Draw a complete Lewis structure of methanol, CH3OH. A complete Lewis structure includes all atoms, shows all bonds, and shows all lone pair electrons. (Note: this Lewis structure should be hand-drawn, NOT copied and pasted from the internet or created with a structure drawing program. Remember that you can draw your structure on paper and then paste a photo of the drawing into this document.) 1b. (1 point) We have discussed many different bonds and interactions up to this point in the course. These include: LDF, dipole-dipole, Hydrogen bonding interactions, covalent bonds, metallic bonds, and ion-ion interactions. Consider methanol in the liquid state. Using the interactions we have discussed in this class, list all intermolecular forces that exist in liquid methanol. 1c. (2 points) Use Lewis structures to draw a molecular level picture of methanol, CH3OH, in the liquid state. You should draw three methanol molecules. Show and label the strongest intermolecular force present in liquid methanol. If no intermolecular forces exist, write “none”. 1d. (2 points) Use Lewis structures to draw a molecular level picture of methanol, CH3OH, in the gaseous state. You should draw three methanol molecules. Show and label the strongest intermolecular force present in gaseous methanol. If no intermolecular forces exist, write “none”. 1e. (2 points) Explain what happens on the molecular level when methanol boils. Be sure to include forces and energy in your explanation. 1f. (2 points) The boiling point of methanol is 65°C. When methanol boils, what is the sign for ΔHsys? Give a molecular level explanation to explain how you determined this sign. 1g. (2 points) The boiling point of methanol is 65°C. When methanol boils, what is the sign for ΔSsys? Give a molecular level explanation to explain how you determined this sign. 1h. (2 points) Use the equation for the change in Gibbs free energy and the signs you determined in parts f and g to explain why methanol boils at temperatures above the boiling point. Include the equation and a written explanation. 1i. (2 points) Use the equation for the change in Gibbs free energy and the signs you determined in parts f and g to explain why methanol does not boil at temperatures below the boiling point. Include the equation and a written explanation.
The Correct Answer and Explanation is:
1a. Lewis structure of methanol, CH₃OH
(Draw the structure by hand or upload a photo if submitting; the description is below.)
- Carbon (C) is single bonded to three hydrogen atoms (H) and one oxygen atom (O).
- The oxygen atom is single bonded to a hydrogen atom and has two lone pairs of electrons.
mathematicaCopyEdit H
|
H — C — O — H
|
H
- Show lone pairs on oxygen: two pairs (••)
1b. Intermolecular forces in liquid methanol:
- London dispersion forces (LDF)
- Dipole-dipole interactions
- Hydrogen bonding (strongest)
Methanol has a polar O–H bond which enables hydrogen bonding between molecules. LDF and dipole-dipole are present in all polar molecules.
1c. Three methanol molecules in liquid
(Draw the Lewis structures and show hydrogen bonds as dashed lines.)
Example:
mathematicaCopyEditH H H H H H
\ | / \ | /
C --- H···O C --- H···O
/ | / |
H O···H H O
| |
H H
Label the dashed line as a hydrogen bond.
Strongest force: Hydrogen bonding
1d. Three methanol molecules in gas
In the gas state, molecules are far apart.
mathematicaCopyEditH H H H H H H H H
\ | / \ | / \ | /
C C C
/ O / O / O
H | H | H |
H H H
Strongest force: Hydrogen bonding (still present but much weaker due to distance)
1e. What happens when methanol boils?
During boiling, energy is added to overcome intermolecular forces, especially hydrogen bonds. Molecules move from a tightly held liquid state to a free-moving gas. No covalent bonds break. Energy is used to separate the molecules, not atoms.
1f. Sign of ΔHsys when methanol boils:
Positive (ΔHsys > 0)
Energy is absorbed from the surroundings to overcome intermolecular forces like hydrogen bonding. This makes boiling an endothermic process.
1g. Sign of ΔSsys when methanol boils:
Positive (ΔSsys > 0)
In the gas phase, methanol molecules have more disorder and greater freedom of motion compared to the liquid. Thus, the system’s entropy increases.
1h. Why methanol boils above 65°C:
Use: ΔG = ΔH – TΔS
Above the boiling point, TΔS > ΔH, so ΔG < 0.
This means the process is spontaneous. Methanol changes to gas because the gain in entropy (ΔS > 0) outweighs the energy cost (ΔH > 0).
1i. Why methanol does not boil below 65°C:
Use: ΔG = ΔH – TΔS
Below the boiling point, TΔS < ΔH, so ΔG > 0.
This makes the process non-spontaneous. There is not enough thermal energy to overcome the hydrogen bonds holding methanol molecules in the liquid phase.
