MCAT Physical : Covalent Bonds

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #4 : Compounds, Molecules, And Bonds

Diffusion can be defined as the net transfer of molecules down a gradient of differing concentrations. This is a passive and spontaneous process and relies on the random movement of molecules and Brownian motion. Diffusion is an important biological process, especially in the respiratory system where oxygen diffuses from alveoli, the basic unit of lung mechanics, to red blood cells in the capillaries.

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Figure 1 depicts this process, showing an alveoli separated from neighboring cells by a capillary with red blood cells. The partial pressures of oxygen and carbon dioxide are given. One such equation used in determining gas exchange is Fick's law, given by:

ΔV = (Area/Thickness) · Dgas · (P1 – P2)

Where ΔV is flow rate and area and thickness refer to the permeable membrane through which the gas passes, in this case, the wall of the avlveoli. P1 and P2 refer to the partial pressures upstream and downstream, respectively. Further, Dgas­, the diffusion constant of the gas, is defined as:

Dgas = Solubility / (Molecular Weight)^(1/2)

Carbon dioxide and oxygen have which type of intra-molecular bonds?

Possible Answers:

non-covalent

covalent

ionic

London-dispersion forces

Correct answer:

covalent

Explanation:

Note how this question has little to do with the passage. Often you will see questions like this on the MCAT. This is a simple, straightforward vocabulary question. The bonds between each compound are covalent. One might get confused and misunderstand the question to read inter-molecular bonds—those between different molecules—as opposed to the bonds within a molecule.

Example Question #5 : Compounds, Molecules, And Bonds

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

Water is commonly called the universal solvent, in part due to its relatively strong molecular dipole moment. What is true of the oxygen atom in water's O-H bonds?

Possible Answers:

The oxygen atom forms the positive pole in water's dipole

The oxygen atom is sp2 hybridized

The oxygen atom reduces the hydrogen atom with which it shares a bond

The oxygen atom has two lone pairs

The oxygen atom will reversibly break its covalent bond with hydrogen in order to dissolve added solutes

Correct answer:

The oxygen atom has two lone pairs

Explanation:

The oxygen atom in water has two lone pairs, though they are not as often drawn into depictions of the molecule as they are in nitgrogenous molecules, such as ammonia. They, along with the two bonds formed with hydrogen in stable water, make up the satisfied octet of oxygen.

Example Question #1 : Covalent Bonding

Which is not a property of covalent compounds?

Possible Answers:

The elements involved generally have large differences in electronegativity

Many are not soluble in water, but are soluble in nonpolar liquids

They are poor electrical conductors

They contain bonds formed by sharing one or more pairs of valence electrons between atoms

They have low melting and boiling points

Correct answer:

The elements involved generally have large differences in electronegativity

Explanation:

The incorrect statement is a property of ionic compounds rather than covalent. Recall that electronegativity is a measure of the ability of an atom to draw electrons to itself. Ionic compounds are formed by elements with very different electronegativities, since elements with different electronegativities will tend to form positive and negative ions (that is, they give away or gain electrons easily). In contrast, covalent bonds are formed by elements which are close in electronegativity and could exist as stable free molecules. All other statements are true of covalent compounds.

Example Question #9 : Compounds, Molecules, And Bonds

Which of the folling carbon-carbon bonds exhibits the stongest bond?

Possible Answers:

Ethyne because the bond length between these carbons is the shortest

The bond strength between each of the carbons in these molecules are the same

Ethane because the bond length between these carbons is the longest

Ethene because double bonds are stronger than single and triple bonds

Correct answer:

Ethyne because the bond length between these carbons is the shortest

Explanation:

The suffix -yne tells us that this is an aklyne and contains a carbon-carbon triple bond. Ethyne has the stongest carbon-carbon bond because it has the shortest bond length. It is important to remember that as the bond length gets shorter, the bond strength increases, and vice versa. 

Example Question #1 : Covalent Bonds

What type of bond does Ohave?

Possible Answers:

Polar covalent bond

Ionic bond

Nonpolar covalent bond

Polar ionic bond

Correct answer:

Nonpolar covalent bond

Explanation:

O2 has a nonpolar covalent bond. Nonpolar covalent bonds are bonds formed between atoms that have the same (or nearly the same) electronegativity. Since both oxygen atoms have the same electronegativity, they will have a nonpolar covalent bond between them. 

Example Question #1 : Covalent Bonds

What type of bonds form between the two hydrogen atoms and one oxygen atom within a single water molucule?

Possible Answers:

Nonpolar covalent bond

Ionic bond

Hydrogen bond

Polar covalent bond

Correct answer:

Polar covalent bond

Explanation:

The intramolecular bonds within a water molecule are polar covalent bonds. O-H bonds are polar covalent since there is more electron density around the oxygen atom than there is around the hydrogen atom, making the oxygen atom slightly negative and the hydrogen atom slighty positive. This polarity allows for intermolecular hydrogen bonding; water does not exhibit intramolecular hydrogen bonding.

This bond is not ionic since the oxygen does not completly steal the electron away from the hydrogen (the electronegativity difference between the two atoms is between 0.4 and 1.7 on the Pauling scale).

Example Question #2 : Covalent Bonds

Which of the following molecules contains polar, covalent bonds?

I. 

II.

III.

IV.

V.

Possible Answers:

II, IV

I, II, IV, V

III, IV, V

II, IV, V

II, III, IV, V

Correct answer:

II, IV, V

Explanation:

 is an ionic compound, while  is a nonpolar covalent compound. Remember that polarity results from a difference in the electronegativities of the atoms involved in the bond. Too great of a difference will result in an ionic bond; two of the same atoms will have zero difference, resulting in a nonpolar bond.

, , and  contain polar covalent bonds. In the first two, oxygen will carry a slight negative charge, leaving sulfate and phosphorus with slight positive charges. In , chlorine will carry a slight negative charge, leaving the carbon slightly positive. In  and , symmetry helps to balance the polar bonds, resulting in an overall nonpolar molecule, even though the individual bonds are nonpolar.

Example Question #4 : Covalent Bonds

When covalent bonds are formed __________.

Possible Answers:

atomic species share inner and outer shell electrons

atomic species either surrender or gain electrons

atomic species share electrons, but not necessarily equally

atomic species share electrons equally

atomic species share their outermost electrons, allowing them to conduct electricity

Correct answer:

atomic species share electrons, but not necessarily equally

Explanation:

Covalency is a form of electron sharing that lets an atom fulfill the octet rule. The sharing may be unequal, in which case the more electronegative species more strongly attracts the electrons than the weaker, less electronegative species, creating a polar covalent bond. In the Lewis dot model, however, we draw the electrons in freeze-frame, equally distributed between the various atoms.

Example Question #5 : Covalent Bonds

Which of the following represents a triple bond?

Possible Answers:

Each of these combinations is possible

Two sigma bonds and one pi bond

Three sigma bonds

Three pi bonds

One sigma bond and two pi bonds

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 #6 : 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.

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