MCAT General Chemistry Question 37: Answer and Explanation

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Question: 37

7. Despite the fact that both C2H2 and HCN contain triple bonds, the lengths of these triple bonds are not equal. Which of the following is the best explanation for this finding?

  • A. In C2H2, the bond is shorter because it is between atoms of the same element.
  • B. The two molecules have different resonance structures.
  • C. Carbon is more electronegative than hydrogen.
  • D. Nitrogen is more electronegative than carbon.

Correct Answer: D

Explanation:

Bond lengths decrease as the bond order increases, and they also decrease with larger differences in electronegativity. In this case, because both C2H2 and HCN have triple bonds, we cannot compare the bond lengths based on bond order. We must then rely on other periodic trends. The bond length decreases when moving to the right along the Periodic Table's rows because more electronegative atoms have shorter atomic radii. The nitrogen in HCN is likely to hold its electrons closer, or in a shorter radius, than the carbons in C2H2.

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