Chemistry Depth Study
Inquiry Question:
How does the concentration of hydrochloric acid affect the rate of its reaction with calcium carbonate (marble chips)?
Whenever dealing with acids both inside and outside of the lab, the same question always arises: “Does the concentration of the acid really make that much of a difference on the reaction?”. In society, acids are used in many different fields and areas. Some of these include food and hospitality, agriculture, mechanics and engines, cleaning, and many more. The question of what concentration to use continuously arises. In many areas, an exact balance of strength is required. This is because an amount that is strong enough to cause a reaction is required, yet it cannot be too strong so that it doesn't damage other parts of the system. An example of this is acids in the stomach. The acid that is located in a human’s stomach is essential in breaking down food, and it must be strong enough to be able to break different foods, however, if acidity gets too high, medication is required as it can damage the body. The question of how does concentration affect reaction time is relevant to society, as it can improve efficiency. The faster the reaction time, the more time is saved. In today’s society, businesses are constantly searching for ways to improve their efficiency. A deep knowledge on the impacts of concentration on the rates of different reactions can create significant improvements in the efficiency of the production of different goods including foods and engines, which are both heavily reliant on reactions involving acids.
Background Research:
Concentration is a measure of how many available acid ions are present within a solution. This means how many unreacted “parts” of the acid are present in a solution. This can be measured in several different units, including; parts per million, moles per litre and percentage. More simply, concentration is a representation of the ratio between the solute and the solvent in a solution.(sciencing.com)
The reaction rate of a reaction is the speed at which a chemical reaction occurs. Different chemical reactions occur at different rates. There are many different factors which influence the rate at which a reaction will occur. Some of these include; the type of reaction, the temperature, the surface areas of reactants, and many more. Generally, a chemical reaction involves the breaking and reformation of chemical bonds within substances. This means that the rate of reaction is the rate at which the bonds are broken and then formed again. (https://www.britannica.com/science/reaction-rate)
The concentration of an acid is a key factor when considering reaction rates. This is because the higher the concentration, the more acid particles which can react are present. This concept relates to the Kinetic Particle Theory, which states that matter consists of small particles which are constantly in motion. Since a reaction occurs when particles collide with each other, breaking and reforming bonds, the higher the concentration of the reactants, the faster the reaction rate. The reason for this is that since more particles that are present, the likelihood of a collision occurring is increased considerably. An increase in the number of collisions that occur, leads to a general increase in the number of successful collisions. Successful collisions are ones where bonds are indeed broken and reformed. By adding more of the reactants in the same volume, the likelihood of collisions are increased exponentially. (https://www.le.ac.uk/se/centres/sci/selfstudy/particle01.html)
Figure 1: Influences of temperature and concentration on the rate of reaction.
(http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway_pre_2011/rocks_metals/7_faster_slower3.shtml)
Aim:
To determine the effect that the concentration of hydrochloric acid has on the rate of its reaction with calcium carbonate.
Hypothesis:
The greater the concentration of the acid, the faster the rate of reaction with calcium carbonate. Assuming that all variables are controlled specifically temperature and the surface area of the reactants, the acids of greater concentrations will react with the marble chips at a faster rate.
Risk Assessment
Risk
Consequence
Precaution
Broken Glassware
Cuts and lacerations which could result in pain and infection.
Take care when handling glassware and contact teacher if any breaks occur.
Strong Acids
Can splash into eyes leading to damaged eyesight and even loss of sight.
Ensure that safety goggles are worn at all times when dealing with acid.
Spills of liquids
Can cause people to slip resulting in injuries such as broken bones.
Clean up any spills immediately and alert teacher and others in proximity.
Electrical equipment being used near liquids
Electrocution and damage to equipment
Keep electrical equipment away from the liquids and clean up any spills that occur near the equipment
Apparatus:
Electronic Balance
4 Molar Hydrochloric Acid
Calcium Carbonate
50 mL beaker
10 mL measuring cylinder
Distilled Water
Stopwatch
Method 1 (Reaction):
Measure out 1 gram of calcium carbonate chips using the electronic balance. (Allow for a margin of error of 0.01 grams)
Place the gram of calcium carbonate in a 50 mL beaker and place on the electronic balance.
Using the measuring cylinder, measure 10 mL of 4 molar Hydrochloric acid.
Pour the hydrochloric acid into the beaker containing the calcium carbonate.
As soon as all the acid is in the beaker, re-zero the balance and start the stopwatch.
Every 30 seconds record how much mass has been lost (will appear as a negative on the balance).
Record results in results table until 4 minutes have passed.
Rinse out beaker.
Repeat steps 1-8 3 times and then calculate an average.
Repeat steps 1-9 using different concentrations of acid. Use concentrations of 2, 2.5, 3, 3.5 and 4 molar acid. (See Method 2 for instructions on how to dilute the acid)
Method 2 (Diluting Acid):
Refer to the table below and pour the correct amount of four molar hydrochloric acid into the measuring cylinder.
Add the correct amount of distilled water to the acid. Ensure that the final amount of the solution is 10 mL.
Use the dilute acid in the reaction and then rinse out measuring cylinder.
Concentration (moles/litre)
4 Molar Hydrochloric Acid (mL)
Distilled Water (mL)
4.0
10.0
0.00
3.5
8.75
1.25
3.0
7.50
2.50
2.5
6.25
3.75
2.0
5.00
5.00
Independent and Dependent Variables:
The independent variable in this experiment will be the concentration of the Hydrochloric acid used. This is the only variable that will be changed in order to obtain results. This will be measured and controlled by using mathematics to calculate the correct ratio of water: acid in order to achieve the correct concentrations.
The dependent variable in this experiment will be the amount of mass lost as gas over the course of the reaction. This will be measured by zeroing the electronic balance and recording how much mass is lost as gas. The amount of gas lost will represent the rate of reaction.
Controlled Variables:
In order for the experiment to be valid, there are many variables which must be controlled and kept the same over all the tests. It is important that variables are controlled as they can significantly influence and impact the results obtained. The variables that had to be controlled included; temperature, amount of reactants, type of acid, measuring equipment, and the cleanliness of the equipment.
Temperature was difficult to control, however it was controlled to the furthest extent it could. In order to avoid fluctuations in temperature, the experiment was conducted inside a closed lab, which meant that the temperature was kept relatively constant. It was however impossible to control exact temperatures as factors such as air conditioning and open windows were out of control.
The amount of reactants was controlled by measuring the mass of the calcium carbonate and keeping it constant, and the amount of acid was controlled by using a measuring cylinder in order to ensure that exactly 10 mL of acid was tested.
The acid that was used was the same 4 molar acid. Even when diluting the acid the same original acid was used. This was done to avoid any impurities or contaminations from impacting and altering the results. Similarly, calcium carbonate from the same container was used.
The measuring equipment that was used was kept the same. The same electronic balance was used to measure the mass of the calcium carbonate and the mass lost during the reaction. This was done to avoid any small differences between readings of different balances. The same measuring cylinder was also used in order to ensure that any imperfections and differences in scales did not affect the results.
It was important that the equipment was cleaned after every use. Specifically the measuring cylinder and the beaker in which the reactions occurred. This was because there often remained leftovers in the glassware which would have impacted results. In order to control this, after every use the glassware was rinsed in water and the dried with a paper towel.
Calculations:
Before conducting the experiment some calculations had to be done. In order to determine how much of each reactant to use, an understanding of which one would be the limiting reagent was required. In order to calculate this, an estimate of how much would be used was taken (1 gram of calcium carbonate and 10 mL of hydrochloric acid), and the calculation of the number of moles was then done to determine whether these quantities were realistic and practical to use in the experiment. The objective was for the calcium to be the limiting reagent. The reason for this was that it was easier to observe when all of the calcium carbonate had reacted in the acid, as opposed to when the acid was completely used up in the reaction with the calcium carbonate. The following calculations were used to determine whether the estimated quantities were appropriate:
Another set of calculations that needed to be done was the dilution of the acid. This was done before the experiment was conducted to ensure that the correct quantities of acid and water were used to form each different concentration of hydrochloric acid that were required. The following calculations were used to determine the quantities of acid required, and were used to formulate the following table:
Concentration (moles/litre)
4 Molar Hydrochloric Acid (mL)
Distilled Water (mL)
4.0
10.0
0.00
3.5
8.75
1.25
3.0
7.50
2.50
2.5
6.25
3.75
2.0
5.00
5.00
Results:
Trial 1
Change in Mass(g) over time(min)
Concentration (Mol/L)
0.5 min
1 min
1.5 min
2 min
2.5 min
3 min
3.5 min
4 min
4
0.05
0.10
0.17
0.21
0.25
0.28
0.33
0.36
3.5
0.03
0.06
0.10
0.13
0.16
0.19
0.21
0.23
3
0.00
0.03
0.07
0.12
0.15
0.19
0.22
0.24
2.5
0.00
0.01
0.04
0.08
0.10
0.13
0.15
0.18
2
0.00
0.00
0.02
0,09
0.06
0.09
0.10
0.12
Trial 2
Change in Mass(g) over time(min)
Concentration (Mol/L)
0.5 min
1 min
1.5 min
2 min
2.5 min
3 min
3.5 min
4 min
4
0.04
0.09
0.14
0.18
0.23
0.26
0.29
0.32
3.5
0.02
0.06
0.11
0.16
0.20
0.23
0.26
0.28
3
0.02
0.05
0.09
0.12
0.15
0.18
0.20
0.23
2.5
0.02
0.05
0.09
0.13
0.16
0.19
0.22
0.24
2
0.00
0.00
0.00
0.01
0.03
0.06
0.08
0.10
Trial 3
Change in Mass(g) over time(min)
Concentration (Mol/L)
0.5 min
1 min
1.5 min
2 min
2.5 min
3 min
3.5 min
4 min
4
0.03
0.08
0.14
0.18
0.22
0.26
0.29
0.31
3.5
0.02
0.06
0.10
0.15
0.19
0.22
0.25
0.27
3
0.01
0.06
0.08
0.12
0.15
0.18
0.20
0.23
2.5
0.00
0.04
0.09
0.12
0.15
0.17
0.20
0.21
2
0.00
0.00
0.01
0.03
0.07
0.09
0.11
0.13
Average
Change in Mass(g) over time(min)
Concentration (Mol/L)
0.5 min
1 min
1.5 min
2 min
2.5 min
3 min
3.5 min
4 min
4
0.04
0.09
0.15
0.19
0.24
0.27
0.30
0.33
3.5
0.02
0.06
0.11
0.15
0.19
0.22
0.24
0.26
3
0.01
0.04
0.08
0.12
0.15
0.18
0.21
0.23
2.5
0.01
0.04
0.07
0.11
0.14
0.17
0.20
0.21
2
0.00
0.00
0.01
0.04
0.04
0.07
0.09
0.11
Acid
Mass of CO2 Lost per Minute Over the 4 Minutes Tested (g)
4.0 Molar
0.0825
3.5 Molar
0.065
3.0 Molar
0.0575
2.5 Molar
0.525
2.0 Molar
0.0275
Conclusion:
As a general rule, the more concentrated the acid, the greater the reaction rate. This rule applies considering all other variables are controlled and their impacts on results are accounted for. According to the results of the experiment, reactions between hydrochloric acid of higher concentrations with calcium carbonate occurred considerably faster than reactions of less concentrated hydrochloric acid with the same calcium carbonate. This implies that an acid with a higher concentration has a faster reaction rate than an acid with a lower concentration.
Discussion
The method used in the experiment could be regarded as relatively valid. Important variables such as the amount of reactants and the concentration of the acid tested were checked multiple times before being used in the experiment. This was done to ensure that parallax error in the measuring and dilution of the acid was minimised. The way in which this was done was by getting down to the same level as the increments on the measuring cylinder. Similarly, the method involved the cleaning of equipment between tests. This guaranteed that any residue and leftovers from previous tests would not affect the results of the experiment. On the other hand, there were some problems with the method which contributed to some error in the results obtained. One of these problems were that the calcium carbonate was only measured in terms of mass and factors such as exposed surface area and purity were not considered. One of the key factors affecting reaction rate is surface area. The greater area that is exposed to the acid, the more particles can react simultaneously. This means that the overall reaction will occur at a faster rate. The fact that surface area was not considered when conducting the experiment meant that a relative large influence on the results was ignored. Although the fact that surface area was not taken into account could have lead to slightly inaccurate results, it would have been almost impossible to control with the limited equipment that was accessible. Due to the fact that the calcium carbonate was present as small marble chips of varying shapes and sizes, it would have been extremely difficult to calculate the surface area for just one test, let alone maintain a constant surface area over every trial in the experiment. Another problem with the validity of the experiment was that the stopwatch was only started after all of the acid was poured into the beaker containing the calcium carbonate. This meant that the reaction had already started for a short amount of time before the timing was started and the scale was zeroed. Although the impact for this error was very minimal, it could have contributed to a slight deviation from the actual results. It would have been difficult to change this limitation of the method however, as an automatic system would have been required to completely eliminate reaction rate, and using the equipment available, a method as such would have been impossible to replicate. Overall, the experiment did test the hypothesis, and did provide results to support the background research completed on the topic. This implies that the chosen method was successful in testing the problem, and was therefore valid to an adequate extent.
The accuracy of the experiment could be described as very good. The equipment that was used to measure the equipment was kept the same to ensure that any limitations of a specific measuring cylinder or electronic balance would not result in an alteration of results. The electronic balance that was used provided measurements which were accurate and precise to the nearest 0.01 grams. This meant that in a measurement of 1.00 grams (which was that of the calcium carbonate) the greatest possible error in terms of measurement of the reactant could have been 1 percent of its actual mass. The measuring cylinder which was used to measure the quantity of acid tested and to dilute the acid was marked in increments of 0.2 mL. This meant that the measurements were precise to the nearest tenth of a milliliter. This meant that The largest error that was made assuming that parallax error was not a factor was a 1% deviation from the intended quantity. An issue regarding the accuracy of the experiment was errors made by the people taking measurements. These errors were in the form of parallax error when measuring the volume of the acid and the difficulty in taking the reading for the volume with a relatively large meniscus. Another measurement related error was the reaction time in starting the stopwatch and zeroing the balance exactly when all the acid was poured into the beaker.
The reliability of the experiment could be described as being good. The reliability of the experiment can be assessed in a few different ways. The first way would be by looking at the results obtained every test and comparing them to when the experiment was repeated. Since the experiment was repeated three times, it is possible to obtain a general insight into the reliability. Based on the results it can be seen that the variation between different trials was relatively small variation, with the largest being 0.03 grams from the average. This meant that the results were quite similar every time the experiment was repeated, implying that the method used was reliable and was kept the same. On the other hand however, the graphs of the results did not show a particularly convincing trend linking the different concentrations and the amount of mass lost as carbon dioxide. The fact that the results did not lie very close to the trend line in all of the graph gives the impression that some variables were not adequately controlled between tests.
There are several different ways in which the experiment could have been improved. The first improvement which could have been made was using more accurate and precise measuring equipment. This would have provided both more accurate results, as more precise equipment could have provided results correct to more significant figures, as well as minimising errors such as reaction time and parallax error. Another way in which the experiment could have been improved was by better controlling some variables. One variable which should have been better controlled was surface area of the calcium carbonate. The influence that surface area of the calcium carbonate had on the results of the experiment is difficult to assess, however based on research, it is one of the main components affecting reaction rate. By better controlling the surface area of the reactants, and maintaining a constant area that is exposed to the acid, the results can be made more relevant, and the experiment can be made more valid. Another variable that could have been better controlled to produce a more valid and accurate experiment was temperature. The experiment was carried out in the lab, where the temperature it was not possible to maintain a specific temperature accurately. Heat causes particles to vibrate and move more. This causes collisions of particles to be stronger as they involve more energy. This increases the likelihood of successful collisions, and in turn increases the reaction rate. Since the trials were carried over several days and times, the temperature of the room varied slightly between each test. The degree to which these differences in temperature affected the results are difficult to conclude, however they would for sure have caused some fluctuations in the results, even if the difference was relatively negligible. The experiment could be improved in many different ways, however considering the accessible equipment, the results reflected a well performed, and functional experiment. There could have been improvements by better controlling variables and using more precise equipment, however as a whole, the hypothesis was tested and confirmed, and the experiment answered the inquiry question and provided a deeper insight into the problem.
Bibliography:
Bbc.co.uk. (2018). BBC – GCSE Bitesize: Effect of temperature and concentration. [online] Available at: http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway_pre_2011/rocks_metals/7_faster_slower3.shtml [Accessed 5 Aug. 2018].
Encyclopedia Britannica. (2018). reaction rate | Facts & Formula. [online] Available at: https://www.britannica.com/science/reaction-rate [Accessed 5 Aug. 2018].
Le.ac.uk. (2018). Particle Theory – home. [online] Available at: https://www.le.ac.uk/se/centres/sci/selfstudy/particle01.html [Accessed 5 Aug. 2018].
Rsc.org. (2018). The effect of concentration on reaction rate- Learn Chemistry. [online] Available at: http://www.rsc.org/learn-chemistry/resource/res00000743/the-effect-of-concentration-on-reaction-rate?cmpid=CMP00000821 [Accessed 7 Aug. 2018].
Sciencing. (2018). Difference Between Strength & Concentration. [online] Available at: https://sciencing.com/difference-between-strength-concentration-8601963.html [Accessed 5 Aug. 2018].