Aim for FiveChemistry

Short free-response question

Resonance in the cyanate ion

  • Unit 2
  • 4 points
  • About 9 minutes

You can use a calculator on this question, just like on exam day.

A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.

The question

The cyanate ion, OCN⁻, has a carbon atom as its central atom, bonded to one oxygen atom and one nitrogen atom.

Suggested time: 9 minutes

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Part (a)

1 point

Describe a complete Lewis electron-dot diagram for OCN⁻ in which the carbon atom forms a triple bond to the nitrogen atom. State each bond, every lone pair, and the formal charge on each atom.

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Part (b)

1 point

Describe a second valid resonance structure for OCN⁻ in which the carbon atom forms two double bonds. State every lone pair and the formal charge on each atom.

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Part (c)

1 point

Which of the two resonance structures from parts (a) and (b) contributes more to the actual structure of OCN⁻? Justify your answer.

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Part (d)

1 point

Predict the O–C–N bond angle in OCN⁻. Justify your prediction.

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