Yezi Cho
1. Introduction
Intrigued by how acidic rain can cause the erosion of rocks, statues, and buildings, I became interested in the mechanism behind this phenomenon. I found out that limestone structures, which contain calcium carbonate, are subject to reacting with acidic rain. When acidic solutions containing hydronium ions are in contact with calcium carbonate, there is a neutralization reaction that produces water and carbon dioxide. In this experiment, calcium carbonate was reacted with hydrochloric acid, which produces calcium chloride, water, and carbon dioxide. Using the mass loss representing production of carbon dioxide gas, I aim to explore the research question “To what extent does temperature (20°C, 24°C, 28°C, 32°C, 36°C) affect the average rate of reaction for 2 minutes between calcium carbonate and hydrochloric acid, measured by mass loss due to carbon dioxide production?”.
2. Background
2.1. Reaction
Calcium carbonate (CaCO3) is an ionic compound consisting of a calcium cation (Ca2+) and a carbonate (CO32-), a polyatomic anion. Having a small solubility product (Ksp = 7.5 x 10-9), the ionic compound is insoluble in water. However, it is soluble in acidic solutions, where the hydrogen ions from the acid react with carbonate ions.
Hydrochloric acid (HCl) is an inorganic acid that is highly reactive and corrosive. It is a strong acid, with a Ka value of 1.3 x 106 (“Table of Acid and Base Strength”). As a monoprotic acid, it can donate one proton per molecule. When hydrogen chloride is dissolved in water, it exists in the form of hydronium cations (H3O+) and chloride anions (Cl-).
HCl (aq) + H2O (l) → H3O+ (aq) + Cl- (aq) Ka = 1.3 x 106
The reaction in this investigation is a double replacement reaction between two compounds. Generally, metal carbonates react with acids to form salt, water, and carbon dioxide. In this reaction, the hydrogen ion (H+) produced by the dissolution of hydrochloric acid reacts with the carbonate ion (CO32-) from calcium carbonate, producing carbonic acid (H2CO3) and dissolved calcium chloride (CaCl2).
2HCl (aq) + CaCO3 (s) → CaCl2 (aq) + H2CO3 (aq)
2H+ (aq) + CO32- (aq) → H2CO3 (aq) (Net ionic equation)
However, the carbonic acid (H2CO3) is highly unstable, so it immediately decomposes into water and carbon dioxide.
H2CO3 (aq) → CO2 (g) + H2O (l)
Thus the net chemical reaction is as follows:
CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
CO32- (aq) + 2H+ (aq) → CO2 (g) + H2O (l) (Net ionic equation)
2.2. Collision theory
Molecules in a liquid state or aqueous solution are constantly moving with an average speed that depends on temperature. At higher temperatures, the molecules move with greater velocity, thus having greater kinetic energy (K.E = (½)mv2). According to collision theory, molecules need to collide in the correct orientation and with sufficient energy above activation energy for the reaction to be successful. As temperature increases and molecules’ kinetic energy increases, there is a greater frequency of collisions and a greater fraction of molecules with sufficient energy (> Ea) to react.
In this experiment, at higher temperatures, the hydrogen ions (H+) will collide more frequently with carbonate ions (CO32-) and with more force. Thus there will be more successful collisions forming bonds between H+ and CO32- to produce carbonic acid (H2CO3), which quickly breaks down into water and carbon dioxide. In this investigation, the carbon dioxide gas is allowed to escape, causing the mass of the reaction mixture to decrease. Thus the average rate of reaction for two minutes, measured by decrease in mass over time, will be higher using higher temperatures of solution.
Figure 1. Maxwell-Boltzmann distribution at a higher temperature (B, 2023)
Figure 1 shows a Maxwell-Boltzmann distribution representing the fraction of molecules with a given kinetic energy at two different temperatures. At a higher temperature (shown by the red curve), the distribution flattens out and the peak of the curve shifts to the right. There is a greater fraction of molecules above activation energy (indicated by shaded red area) for the reaction to be successful. At a lower temperature (shown by the blue curve), the average kinetic energy is lower and there is a smaller proportion of molecules above activation energy that can react (indicated by shaded blue area).
In this experiment, at higher temperatures, there is a greater fraction of hydrogen ions and carbonate ions with sufficient energy (> Ea) than at lower temperature. There will be more successful collisions, resulting in a greater mass of carbon dioxide produced and a higher calculated rate of reaction.
3. Hypothesis
As temperature increases, the rate of reaction between hydrochloric acid solution and calcium carbonate will increase, measured by a mass loss due to the production of carbon dioxide gas.
Independent variable: temperature of HCl solution (20°C, 24°C, 28°C, 32°C, 36°C)
Dependent variable: mass loss due to production of carbon dioxide
Controlled variables
- Mass of CaCO3: Increasing the mass of CaCO3 increases the moles of CaCO3 available to react, resulting in more frequent collisions between CaCO3 and HCl. There would be more frequent successful reactions, resulting in a larger amount of CO2 produced within a given period of time and a larger calculated rate of reaction. Accurately measuring a consistent mass of CaCO3 using a digital scale for each experiment would ensure that the average rate of reaction for 2 minutes depends on the independent variable, temperature.
- Volume of HCl: A larger volume of HCl would have little effect on the initial rate of reaction, as the mass of CaCO3 and concentration of HCl is constant. However, after some time, the average rate of reaction will differ for different volumes of HCl, as HCl is the limiting reactant (10.0 cm3 of 0.5 mol dm-3 HCl = 0.005 mol HCl, 2.0 g CaCO3 = 0.020 mol CaCO3). Increasing the volume of HCl increases the moles of HCl available to react with excess CaCO3, thus increasing the amount of carbon dioxide produced over a longer period of time. Therefore, to compare the average rate of reaction for 2 minutes, a consistent volume of HCl should be used for each trial, accurately measured using a graduated cylinder.
- Concentration of HCl solution: Increasing the concentration of reactant HCl will allow for more frequent collisions (and successful reactions) between carbonate ions and hydrogen ions, increasing the average rate of reaction for 2 minutes. Additionally, increasing HCl concentration while keeping the volume consistent also leads to more moles of HCl available to react with excess CaCO3, increasing the average rate of reaction. Thus the same concentration of HCl solution, 0.5 mol dm-3, should be used for all trials.
- Time: The time passed since the reaction started until the mass loss was recorded should be consistent. The more time has passed, the more moles of hydrochloric acid and calcium carbonate would have reacted, leading to a greater mass loss due to carbon dioxide production. In this experiment, the reaction was allowed to occur for two minutes for all trials, using a stopwatch for greater accuracy.
4. Materials and Methods
4.1. Chemicals and apparatus
50.0 g of CaCO3 powder
250.0 cm3 of 0.5 mol dm-3 HCl solution
Digital scale (± 0.01 g)
10 cm3 graduated cylinder (± 0.5 cm3)
Thermometer (± 0.05 °C)
Test tubes
Spatula
Styrofoam cup
Stopwatch
4.2. Safety
Hydrochloric acid is a highly corrosive acid that can cause irritation, burns, and damage to the skin, eyes, and respiratory tract upon direct contact or inhalation. 0.5 mol dm-3 HCl is moderately hazardous and requires careful handling and disposal procedures to minimize risks to health and safety. Gloves, safety goggles, and a lab coat should be worn to prevent direct contact with the acid. Neutralizers (such as a weak alkaline solution) should be in hand to be used in the case of skin irritation. The experiment should be performed in a well-ventilated area or under a fume hood to minimize inhalation of acid vapors. After use, the acidic solution should be neutralized and diluted before disposal (and/or disposing it into a waste container).
Calcium carbonate (CaCO3) is generally considered to be a low-hazard compound under normal conditions of use. It is generally non-toxic and non-irritating to the skin and eyes; it is not flammable/combustible and does not pose an explosion hazard under normal conditions. However, inhaling fine CaCO3 particles over a prolonged period can irritate the respiratory tract.
4.3. Procedure
Scoop CaCO3 powder using a spatula to measure 2.0 g on a weighing paper using a digital scale.
Measure 10 cm3 of 0.5 mol dm-3 HCl solution using 10 cm3 graduated cylinder. Transfer the HCl solution into a test tube.
Set up a water bath at 20°C by adding hot water to a 250 cm³ beaker. Use a thermometer to monitor and adjust the temperature as needed.
Place the beaker with 20°C water in a styrofoam cup to minimize heat loss.
Place the test tube with HCl solution in the water bath. Use a thermometer to ensure the HCl solution reaches the desired temperature of 20°C.
Add CaCO3 powder to the HCl solution using a funnel. Swirl the test tube 5 times to ensure thorough mixing of the reactants.
Immediately place the styrofoam cup (with the beaker and test tube) on the digital scale, and start the stopwatch.
Record the mass using the digital scale every 10 seconds for two minutes.
Repeat steps 1-7 using a water bath set to 24°C, 28°C, 32°C, and 36°C. Obtain 5 trials for each temperature.
5. Results
Table 1: Average Mass Loss of CaCO3 (g) at Different Temperatures
Temperature (°C) | Trial 1 | Trial 2 | Trial 3 | Trial 4 | Trial 5 | Average Mass Loss (g) |
20 | 0.75 | 0.78 | 0.74 | 0.76 | 0.77 | 0.76 |
24 | 0.89 | 0.87 | 0.88 | 0.90 | 0.91 | 0.89 |
28 | 1.03 | 1.01 | 1.05 | 1.02 | 1.04 | 1.03 |
32 | 1.16 | 1.15 | 1.17 | 1.18 | 1.19 | 1.17 |
36 | 1.30 | 1.28 | 1.29 | 1.31 | 1.32 | 1.30 |
6. Conclusion
The results indicate a positive correlation between temperature and average mass loss due to carbon dioxide production. The average mass loss increased from 0.76 g at 20°C to 1.30 g at 36°C, demonstrating that higher temperatures enhance the rate of reaction between hydrochloric acid and calcium carbonate.
The findings of this experiment support the hypothesis that increasing the temperature of hydrochloric acid solution accelerates the reaction rate with calcium carbonate, as measured by the mass loss due to carbon dioxide production. The data collected indicates a clear trend: as the temperature increases, the average mass loss correspondingly increases, reflecting the increased kinetic energy and collision frequency among reactant molecules, which aligns with the principles of collision theory. The results confirm that temperature is a significant factor affecting reaction rates in acid-carbonate reactions.
7. Works Cited
B, A. (2023, February 7). Interpretation of Maxwell Boltzmann Distribution - Thermodynamics -PSIBERG. PSIBERG. https://psiberg.com/maxwell-boltzmann-distribution/#google_vignette