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

6. Thermodynamics and Kinetics

Entropy

Organic Chemistry

6. Thermodynamics and Kinetics

Entropy

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6:18 minutes

Problem 7a

Textbook Question

When ethene is mixed with hydrogen in the presence of a platinum catalyst, hydrogen adds across the double bond to form ethane. At room temperature, the reaction goes to completion. Predict the signs of ΔH° and ΔS° for this reaction. Explain these signs in terms of bonding and freedom of motion.

Verified step by step guidance

Step 1: Analyze the reaction. The reaction involves the addition of hydrogen (H₂) to ethene (C₂H₄) in the presence of a platinum catalyst, resulting in the formation of ethane (C₂H₆). This is a hydrogenation reaction where the double bond in ethene is converted into a single bond in ethane.

Step 2: Predict the sign of ΔH°. The reaction involves breaking the π bond in the double bond of ethene and forming two new C-H σ bonds in ethane. Since σ bonds are stronger and more stable than π bonds, the reaction releases energy. Therefore, ΔH° (enthalpy change) is negative, indicating an exothermic reaction.

Step 3: Predict the sign of ΔS°. The reaction starts with two reactant molecules (ethene and H₂) and produces one product molecule (ethane). This reduction in the number of molecules leads to a decrease in the system's randomness or disorder. Therefore, ΔS° (entropy change) is negative.

Step 4: Explain ΔH° in terms of bonding. The breaking of the weaker π bond in ethene and the formation of stronger C-H σ bonds in ethane result in a net release of energy, making the reaction exothermic (negative ΔH°).

Step 5: Explain ΔS° in terms of freedom of motion. The reactants (ethene and H₂) have more freedom of motion as separate molecules compared to the single product (ethane). This reduction in molecular freedom leads to a decrease in entropy (negative ΔS°).

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Key Concepts

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

Enthalpy Change (ΔH°)

The enthalpy change (ΔH°) of a reaction indicates whether the reaction is exothermic or endothermic. In the case of the hydrogenation of ethene to form ethane, the reaction releases energy as new C-H bonds are formed, resulting in a negative ΔH°. This signifies that the products are more stable than the reactants due to the formation of stronger bonds.

Entropy Change (ΔS°)

Entropy change (ΔS°) measures the disorder or randomness in a system. In the hydrogenation reaction, two gaseous reactants (ethene and hydrogen) combine to form one gaseous product (ethane), leading to a decrease in the number of gas molecules. This results in a negative ΔS°, indicating a reduction in the system's disorder as the reaction proceeds.

Catalysis

Catalysis involves the use of a catalyst to increase the rate of a chemical reaction without being consumed in the process. In this reaction, platinum acts as a catalyst, facilitating the addition of hydrogen across the double bond of ethene. The presence of the catalyst lowers the activation energy required for the reaction, allowing it to proceed more efficiently at room temperature.

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Related Practice

Textbook Question

Calculate ∆G°, ∆H°, and ∆S° for the following acid–base reactions. Rationalize the value of ∆H° based on the structure of the conjugate bases. [Assume T = 298 K.]

(b)

Textbook Question

When ethene is treated in a calorimeter with H₂ and a Pt catalyst, the heat of reaction is found to be –137 kJ/mol (–32.7 kcal/mol), and the reaction goes to completion. When the reaction takes place at 1400 K, the equilibrium is found to be evenly balanced, with K_eq =1. Compute the value of ΔS for this reaction.

Textbook Question

For the following values of ∆H° , ∆S°, and T, tell whether the process would be favored.

(a) ∆H° = -15 kcal/mol ; ∆S° = +37 cal/mol•K ; T = 273 K

Textbook Question

For each reaction, estimate whether ΔS° for the reaction is positive, negative, or impossible to predict.

(a)

Textbook Question

For each reaction, estimate whether ΔS° for the reaction is positive, negative, or impossible to predict.

(b)

Textbook Question

For each reaction, estimate whether ΔS° for the reaction is positive, negative, or impossible to predict.

(c)