Intermolecular Forces and Liquids and Solids: Predict Melting Point
Question
The melting point of H2O(s) is 0°C. Would you expect the melting point of H2S(s) to be −85°C, 0°C, or 185°C? Explain your answer.
Show/Hide Answer
−85°C
Water has hydrogen bonds which are a stronger intermolecular force. Therefore, H2S must melt at a lower temperature since it has dipole-dipole intermolecular attractions, which are weaker than hydrogen bonds.
Refer to Section 6.1: Intermolecular Forces (1).
Strategy Map
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1. Identify the difference between the 2 molecules.
Show/Hide HintRecall the Lewis Structures of both molecules. Refer to 4.4: Lewis Symbols and Structures (2). |
2. Identify what intermolecular forces each molecule has.
Show/Hide HintThey do not have the same intermolecular forces. Just because there are hydrogen atoms in both molecules does not mean they both have hydrogen bonds. |
Solution
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Answer: −85°C
Water has hydrogen bonds between the molecules which are a stronger intermolecular force, so it takes more energy to break some intermolecular attractions and change from a solid to a gas. We expect H2S to melt at a lower temperature than H2O because it has dipole-dipole intermolecular attractions, which are weaker than hydrogen bonds.
Guided Solution
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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.
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This question is a theory type problem where you must identify the intermolecular forces of the given molecules and decide if the melting point of one will be lower, the same, or higher than the other.
Show/Hide ResourceRefer to Section 6.1: Intermolecular Forces (1). |
| The melting point of H2O(s) is 0°C. Would you expect the melting point of H2S(s) to be −85°C, 0°C, or 185°C? Explain your answer. |
Hydrogen bonding will occur when a hydrogen atom is bonded to what other atoms?
Show/Hide Don’t Forget!Recall that hydrogen bonding occurs when a hydrogen atom is bonded to fluorine, oxygen, and nitrogen. |
Since the molecules have different atoms, you can expect that the molecules do not have the same melting point.
Show/Hide Think About This!The molecule with the stronger intermolecular force will have the higher melting point. |
| Complete Solution |
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| Identify the intermolecular attractions in each molecule:
Although both H2O and H2S have hydrogen atoms, sulfur is not one of the very electronegative atoms; therefore, H2S does not contain hydrogen bonds, it contains dipole-dipole attractions. Show/Hide Watch Out!Students often see a hydrogen atom and think of a hydrogen bond, but they must remember that hydrogen bonds only occur between hydrogen and specific atoms. |
| Compare the strength of the intermolecular attractions:
Dipole-dipole attractions are not as strong as hydrogen bonds, meaning the melting point of H2S must be lower than the melting point of H2O. |
| Answer: The melting point of H2S is expected to be −85°C.
Although both H2O and H2S have hydrogen atoms, sulfur is not one of the very electronegative atoms; therefore, H2S does not contain hydrogen bonds, it contains dipole-dipole attractions. |
| Dipole-dipole attractions are not as strong as hydrogen bonds meaning the melting point of H2S must be lower than the melting point of H2O. |
Check Your Work
We expect H2S’s melting point to be less than that of water, and −85°C is less than the 0°C melting point of water. Some intermolecular attractions must be broken to melt a substance in order to change it form a solid to a liquid. Because of this, molecules with stronger intermolecular attractions will melt a a higher temperature and molecules with weaker intermolecular attractions will melt at a lower temperature. Since H2S has dipole-dipole intermolecular attractions, which are weaker than the stronger hydrogen bonds that H2O contains, H2S must have a lower melting point than H2O.
Does your answer make chemical sense?
Show/Hide Answer
The 2 molecules do not have the same intermolecular forces.
Hydrogen bonding is caused by the very strong electronegativity difference between hydrogen and 1 of the highly electronegative atoms. The strong difference between the 2 causes the hydrogen atom to have a very strong partial positive charge.
Although the bond between hydrogen and sulfur is strong, it is only a dipole-dipole attraction, meaning the forces between 2 H2O molecules is stronger than the forces between 2 H2S molecules. Since H2S has a weaker force, its melting point will require less energy and, thus, will be lower than H2O.
PASS Attribution
- LibreTexts PASS Chemistry Book CHEM 1500 (3).
- Question 6.E.15 from LibreTexts PASS Chemistry Book CHEM 1500 (4) is used under a CC BY-NC 4.0 license.
- Question 6.E.15 is question 10.E.1.22: Q10.1.15 from LibreTexts Chemistry 1e (OpenSTAX) (5), which is under a CC BY 4.0 license.
- Question 10.E.1.22: Q10.1.15 is question 15 from OpenStax Chemistry 2e (6), which is under a CC BY 4.0 license. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction
References
1. OpenStax. 6.1: Intermolecular Forces. In CHEM 1500: Chemical Bonding and Organic Chemistry; LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM1500%3A_Chemical_Bonding_and_Organic_Chemistry/06%3A_Intermolecular_Forces_and_Liquids_and_Solids/6.01%3A_Intermolecular_Forces.
2. OpenStax. 4.4: Lewis Symbols and Structures. In CHEM 1500: Chemical Bonding and Organic Chemistry; LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM_1500%3A_Chemical_Bonding_and_Organic_Chemistry/04%3A_Chemical_Bonding_I-_Basic_Concepts/4.04%3A_Lewis_Symbols_and_Structures.
3. Blackstock, L.; Brewer, S.; Jensen, A. PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500.
4. Blackstock, L.; Brewer, S.; Jensen, A. 6.3: Question 6.E.15 PASS – Predict Melting Point. In PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500/06%3A_Intermolecular_Forces_and_Liquids_and_Solids/6.03%3A_Question_6.E.15_PASS_-_predict_melting_point.
5. OpenStax. 10.E: Liquids and Solids (Exercises). In Chemistry 1e (OpenSTAX); LibreTexts, 2022. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/10%3A_Liquids_and_Solids/10.E%3A_Liquids_and_Solids_(Exercises).
6. Flowers, P.; Theopold, K.; Langley, R.; Robinson, W. R. Ch. 10 Exercises. In Chemistry 2e. OpenStax, 2019. https://openstax.org/books/chemistry-2e/pages/10-exercises.
