6.2: Question 6.E.11 PASS - arrange in order of increasing boiling point
- Page ID
- 452263
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Arrange each of the following sets of compounds in order of increasing boiling point temperature:
a) HCl, H2O, SiH4
b) F2, Cl2, Br2
c) CH4, C2H6, C3H8
- Answer
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a) SiH4 < HCl < H2O
b) F2 < Cl2 < Br2
c) CH4 < C2H6 < C3H8
- Strategy Map
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Step Hint 1. Identify what intermolecular forces are acting on each of the molecules in question. See LibreText 6.1 Intermolecular Forces 2. Rank the molecules based on their intermolecular forces (the stronger the forces, the higher the boiling point) Intermolecular 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. When comparing nonpolar 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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a) HCl, H2O, SiH4
SiH4 dispersion forces
HCl dipole-dipole attractions + dispersion forces.
H2O hydrogen bonding + dispersion forces
Answer: SiH4 < HCl < H2O
b) F2, Cl2, Br2
F2 dispersion forces
Cl2 dispersion forces
Br2 dispersion forces
The largest molecule has the highest boiling point temperature.
Answer: F2 < Cl2 < Br2
c) CH4, C2H6, C3H8
All are nonpolar with dispersion forces.
The larger the molecule (the larger the surface area), the higher the boiling point temperature.
Answer: CH4 < C2H6 < C3H8
- Guided Solution
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Download Guided Solution as a pdf
Guided Solution Hint This is a theory type problem where you will rank the given compounds by their properties (boiling point). This can be done by evaluating the strength of their intermolecular forces. Arrange each of the following sets of compounds in order of increasing boiling point temperature:
a) HCl, H2O, SiH4
b) F2, Cl2, Br2
c) CH4, C2H6, C3H8
How is boiling point impacted by intermolecular forces?
The stronger the intermolecular forces, the more energy required to break the intermolecular forces and transition the compound between the liquid phase and the gas phase.
The stronger the intermolecular forces are, the more energy it would require to boil.
Recall the comparative strength of the intermolecular forces. Intermolecular forces ranked from strongest to weakest:
Hydrogen Bonding > Dipole-Dipole Forces > Dispersion Forces
If the molecules are impacted by the same types of intermolecular forces, which one will have the higher boiling point temperature? If the molecules in question all have the same intermolecular forces, their boiling point temperatures will be dependent on the chemical properties of the molecule. For nonpolar molecules the size of the molecule will change its boiling point, the larger the molecule (molar mass/atomic radius/surface area) the higher the boiling point temperature. Complete Solution:
- Identify all forces acting in each molecule.
- Bolded are the most dominant forces in the molecule (keeping in mind that hydrogen bonding is a type of dipole-dipole attraction).
- Hydrogen bonding is the strongest force, followed by dipole-dipole attractions and the weakest force is dispersion forces.
Dispersion forces act in all molecules, if they are the only forces acting between the molecules, a “tie breaker” is used to determine the boiling point temperature.
a) HCl, H2O, SiH4
SiH4 dispersion forces
HCl dipole-dipole attractions + dispersion forces.
H2O hydrogen bonding + dispersion forces
Answer: SiH4 < HCl < H2O
b) F2, Cl2, Br2
F2 dispersion forces
Cl2 dispersion forces
Br2 dispersion forces
All forces are the same type therefore it goes based on size. The largest atom has the highest boiling point temperature.
Answer: F2 < Cl2 < Br2
c) CH4, C2H6, C3H8
All are nonpolar with dispersion forces. All molecules have the same elements but differ in the numbers of C and H and the molecule size.
The larger the molecule (larger size, molar mass and surface area), the higher the boiling point temperature.
Answer: CH4 < C2H6 < C3H8
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)
Why does this answer make chemical sense?
All molecules have intermolecular forces which determine the phase of a substance. Solids have the most intermolecular forces, liquids have less and gases have the least. To transition between phases intermolecular forces must be formed or broken. A boiling point is the transition between a liquid state and a gaseous state which requires the breaking of intermolecular forces. Breaking intermolecular forces requires energy (heat) and the amount of energy is determined by the intermolecular forces themselves. The stronger the force, the more energy is required.
Hydrogen bonds are the strongest due to them being between two very polar molecules. Dipole-Dipole attractions are slightly weaker, but still stronger than dispersion forces as the molecules in a dipole-dipole attraction both have a constant partial charge. Dispersion forces are the weakest and occur in all molecules due to the movement of electron clouds. Within dispersion forces, molecules with higher surface areas will have stronger attractions than smaller molecules due to there being more space for the molecules to “cling” to one another. (Think about Velcro strips, the larger strip will have a better grip than the smaller one due to its larger surface area).
'<' means 'less than'
Make sure you are using this representation correctly.
(question source from page titled Liquids and Solids (Exercises) https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/10%3A_Liquids_and_Solids/10.E%3A_Liquids_and_Solids_(Exercises), shared under a CC BY 4.0 license, authored, remixed, and/or curated by OpenStax, original source https://openstax.org/books/chemistry-2e/pages/10-exercises, Access for free at https://openstax.org/books/chemistry/pages/1-introduction)