Law of Definite Proportions
The objective of this lab is to demonstrate the Law of Definite Proportions by synthesizing a binary compound of copper and iodine. The lab focuses on the use of tools and equipment, safe laboratory practices, understanding chemical reactions and quantities, understanding molecules and compounds, and recording experimental results.
The discussion introduces Dalton's atomic theory, which states that atoms of a given substance have the same weight and that compounds are formed by combinations of atoms. The Law of Definite Proportions supports these concepts, stating that in binary compounds formed by conventional covalent or ionic bonding, the elements always combine in the same weight percentages.
In this experiment, copper and iodine are combined to form a compound. Copper wire is used as the copper source, and iodine is provided as a solid that readily sublimes into vapor. When the iodine vapor comes into contact with the copper wire, a chemical reaction occurs, resulting in the formation of a white solid compound that adheres to the surface of the wire. The increase in weight of the copper wire after the compound forms is due to the added weight of the iodine atoms.
The chemical properties of copper atoms in the compound are different from those in the metal. The copper atoms in the compound can react with sodium thiosulfate solution, while unreacted copper does not. This property allows the compound to be dissolved while the unreacted copper remains intact. By removing the compound from the copper wire, the weight of the copper wire decreases, corresponding to the weight of the copper atoms that were in the compound.
According to the Law of Definite Proportions, the weight ratio of iodine to copper in the compound should be consistent in multiple syntheses. The lab procedure involves conducting the synthesis multiple times and determining the weight of iodine per gram of copper in each synthesis. The values are calculated to verify if the techniques used in the experiment are accurate and consistent.
The lab procedure includes steps such as preparing the copper wire, cleaning it with nitric acid and water, and weighing it before and after the compound formation. The copper wire is exposed to iodine vapor, and the coated wire is carefully weighed. The coated wire is then immersed in a sodium thiosulfate solution to remove the compound, rinsed, and weighed again. These measurements are repeated multiple times to ensure accuracy.
In the notebook, the weight of iodine per gram of copper is calculated for each synthesis. The values are analyzed to evaluate the consistency of the results and to determine the empirical formula of the compound formed.
Through this lab, students gain hands-on experience in conducting chemical reactions, measuring quantities, and applying the Law of Definite Proportions. They learn to use various tools and equipment, practice safe laboratory procedures, and record experimental data accurately. The lab emphasizes the importance of precision and accuracy in chemical analysis and reinforces key concepts related to atoms, compounds, and chemical composition.
Objective
· Use the tools and equipment necessary for basic scientific analysis and research
· Demonstrate safe practices in the Chemical Laboratory
· 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
· 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
· 1m #24 copper wire
· Test Tube
· 2M Nitric Acid
· Acetone
· Iodine
· Heat Gun
· 0.5 M Sodium Thiosulfate
Discussion
Dalton's atomic theory encompassed the ideas that atoms of a particular substance have identical weights and that compounds are formed through combinations of atoms. The Law of Definite Proportions, supported by experimental evidence, further substantiates these concepts. It consistently reveals that binary compounds formed by conventional covalent or ionic bonding always contain the same weight percentages of the two elements involved in the chemical combination to create the compound. The objective of this experiment is to synthesize a binary compound multiple times and determine its empirical formula.
In this experiment, we will combine two elements: copper and iodine. Copper, a soft metal, will be utilized in the form of a fine wire, while iodine is a solid that readily sublimes, meaning it converts to vapor upon gentle heating. The iodine vapor will be brought into contact with the copper wire, resulting in a chemical reaction that forms a new compound. The compound will appear as a white solid adhering to the surface of the copper wire. Consequently, the weight of the copper wire will increase due to the presence of the compound. It's important to note that the additional weight solely arises from the iodine atoms in the compound, as the copper atoms in the compound originally constituted part of the weight of the copper wire. Essentially, the copper atoms transition from associating with other copper atoms in the metal to associating with iodine atoms in the compound.
The chemical properties of copper atoms in the compound differ from those found in the metal. Specifically, copper atoms in the compound will react with a solution of sodium thiosulfate to produce a new water-soluble substance containing the copper atoms (the copper wire itself does not react with sodium thiosulfate solution). Consequently, the copper compound dissolves while the unreacted copper does not. As a result, when the compound coating is removed, the copper wire will weigh less than its original weight because some copper atoms have been removed from the surface. In fact, the difference in weight precisely corresponds to the weight of the copper atoms that combined with iodine atoms to form the white solid compound. Therefore, the weight gained by the copper wire upon being coated with the compound is attributed to the iodine atoms in the compound, while the weight lost when the compound is stripped off is attributed to the copper atoms in the compound.
According to the Law of Definite Proportions, regardless of the weight of copper and iodine combined to form the compound, the ratio of iodine's weight to copper's weight remains constant in all cases. To experimentally test this, follow the procedure outlined below at least three times and analyze the data in terms of the weight of iodine that reacts with one gram of copper in each compound synthesis.
Procedure
1. Take a 1-meter long piece of #24 copper wire. Wrap the wire tightly around a pen or pencil, creating a close spiral. Leave approximately 5 cm of straight wire as a handle. Bend the end of the handle slightly to allow the coil of wire to be suspended from the top rim of a test tube.
2. Briefly dip the copper coil into a dilute solution of nitric acid (2 M), for around fifteen seconds. Then, rinse the coil with distilled water. Repeat this process of dipping the coil in dilute nitric acid and washing it with distilled water. Subsequently, immerse the coil in acetone and allow it to air-dry. Use an analytical balance to weigh the copper coil by suspending it from a hook just below the balance pan hanging point. Record the weight to the nearest 0.1 mg.
3. Within the laboratory hood, locate test tubes (25 x 150 mm) containing iodine being warmed, causing iodine vapors (note: iodine solid or vapors are toxic) to rise near the top of the test tube. Adjust the length of the handle and the hook on the copper wire coil so that the bottom of the coil is at least 3 cm above the bottom of the test tube. Insert the coil into the test tube and hang it by hooking it on the edge. Allow the coil to remain in contact with the iodine vapor for a minimum of two minutes. Carefully remove the wire, taking care not to shake or jar it excessively to prevent the loss of the coating. If needed, eliminate excess iodine crystals by holding the coated wire in the warm air from a heat gun, ensuring minimal vibration of the coil. Weigh the coated copper wire on the analytical balance, rounding the measurement to the nearest 0.1 mg.
4. Submerge the coated copper coil in a 0.5 M sodium thiosulfate solution for approximately two minutes, occasionally swirling the solution. Ensure that the entire coating is covered by the thiosulfate solution. Once the bright surface of the copper reappears, remove the coil from the solution and rinse it with distilled water. Then, dip the coil in acetone and allow it to air-dry. Weigh the copper coil on the analytical balance, rounding the measurement to the nearest 0.1 mg.
5. Calculate the weight of iodine per gram of copper in your notebook to verify the accuracy of your techniques.
6. Repeat steps 3 and 4 two or more times. Calculate the weight of iodine per gram of copper for each of the three experiments and present these values to the instructor.
7. Determine the formula for the compound that was formed.
Report Sheet for Law of Definite Proportions
Name_______________________________________ Date___________________________
Lab / Section_________________________________
Questions for
1. State the purpose of this lab.
1. The copper wire stripped of its coating of the compound will weigh LESS than the original copper wire? True / False
2. The newly formed compound will weigh LESS than the original copper wire? True / False
3. The ratio of the Mass of iodine to the Mass of copper in the compound should be
a. 1:1
b. 2:1
c. 1:2
4. The ratio of moles of the copper to the iodine should be
d. 1:1
e. 2:1
f. 1:2
Review Questions
1. What is the objective of this lab?
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2. Name two tools or equipment used in this lab.
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3. Why is it important to demonstrate safe practices in the chemical laboratory?
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4. Explain the Law of Definite Proportions and its relevance to this experiment.
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5. How is the compound formed in this experiment?
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6. What are the chemical properties of copper atoms in the compound compared to copper in its pure form?
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7. Describe the process of decantation and its purpose in this experiment.
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8. What is the purpose of filtration in this lab? Explain how it works.
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9. How does coagulation help in separating suspended particles in a solution?
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10. What is the purpose of distillation in this experiment? Describe the distillation apparatus used.
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11. Explain the concept of sublimation and how it is used to separate solids.
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12. How does extraction work in this lab? Provide an example of a substance that can be extracted using this method.
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13. What is the significance of determining the empirical formula in this experiment?
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14. What is the function of nitric acid in preparing the copper wire?
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15. Why is it important to rinse the copper coil with distilled water after exposure to iodine vapor?
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16. What is the purpose of immersing the coated copper coil in a sodium thiosulfate solution?
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17. How does sodium thiosulfate solution help in removing the compound from the copper wire?
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18. Why is it necessary to repeat the synthesis of the compound multiple times in this experiment?
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19. How do you calculate the weight of iodine per gram of copper?
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20. How can the results of this experiment confirm the Law of Definite Proportions?
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