Marble Chips and Acid
I am going to conduct an investigation to determine how changing the concentration of an acid affects the rate of reaction in an acid-carbonate reaction.
When marble chips (Calcium Carbonate) are added to Hydrochloric acid a reaction takes place. Gas is given off as a product of this reaction. The equation for the reaction is: -
2HCl + CaCO3 --> CaCl2 + H2O + CO2
I am going to use the fact that the speed of the reaction can be determined by measuring the amount of gas given off, as carbon dioxide is a product of the reaction.
My prediction is that increasing the concentration (molarity) of the acid will increase the speed of reaction. This is because with a higher concentration of acid there are more acid molecules per unit volume, this means that there are more acid molecules available to collide, thus there will be more collisions, this means there will be more successful collisions, hence the reaction will take place faster.
The apparatus required for this experiment is: -
* A Clamp stand
* A Basin of water
* A measuring cylinder
* A rubber tube
* A sidearm flask
* An accurate stop clock
The apparatus must be set up as shown in this diagram: -
To work out what size marble chips to use in the investigation, I did some preliminary work. I needed to know what size marble chips/powder would give the best reaction times in the acid. I added four different marble chip sizes (9-12mm, 6mm, 2-4mm, powder) to an equal amount of 2M acid and measured how much gas was given off after 1 minute, 2 minutes and 3 minutes. I recorded my results in this table.
Volume of
gas given
off (cm3)
Grain size (mm)
After
min
After
2min
After
3min
9-12
80
210
300+
6
80
300+
empty
2-4
90
330
empty
Powder
94
empty
empty
From these results I have determined that 9-12mm is the best Grain size to use. This is because it is the only one that does not empty the measuring cylinder after 3 minutes. Thus it is the only grain size that will give results for all molarities after 3 minutes. If it does not empty the measuring cylinder after 3 minutes with 2M acid it will not empty the cylinder with any other molarities. I used 2M acid for my preliminary work as it is the highest molarity I will be using in my experiment and I expect this to be the fastest and hence produce the most gas, therefore, if a grain size gives results with 2M it will give results with all my other molarities.
(I used the same method for my preliminary experiment as I will use for my final experiment, this is described below)
This is the method for which I will conduct my experiment (protective goggles must be worn throughout the experiment as it involves using acid): -
) First I will set up my apparatus as shown in the diagram above.
2) Next I will put 50cm3 of 2M acid into the sidearm jar.
3) Then I must carefully weigh 5grams (I allowed a +/-0.1g margin of error) of 9-12mm marble chips and add ...
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(I used the same method for my preliminary experiment as I will use for my final experiment, this is described below)
This is the method for which I will conduct my experiment (protective goggles must be worn throughout the experiment as it involves using acid): -
) First I will set up my apparatus as shown in the diagram above.
2) Next I will put 50cm3 of 2M acid into the sidearm jar.
3) Then I must carefully weigh 5grams (I allowed a +/-0.1g margin of error) of 9-12mm marble chips and add them to the acid. Then quickly place the bung firmly into the mouth of the flask to prevent gas from escaping and causing a loss of accuracy. As soon as the marble chips are added to the acid, the stop clock must be started, this must be done concurrently to avoid loss of accuracy.
4) At 1-minute intervals (from 1 minute to 3 minutes) I will measure where the water meniscus has reached on the measuring cylinder and record these results. Where the meniscus has reached shows the amount of gas produced because the CO2 displaces the water in the measuring cylinder.
5) I will then repeat steps 1-4 and record the results again so I will be able to take an average for more accuracy. Taking averages aids in maintaining accuracy because it helps to reduce errors.
6) I will then repeat steps 1-5 but using 1.6M, 1.2M, 0.8M, and 0.4M acids.
There are several factors that could affect the experiment and make it an unfair test; I must take precautions to ensure that it is a fair test.
'The five main factors which influence the rate of reaction are:
* The temperature at which the reaction takes place. Reactions generally go faster when the temperature is raised. An increase in temperature makes the particles move faster. This means that they will hit one another harder and more often. Both these effects will increase the chance of a chemical reaction.
* The concentration of the reacting substances. Increases in the concentration usually increase reaction rates. An increase in concentration (or pressure in the case of gases) means that the particles will be more crowded together, so they will collide more often, and the chances of a reaction taking place are greater. (*)
* The use of catalysts and inhibitors. Catalysts speed reactions up. Inhibitors slow them down. Catalysts work in a number of ways, but a common one is for the catalyst to act as a kind of framework which holds the reacting particles together for just a little longer than they would during a normal collision. This provides extra time for the reaction to take place.
* The presence of light. Ultraviolet light, especially, may speed up a chemical reaction. Light has a different effect. Most frequently it breaks up a particle like a molecule or an ion into smaller particles, which are more reactive than the particles originally present.
* The size of particles that are reacting. Reactions involving small particles of solid are often very fast. The smaller the particles in a given amount of substance, the greater the area of surface exposed for chemical reaction'
Source: Colin Johnson's Chemistry for GCSE pg. 54-55
*This supports my prediction as stated above
To ensure that temperature does not affect my experiment, I will make sure all the apparatus, chemicals and reactants are maintained at room temperature throughout the experiment. I will carry the entire practical out in a single 1-hour chemistry lesson so that room temperature will not change dramatically during the course of the experiment. This will also aid in maintaining the atmospheric pressure that could also affect the accuracy of the results. Temperature not only affects the rate of reaction but along with atmospheric pressure it could also affect the volume of gas in the measuring cylinder. The molecules in gas are further apart when heated so the volume of the gas will be greater thus more water will be displaced by a lesser amount of CO2 molecules. Atmospheric pressure also affects the volume of gas by making the molecules move closer and further apart.
To make the quality of the results as good as possible I must make sure the concentrations of the acids are measured as accurately as I can.
To ensure a fair test I must also make sure that the sidearm flask is not disturbed, as any movement of the flask will result in more collisions taking place between the acid and Calcium Carbonate, thus speeding up the reaction.
I must replace the marble chips each time the method is restarted as during the reaction the marble chips are being constantly eaten away, this means that the grain size is being reduced. If the surface area of the grains is larger/smaller then there will be less/more molecules available for collision, thus the amount of successful collisions is affected, therefore the rate of reaction will be different.
The following table shows the results I recorded from the practical.
Concentration of acid
(molar)
Ratio acid/water (cm3)
Mass of marble (g)
Volume of gas after 1min (cm3)
Volume of gas after 2min (cm3)
Volume of gas after 3min (cm3)
2.0
50-0
5.032
5.012
98.0
10.0
92.0
215.0
290.0
300.0
Average
04.0
203.5
295.0
.6
40-10
5.015
4.960
89.0
83.0
80.0
76.0
257.0
247.0
Average
86.0
78.0
252.0
.2
30-20
5.049
4.937
49.0
80.0
10.0
36.0
70.0
206.0
Average
64.5
23.0
88.0
0.8
20-30
4.934
4.917
45.0
25.0
93.0
62.0
22.0
99.0
Average
35.0
77.5
10.5
0.4
0-40
5.013
5.001
2.0
4.0
0.0
2.0
20.0
26.0
Average
3.0
1.0
23.0
From my graph I have determined that there is a positive correlation between the amount of time taken and the volume of gas produced. These two variables are also proportional. This means that not only does 1 value increase as the other increases, it also means that as 1 doubles the other doubles (I can tell this as all the molarities gave straight lines). I have also realised that there are no seriously anomalous results that need to be commented on, this shows that my experimental procedure was fairly accurate.
The gradient of each line represents the speed of the reaction (volume of gas released per minute). This is because CO2 is a product of the reaction so the more that is released the faster the reaction is taking place.
As the molarity of the acid increases, so does the gradient of the line. This means that as the molarity increases, so does the speed of the reaction. This means that my prediction was correct.
I would expect there to be a positive correlation between the
molarity of the acid and the speed of the reaction. I can find out if this is true by working out the gradient (speed of reaction) for molarity 1.6, and molarity 0.8. If there was a positive correlation, the gradient of molarity 1.6 would be double that of molarity 0.8. I worked out the gradients by dividing the amount of gas produced after 3 minutes by 3 (y divided by x). The gradients for the molarities were: -
Molarity
Gradient (x/y)
0.4
7.6
0.8
38.8
.2
62.6
.6
84.0
2.0
97.3
After plotting these results on a graph it becomes obvious that there is a positive correlation between the concentration of the acid and the speed of reaction. Although there is an apparent straight line, a few of the points are not directly on the line. This explains why the gradient for 1.6M is not exactly double 0.8M. The point for 0.4M was clearly anomalous. This is because the measuring cylinder I was provided with was unable to determine small values accurately, although my results were accurate enough to give me a straight line on the other graph, the errors were serious enough to make the results obviously anomalous on this graph.
The fact that this graph has a positive correlation also proves my prediction was correct.
Overall the experimental method worked quite well. The accuracy of the results was satisfactory. However, there are several ways the experiment could be improved: -
* I could ensure the temperature for the practical was more consistent by using a water bath. I would need to keep the sidearm flask and the acids used for the experiment submerged in the water bath too ensure they all maintain the same temperature. Obviously I cannot keep the measuring cylinder submerged, however by collecting the bubbles of CO2 through the warm water and the fact the cylinder is in contact with the water in the water bath it should improve the accuracy noticeably.
* To ensure that the concentration of the acids is accurate, they could be titrated against a known alkaline (Colin Johnson Chemistry for GCSE pg.58), this would give the exact molarity of the acid,
meaning that I could compensate when plotting graphs etc.
* To make sure the marble chips are of consistent size and shape, I could pass the chips through a sieve of a known diameter. This would mean that the surface areas of the chips are all similar.
* To ensure that no gas escapes before the bung is inserted into the sidearm flask, there are 2 things that could be done
. After the grains had been placed into the flask and the bung inserted, the acid could be injected into the flask through the bung.
2. A device such as depicted below could be used
If the device is tipped, the acid will fall onto the marble chips and the reaction will begin without the loss of any gas (CO2)
