To investigate the effects of change of concentration on rate of reaction - between Calcium Carbonate and Hydrochoric Acid
To investigate the effects of change of concentration on rate of reaction - between Calcium Carbonate and Hydrochoric Acid
Planning
Aim: To accurately investigate the affect of concentration of acid on the rate of reaction between Hydrochloric Acid and Calcium Carbonate.
Theory
Carbonates are compounds made of a metal cation and a carbonate anion. Calcium carbonate is an example of a carbonate. All carbonates react with acids to give off carbon dioxide.
Calcium Carbonate (CaCo3) is a white, insoluble solid that occurs naturally as limestone, chalk, marble and calcite.
In the case of this investigation the reaction between dilute Hydrochloric Acid and dry Calcium Carbonate the formula equation is:
CaCO3(s) + 2HCl (aq) CaCl3 (aq) + CO2 (g) + H2O (l)
The Rate of Chemical Reaction can be defined as "the time it takes for an amount of product to appear or for an amount of reactant to disappear." In this reaction we will be studying the amount of carbon dioxide that has 'appeared' over a set time period.
Before a reaction can occur a number of things must happen:
* Particles must collide before they can react
* There must be enough energy between the particles for bonds to break
* They must collide at the appropriate angle
In liquids like HCl, particles are in constant motion, but only some collisions are successful. For a reaction to occur between molecules, bonds must be broken before they can form new bonds. For this to occur there must be energy involved. This is called the activation energy.
Concentration - the measurement of a solute dissolved in a solvent expressed in moles (see below) per dm¯³
Mole (Mol) -The SI unit of the amount of a substance. One mole contains the same number of particles as there are atoms in 12 grams of the carbon-12 isotope.
The variables that help speed reactions (temperature, concentration, surface area, and catalysts) do so because they are able to lower the activation energy amounts required.
Concentration of one or more reactants changes the rate of reaction because when there are more particles in a given volume, there is more chance of collisions between particles occurring.
Increased concentration = more particles
Increased number of particles = more collisions
Increasing the number of collisions = more successful collisions
More successful collisions = rate of reaction increases
Other independent variables that affect the rate of reaction are:
Temperature: Increasing the temperature in the reaction means that more particles have the
activation energy. This means that more collisions are successful, thus the rate of reaction increases.
Surface Area and Particle Size: Only the particles on the surface of a solid are exposed to collisions.
Breaking up the solid makes lots of smaller solids but the surface area overall is increased. More collisions can occur with the new amounts of exposed surface, increasing rate of reaction.
Catalysts: A catalyst allows reactions to occur with less activation energy required, without getting involved in the reaction. With the lower requirements for activation energy more successful reactions will occur. (In our investigation, catalysts do not apply.)
Experiment Plan
My hypothesis based on the theory on the previous pages, is that as concentration of the HCl acid is increased, the rate of reaction will also increase.
Concentrated HCl acid will react much faster and produce more carbon dioxide than diluted acid. (See concentration theory)
I will test this hypothesis by reacting different ...
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Catalysts: A catalyst allows reactions to occur with less activation energy required, without getting involved in the reaction. With the lower requirements for activation energy more successful reactions will occur. (In our investigation, catalysts do not apply.)
Experiment Plan
My hypothesis based on the theory on the previous pages, is that as concentration of the HCl acid is increased, the rate of reaction will also increase.
Concentrated HCl acid will react much faster and produce more carbon dioxide than diluted acid. (See concentration theory)
I will test this hypothesis by reacting different molar concentrations of Hydrochloric Acid with set amounts of calcium carbonate chips. The Carbon Dioxide will be collected over a certain length of time. The amount of carbon dioxide collected in each experiment will give an indication to the rate of reaction.
I expect my graphs to look similar to this sketch graph:
I intend to repeat each experiment twice and compare the figures, the results will be plotted graphically to show their relationships.
The graph is expected to have a curve where there is a large increase in the amount of gas produced per each mole, while slowing and not producing extra excess as the larger the molar the concentration. This is because no matter how much the molar concentrations of HCl increase, past a certain critical point the HCl particles are not exposed to enough Calcium Carbonate particles to react and produce CO2 .
(see reasons for graph shape in the analysing data and conclusion secttion for more detail)
I will use molar concentrations of HCl starting from 0 moles (control) to a 4 molar solution.
INDEPENDENT VARIABLE: Concentration of Acid
DEPENDANT VARIABLE: Rate of reaction
To stop the other variables affecting the experiment and making the test inaccurate and unfair, these precautions will be taken:
Surface Area of Reactant:
This variable can be avoided by making sure that all the Calcium Carbonate marble chips have the same surface area.
Note: the marble chips provided by the Chemistry laboratory at school are of various shapes and sizes. This means that although mass can be the same, surface area is different for each chip. This variable cannot be accurately controlled, but to try to make the chips as uniform as possible, I will carefully select chips of a similar mass and size.
Temperature:
To avoid this variable, the experiment will be done in an environment where temperature is kept at a constant.
Catalyst:
No catalyst will be used in this experiment.
Other than these variables, it must also be noted that the volumes of acid used will be the same (10ml) and the HCl will come from the same source.
In my experiment the following equipment will be used:
Boiling Tubes: These will be where the reactant will react together. A boiling tube was chosen over a
test tube because it can contain far more material.
Bungs with glass Delivery Tube: These will be fitted firmly across the top of the boiling tube, to make
sure that no carbon dioxide is lost during the experiment. The delivery tube will be used to connect the boiling tube to the gas syringe. This is so that the carbon dioxide can be measured.
Gas Syringe: This is used to measure the volume of gas
Thermometer: This will be used to find out the room temperature of the lab where the experiment will
take place. This can then help make the comparison between resistivity of constantan at room temperature to the resistance we find in our experiments.
Stop Watch: This will be used to time the experiment; to ensure accurate recording over a lapsed
time, of the amount of Carbon Dioxide emitted by the reaction.
Balance: To make sure that the marble chips are as similar as possible, all the chips will be weighed.
Because the balance is electronic, there can be no reading error, though zero error must be noted. The balance was used, as it is accurate up to one-tenth of a gram
Clamps and Stand: These will be used to hold the apparatus (the gas syringe) in place during the
experiment.
SAFETY:
Although we are using only dilute lab Hydrochloric Acid, it must be noted that care should be given with the usage. Care must be taken to not allow the acid to touch the skin, eyes e.t.c and hands must be thoroughly washed after the experiments completion.
TABLES for this experiment will look like this:
This is because this is the neatest and clearest way to present recorded information.
Molar Concentration
CO2 Collected (cm³) 1
CO2 Collected (cm³) 2
Average CO2 Collected (cm³)
Mass 1
(g)
Mass 2
(g)
Average Mass (g)
0
0.5
.5
2
2.5
3
3.5
4
THE PROCEDURE:
* First, put the equipment in place as shown in the diagram. Make sure there are no holes where the C02 can escape.
* Next weigh all the marble chips. Put the chips into pairs with the ones who have an equal mass to themselves. These pairs will be the ones who will be compared for each reaction.
* Measure the temperature of the acid.
* Take the first marble chip and put it into the boiling tube, which has already been filled with 0.5M of HCl. Quickly place the bung over the top and start timing.
* After one minute record how much CO2 has entered the gas syringe.
* Do the same with its 'sister' chip.
* Repeat the experiment for all the chips but at different concentrations of solution (see chart above).
ERRORS
All experimental measurements are subject to some errors, other than those caused by carelessness. The most common errors, which occur, are parallax errors, zero errors and reading errors.
Parallax Errors: The error, which occurs when the eye is not placed directly opposite a scale when a reading is being taken.
This must be taken in to account when measuring the CO2 level.
Zero Error: The error, which occurs when a measuring instrument does not indicate zero when it should. If this happens, the instrument's inaccurate "zero reading" should be subtracted or added from any other reading taken.
This must be taken into account when using the balance.
Reading Error: The error due to the guesswork involved in taking a reading from a scale when the reading lies between the scale divisions.
Obtaining Evidence
Molar Concentration
CO2 Collected (cm³) 1
CO2 Collected (cm³) 2
Average CO2 Collected (cm³)
Mass 1(g)
Mass 2(g)
Average Mass (g)
0
0
0
0
.0
.2
.1
0.5
.0
.5
.25
.1
.1
.1
.0
6.0
7.0
6.50
.2
.2
.2
.5
2.0
2.0
2.0
.1
.3
.2
2.0
2.0
4.0
3.0
.3
.2
.25
2.5
3.0
4.0
3.5
.2
.1
.5
3.0
4.0
4.0
4.0
.0
.0
.0
3.5
4.0
5.0
4.5
.2
.1
.15
4.0
5.0
5.0
5.0
.2
.2
.2
This table shows the relationships between Molar concentrations, amount of CO2 collected and the mass of the marble chips. The mass was taken to show that overall, the marble chips were similar, even if their exact surface area could not be defined/calculated accurately.
For the following graphs, the mean amount of CO2 gas was used on the y-axis so as to make the findings as accurate as possible.
Analysis and Conclusion
Graph one shows the relationship between volumes of CO2 gas produced against Molar Concentration of HCl. This graph starts with a steep incline until about 1.5 moles - 2 moles where the gradient shifts and flattens out. Both segment lines of the graph are proportional. The probable reason for the change in the proportional values at 1.5-2 moles, is that the chips are not exposed sufficiently (remember that the marble is a chip piece not powder) CaCO3 particles to react with the extra particles of HCl added each time with the more concentrated solutions. In fact it reaches a critical point (the curve between 1.5 moles and 2 moles) where the bulk of the reaction that can be completed by the amount of reactants has finished. The line points after the critical point continues to rise at a steady rate, indicating that the reaction has slowed down (this is because the amount of CO2 gas represents - more or less - the rate of reaction). The higher quantities of particles provided by higher concentrations still increases the chance for successful collisions, thus reactions, during the experiment.
Graph two shows the amount of CO2 gas produced at various molar concentrations. The x-axis (which shows time) is not relevant as it just represents the fact that all were timed from 0 seconds to 60 seconds.
The lines used to join the plotted points to the 0 point also have no relevance as they represent the origins of each point. It cannot be said, in this experiment, if the lines actually should be linearly proportional or not.
This graph helps prove the point that as the molar concentration of HCl is increased; the rate of reaction is also increased.
Rate of reaction is in direct relationship to time and amount of product produced. The more of a product produced in a set time, the faster the rate of reaction. From the graph it is clear that 4 moles (which is the highest concentration) produced the most gas in the set time. The rate of reaction for the greatest concentration is highest. The same is also true for the smallest concentration, which has the least rate of reaction.
Thus it can be concluded that as the molar concentration increases, the rate of reaction also increases, even if by small amounts.
EVALUATION
This experiment went very well. The background theory supported the hypothesis, which was supported by the experimental results. During the experiment, there were no anomalous results, and the change in the graph curve/gradient was explained through scientific knowledge.
Various sources were used in compiling background information, making the theory sounder and arguably more accurate. The experiment itself was conducted in a professional manner, trying to make the results as accurate as possible by making the experiment fair. Controlling all the independent variables and repeating the experiment twice in order to check if there were any anomalous results ensured this.
If the experiment could be extended; then investigations on how much CO2 is lost by dissolving into the water within the acid could be pursued. This would give increasingly accurate results, especially for the low concentration solutions, which have more water to dissolve the carbon dioxide into.
If the experiment could be repeated then there would be some minor changes made. Firstly, -because the surface area of the marble chips cannot be measured and thus cannot be controlled - powdered calcium carbonate could be used. This would mean that the surface areas exposed would be more uniform.
The timer is also affected as it is pressed by people whose reactions may not be fast enough to start immediately from when the reaction first started. This is important, as there may be minor changes in the amount of carbon dioxide produced in the time set.
Overall, it must be said that the experiment was highly accurate and safely carried out. It proved the original prediction/hypothesis: As concentration of the HCl acid is increased, the rate of reaction will also increase.
