Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

9. Alkenes and Alkynes

Hydrogenation of Alkynes

Organic Chemistry

9. Alkenes and Alkynes

Hydrogenation of Alkynes

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3:52 minutes

Problem 28

Textbook Question

When sodium generates electrons in the presence of ammonia, these electrons persist in solution, giving the blue color. However, electrons do not persist when sodium is added to water. Why?

Verified step by step guidance

Understand the behavior of sodium in both ammonia and water: Sodium (Na) is a highly reactive alkali metal. When it reacts with ammonia (NH₃), it generates solvated electrons, which are free electrons stabilized by the ammonia molecules. These solvated electrons are responsible for the blue color of the solution. In contrast, when sodium reacts with water (H₂O), the reaction is much more vigorous and does not produce solvated electrons.

Analyze the reaction of sodium with ammonia: Sodium reacts with liquid ammonia to form sodium amide (NaNH₂) and solvated electrons. The reaction can be represented as:

Na + {NH}_{3} \to Na {NH}_{2} + e ⁻

. The solvated electrons are stabilized by the ammonia molecules, allowing them to persist in solution.

Examine the reaction of sodium with water: Sodium reacts with water to form sodium hydroxide (NaOH) and hydrogen gas (H₂). The reaction is highly exothermic and can be represented as:

Na + H_{2} O \to Na OH + H_{2}

. In this reaction, the electrons from sodium are immediately consumed in the formation of hydroxide ions (OH⁻) and hydrogen gas, leaving no free electrons to persist in solution.

Compare the stabilization of electrons in ammonia versus water: In ammonia, the solvated electrons are stabilized by the polar nature of ammonia molecules, which form a solvation shell around the electrons. In water, however, the electrons are not stabilized because they are rapidly consumed in the chemical reaction to form hydroxide ions and hydrogen gas. This difference in stabilization explains why electrons persist in ammonia but not in water.

Conclude the reasoning: The key difference lies in the nature of the reactions and the stabilization of electrons. In ammonia, the reaction produces solvated electrons that are stabilized by the solvent, resulting in the blue color. In water, the reaction is too vigorous, and the electrons are immediately consumed, preventing their persistence in solution.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electron Generation and Persistence

In the context of sodium and ammonia, sodium donates electrons, which can remain in solution due to the solvent's properties. Ammonia, being a polar solvent, stabilizes the generated electrons, allowing them to persist and contribute to the blue color observed. This contrasts with water, where the high dielectric constant and reactivity lead to rapid electron consumption.

Solvent Effects on Chemical Reactions

The choice of solvent significantly influences the behavior of solutes. In ammonia, the solvent's ability to stabilize charged species allows for the retention of free electrons. In contrast, water's strong hydrogen bonding and high polarity facilitate the rapid reaction of sodium with water, leading to the formation of sodium hydroxide and hydrogen gas, which consumes the electrons.

Sodium's Reactivity with Water vs. Ammonia

Sodium is highly reactive with water, resulting in an exothermic reaction that produces sodium hydroxide and hydrogen gas. This reaction is vigorous and leads to the immediate consumption of electrons. In ammonia, however, the reaction is less vigorous, allowing for the stabilization of electrons in solution, which is why the blue color persists in ammonia but not in water.

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