaction will not continue forever unless we add more reacting molecules to the flask. Therefore all the graphs should slowly curve and end at the same weight loss. The highest concentration of acid will reach this weight loss the quickest as the reaction will be the faster as there will be a greater chance for collisions to occur.
Predicted Graph: Figure 2.
Equipment
- 100cm³ conical flask- this will be used for the reaction to take place because it is large enough to hold the reaction
- Measuring cylinders- to measure appropriate amount of liquids in.
- Balance- to weigh the reactions.
- Cotton Wool- this will be placed on top of the conical flask and used to stop any acid or reactants from escaping.
- Stopwatch- to indicate how long the reaction has been going on for.
- Marble chips
- Hydrochloric Acid.
Diagram
Method.
- Weigh out the correct amount of calcium carbonate (15g)
- Measure out correct amount and concentration of HCl so the HCl and water = 50g.
- Weigh the marble chips and the cotton wool and record this weight.
- Next weigh the flask with the acid in.
- Add the two weights together, this is the starting weight of the experiment.
- Drop the marble chips in the acid, start stopwatch and bung the conical flask with cotton wool.
After set amount of time (15 seconds) record the weight on the balance.
7) Repeat experiment twice for each concentration, this is too limit inaccuracies and for our results to be as accurate as possible as we will be able to take averages.
Preliminary experiments.
Before we collected conducted our main experiment we had to identify how many grams of marble chips would be sufficient to cause a recordable mass loss for each percentage of acid. We experimented with different weights of marble chip. Below are the results we gained.
1g of marble chips.
After a minute it was very clear that we needed a lot more than 1gram of marble reacting with the acid otherwise our results would tell us nothing and the weight would stay pretty much the same.
6g of marble chips.
10g of marble chips.
From these preliminary results we decided to use 15 grams of marble chips for our final experiments this was because with this weight we can be sure that there will be a recordable mass loss on each results for every concentration of acid. With both 6 grams and 10 grams the mass loss was not significant enough. This is because in these experiments we used 100% acid and although the mass did decrease when we used lower concentrations of acid the mass loss would be very small.
In our preliminary experiments we also had to decide which concentrations of acid we would experiment with. We found that with concentrations of 20% acid the mass loss wasn’t significant enough, this is proven in the results table below.
10g of marble chips.
From the preliminary experiments we found that for our main experiment we would use 15 grams of marble chips and use acid from 50% to neat concentration. We are confident that this will provide use with useful results and we will able to draw conclusions from the results.
Results:
When plotting my graph I found it hard to plot the 70% concentration points as they were very close to the 60% concentration marks. I think the results was anomalous as sometimes unlike with all the other results the 70% concentration mass loss was under the 60% concentration mass loss and the results should not have done this. Therefore in the table below, that I used to plot my graph, I have not included the results for 70% concentration.
Conclusion:
My graphs show a definite trend in the results. Firstly it is clear that the higher the concentration of acid the greater the loss in mass. We can see this as at 50% concentration the total mass loss was 0.385 grams and at 100% the total mass loss was 1.345. This happened because when there is a higher concentration of acid there are more reacting particles present per set volume which means the reaction rate of reaction will be faster, CO2 will be produced more quickly. I expected this to happen. In my prediction I also said that the increase in the concentration should be directly proportional to the increase in total mass loss. This is because by doubling the number of hydrochloric acid molecules present the chance of a collision should be doubled, as there is now twice the possibility of a collision taking place initially. From the graph showing the total mass loss at each concentration it is clear that these two factors are going up proportionally, this is because the line is straight. However, I don’t think that this would have happened if we had left the experiments for a longer time then the line of this graph would be vertical as each concentration would eventually loose the same amount of mass. On the graph that plots time with mass loss it is clear that each line is straight and this means that the reaction was still occurring even after 2 and a half minutes. In the 100% concentration the line does seem to be more curved and mass loss would reach a peak and then stop. On and then perhaps the final two or three results would have produced a more curved graph rather than this graph which is fairly straight. My predicted graphs showed this happening but this did not happen with my graph as we didn’t leave our experiment for long enough. The mass loss would eventually stop decreasing because eventually all the possible reacting molecules will have reacted together and C02 will stop being produced.
Evaluation
The results that were derived from the experiments showed a pattern that supported my original prediction and results that can be explained using accurate scientific knowledge and understanding. However I was surprised that the reaction showed little sign of slowing down. I think that for further work on this experiment I would let the reaction be left for longer, this would prove that there is a maximum amount of CO2that could be produced from a certain amount of Calcium carbonate and acid. My predicted graph showed what would happen if we did leave the experiment for a longer time and this is what ours should have looked like. We should have varied the time we left our reaction to take place as part of our preliminary experiments but we thought that 2 minutes 30 seconds would be enough time, we have learnt from the experiment that this is not so. This does not mean our results are wrong though they are just unable to prove eventually the mass will stay the same, it will stop decreasing. I am unsure how long it would take 50% acid concentration to finish reacting. It may not be practical to leave the experiments running for this to have happened but as I say if I had a long time to repeat this experiment I would leave the experiment running for longer than my graphs would be more similar to those predicted.
The result for 80% concentration and the lower range results of the 70% concentration I think are anomalous results, as they don’t quite fit the lines of best fit drawn of each graph. For 80% the overall mass loss wasn’t as big as it should have been. I am not sure how we got an inaccurate result but it may have been down to human error. We may have measured the marble chips wrongly and perhaps didn’t put 15 grams of chips in either experiment. It is strange though as all results for 80% are similar. Another possibility is because the surface was different for each chip the total surface area was different this would be cause a slight difference in mass loss. Having said this I think we gained more than enough evidence to support a firm conclusions that backs up my prediction. All the other lines fitted perfectly and showed things that could be explained.
Also for further work on this experiment I would extend the concentration range from our experiment I can not certain that if the concentration keeps increasing the rate of reaction. To do this experiment I would have to use hydrochloric acid stronger than 2 molars. These reactions would have to conducted very carefully though as hydrochloric acid of this concentration can be very dangerous.
Bibliography.
Figure 1 and 2: NEAB: Material and their properties. Edited by Richard Parsons.
Information and research taken from various websites.
Background information from lessons and from Letts Science by Graham Hill.
Information and research taken from Encarta 97.
