GRE Subject Test: Chemistry : Molecules and Compounds

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #11 : Bonds And Forces

Which of the following represents a triple bond?

Possible Answers:

Three pi bonds

Two sigma bonds and one pi bond

One sigma bond and two pi bonds

Three sigma bonds

Each of these combinations is possible

Correct answer:

One sigma bond and two pi bonds

Explanation:

A sigma bond is a single covalent bond, involving an electron pair located between the two bonding atoms. A pi bond occurs when the p orbitals above and below the bonding atoms overlap, or when the p orbitals to the left and right overlap. In any covalent bond, the first bond formed is a sigma bond and any additional bonds must be pi bonds. Initial orbital overlap always comes from the sigma, or s, subshell; subsequent overlap comes from the pi, or p, subshells.

Example Question #1 : Covalent Bonds

Which of the following molecules contains the shortest bond between nitrogen and oxygen?

Possible Answers:

Correct answer:

Explanation:

This question is testing your ability to draw Lewis dot structures and your knowledge of how resonance effects bond length. The N-O bond with the greatest pi-bond character will be the shortest; thus, we are looking for a double- or triple-bond between nitrogen and oxygen.

Hydroxylamine () only contains single bonds, which have the least pi-bond character.

The nitrite and nitrate ions both have a double bond between the nitrogen and oxygen, but also one or more single bonds between these elements. This means that, because of resonance, the N-O bonds in these molecules will be averaged and our average bond order will be somewhere between single and double. Nitrate will have one double bond and two single bonds, for an average bond order of 1.33. Nitrite has one single bond and one double bond, for a bond order of 1.5.

The nitrosyl ion, however, will contain a triple bonds between the nitrogen and oxygen atoms, giving it the greatest pi-bond character. This bond will contain more energy and be shorter than the bonds in the other answer options.

Example Question #2 : Covalent Bonding

Screen shot 2015 09 24 at 6.13.37 pm

Using the values from the given table, which single bond would be the most polar?

Possible Answers:

Correct answer:

Explanation:

Electronegativity of a chemical bond can be determined by calculating the electronegativity differences of the elements in a chemical bond. For example, using the values in the table given, the electronegativity differences between the bond are given below:

Based on the calculations above, we can see that the  bond has the highest difference and therefore has the highest electronegativity.

Example Question #1 : Hydrogen Bonding

Which of the following molecules is not capable of hydrogen bonding?

Possible Answers:

Methanal

Ammonia

Hydrofluoric acid

Ethanol

Correct answer:

Methanal

Explanation:

A molecule is capable of hydrogen bonding when a hydrogen atom is attached to a nitrogen, oxygen, or fluorine atom in the molecule. Methanal consists of a central carbon bound to two hydrogen atoms and a double bond with an oxygen atom. The hydrogen atoms are bound only to the carbon, and thus cannot form hydrogen bonds.

Hydrofluoric acid, ethanol, and ammonia are all capable of hydrogen bonding.

Example Question #2 : Hydrogen Bonding

Which compound exhibits hydrogen bonding?

Possible Answers:

Screen shot 2015 10 27 at 5.29.48 pm

Correct answer:

Explanation:

Hydrogen bonding occurs between a hydrogen atom and an atom that has a high electronegativity such as fluorine, oxygen, or nitrogen. In this case, the only compound that exhibits hydrogen bonding is water, which has a hydrogen bonded to an oxygen atom. Note that in most cases hydrogen bonding is intermolecular, but in some cases, hydrogen bonding can occur intramolecularly.

Example Question #1 : Other Intermolecular Forces

Rank the following compounds in terms by increasing boiling point, starting with the lowest boiling point first.

I.     1-pentanol

II.    n-pentane

III.   2,2-dimethylpropane

IV.  (R)-4-hydroxypentanoic acid.

Possible Answers:

III < II < IV < I

IV < I < II < III

II < III < I < IV

II < III < IV < I

III < II < I < IV

Correct answer:

III < II < I < IV

Explanation:

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 #1 : Other Intermolecular Forces

Which molecule experiences only London dispersion forces, and no other intermolecular interactions?

Possible Answers:

Ammonia

Ethanol

Liquid bromine

Acetone

Correct answer:

Liquid bromine

Explanation:

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 #2 : Other Intermolecular Forces

Which of the following solvents is the most polar?

Possible Answers:

Screen shot 2015 10 28 at 6.41.13 am

Correct answer:

Explanation:

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 #1 : Other Intermolecular Forces

Which solvent is miscible with water?

Possible Answers:

Cyclohexane

Pentane

Correct answer:

Explanation:

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.

Example Question #1 : Help With Lewis Diagrams

Which answer option correctly depicts the Lewis dot structure of sodium chloride?

Possible Answers:

4

5

3

1

2

Correct answer:

1

Explanation:

When drawing a Lewis dot structure, we are always trying to reach an electron count where all atoms involved are stable and (usually) have full octets. We are also trying to estabilsh a structure in which we have the smallest formal charge possible. The general rule is first to draw out all of the elements involved and their valence electrons, then start piecing them together trying to reduce the formal charge and get all elements involved to an octet. There are a couple exceptions to the octet rule.  

Sodium and chlorine form an ionic bond, meaning that one atom will donate an electron and the other will receive it. This gives each atom a charge. Chlorine has seven valence electrons, while sodium has one valence electron. For each atom to arrive at an octet, sodium will need to lose one electron and chlorine will need to gain one electron. This would give chlorine a negative charge, and sodium a positive charge.

Thus, the answer is a sodium with a positive charge (due to one lost electron) and a chlorine with eight electrons and a negative charge (due to one electron gained). 

 

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