Multiple Choice
Which phrase best describes a polar molecule in terms of its molecular polarity and distribution of charge?
13. Liquids, Solids & Intermolecular Forces
Molecular Polarity
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Which of the following best describes the polarity of the molecule C_2H_2Br_2?
A
C_2H_2Br_2 is always polar, regardless of its structure.
B
C_2H_2Br_2 is always nonpolar, regardless of its structure.
C
C_2H_2Br_2 is polar if the two Br atoms are on the same side of the molecule (cis isomer), but nonpolar if they are on opposite sides (trans isomer).
D
C_2H_2Br_2 is nonpolar if the two Br atoms are on the same carbon (cis isomer), but polar if they are on opposite carbons (trans isomer).
Verified step by step guidance1
Identify the molecular structure of C_2H_2Br_2, which is a substituted ethene (alkene) with two bromine atoms and two hydrogen atoms attached to the carbon-carbon double bond.
Understand that C_2H_2Br_2 can exist as geometric isomers: the cis isomer (both Br atoms on the same side of the double bond) and the trans isomer (Br atoms on opposite sides).
Recall that polarity depends on the vector sum of bond dipoles; bromine is more electronegative than hydrogen, so C-Br bonds are polar, while C-H bonds are less polar.
Analyze the cis isomer: since both Br atoms are on the same side, their dipoles add up, resulting in a net molecular dipole moment, making the molecule polar.
Analyze the trans isomer: the Br atoms are on opposite sides, so their dipoles cancel each other out, resulting in no net dipole moment, making the molecule nonpolar.
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