Kinetics
Michelle_Michy
Examplethathasbeensubmittteddontcopyanyinformation.docx
Quantitative Analysis- Physical Chemistry
Aim: To investigate the effect of increasing the concentration of hydrochloric acid on the reaction between hydrochloric acid and marble chips.
Introduction :
Before introducing the experiment, we should first fully comprehend what is rate and how to measure it. The rate of the reaction is the speed with which reactants disappear and products form for a particular reaction. Mathematically, the rate of the reaction can be measured through the rate law.
rate=k[A]m[B]n
This expression is the rate law for the general reaction above, where k is the rate constant. Multiplying the units of k by the concentration factors raised to the appropriate powers give the rate in units of concentration/time.
The dependence of the rate of reaction on the reactant concentrations can often be expressed as a direct proportionality, in which the concentrations may be raised to be the zeroth, first, or second power. The exponent is known as the order of the reaction with respect to that substance. In the reaction above, the overall order of reaction is given by the following:
order=m+n (Chemistry LibreTexts, 2019)1
(Chemistry-helper.blogspot.com, 2019)2
In the following reaction, hydrochloric acid reacts with calcium carbonate to form calcium chloride, water, and carbon dioxide. We used marbles as a reactant since they are metamorphic rocks composed of calcium carbonate. This reaction is a neutralization reaction.
The rate of the reaction can be measured through multiple tests, where in this case we will measure the loss in mass as the gas is evolving, since carbon dioxide can cause a significant decrease in mass due to its density.
Prediction:
Variables:
|
Independent Variable |
Dependent Variable |
|
Concentration of hydrochloric acid (mol/dm3)
|
Mass of carbon dioxide (grams) |
|
Other controlled variables: Time between each interval (30 seconds) Temperature (oC) Pressure (kPa) The size and number of the marbles (grams) (surface area)
|
Materials:
|
Hydrochloric acid (2 mol/dm3) (En.wikipedia.org, 2019)4 |
5 Weighing boats |
|
Marble chips (small balls) |
Top-pan balance capable of weighing to three decimal places |
|
Distilled water |
Stop watch |
|
2 Measuring cylinders (25ml) |
Eye protection |
|
5 Conical flasks (100ml) |
Spatula |
|
Cotton wool |
|
Safety Precautions:
|
SAFETY PRECAUTION |
REASON(S) |
|
1. Wear lab coat
|
To prevent any corrosive material from contacting our clothes and leading to burns |
|
2. Wear safety goggles
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To avoid any irritations from calcium carbonate and calcium chloride |
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3.Wear nitrile gloves
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To protect hands from corrosive material like hydrochloric acid |
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4.Handling of spillages
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To avoid any possible contact with the solutions |
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5.Disposal of materials
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To avoid atmospheric and environmental pollution |
Apparatus Diagram(s):
Instructions:
1. A specific volume of acid was measured with a measuring cylinder and added it to a conical flask. Then a specific volume of water was measured with a different measuring cylinder and added it to the same conical flask. this step was repeated 5 times with different volumes of hydrochloric acid and water making 5 different flasks with different concentrations as shown in the following table.
|
Molar concentration |
V of acid (cm3) |
V of water (cm3) |
Flask Number |
|
0.4 |
5 |
20 |
1 |
|
0.8 |
10 |
15 |
2 |
|
1.2 |
15 |
10 |
3 |
|
1.6 |
20 |
5 |
4 |
|
2.0 |
25 |
0 |
5 |
2. 2.5g of marble chips were weighed in a weighing boat through the top-pan balance.
3. Conical flask 1 was loosely plugged with a cotton wool, and weighed.
4. The weight was neglected after pressing tare.
5. The cotton wool was then removed to add 2.5g of marble chips. The stopwatch started immediately after and the cotton wool was placed back.
6. The mass was recorded every 30 seconds for minutes (10 readings).
7. Steps 2 till 6 were repeated 4 times with the rest of the conical flasks.
Results:
|
|
Mass (g) at different concentrations (mol.dm-3) |
||||
|
Time(s) |
0.4 |
0.8 |
1.2 |
1.6 |
2.0 |
|
0
|
2.500 |
2.500 |
2.500 |
2.500 |
2.500 |
|
30
|
2.485 |
2.434
|
2.398 |
2.371 |
2.350
|
|
60
|
2.470 |
2.372 |
2.368 |
2.250 |
2.214
|
|
90
|
2.458
|
2.327 |
2.309 |
2.149 |
2.092 |
|
120
|
2.458 |
2.297 |
2.257 |
2.072 |
1.995 |
|
150
|
2.452
|
2.265 |
2.210 |
2.011 |
1.920 |
|
180
|
2.451 |
2.252 |
2.193 |
1.958 |
1.863 |
|
210
|
2.441 |
2.239
|
2.133 |
1.920 |
1.820 |
|
240
|
2.447 |
2.238 |
2.124 |
1.886 |
1.782 |
|
270
|
2.442 |
2.238 |
2.120 |
1.860 |
1.753 |
|
300
|
2.440 |
2.238 |
2.082 |
1.837 |
1.729 |
|
Total loss (g)
|
0.06 |
0.262 |
0.418 |
0.663 |
0.771 |
|
|
Mass loss(g) at different concentrations (mol/dm3) |
||||
|
Time Intervals(seconds) |
0.4 |
0.8 |
1.2 |
1.6 |
2.0 |
|
0 |
0 |
0 |
0 |
0 |
0 |
|
30 |
0.030 |
0.066 |
0.102 |
0.129 |
0.15 |
|
60 |
0.035 |
0.128 |
0.132 |
0.25 |
0.286 |
|
90 |
0.042 |
0.173 |
0.191 |
0.351 |
0.408 |
|
120 |
0.042 |
0.203 |
0.243 |
0.428 |
0.505 |
|
150 |
0.048 |
0.235 |
0.29 |
0.489 |
0.58 |
|
180 |
0.049 |
0.248 |
0.307 |
0.542 |
0.637 |
|
210 |
0.059 |
0.261 |
0.367 |
0.58 |
0.68 |
|
240 |
0.053 |
0.262 |
0.376 |
0.614 |
0.718 |
|
270 |
0.058 |
0.262 |
0.38 |
0.64 |
0.747 |
|
300 |
0.06 |
0.262 |
0.418 |
0.663 |
0.771 |
|
Concentration (mol/dm3) |
Initial Rates (x10-3 mol/dm3.s) |
|
0 |
0 |
|
0.4 |
0.5 |
|
0.8 |
2.2 |
|
1.2 |
3.4 |
|
1.6 |
4.3 |
|
2 |
5 |
Discussion:
The first graph shows the mass loss of the marble chips (CaCO3) in different concentrations with respect to time. The graph shows that as the concentration increases from 0.4 till 2 mol/dm^3, the mass loss increased from 0.06 till 0.771 grams. That mass loss of the marble chips is the same mass produced for carbon dioxide.
The second graph is to show how to get the initial rate for one of the concentrations (0.8mol/dm^3).
To do so, we draw a tangent on the curve of the variation of mass loss at the initial
point so that we can calculate the gradient which is the rate. The tangent is shown in the second graph where the rate is 2.2 (mol/dm3.s).
We do the same thing for the rest of the graphs and get the remaining rates. The third graph shows the concentrations and their corresponding rates.
The graph shows a straight line which means they are directly proportional. Hence based on this graph, the order of the reaction is first. We could also see how doubling the concentration, doubles the rate which verifies that the order is first.
The reaction is: 2HCl +CaCO3 => CaCl2 + CO2 + H2O
This reaction is also an exothermic reaction, where the heat is released to the surroundings and that can be detected by a thermometer.
For a chemical reaction to occur, the particles of the reactants have to collide with sufficient energy to react. According to collision theory, if there is more concentration, there will be more particles available to collide hence the chance of successful collisions is much higher. This will lead to faster disappearance of the marble chips and faster production of carbon dioxide.
The results were as predicted since the more concentration of the reactant, the rate will increase due to them being directly proportional.
Evaluation:
It is not safe to say that the experiment is reliable since we only did it once. The experiment has to be done more than once in order to check whether its reliable or not. However, the experiment done is used in most curriculums and is trusted by many educational institutions.
An alternative to checking the rate is measuring the volume of the gas using a gas syringe.
Using a cotton wool could be risky if not put correctly, because that might affect the mass of carbon dioxide by it escaping from the flask. Instead, we could have used a cork which would guarantee that the gas cannot escape, making the results more accurate and the experiment more precise. Even though carbon dioxide is dense, we could have used a gas syringe to check its volume which would have also been accurate. A colorimeter may be used if one of the constituents are coloured.
The size of the marble chips is not constant, and the surface area is a factor that might affect the rate, hence this explains the small fluctuations in the graphs.
Getting the half-life would have been a suitable method to also check the order of the reaction.
Conclusion:
As the concentration of a reactant increases, the rate increases. This means the reactant disappears faster and the product is formed faster. The rate of the reaction can be deduced through measuring the mass of the product and studying its changes. The graph between the concentrations and the rates shows a straight line which shows that the reaction is of first order with respect to hydrochloric acid.
Referencing:
1. Chemistry LibreTexts. (2019). The Rate Law. [online] Available at: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/Rate_Laws/The_Rate_Law [Accessed 28 Apr. 2019].
2. Chemistry-helper.blogspot.com. (2019). 16. Kinetics HL. [online] Available at: http://chemistry-helper.blogspot.com/2011/04/distinguish-between-terms-rate-constant.html [Accessed 28 Apr. 2019].
3. En.wikipedia.org. (2019). Hazard symbol. [online] Available at: https://en.wikipedia.org/wiki/Hazard_symbol [Accessed 30 Apr. 2019].
4. En.wikipedia.org. (2019). Corrosive substance. [online] Available at: https://en.wikipedia.org/wiki/Corrosive_substance [Accessed 30 Apr. 2019].
