Explain why a. H 2 O (100 °C) has a higher boiling point than CH 3 OH 165 °C2. b. H 2 O (100 °C) has a higher boiling point than NH 3 (-33 °C).

Hi, everybody. Welcome back. Let's take a look at our next problems. It says explain the difference in boiling points in the following pairs of compounds. And we have two pairs, one and two. Number one says the boiling point of ethanol, 78 °C is lower than methane diol, 192 °C. And then number two, the boiling point of Ethan amine 17 °C is lower than ethanol, 78 °C. And we have four different answer choices with different explanations for each. We'll walk our way through them after we look at these compounds and think about what might make these differences. So for number one, our two molecules are ethanol and methane diol. Of course, ethanol will have two carbons and one alcohol group. So drawing a skeletal structure here, 12 carbons and then one of the carbons has a bond to an alcohol group. Methane diol has only one carbon and two alcohol groups. So we had the sort of funky structure, just one carbon and then a line going to oh on one side and a line going to oh on the other side. So similar size molecules but one of these has one alcohol group and one of these has two alcohol groups. But when we think about boiling points, a lower boiling point, I'll put BP means that there are lower intermolecular forces in the molecule. So what would lead to um ethanol having lower intermolecular forces than methane diol? But what kind of intermolecular forces are there in these molecules? Well, we have alcohol. Oh so the strongest intermolecular force will be hydrogen bonding. Well, both molecules have hydrogen bonding. But what's the difference? The uh methane diol has twice as many alcohol groups as two alcohol groups to one alcohol group. So there will be more hydrogen bonding happening in methane diol. So methane diol has more hydrogen bonding and therefore stronger intermolecular forces and higher boiling point. So that's our explanation for number one. So we can look at our answer choices and rule some out uh choice. A again, in terms of the way we're working through this problem, we could work through both and then look through them. I like to approach these as if you know, how would I handle this on a test? And I might try and see if, can I eliminate answer choices in case I were really in a hurry and had to guess and move on or weren't sure about part two. Sometimes it's good to see. Do you eliminate things right away? So choice A for number one says methane dial has a higher molar mass. Well, it will have a higher molar mass because the oxygens have are more massive than carbon. But that isn't the reason for the boiling point difference. So choice or number one has the reason here is incorrect. So choice A can't be our answer. Choice. B for number one says the molecules of methane diol have more hydrogen bonds between them. And this is correct due to those two alcohol groups. So number one is the correct explanation. So B can stay choice. C number one says methane dial has a higher molar mass. So we cross that out and then choice D says number one molecules of methane diol have more hydrogen bonds between them. So that's correct. So we're going to be between choice B and choice D. So now let's look at number two, number two says the boiling point of F an amine. So that's going to be two carbons and an amine. So we've got carbon one, carbon two and then a bond to NH two is lower than ethanol. So we already have that there. So we've got the same number of carbons, but we have this oh group instead of NH two group. So again, we know that the han amine must have lower intermolecular forces. So why the difference? Well, we have h bonding available. In both, in both cases, we have a bond between a more electron atom and hydrogen, which is what makes hydrogen bonding possible. Unlike the previous problem, they both just got the one electron atom hydrogen pair. But what's the difference between oxygen and nitrogen? Well, oxygen is more electron then nitrogen. So that gives it a greater electron negativity difference with hydrogen. And therefore the charge separation with hydrogen will be larger in the oh bond and making the hydrogen bond stronger, we'll put hydrogen bonds stronger on the alcohol that makes gives it the higher boiling point. But when we look at our reasons, we have in answer B, number two, the molecules of ethanol form stronger hydrogen bonds between them. So there we go explanation for number two is correct. Also, so choice B must be our answer to be thorough. Look over at number uh choice D it says number two, ethanol has a higher molar mass. But again, this wouldn't be any sort of reason for the difference in boiling point or at least not a significant difference. So we have, even though it will because oxygen is more massive than nitrogen, let's say not the reason. So choice D incorrect. So there we have it the difference in boiling points in the following pairs of compounds. Uh The answer choices B for number one, the molecules of methane diol have more hydrogen bonds between them. And for number two, the molecules of ethanol form stronger hydrogen bonds between them. See you in the next video.

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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

2. Molecular Representations

Intermolecular Forces

Organic Chemistry

2. Molecular Representations

Intermolecular Forces

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Problem 34a,b

Textbook Question

Explain why
a. H₂O (100 °C) has a higher boiling point than CH₃OH 165 °C2.
b. H₂O (100 °C) has a higher boiling point than NH₃ (-33 °C).

Verified step by step guidance

Step 1: Begin by understanding the concept of boiling point. The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure, allowing the substance to transition from liquid to gas. Boiling points are influenced by intermolecular forces such as hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Step 2: Compare the boiling points of H2O and CH3OH. Both molecules can form hydrogen bonds due to the presence of highly electronegative atoms (oxygen) bonded to hydrogen. However, H2O can form stronger and more extensive hydrogen bonding networks because each water molecule has two hydrogen atoms and two lone pairs on oxygen, allowing it to act as both a hydrogen bond donor and acceptor. CH3OH, on the other hand, has only one hydrogen atom available for hydrogen bonding and fewer lone pairs. This results in weaker intermolecular forces in CH3OH compared to H2O, leading to a lower boiling point for CH3OH.

Step 3: Compare the boiling points of H2O and NH3. Both molecules can form hydrogen bonds, but the extent and strength of hydrogen bonding differ. H2O has a highly electronegative oxygen atom with two lone pairs, enabling it to form stronger hydrogen bonds. NH3 has a less electronegative nitrogen atom and only one lone pair, which limits the strength and number of hydrogen bonds it can form. Additionally, the molecular geometry of NH3 (trigonal pyramidal) further reduces the efficiency of hydrogen bonding compared to the bent geometry of H2O. These factors contribute to H2O having a higher boiling point than NH3.

Step 4: Highlight the role of molecular polarity and electronegativity. H2O is a highly polar molecule due to the significant electronegativity difference between oxygen and hydrogen, as well as its bent geometry. This polarity enhances the strength of hydrogen bonding. CH3OH and NH3 are less polar in comparison, which reduces the strength of their intermolecular forces and contributes to their lower boiling points.

Step 5: Summarize the key factors influencing boiling points. The boiling point of H2O is higher than CH3OH and NH3 due to its ability to form stronger and more extensive hydrogen bonding networks, its higher molecular polarity, and the electronegativity of oxygen compared to nitrogen. These factors collectively result in stronger intermolecular forces in H2O, requiring more energy to overcome during the phase transition from liquid to gas.

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Key Concepts

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

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen or nitrogen. In water (H2O), each molecule can form up to four hydrogen bonds, leading to a higher boiling point due to the significant energy required to break these interactions. In contrast, while CH3OH also forms hydrogen bonds, its structure and fewer hydrogen bonds result in a lower boiling point.

Molecular Weight and Boiling Point

Molecular weight can influence boiling points, but it is not the sole factor. Generally, larger molecules with greater molecular weights have higher boiling points due to increased van der Waals forces. However, in the case of H2O and NH3, the strong hydrogen bonding in water outweighs the effects of molecular weight, resulting in a higher boiling point despite NH3 having a lower molecular weight.

Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules, which significantly affect physical properties like boiling points. Water exhibits strong hydrogen bonding, while ammonia (NH3) has weaker hydrogen bonds due to its lower electronegativity compared to oxygen. This difference in intermolecular forces explains why H2O has a much higher boiling point than NH3, despite NH3 being a polar molecule.

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Explain whya. H2O (100 °C) has a higher boiling point than CH3OH 165 °C2.b. H2O (100 °C) has a higher boiling point than NH3 (-33 °C).

Key Concepts

Hydrogen Bonding

Molecular Weight and Boiling Point

Intermolecular Forces

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Explain why
a. H₂O (100 °C) has a higher boiling point than CH₃OH 165 °C2.
b. H₂O (100 °C) has a higher boiling point than NH₃ (-33 °C).