Preliminary work
In my preliminary work I will need to find out the best method to use for my experiment. There are several methods I could use, they are: Water displacement, pH reading, change in mass and appearance. I will also need to find the ideal volume of acid and the ideal mass of marble chips.
pH method:
- First I will measure 1g of small marble pieces on a balance accurate to 3 decimal places.
- Place the chips into a 100 cm conical flask.
- Measure 20 cm of 2M hydrochloric acid with a 25 cm measuring cylinder.
- Measure the pH of the acid with a digital pH scale.
- Pour the acid onto the marble pieces and monitor the pH every 30 seconds.
Water displacement method:
- Repeat first procedure up to and including number 3.
- Fill a tub up with water as well as a 100 cm measuring cylinder.
- Place the measuring cylinder upside down in the tub without spilling any water.
- Place the rubber tubing from the conical flask inside the bottom of the measuring cylinder.
- Add the acid to the chips and place a bung in the conical flask.
- Measure the volume of gas in the measuring cylinder every 30 seconds.
Appearance method:
- Repeat first procedure up to and including number 3.
- Add the acid into the conical flask and time how long the chips take to disappear.
Change in mass method:
- Repeat first procedure up to and including number 3.
- Place the conical flask onto the balance accurate to 3 decimal places.
- Add the acid and measure the mass.
- Record the mass every 30 seconds.
Results – Change in mass – Exp.1: 0.109g in 180 seconds
Exp.2: 0.104g in 180 seconds
The mass decreased by measurable amounts as the reaction took place.
Water displacement – Exp.1: Unable to measure (too rapid)
Change in pH – Exp.1: 2.5 in 180 seconds
Exp.2: 1.2 in 180 seconds
The pH gradually went down as the reaction took place.
Appearance – Exp.1: 6 minutes plus. (Too long to finish)
From these experiments I have found the following:
- The pH method was too unreliable and no conclusions could be made from the results as sometime the readings went up when they should have gone down and any change was too small to be of value.
- For the water displacement method I found that after around a minute and a half the cylinder had nearly filled up with gas and not enough results could be found to make a decent conclusion.
- For the appearance method I found that the reactions were taking too long and that I would not be able to complete the experiment in the time given.
- I was able to get a full set of results and to the best accuracy for the change in mass method so I have come to the conclusion that this is the best method to use.
I will now repeat the change in mass method but this time I will use 10 cm of Hydrochloric acid instead of 20.
I found that the percentage error is less for 20 cm so I will use this volume for my experiment.
I also repeated the experiment with 0.5g of marble chips instead of 1g.
I found that the percentage error is less for 0.5g so I will use this mass for my experiment.
N.b. in an experiment for it to be classified as accurate the total errors must be less than 10%. The reliability is how small or large the range is.
E.g. Exp.1 - 125, 126, 127 Avg. 126
Exp.2 – 136, 126, 116 Avg. 126
Experiment 1 is more reliable as it has a much smaller range.
Plan
First I will measure 0.5g of small marble pieces on a balance accurate to three decimal places then I will place the chips into a 100 cm conical flask. Measure 20 cm of 5M hydrochloric acid with a 25 cm measuring cylinder. Place the conical flask onto the balance accurate to three decimal places then add the hydrochloric acid and record the starting mass. Continue to record the mass every 30 seconds for 120 seconds. Repeat the experiment for 4, 3, 2, 1 M acid. Repeat the entire experiment two more times so that we can make a better conclusion from the results.
I must make sure that I am wearing safety goggles at all times so that my eyes are protected from any acid. I must make sure that I do not spill any acid as it is corrosive and could cause damage to me and other people. If I or anyone else comes into contact with the acid they should wash the area thoroughly with water and report the incident. To make the test fair I should use the same set of equipment, especially the balance as others may have slightly different readings.
Equipment
-Balance accurate to 3 decimal places.
-A conical flask. (100 cm )
-A stopwatch
-Marble chips
-Hydrochloric acid (1-5 M)
-Safety goggles
Results
Conclusion
From this experiment I have found that the higher the concentration the higher the change in mass. This is because more acid molecules are colliding with the calcium carbonate molecules and therefore producing more carbon dioxide that can escape. The result of more carbon dioxide escaping means that the mass goes down faster i.e. the rate reaction speeds up. The graph shows that the concentration of acid is indirectly proportional to the change in mass. It also appears that the change in mass goes up by less each time apart from the 5M value. On the basis of my results, my theory that if you double the concentration you double the rate of reaction cannot be true except for the first two concentrations as the averages are roughly doubled. I believe the reason why the change in mass ‘slows down’ is because there is too little surface area for the acid molecules to react with so therefore eventually the values will level out as the concentration gets higher. If I was to have a larger surface area of marble then I believe that the rate of reaction will double as you double the concentration, as my results started off doing so, but after that the surface area was just too small to allow the concentration to make much of a difference.
Evaluation
If I were to repeat my experiment I would increase the surface area of the marble chips to further investigate my theory so that the higher concentrations of acid can take advantage of the extra particles available.
Most of my results were very reliable as the error bars are very small but a couple did have fairly larger error bars, which tells us that those results may not be very reliable. The results were recorded to 3 decimal places which is necessary to provide the best accuracy for the experiment. There is one point on the graph that does not fit the pattern. Where as the difference between all the points before are gradually decreasing this point goes up, but the error bars for this point were fairly large so it is an unreliable piece of data. I think the method was suitable and worked well as overall I received a good set of data. I think my results have given me enough evidence to support my conclusion but I think that the experiment should be repeated so that I can investigate my original prediction better than in this experiment by changing the surface area of marble chips.
I could also do further work by changing the chemical used, such as magnesium instead of calcium carbonate, to prove that the idea of the higher the concentration the faster the rate of reaction, works in the same way as in this experiment.
To further improve this experiment I could conduct it in a place where such an accurate balance is less likely to be affected by foreign materials from the environment. The anomalies in the experiments for the 4 and 5M concentrations may be caused by the different amounts of surface area available and that some of the chips may have had slightly larger surface areas than in the other experiments which meant that more acid molecules could collide with the marble molecules. With a smaller concentration this would make very little difference but as the concentration gets to this level there are many more particles to take advantage of the extra surface area provided. It is very hard to keep the surface area the same with the given resources.
