Hydrates
The objective of this lab is to investigate the properties of hydrates and their behavior upon heating. Through this experiment, students will use tools and equipment, demonstrate safe laboratory practices, and apply their understanding of electrons in atoms, chemical bonding, chemical reactions, quantities in chemical reactions, molecules, compounds, and chemical composition. They will also practice the proper use of exponential notation and significant figures, record their observations, and use scientific reasoning to evaluate the physical phenomena involved.
The discussion explains that water can be associated with chemical compounds either as absorbed water on the surface or chemically bound in larger amounts within certain compounds called hydrates. Hydrates have a specific number of water molecules as an integral part of their crystalline structure. The water within hydrates is referred to as water of hydration, and the salts themselves are known as hydrates. The bonding between water molecules and the ions in the compound contributes to the stability of hydrates.
Hydrates can undergo changes when exposed to different conditions. Some hydrates effloresce, meaning they lose water of hydration when exposed to air, while others are deliquescent, absorbing water from the atmosphere and eventually dissolving in their water of hydration. The stability of a hydrate depends on the relative vapor pressure of the water in the air compared to the vapor pressure of the hydrate.
The lab procedure consists of two parts:
Part A focuses on observing efflorescence and deliquescence of various chemical compounds placed on watch glasses. Students determine which salts deliquesce, which effloresce, and which show neither efflorescence nor deliquescence. They record their observations in their notebooks.
Part B involves the determination of the formula of an unknown crystalline hydrate. A clean, dry evaporating dish and watch glass are weighed. A sample of the unknown hydrate is placed in the dish, and the combined weight is recorded. The dish is then heated gradually, ensuring no moisture is visible on the watch glass. After cooling, the dish is reweighed. This heating, cooling, and weighing process is repeated until two consecutive weighings agree within 1 mg. The report sheet is completed, and the number of moles of water per mole of anhydrous salt is calculated. The formula of the hydrate is reported as M ∙ XH20, with X representing the moles of water calculated.
In their notebooks, students are expected to write the equations for the observed reactions when heating the Cobalt(II) Chloride (CoCl2 ∙ 6H20) and adding water. They are also asked to consider how this salt could be used as a moisture indicator.
This lab provides students with an opportunity to observe and analyze the behavior of hydrates, reinforcing concepts related to chemical bonding, composition of matter, chemical reactions, and stoichiometry. It also promotes skills in using laboratory equipment, practicing safe procedures, recording data, performing calculations, and drawing conclusions based on scientific reasoning.
Objective
· Use the tools and equipment necessary for basic scientific analysis and research
· Demonstrate safe practices in the Chemical Laboratory
· Demonstrate an understanding of Electrons in Atoms and the Periodic Table and Chemical Bonding
· Demonstrate an understanding of Chemical Reaction and Quantities in Chemical Reactions
· Demonstrate an understanding of Molecules, Compounds and Chemical Composition
· Demonstrate the proper use of Exponential Notation and Significant Figures
· Demonstrate an understanding of the composition of matter and energy
· Record the results of investigation through writing
· Use scientific reasoning to evaluate physical and natural phenomena
· Identify the unifying themes of the scientific field of study
Materials
· Cobalt (II) Chloride (CoCl2 ∙ 6H20)
· Test Tube
· unknown crystalline hydrate
· wire triangle
· ring stand
· Bunsen burner
· Watch glass
Discussion
Water exhibits a strong attraction towards various compounds due to its polar nature and electronic structure. When water interacts with solid chemical compounds, it can exist in two forms: absorbed on the surface of crystals, which can be removed by gentle heating, or chemically bound in larger quantities within certain compounds, typically ionic salts. Consequently, many salts contain a specific number of water moles as an essential part of their crystalline structure. For instance, copper (II) sulfate crystallizes as CuSO4 ∙ 5H20, magnesium sulfate as MgSO4 ∙ 7H20, and borax has the composition Na2B407 ∙ 10H20. The water integrated into the crystal structure is referred to as water of hydration, and such salts are known as hydrates. Recent developments propose that this water arises from the bonding between the water molecule and the cation, along with the strong electrostatic attraction between the water molecule and the anion.
When hydrates are heated, they decompose and release water vapor, resulting in the formation of an anhydrous salt. During the loss of water of hydration from a hydrate, the compound may undergo color changes that correspond to different hydrates formed by the salt. Less stable hydrates tend to lose water of hydration at room temperature, creating a specific vapor pressure. These hydrates are called efflorescent, as they lose water of hydration when exposed to air. On the other hand, some ionic compounds can absorb so much water from the atmosphere that they eventually dissolve in their water of hydration; such substances are referred to as deliquescent.
Moderately stable hydrates may exhibit efflorescence or deliquescence upon exposure to air. Deliquescence occurs when the water vapor pressure in the air exceeds the vapor pressure of the hydrate, while efflorescence occurs when the conditions are reversed.
Certain hydrates are exceptionally stable, to the point that heating leads to the decomposition of the salt rather than dehydration. An example of this is hydrated aluminum chloride, AlCl3 ∙ 6H20, which can decompose in different ways depending on the prevailing conditions.
The following equations illustrate two possible decomposition reactions for this salt, depending on the specific conditions involved.
AlCl3 ∙ 6H20 → AlOCl + 2HCl + 5H20
2AlCl3 ∙ 6H20 → Al203 ∙ 3H20 + 6HCl + 6H20
Experimental Procedure
A. Efflorescence and Deliquescence
a. In your laboratory, several chemical compounds have been placed on watch glasses for observation. Each compound is accompanied by its correct formula and name. After approximately one hour, perform the following tasks:
i. Determine which salts have deliquesced.
ii. Identify the salts that have effloresced.
iii. Note the salts that neither effloresced nor deliquesced.
b. Take 2 or 3 crystals of Cobalt(II) Chloride (CoCl2 ∙ 6H20) and place them in a clean, small test tube. Heat the test tube until the color change appears to be complete. Allow the contents to cool down and leave them exposed to the atmosphere for about 30 minutes. If little or no change is observed, add 1 or 2 drops of water. Record your observations in your notebook.
c. Write the equations for the reactions observed during the heating of the salt and when water is added. Explain how this salt can be used as a moisture indicator.
d. Note: An efficient student would perform Part 2 while conducting various cooling periods in the next experiment.
B. Determination of the Formula of a Hydrate
a. Use an analytical balance to weigh a clean and dry evaporating dish along with a watch glass, with the nearest accuracy of 0.1 mg. Place 1-2 g of an unknown crystalline hydrate (provided by the instructor) in the weighed dish and reweigh it to the nearest 0.1 mg. Place the dish on a wire triangle (or gauze) supported by a ring stand or tripod, and cover it with the watch glass (refer to Figure 11).
b. Heat the dish slowly using a small colorless flame for approximately 15-20 minutes, until no moisture is visible on the watch glass. If the hydrate melts, heat it slowly to prevent splattering of the salt. Be cautious not to let the flame touch the watch glass, as it may break.
c. lAllow the dish to cool down to room temperature and weigh it immediately. Reheat the dish for about 5 minutes, being careful not to overheat and cause decomposition of the salt.
d. Cool the dish again and reweigh it. Repeat the process of heating, cooling, and weighing until two consecutive weighings agree within 1 mg (0.001 g). Complete the report sheet and calculate the number of moles of water per mole of anhydrous salt. Report the formula of the hydrate as M ∙ XH20, where X represents the calculated moles of water. Show your calculations.
Figure 11 Experimental apparatus for the dehydration of a hydrate
Report Sheet for HYDRATES
Name_______________________________________ Date___________________________
Lab / Section_________________________________
Unknown Number: ________________
Questions for HYDRATES
1. State the purpose of this lab
2. Hydrates decompose on heating by releasing water to produce a(n)
___________________ and ___________________
3. In the lab there will be 3 different salts on the Top Loading balances. Name these salts, record their weight at the start of lab, and record their final weight after one hour.
4. In the following hour of measuring the salts, determine which salts either stayed the same, effloresced, or deliquescented
5. Why is it important to allow to cool to room temperature before weighing?
Review Questions
1. What is a hydrate? Provide an example of a common hydrate.
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2. Explain the difference between efflorescence and deliquescence in hydrates.
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3. Why does water have a strong attraction for many compounds?
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4. What is the purpose of heating a hydrate in the lab? What changes are expected to occur?
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5. Write the balanced chemical equation for the decomposition of Cobalt(II) Chloride hexahydrate (CoCl2 ∙ 6H2O) upon heating.
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6. How could Cobalt(II) Chloride be used as a moisture indicator?
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7. Describe the properties and behavior of hydrates when exposed to different environmental conditions.
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8. Why is it important to use a wire triangle or gauze when heating the evaporating dish in Part B of the lab?
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9. What is the significance of weighing the evaporating dish and watch glass before and after heating the hydrate?
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10. What precautions should be taken when handling concentrated nitric acid in the lab?
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11. Explain the concept of water of hydration and its role in the stability of hydrates.
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12. How does the number of water molecules in a hydrate affect its chemical formula?
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13. What is the difference between absorbed water and chemically bound water in a hydrate?
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14. Compare and contrast the behaviors of stable, moderately stable, and less stable hydrates.
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15. Describe the color changes that may occur as a hydrate loses water of hydration.
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16. How would you differentiate between a deliquescent substance and an efflorescent substance in the lab?
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17. What factors determine whether a hydrate will effloresce or deliquesce?
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18. Discuss the possible reasons for a hydrate melting upon heating instead of releasing water.
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19. Why is it important to repeat the heating, cooling, and weighing process until two consecutive weighings agree within a certain tolerance?
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20. Analyze the significance of determining the formula of an unknown crystalline hydrate and how it contributes to our understanding of chemical composition and stoichiometry.
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