MCAT Physical : Intermolecular Forces
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Example Question #871 : Mcat Physical Sciences
A student mislabels three jars containing three different molecules. The student frantically tries to find the identity of the molecules in each jar. He knows that the three possible molecules are methanol (
The molecule with the highest boiling point will contain the greatest number of __________.
hydrogen bonds because they are the strongest intramolecular forces
covalent bonds because they are the strongest intermolecular forces
covalent bonds because they are the strongest intramolecular forces
hydrogen bonds because they are the strongest intermolecular forces
hydrogen bonds because they are the strongest intermolecular forces
"Inter-" means between and "intra-" means within. Intermolecular forces are forces that exist between separate molecules and intramolecular forces exist within or inside a single molecule.
Remember that boiling point and melting point depend on the intermolecular forces, not intramolecular forces. Boiling and melting involve separation of molecules from one another. You will need a higher boiling and melting point if the forces between the molecules (intermolecular forces) are strong. The strongest form of intermolecular force is the hydrogen bond. Hydrogen bonds are intermolecular forces that occur between a hydrogen atom in one molecule and a nitrogen, oxygen, or fluorine atom on another molecule. The molecule with the highest boiling point will have the greatest amount of hydrogen bonds, since this would result in the strongest intermolecular interactions.
Covalent bonds are bonds within the molecule (intramolecular forces) and do not have any effect on the boiling point. Hydrogen bonds can form within a molecule, generating intramolecular forces, but will only do so under certain conditions. The formation of intramolecular hydrogen bonds will not affect boiling point unless the ability to form intermolecular hydrogen bonds is inhibited by this process.
Example Question #41 : Compounds, Molecules, And Bonds
Which of the following compounds has the highest boiling point?
All options have hydrogen attached to an electronegative atom. This difference in electronegativity gives hydrogen a partially positive charge, which allows it to become attracted to neighboring molecules with partially negative charges. This intermolecular force is called hydrogen bonding, and takes place when hydrogen is attached to nitrogen, oxygen, or fluorine.
Fluorine is the most electronegative atom out of the options, meaning that the hydrogen has the strongest partially positive charge in hydrofluoric acid. As a result, it will have the strongest attraction to neighboring molecules. Larger intermolecular forces generally result in higher boiling points. This strong attraction gives it the highest boiling point.
Hydrogen sulfide is the only given compound that does not exhibit hydrogen bonding. This means it will have the weakest intermolecular interactions and, as expected, this results in the lowest boiling point of the given compounds.
Example Question #41 : Compounds, Molecules, And Bonds
Which of the following is not true of hydrogen bonds?
They are stronger than van der Waals interactions
They may occur between different molecules or within a single molecule
They explain many of the properties of water
They are weak bonds that are easily formed and broken
They can form between molecules with either polar or nonpolar covalent bonds
They can form between molecules with either polar or nonpolar covalent bonds
Hydrogen bonds are only formed between molecules with polar covalent bonds, and not in nonpolar moelcules. They result from the electromagnetic attraction between hydrogen (which is slightly positively charged) and an atom of opposite (negative) charge, namely the negatively charged end of a polar molecule. All the other statements are accurate.
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