The most significant factor (intermolecular forces) which is responsible for the higher boiling point of mercaptoethanol, C2H5SH, compared to ethane, C2H6, is the: A) higher molecular mass B) larger size C) stronger dispersion forces D) dipolar forces E) hydrogen bonding forces
The Correct Answer and Explanation is:
The correct answer is E) hydrogen bonding forces.
Here’s why:
Mercaptoethanol (C2H5SH) has a much higher boiling point compared to ethane (C2H6) primarily due to the presence of hydrogen bonding between its molecules. Let’s break this down:
- Mercaptoethanol (C2H5SH) contains a hydroxyl group (-OH) and a thiol group (-SH). These groups are capable of forming hydrogen bonds, which are strong intermolecular forces that occur when a hydrogen atom, bonded to a highly electronegative atom (like oxygen or sulfur), interacts with another electronegative atom (such as oxygen, nitrogen, or sulfur) from a neighboring molecule. The hydrogen bonding in mercaptoethanol is much stronger than the weaker forces in ethane.
- Ethane (C2H6), on the other hand, is a simple alkane with only nonpolar covalent bonds and no functional groups capable of forming hydrogen bonds. Its intermolecular forces are mainly dispersion forces (also called London forces), which are much weaker than hydrogen bonds.
- Why hydrogen bonding matters: Hydrogen bonds require more energy to break than dispersion forces. This means that a substance with hydrogen bonding, like mercaptoethanol, will generally have a higher boiling point than a substance with only dispersion forces, like ethane.
- Other factors:
- A) Higher molecular mass: While mercaptoethanol is heavier than ethane, the difference in boiling points is more due to the hydrogen bonding than the mass.
- B) Larger size: The size of the molecules can influence boiling points, but it doesn’t explain the significant difference observed here.
- C) Stronger dispersion forces: Dispersion forces are present in both molecules, but they are not as strong in ethane as the hydrogen bonds in mercaptoethanol.
- D) Dipolar forces: Mercaptoethanol does have a polar functional group, but it is hydrogen bonding, not just dipole-dipole interactions, that is the key force behind its higher boiling point.
In summary, the hydrogen bonding forces in mercaptoethanol are the primary reason for its significantly higher boiling point compared to ethane.
