1. Describe four types of intermolecular interactions; sketch attractions betwee
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Question
1. Describe four types of intermolecular interactions; sketch attractions between molecules (somehow communicate what the attraction is). Which of these is the strongest? Which is the weakest? What can a substance’s phase at room temperature tell us about the intermolecular interactions?
2. What types of substances can hydrogen bond? What is special about the hydrogen bond?
3. Define each of the following: vaporization; condensation; sublimation; deposition; melting (fusion); freezing. Which of these are exothermic? Which are endothermic?
4. Define the “heat of fusion” (Hfus) and “heat of vaporization” (Hvap). Why do these processes require an input of energy? What is the energy used for?
Explanation / Answer
1. IONIC FORCES:
The forces holding ions together in ionic solids are electrostatic forces. Opposite charges attract each other. These are the strongest intermolecular forces. Ionic forces hold many ions in a crystal lattice structure.
2. DIPOLE FORCES:
Polar covalent molecules are sometimes described as "dipoles", meaning that the molecule has two "poles". One end (pole) of the molecule has a partial positive charge while the other end has a partial negative charge. The molecules will orientate themselves so that the opposite charges attract principle operates effectively.
In the example on the left, hydrochloric acid is a polar molecule with the partial positive charge on the hydrogen and the partial negative charge on the chlorine. A network of partial + and - charges attract molecules to each other.
3. HYDROGEN BONDING:
The hydrogen bond is really a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine.
In other words - The hydrogen on one molecule attached to O or N that is attracted to an O or N of a different molecule.
3. HYDROGEN BONDING:
Link to more extensive discussion: Hydrogen Bonding
The hydrogen bond is really a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine.
In other words - The hydrogen on one molecule attached to O or N that is attracted to an O or N of a different molecule.
4. INDUCED DIPOLE FORCES:
Forces between essentially non-polar molecules are the weakest of all intermolecular forces. "Temporary dipoles" are formed by the shifting of electron clouds within molecules. These temporary dipoles attract or repel the electron clouds of nearby non-polar molecules.
The temporary dipoles may exist for only a fraction of a second but a force of attraction also exist for that fraction of time. The strength of induced dipole forces depends on how easily electron clouds can be distorted. Large atoms or molecules with many electrons far removed from the nucleus are more easily distorted.
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