[VISUAL] The structural formulas for two isomers of 1,2-dichloroethene are shown above. Which of the — States of Matter Chemistry Question

STEPS:

1. Understand the relationship between intermolecular forces (IMFs) and equilibrium vapor pressure:
Vapor pressure is a measure of the pressure exerted by a gas in equilibrium with its liquid phase. The ease with which molecules escape from the liquid surface into the vapor state is inversely related to the strength of the attractions holding them together. Liquids with weaker overall IMFs have a higher vapor pressure because fewer energy barriers exist to prevent molecules from escaping into the gas phase.

2. Analyze the symmetry and polarity of the *trans*-isomer:
In the *trans*-isomer, the chlorine atoms are positioned on opposite sides of the carbon-carbon double bond. The individual carbon-chlorine ($\text{C–Cl}$) bond dipoles are polar and equal in magnitude, but because they point in geometrically opposite directions, they cancel each other out completely. As a result, the *trans*-isomer has a net molecular dipole moment of zero and is nonpolar, meaning it experiences only London dispersion forces in the liquid state.

3. Analyze the symmetry and polarity of the *cis*-isomer:
In the *cis*-isomer, the chlorine atoms are on the same side of the double bond. This asymmetrical distribution means that the individual polar bond dipoles do not cancel each other out, giving the molecule a net molecular dipole moment. Consequently, the *cis*-isomer experiences dipole-dipole interactions in addition to London dispersion forces.

4. Compare the overall IMF strengths of the two isomers:
Because both molecules are isomers, they share the same molecular formula and molar mass, giving them nearly identical London dispersion force strengths. However, the *cis*-isomer has the additional, stronger attraction of permanent dipole-dipole forces, which makes its overall IMFs stronger than those of the *trans*-isomer.

5. Determine which isomer has the higher vapor pressure:
Because the *trans*-isomer is held together only by weaker London dispersion forces, its molecules can easily escape the surface of the liquid compared to those of the *cis*-isomer. Therefore, the _trans_-isomer has the higher equilibrium vapor pressure at 20°C. This matches Option D.

---

WHY_OTHERS_WRONG:

• Option A is incorrect: It incorrectly concludes that the *cis*-isomer has a higher vapor pressure. Because the *cis*-isomer has stronger intermolecular attractions (dipole-dipole interactions), it holds its molecules more tightly in the liquid phase, resulting in a *lower* vapor pressure than the *trans*-isomer.
• Option B is incorrect: This option incorrectly assigns the types of intermolecular forces present, claiming that the *cis*-isomer is nonpolar (only London dispersion forces) and the *trans*-isomer is polar (dipole-dipole interactions). This is the exact opposite of their true molecular structures and polarities.
• Option C is incorrect: While it correctly identifies the *trans*-isomer as having the higher vapor pressure, it provides the incorrect structural reasoning by claiming that the *trans*-isomer has dipole-dipole forces and the *cis*-isomer has only London dispersion forces.