All GRE Subject Test: Chemistry Resources
Example Question #1 : Other Intermolecular Forces
Rank the following compounds in terms by increasing boiling point, starting with the lowest boiling point first.
IV. (R)-4-hydroxypentanoic acid.
II < III < IV < I
IV < I < II < III
III < II < IV < I
III < II < I < IV
II < III < I < IV
III < II < I < IV
Boiling point is highly dependent on the intermolecular forces of a compound. Compounds with stronger intermolecular forces, larger masses, and less branching will have higher boiling points.
Compounds II and III only exhibit intermolecular London dispersion forces, so they would be the two lowest boiling compounds (weakest intermolecular forces). Because compound III has more branching, these London dispersion forces would be weaker, resulting in a lower boiling point than compound II.
III < II
Compounds I and IV would be higher boiling point compounds because of additional hydrogen bonding (strong intermolecular forces). Compound IV would be the highest boiling because the hydroxy group and carboxylic acid group could BOTH participate in intermolecular hydrogen bonding. In addition, compound IV is more polar (more polarized carbon-oxygen bonds), resulting in greater dipole-dipole attraction as well.
III < II < I < IV
Example Question #2 : Other Intermolecular Forces
Which molecule experiences only London dispersion forces, and no other intermolecular interactions?
Intermolecular forces become relevant when there are partial charge differences between atoms in the molecule, generating polarized bonds. In order for this to happen, the two bonded atoms must have different electronegativities such that one atom pulls the electrons in the bond closer to it.
In liquid bromine , the two bromine atoms have the same electronegativity, so there is no unequal sharing of electrons. As a result, only London dispersion forces are found between bromine molecules.
Ethanol and ammonia are capable of hydrogen bonding, while acetone is capable of dipole-dipole interactions due to the polarized carbon-oxygen bond.
Example Question #3 : Other Intermolecular Forces
Which of the following solvents is the most polar?
Water is the most polar solvent. Polar molecules contain bonds that have charges that are opposite in charge with high electronegativity differences. The oxygen atom in water draws electrons away from the hydrogen atoms causing the oxygen to be more negatively charged and hydrogen atoms to be more positively charged. Water would not be polar without its bent geometry which allows it to have a non zero dipole moment (1.85D), making it a polar molecule. The polarity of water is essential for life as we know it.
Example Question #4 : Other Intermolecular Forces
Which solvent is miscible with water?
Two solvents are miscible if after mixing them, they form a homogeneous mixture. The saying, like dissolves like, is an explanation of why benzene which is non polar will dissolve in a non polar solvent such as pentane. Polar molecules are soluble in polar solvents. For example, water is miscible in alcohols. Ionic substances such as common table salt which is composed of sodium chloride dissolve in water but do not dissolve to any great extent in most organic solvents which are non-polar. If we were to mix the benzene solution with water, we will find that they are not miscible and form a heterogenous mixture. Methanol , is the only polar solvent in the options given and is miscible with water.