Take-home Practice Problems: Intermolecular Forces Consider the following molecules: CH4, NH3, CH3OCH3, and NH4Cl. What type of intermolecular forces do these substances have? CH4: London dispersion forces NH3: London dispersion forces and hydrogen bonding CH3OCH3: London dispersion forces and dipole-dipole forces NH4Cl: London dispersion forces, dipole-dipole forces, and ion-dipole forces Rank these substances by the strength of their intermolecular forces: Strongest forces: NH4Cl Weakest forces: CH4 Rank these substances by their boiling points: Highest boiling point: NH4Cl Lowest boiling point: CH4 Which of these substances would be insoluble in water? Why? CH4 and CH3OCH3 would be insoluble in water because they are nonpolar molecules and water is a polar solvent. NH3 and NH4Cl would be soluble in water because they can form hydrogen bonds with water molecules.
The Correct Answer and Explanation is:
To evaluate the intermolecular forces of each molecule, we need to consider the types of interactions that occur between the molecules and the factors that influence their strengths.
1. Intermolecular Forces:
- CH4 (Methane): CH4 is a nonpolar molecule. The only intermolecular force present here is London dispersion forces (induced dipole-induced dipole), which are the weakest of all intermolecular forces. These forces arise from temporary shifts in electron density in the molecules.
- NH3 (Ammonia): NH3 is a polar molecule due to the difference in electronegativity between nitrogen and hydrogen. It has London dispersion forces and hydrogen bonding. The nitrogen-hydrogen bond allows for hydrogen bonding, a strong intermolecular force, where the hydrogen atoms are attracted to lone pairs of electrons on nitrogen atoms of neighboring molecules.
- CH3OCH3 (Dimethyl Ether): This molecule is polar because the oxygen atom is more electronegative than the carbon atoms. It exhibits London dispersion forces and dipole-dipole forces. The dipole-dipole interactions are stronger than the London forces but not as strong as hydrogen bonding.
- NH4Cl (Ammonium Chloride): NH4Cl is an ionic compound, so it has London dispersion forces, dipole-dipole forces, and ion-dipole forces. The ion-dipole forces (between the NH4+ and Cl-) are much stronger than the other forces in the molecule, giving NH4Cl significantly stronger intermolecular forces.
2. Ranking by Strength of Intermolecular Forces:
The strength of the intermolecular forces follows the order from weakest to strongest:
- Weakest: CH4 (only London dispersion forces)
- Next: CH3OCH3 (London dispersion forces + dipole-dipole)
- Next: NH3 (London dispersion forces + hydrogen bonding)
- Strongest: NH4Cl (London dispersion forces + dipole-dipole + ion-dipole)
3. Ranking by Boiling Point:
The boiling point of a substance is influenced by the strength of the intermolecular forces. Stronger forces require more energy to overcome, leading to higher boiling points.
- Highest boiling point: NH4Cl (strong ion-dipole forces)
- Next: NH3 (strong hydrogen bonding)
- Next: CH3OCH3 (dipole-dipole forces)
- Lowest boiling point: CH4 (weak London dispersion forces)
4. Solubility in Water:
Water is a highly polar solvent, so it will dissolve substances that can interact with it through hydrogen bonding or other polar interactions.
- CH4: CH4 is nonpolar and does not have the capability to form hydrogen bonds or interact with water’s dipoles. It is insoluble in water.
- CH3OCH3: Although CH3OCH3 is polar, it does not form hydrogen bonds with water, and thus it is only slightly soluble in water but generally considered insoluble.
- NH3: NH3 can form hydrogen bonds with water molecules due to its polarity, making it soluble in water.
- NH4Cl: NH4Cl dissociates into NH4+ and Cl-, and the NH4+ ion can form strong ion-dipole interactions with water molecules. Thus, NH4Cl is highly soluble in water.
Conclusion:
- NH4Cl has the strongest intermolecular forces and the highest boiling point.
- CH4 has the weakest intermolecular forces and the lowest boiling point.
- NH4Cl and NH3 are soluble in water, while CH4 is insoluble. CH3OCH3 is only slightly soluble, or considered insoluble.
