MCAT Physical : Compounds, Molecules, and Bonds
Study concepts, example questions & explanations for MCAT Physical
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Example Questions
Example Question #1 : Compounds, Molecules, And Bonds
What type of bond is formed in potassium iodide?
Ionic bond
Polar ionic bond
Nonpolar covalent bond
Polar covalent bond
Ionic bond
Potassium iodide (KI) forms an ionic bond. Potassium and iodine have very different electronegativities. The two atoms would form an ionic bond since ionic bonds form between atoms with a large difference in electronegativity (difference>1.7 using the Pauling scale will result in an ionic bond).
Example Question #1 : Ionic Bonds
Which of the following compounds contain an ionic bond?
Ionic bonds are bonds in which there is a complete transfer of electrons between two elements. They are formed between two elements with a large difference in electronegativity, like a metal and nonmetal. Molecules with similar electronegativities share their electrons and form covalent bonds.
Because bromine has a much higher electronegativity than potassium, it will fully take an electron from potassium to form a complete octet, leaving potassium also with a complete octet.
Example Question #2 : Compounds, Molecules, And Bonds
The electronegativities of certain elements are given below:
Which of the following compounds is most ionic in character?
The degree of ionic character of a compound is determined by comparing the electronegativities of the species involved. The greater the difference in electronegativity, the more ionic the compound.
Here the greatest arithmetic difference is found in yttrium trichloride.
Example Question #1 : Compounds, Molecules, And Bonds
Which of the following is the correct name for the compound 
Potassium oxide
Potassium (II) oxide
Potassium (I) oxide
Dipotassium monoxide
Potassium monoxide
Potassium oxide
The compound 
- the metal component's name does not change, regardless of quanitiy or charge
- the non-metal component's name ends in -ide, and has no prefix
- if the metal is a transition metal, the metal's charge is specified with a Roman numeral
Using these rules, the correct name is potassium oxide.
Example Question #1 : Ionic Bonds
Which of the following most accurately describes an ionic bond?
An attractive force exists between two atoms that have one or more of the same electrons in valence shells.
An attractive force exists between a cation, which has gained electrons, and an anion, which has lost electrons.
Positive and negative poles form on a molecule that is made of two nonmetals with different electronegativities.
An attractive force exists between a cation, which has lost electrons, and an anion, which has gained electrons.
An attractive force exists between a cation, which has lost electrons, and an anion, which has gained electrons.
An ionic bond is an electrostatic force between a positively charged cation (formed when electrons are lost) and a negatively charged anion (formed when electrons are gained). While one answer choice mentions positive and negative, it is actually a description of a polar covalent bond. The easiest way to recognize this is the fact that it specifies a bond between two nonmetals. A metal in generally a component of an ionic compound.
Example Question #1 : Compounds, Molecules, And Bonds
Which of the statements are true about ionic compounds?
Water is an example of an ionic compound
They are formed by molecules that equally share electrons
They are 3D arrays of charged molecules
They are non-polar
They are 3D arrays of charged molecules
Ionic compounds are made of atoms that are held together by ionic bonds. These bonds allow charged particles that have very different electronegativities to stick together, forming 3D crystals. For example, potassium and bromine form potassium bromide, an anti-convulsant. The electrons are shared very disproportionately between the positive and negatively charged particles, with the all the electrons located on the more electronegative atom. Ionic compounds are not non-polar because non-polar is used to describe a molecule whereas ionic compounds are not molecules. Water is an example of a polar molecule but is not an ionic compound since The hydrogens and oxygen share their electrons.
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.
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?
covalent
London-dispersion forces
ionic
non-covalent
covalent
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?
The oxygen atom is sp2 hybridized
The oxygen atom reduces the hydrogen atom with which it shares a bond
The oxygen atom forms the positive pole in water's dipole
The oxygen atom will reversibly break its covalent bond with hydrogen in order to dissolve added solutes
The oxygen atom has two lone pairs
The oxygen atom has two lone pairs
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 Bonds
Which is not a property of covalent compounds?
They have low melting and boiling points
They contain bonds formed by sharing one or more pairs of valence electrons between atoms
Many are not soluble in water, but are soluble in nonpolar liquids
They are poor electrical conductors
The elements involved generally have large differences in electronegativity
The elements involved generally have large differences in electronegativity
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 #1 : Covalent Bonds
Which of the folling carbon-carbon bonds exhibits the stongest bond?
Ethane because the bond length between these carbons is the longest
The bond strength between each of the carbons in these molecules are the same
Ethene because double bonds are stronger than single and triple bonds
Ethyne because the bond length between these carbons is the shortest
Ethyne because the bond length between these carbons is the shortest
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.
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