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MCAT Sample Questions : Physical Sciences
Many chemical bonds are not purely ionic or covalent, but polar covalent.For example, in an HCl bond, chlorine has a greater attraction for electrons than hydrogen does and therefore develops a partial negative charge with respect to the hydrogen atom.
If the partial charges are separated by a known distance, the dipole moment, a measure of the charge separation in a bond or molecule, can be calculated by the following equation.
dipole moment = charge x separation distance
Dipole moments are usually measured in debyes (D), where 1 D = 3.34 x 10-30 coulomb · meter.
The molecular geometry of some simple molecules can be determined based on the presence or absence of a net dipole moment in the molecule.The observed molecular dipole moments of various compounds are reported in Table 1.
Table 1 Dipole Moments for Various Compounds
||Dipole moment (D)
Some sample questions on this passage are as follows:
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MCAT Sample Question Number :
- Which of the following best explains the observed molecular dipole moment of SnBr4?
- Sn and Br have the same effective nuclear charge.
- Sn and Br are the same size.
- sSn attracts electrons more strongly than predicted by its electronegativity.
- The geometry of the molecule causes the bond moments to cancel.
Explanation: The dipole moment of a molecule is the vector sum of all of the bond moments. According to the data in Table 1, the dipole moment of SnBr4 is zero; therefore, its bond moments add to zero or cancel. Thus, answer choice D is the best answer.
- Based on the observed dipole moment of HF in Table 1, what is the observed dipole moment for HCl?
- 0.02 D
- 1.08 D
- 4.22 D
- 8.97 D
Explanation: Table 1 gives the dipole moment of HF as 1.82 D. Chlorine is just below fluorine in the periodic table; therefore, the electronegativity of chlorine, though significant, is less than that of fluorine. Chlorine is less effective than fluorine in creating a separation of charge when bonded to hydrogen, and the dipole moment of HCl is slightly less than that of HF. Thus, answer choice B is the best answer.
- Which of the following best explains why HCl bonds are polar covalent?
- H atoms are smaller than Cl atoms.
- H atoms are more electronegative than Cl atoms.
- Cl and H atoms have equal electronegativities.
- Cl atoms have a greater effective nuclear charge than H atoms do.
Explanation: HCl is polar covalent because H and Cl share a pair of bonded electrons that are more strongly attracted to the chlorine atom. The higher effective nuclear charge (i.e., the charge of the nucleus minus the shielding caused by extranuclear electrons) of chlorine accounts for its greater electronegativity. Thus, answer choice D is the best answer.
- If an O atom is removed from a CO2 molecule, the observed molecular dipole moment will:
- decrease, because a lone pair of electrons will be formed on the carbon.
- decrease, because the net charge will increase.
- remain constant, because the geometry will not change.
- increase, because a charge separation will develop.
Explanation: Carbon dioxide,O=C=O, is linear. Therefore, the two CO dipoles cancel because they are in opposite directions. If one of the oxygen atoms is removed, the resulting CO will have a dipole because the species is linear and comprised of two different atoms. Thus, the dipole moment will change from zero in CO2 to a positive value in CO. Thus, answer choice D is the best answer
- Which of the two compounds, PCl3 or PCl5, can be expected to have a larger dipole moment?
- PCl3, because its geometry is trigonal planar
- PCl3, because its geometry is pyramidal
- PCl5, because its geometry is octahedral
- PCl5, because its geometry is trigonal bipyramidal
Explanation: An analysis of the two structures shows that the bond moments in PCl5 add to zero; whereas, those in PCl3 do not. As shown in the figure, PCl3 is pyramidal not planar. Thus, answer choice B is the best answer.