5 Intermolecular Forces — Arrange Compounds in Order of Increasing Boiling Point
Question
Arrange each of the following sets of compounds in order of increasing boiling point temperature:
- H2O, HCl, SiH4
- Br2, Cl2, F2
- C2H6, CH4, C3H8
Show/Hide Answer
- SiH4 < HCl < H2O
- F2 < Cl2 < Br2
- CH4 < C2H6 < C3H8
Refer to Section 6.1: Intermolecular Forces (1).
Strategy Map
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| Strategy Map Steps |
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| 1. Identify what intermolecular forces are acting on each of the molecules in question. |
2. Rank the molecules based on their intermolecular forces (the stronger the forces, the higher the boiling point).
Show/Hide HintIntermolecular forces ranked from strongest to weakest: Hydrogen Bonding > Dipole-Dipole Forces > Dispersion Forces. |
3. If two or more molecules have the same intermolecular forces, rank them based on the appropriate properties.
Show/Hide HintWhen comparing non-polar molecules, the larger and heavier a molecule is, the stronger the dispersion forces and the higher the boiling point temperature. |
Solution
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1. HCl, H2O, SiH4
- SiH4 — dispersion forces
- HCl — dispersion forces and dipole-dipole attractions
- H2O — dispersion forces and hydrogen bonding
Hydrogen bonding is stronger than dipole-dipole attractions
Answer
SiH4 < HCl < H2O
2. F2, Cl2, Br2
- F2 — dispersion forces, 18 electrons
- Cl2 — dispersion forces, 34 electrons
- Br2 — dispersion forces, 70 electrons
All are non-polar, symmetrical diatomic molecules. The largest molecule, with the most electrons is the most polarizable and has the highest boiling point temperature.
Answer
F2 < Cl2 < Br2
3. CH4, C2H6, C3H8
- CH4 — dispersion forces
- C2H6 — dispersion forces
- C3H8 — dispersion forces
All are non-polar molecules with dispersion forces.
The larger the molecule (longer carbon chains have a larger surface area), the stronger the temporary dipole interactions and the higher the boiling point temperature.
Answer
CH4 < C2H6 < C3H8
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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| Guided Solution Ideas |
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This question is a theory problem where you rank the given compounds by their properties (boiling point). You can do this by evaluating the strength of their intermolecular forces.
Show/Hide Resource
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How is boiling point impacted by intermolecular forces?
Show/Hide Don’t Forget!The stronger the intermolecular forces, the more energy is required to break the intermolecular forces and transition the compound between the liquid and gas phases. The stronger the intermolecular forces are, the more energy it would require to boil. |
Recall the strength of the intermolecular forces.
Show/Hide Don’t Forget!Intermolecular forces = Hydrogen Bonding > Dipole-Dipole Forces > Dispersion Forces |
If the molecules are impacted by the same intermolecular forces, which one will have the higher boiling point temperature?
Show/Hide Think About This!If the molecules in question all have the same intermolecular forces, their boiling point temperatures will depend on the molecule’s chemical properties. The size of the molecule will change its boiling point. The larger the molecule (molar mass/atomic radius/surface area), the higher the temperature. |
| Complete Solution |
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| Identify all forces acting on each molecule.
Hydrogen bonding is the strongest force, followed by dipole-dipole attractions, and the weakest force is the dispersion force. Dispersion forces are temporarily induced dipoles that act in all molecules. If dispersion forces are the only forces present, they determine the boiling point temperature. Dispersion forces are greater when the molecule has more electrons (i.e., more polarizable) or when the molecular structure has a greater surface area. |
| 1. SiH4 < HCl < H2O
SiH4 = Dispersion Forces |
| 2. F2 < Cl2 < Br2
F2 = Dispersion Forces All forces are the same type therefore it goes based on size. The largest atom has the highest boiling point temperature. |
| 3. CH4 < C2H6 < C3H8
All molecules have the same elements but differ in the size. The larger the molecule (larger size, molar mass and surface area), the higher the boiling point temperature. |
Check Your Work
The question asked you to arrange in order of increasing boiling point temperature, so the lowest boiling point is first. The molecules will be arranged in order of weakest intermolecular attractions to the strongest (going from left to right).
Show/Hide Watch Out!
‘<‘ means ‘less than’
Make sure you are using this representation correctly.
Does your answer make chemical sense?
Show/Hide Answer
All molecules have intermolecular forces that determine the phase of a substance:
- Solids have the most intermolecular forces
- Liquids have less than solids
- Gases have the least
Transitioning between phases requires forming or breaking intermolecular forces. A boiling point is the transition between a liquid and gaseous state that requires breaking intermolecular forces.
Breaking intermolecular forces requires energy (heat). The intermolecular forces determine how much energy they require to break them: the stronger the force, the more energy it requires.
Hydrogen bonds are the strongest. This form occurs when hydrogen bonds to a molecule’s most electronegative atoms (oxygen, nitrogen, or fluorine). It exists between adjacent molecules.
Dipole-dipole attractions are slightly weaker. Due to their molecular geometry, the molecules have permanent dipoles. The electron-rich and electron-deficient regions of neighbouring molecules are attracted to each other.
Dispersion forces are the weakest and occur in all molecules due to the temporary displacement of electron clouds. When dispersion forces are the only intermolecular forces present, they can be ranked.
The instantaneous dipole is strongest when there are more electrons, the molecule is more polarizable, or the molecular shape has the greatest surface area.
PASS Attribution
- LibreTexts PASS Chemistry Book CHEM 1510/1520 (2).
- Question 6.E.11 from LibreTexts PASS Chemistry Book CHEM 1500 (3) is used under a CC BY-NC 4.0 license.
- Question 6.E.11 is question 10.E.1.16: Q10.1.11 from LibreTexts Chemistry 1e (OpenSTAX) (4), which is under a CC BY 4.0 license.
- Question 10.E.1.16: Q10.1.11 is question 11 from OpenStax Chemistry 2e (5), 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. Blackstock, L.; Brewer, S.; Jensen, A. In PASS Chemistry Book CHEM 1510/1520; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1510%2F%2F1520.
3. Blackstock, L.; Brewer, S.; Jensen, A. 6.2: Question 6.E.11 PASS – Arrange in Order of Increasing Boiling 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/6.02%3A_Question_6.E.11_PASS_-_arrange_in_order_of_increasing_boiling_point.
4. 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).
5. Flowers, P.; Theopold, K.; Langley, R.; Robinson, W. R. Ch. 10 Exercises. In Chemistry 2e. https://openstax.org/books/chemistry-2e/pages/10-exercises.
