Question

what is the dominant intermolecular force found between formaldehyde molecules, ch₂o?
red = o, black = c, white = h
dipole-dipole forces
ion-dipole forces
hydrogen bonding
dispersion forces

Explanation:

1. First, analyze the structure of formaldehyde (\(CH_2O\)): The molecule has a polar carbon - oxygen double bond (\(C = O\)). The electronegativity of oxygen is much higher than that of carbon, so there is a significant dipole moment in the \(C = O\) bond, making the formaldehyde molecule a polar molecule.
2. Now, consider each type of intermolecular force:

• Ion - dipole forces: These forces occur between ions and polar molecules. Formaldehyde is a neutral molecule, not an ion, so ion - dipole forces are not the dominant intermolecular force here.
• Hydrogen bonding: For hydrogen bonding to occur, a hydrogen atom must be bonded to a highly electronegative atom (F, O, or N) and be in close proximity to another highly electronegative atom with a lone pair of electrons. In formaldehyde (\(H_2C = O\)), the hydrogen atoms are bonded to carbon, not to oxygen (the \(C = O\) bond is a double bond between carbon and oxygen, and the hydrogens are attached to carbon). So, hydrogen bonding is not possible.
• Dispersion forces (London dispersion forces): These are the weakest intermolecular forces and are present in all molecules, polar or non - polar. However, in polar molecules like formaldehyde, there are stronger intermolecular forces (dipole - dipole) than dispersion forces, so dispersion forces are not the dominant force.
• Dipole - dipole forces: Since formaldehyde is a polar molecule (due to the polar \(C = O\) bond), the positive end of one formaldehyde molecule (the carbon side, since oxygen is more electronegative and pulls electron density towards itself) will be attracted to the negative end (the oxygen side) of another formaldehyde molecule. This type of attraction between polar molecules is called dipole - dipole forces, and it is the dominant intermolecular force in formaldehyde.

Answer:

dipole - dipole forces