MCAT Physical : Intermolecular Forces

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

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Example Question #1 : Intermolecular Forces

Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.  

When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.

For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.

When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.

Dipole moment = charge * separation distance

A scientist is investigating the polar nature of several compounds. He compares the vapor pressure of water to the vapor pressure of an assortment of low molecular weight hydrocarbons. What is he most likely to find?

Possible Answers:

The vapor pressure will vary unpredictably between water and each low molecular weight hydrocarbon

The water will likely have higher vapor pressure, owing to its weaker intermolecular bonds

The water will likely have higher vapor pressure, owing to its stronger intermolecular bonds

The water will likely have lower vapor pressure, owing to its weaker intermolecular bonds

The water molecule will likely have lower vapor pressure, owing to stronger intermolecular bonds

Correct answer:

The water molecule will likely have lower vapor pressure, owing to stronger intermolecular bonds

Explanation:

The strong polarity of water relative to hydrocarbons means that water will have a more difficult time breaking out of its liquid phase, and into its gas phase to generate a vapor pressure. Substances with a high vapor pressure generally have weaker intermolecular bonds and a lower boiling point.

Example Question #2 : Intermolecular Forces

Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils.

Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here.

In this law,  is the mole fraction of the solvent, is the vapor pressure of the pure solvent, and  is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.

A scientist is studying a series of compounds at standard conditions. Of the compounds listed below, which is likely to have the highest vapor pressure?

Possible Answers:

Correct answer:

Explanation:

In this example, methane, , has the lowest molecular weight. Hydrocarbons that have the lowest molecular weight have the least opportunity for van der Waals forces to keep them from moving into the gaseous state; thus, they have the greatest tendency to form vapor and have the greatest vapor pressure. Recognize that the greater the intermolecular forces, the higher the boiling point and lower the vapor pressure.

Sodium chloride is a solid salt. Solids do in fact have vapor pressures, but the ionic structure of this salt makes it very low.

Example Question #2 : Intermolecular Forces

Which hydrocarbon has the highest melting point?

Possible Answers:

Correct answer:

Explanation:

Melting points of hydrocarbons are determined by two main factors: length of the carbon chain and degree of saturation. Longer carbon chains will have higher melting points, and chains with more saturated bonds have higher melting points.

Of the given answers, has the longest carbon chain and is fully saturated. It will thus have the highest melting point.

Example Question #3 : Intermolecular Forces

 

 

Rank the following compounds in order of increasing polarity, starting with the most non-polar compound.

,

Possible Answers:

Correct answer:

Explanation:

Polarity is determined by differences in electronegativity between the two atoms involved in a bond. A large difference in electronegativity will result in a more polar compound. Symmetry, however, can balance net polarities in bonds, can cancel the differences.

has tetrahedral geometry, and since the four groups attached to the central silicon atom are identical, this molecule has no net dipole moment due to its symmetry. It is the most non-polar compound.

In comparing  and , recall that carbon, nitrogen, and oxygen are in the same row of the periodic table, but carbon is farther from oxygen than nitrogen. This means there is a greater electronegativity difference in  than , and  is going to be more polar.

Finally, is an ionic compound, so it is going to be the most polar of all four compounds.

Example Question #4 : Intermolecular Forces

Which of the following is a polar molecule?

Possible Answers:

CH4

CCl4

CO2

H2O

Correct answer:

H2O

Explanation:

Of the answers, only H2O has a net dipole moment, making water the polar molecule. All the other molecules have balanced structures and no difference in electronegativity between side groups. 

Example Question #5 : Intermolecular Forces

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 (), dichloromethane (), and propane (). At room temperature, he observes that one of the jars contains a gas, whereas the other two jars contain liquids. He then finds the boiling point of each jar. The molecule from jar A has a boiling point of , jar B has a boiling point of , and jar C has a boiling point of . Based on his findings he is able to determine the identity of the molecules in each jar.

What is the main cause of dipole-dipole interactions?

Possible Answers:

Differences in electronegativity between atoms that result in uneven sharing of electrons

Differences in atomic size between atoms that changes the overall charge of the molecule

Differences in atomic size between atoms that result in uneven sharing of electrons

Differences in electronegativity between atoms that changes the overall charge of the molecule

Correct answer:

Differences in electronegativity between atoms that result in uneven sharing of electrons

Explanation:

Dipole-dipole interactions are intermolecular forces that result from attraction of partial charges of atoms. Partial charges are caused by uneven sharing of electrons between atoms. For example, a covalent bond between a hydrogen and a chlorine atom will cause uneven electron sharing between the two atoms. Chlorine, a more electronegative atom, will attract the electrons closer than hydrogen; therefore, the chlorine atoms will have a partial negative charge whereas the hydrogen atom will have a partial positive charge.

Molecules containing partial charges, such as , are called dipoles. When dipoles are added into solution, the partial charges attract one another and form dipole-dipole interactions. In an  solution the partial positive charge of hydrogen from one  molecule will interact with the partial negative charge of chlorine from another  molecule and form a dipole-dipole interaction.

Remember that the differences in electronegativity doesn’t change the overall charge of the molecule. It just gives rise to partial charges that result from the relative location of electrons between the two atoms.

Example Question #6 : Intermolecular Forces

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 (), dichloromethane (), and propane (). At room temperature, he observes that one of the jars contains a gas, whereas the other two jars contain liquids. He then finds the boiling point of each jar. The molecule from jar A has a boiling point of , jar B has a boiling point of , and jar C has a boiling point of . Based on his findings he is able to determine the identity of the molecules in each jar.

Dipole-dipole interactions can be observed in molecules of:

I. Methanol

II. Dichloromethane

III. Propane

Possible Answers:

III only

I and II

II only

II and III

Correct answer:

I and II

Explanation:

Dipole-dipole interactions occur between a partial positive and negative charge. Since partial charges arise from electronegativity differences, you are looking for molecules that contain atoms with different electronegativities.

Methanol molecules contain oxygen and hydrogen, which have very different electronegativities. Methanol molecules form dipole-dipole interactions between the partially positive hydrogen and the partially negative oxygen. This bond is also called a hydrogen bond. Hydrogen bonds are an extreme type of a dipole-dipole interaction.

Similarly, dichloromethane molecules contain chlorine and carbon atoms (very different electronegativities). Dichloromethane can form dipole-dipole interactions between partially negative chlorine atoms and partially positive carbon atoms.

Finally, propane contains only carbon and hydrogen, which have similar electronegativities. There are no dipole-dipole interactions in propane because there are no partial charges. The best answer is I and II.

Example Question #7 : Intermolecular Forces

Which of the following molecules contain intramolecular hydrogen bonds? 

Possible Answers:

 Para-nitrophenol

Acetone

Dimethyl ether

Hydrochloric acid

Ortho-nitrophenol

Correct answer:

Ortho-nitrophenol

Explanation:

The question is asking for intramolecular hydrogen bonds, meaning which of the following molecules will contain hydrogen bonds between the atoms within a single molecule. Hydrogen bonds exist only between a hydrogen and a nitrogen, oxygen, or flourine. Although acetone and dimethyl ether contain an oxygen that can make hydrogen bonds, the molecules themselves do not contain hydrogen bonds. These compounds form intermolecular hydrogen bonds only.

Para-Nitrophenol, similarly, will form intermolecular hydrogen bonds. The para positioning of substituents prevents them from interacting within a single molecule. Ortho-nitrophenol allows for such itneractions by having substituents on adjacent carbons. The hydrogen of the phenol and the oxygen of the nitro will form a hydrogen bond within a single molecule, therefore, ortho-nitrophenol is the only molecule present that contains intramolecular hydrogen bonds since it can form hydrogen bonds within itself. 

Example Question #8 : Intermolecular Forces

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 (), dichloromethane (), and propane (). At room temperature, he observes that one of the jars contains a gas, whereas the other two jars contain liquids. He then finds the boiling point of each jar. The molecule from jar A has a boiling point of , jar B has a boiling point of , and jar C has a boiling point of . Based on his findings he is able to determine the identity of the molecules in each jar.

If methanol was added to a solution containing ammonia, which of the following hydrogen bonds will be the strongest?

Possible Answers:

There will be no hydrogen bonds formed because methanol can’t form hydrogen bonds

Bond between the nitrogen from ammonia and the oxygen from methanol

Bond between the nitrogen from ammonia and the hydrogen from the hydroxyl group of methanol

Bond between the hydrogen from ammonia and the oxygen from methanol

Correct answer:

Bond between the nitrogen from ammonia and the hydrogen from the hydroxyl group of methanol

Explanation:

A hydrogen bond forms between a hydrogen bond donor (hydrogen) and a hydrogen bond acceptor (nitrogen, oxygen, or fluorine). The strength of a hydrogen bond can be determined by examining the acidity of the hydrogen and basicity of the acceptor. A hydrogen is more acidic when it is attached to a more electronegative atom. This occurs because the electronegative atom pulls the electron density towards itself, making it easy for the hydrogen to act as a leaving group (weaker bond).

A hydrogen on fluorine is the most acidic, and a hydrogen on nitrogen is the least acidic (of the hydrogen bonging possibilities). Basicity of the acceptor is also important in determining the strength of hydrogen bond. A more basic molecule will make the hydrogen bond stronger. Nitrogen forms the strongest hydrogen bonds, whereas fluorine forms the weakest hydrogen bonds.

In our case, the strongest bond will occur between the hydrogen from the hydroxyl group of methanol (most acidic donor) and the nitrogen from ammonia (most basic acceptor).

Example Question #10 : Intermolecular Forces

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 (), dichloromethane (), and propane (). At room temperature, he observes that one of the jars contains a gas, whereas the other two jars contain liquids. He then finds the boiling point of each jar. The molecule from jar A has a boiling point of , jar B has a boiling point of , and jar C has a boiling point of . Based on his findings he is able to determine the identity of the molecules in each jar.

Which of the three molecules cannot participate in hydrogen bonding?

I. Methanol

II. Dichloromethane

III. Propane

Possible Answers:

II and III

III only

I and III

I only

Correct answer:

II and III

Explanation:

Hydrogen bonding is an intermolecular force that occurs between a hydrogen bond donor (hydrogen) and a hydrogen bond acceptor (nitrogen, oxygen, or fluorine). The bond is a result of the electromagnetic attraction of the partial positive charge on the hydrogen atom to the partial negative charge on nitrogen, oxygen, and fluorine.

To answer this question you need to look at the chemical makeup of each molecule and determine if the molecule contains the appropriate atoms. Methanol contains oxygen and hydrogen; therefore, methanol molecules can form hydrogen bonds. Dichloromethane and propane contain hydrogen, but they don’t contain nitrogen, oxygen, or fluorine; therefore, they can’t form hydrogen bonds.

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