Below is the formula for the reaction
CaCO3 +2HCl CaCl2 + H2O +CO2
The collision theory
The collision theory simply states two things are needed for a reaction to take place:
- Particles must collide before they can react.
- The colliding particles must, between them, have enough energy to cause bonds to break.
The collision theory therefore leads us to the following statements:
- Higher concentration means a larger number of particles in a volume. If there are more particles, there will be more collisions. More collisions mean a larger number of successful collisions. Therefore the rate of reaction increases. We can take this further to say that if concentration doubles, rate of reaction doubles, so rate of reaction is directly proportional to concentration, because if the number of particles doubles, the number of collisions will double.
- The temperature of something is the measure of the average kinetic energy of the particles in it. If the average kinetic energy of the particles increases, then more particles will have enough energy to collide successfully. We can take this further to say that if temperature doubles, rate of reaction doubles, so rate of reaction is directly proportional to temperature.
- A greater surface area means a larger number of particles are exposed; therefore there will be a larger number of collisions between particles. We can take this further to say that if surface area doubles, rate of reaction doubles, so rate of reaction is directly proportional to surface area.
- A catalyst can speed up the rate of reaction; the catalyst lowers the amount of energy needed for a successful collision, therefore in the presence of a catalyst, e.g. an enzyme, more collisions will be successful.
Apparatus
Marble chips
Side arm conical flask
Delivery tube
50cm3 beaker
Standard range thermometer
2 molar hydrochloric acid
Safety goggles & apron (if possible)
Gas syringe
Stopwatch
Top pan balance
25cm3 measuring cylinder
Distilled water
Retort stand and clamp
Diagram
Method
1.Set up the apparatus as shown in the diagram.
2.Measure 20cm3 of hydrochloric acid in a 25cm3 measuring cylinder.
3. Then weigh 10 grams of marble chips, which have a similar surface area.
4. Put the marble chips into the side arm flask.
4. Pour the Hydrochloric acid into the side armed flask without it touching the side arm as when it gets wet this stops the gas syringe working properly.
5.Then quickly place the bung on the flask and time the reaction.
6. After every 15 seconds record the result off the gas syringe, do this for 1 minute.
7. Record the results found in a table.
8. Repeat steps 1-7 for each concentration (these are 2 molar, 1.5 molar, 1 molar, o.5 molar and as a control- 0 molar. These can be obtained by diluting the 2 molar solution in the following ways: for 1.5 molar mix 15cm3 of hydrochloric acid with 5 cm3 of water, for 1molar, mix)
9. To ensure the accuracy of these results repeat steps 1-8 up to three more times.
Safety
- Wear safety goggles to protect your eyes.
- Read instructions of how to do the experiment before starting.
- Read labels on chemical bottles before using, as hydrochloric acid is an irritant.
- Replace bungs straight away.
- Wash of any spillages on the skin with water immediately.
6. Pour the solution produced through the sieve, collecting the
marble chips, and therefore not blocking the sink (remember never to throw solids down the drain, always dispose of them properly)
7. always remember to clean all spillages after the experiment is over.
Keeping the other variables constant
To keep the other variables such as the surface area and mass of the marble chips constant I will have to use my own judgement and a top pan balance. I will weigh out 10 grams of marble chips before each test and keep this mass constant. I will keep the time constant by timing each reaction for one minute.
Preliminary work
The preliminary work helped me to improve the method for the investigation and also gave me a few ideas on how the results would turn out, which in turn helped me to decided on which concentrations to use. The objective of the preliminary work was to find out approximately what concentration sped up the rate of the reaction.
For the preliminary work I tested how much carbon dioxide was produced by the reaction between calcium carbonate and Hydrochloric acid at different concentrations.
After my preliminary experiments I decided that a two changes had to be made to my method. Firstly, I felt that the concentration should be varied using a whole number ratio as otherwise it is do difficult to be precise. The other problem I felt was that the method in which I measured the reaction rate was very accurate but measurements should be taken every 15 or 20 seconds not 10 because while writing down the results the next time interval could pass. Also as the experiment was progressing I learned that due to an increase in temperature the experiment was exothermic, however this change in temperature cannot be easily controlled and therefore forms a possible error in the investigation.
Analysis:
All four graphs rise, which means that the amount of CO2 evolved is proportional to time taken. This can also be seen as the longer you leave the two reactants the more gas will be produced.
However the graph of the 2molar acid solution began to curve at the end this is explained in my conclusions. Rate of reaction: 5.89cm3/second (amount of carbon dioxide at 10 seconds divided by 10 i.e. the gradient of the graphs)
The line on the graph of the 1.5 molar solution is almost perfectly straight, however it is less steep than the graph for the 2molar solution. Rate of reaction: 3.86 cm3/second (amount of carbon dioxide at 10 seconds divided by 10 i.e. the gradient of the graphs)
The graph for the 1molar solution is far less steep than that of the 1.5 molar solution. Also it does not seem to curve off at the end. After 15 seconds the amount of carbon dioxide evolved does not fit the best fit line and therefore does not follow the pattern of the other points on the graph and should be considered and anomaly. Rate of reaction: 1.75 cm3/second (amount of carbon dioxide at 10 seconds divided by 10 i.e. the gradient of the graphs)
The least steep of the four lines is the graph of 0.5 molar hydrochloric acid. This graph has two anomalies. Rate of reaction: 0.073 cm3/second (amount of carbon dioxide at 10 seconds divided by 10 i.e. the gradient of the graphs)
Conclusions:
Before reading this you may want to refer to the background information section.
The 2molar graph is steep due to a fast rate of reaction. This rate is fast because there is a high concentration of HCl and therefore there are more collisions (see collision theory). It is likely that there will be double the rate of reaction to the 1molar due to double the amount of HCl molecules and half the amount of water molecules between the marble chips and HCl molecules. Due to this high rate of reaction the marble chips have become depleted in less than 60 seconds and we have witnessed the fall of the curve of the graph because there was not enough of one of the reactants to allow the reaction to continue at a similar pace.
The other graphs are all less steep due to less reactant molecules and more water molecules.
The anomaly on the graph for 1molar may have been due to the fact that because there is a large amount of the total carbon dioxide produced that dissolved in the water. The amount of carbon dioxide then can no longer dissolve because the solution becomes saturated.
Similarly the first anomaly on the graph for the 0.5 molar solution may have been caused by the same problem that caused the anomaly stated in the previous paragraph. However the second anomaly could have been created by a sudden opening of a large surface area to the hydrochloric acid under circumstances such as the acid corroding through the marble chip into a hollow within it or if a particularly (not mixed up properly) high concentration of HCl within the solution drifted over a piece of marble.
Evaluation:
I believe that the nature of my experiment was accurate in showing how acid rain (Hydrochloric acid) can corrode marble (chips).
I believe that my method was accurate to an extent. However there were possible errors. E.g. there is a small amount of time where gas can be lost between when the solution is put into the side arm conical flask and when the bung is placed into the test tube.
My results may not have been perfectly precise due to dissolved Co2.
If I were able to repeat my experiment I would use a data-logger on my gas syringe, and a more accurate syringe with a larger range as one of my biggest errors came from having to reset the syringe during the experiment if the maximum amount was exceeded.
For further work I would wish to be able to repeat the experiment with the improvements I have stated and then try to do the same thing with different forms of calcium carbonate and then with different acids (e.g. sulphuric acid)
