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Resonance Structures Questions

In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures into a resonance hybrid in valence bond theory.

Definition: Resonance structures are sets of Lewis structures that describe the delocalization of electrons in a polyatomic ion or a molecule.

In many cases, a single Lewis structure fails to explain the bonding in a molecule/polyatomic ion due to the presence of partial charges and fractional bonds in it. In such cases, resonance structures are used to describe chemical bonding.

Resonance Structures Chemistry Questions with Solutions

Q1. Which is the least stable resonance structure among the following?

Resonance Structures Questions 1

 

Correct Answer– (a.) Which is the least stable resonance structure among the following

Explanation– The charge separation between like charges is minimal, which increases and decreases stability.

Q2. How many resonance structure will N3– will have?

a.) 1

b.) 2

c.) 3

d.) None of the above

Correct Answer– (c.) 3

Q3. The number of resonance structures that NO2 has is/are?

a.) 1

b.) 2

c.) 3

d.) None of the above

Correct Answer– (b.) 2

Q4. The number of resonance structure in ozone are-

a.) 1

b.) 2

c.) 3

d.) 4

Correct Answer– (b.) 2

Explanation–Resonance Structures Questions 2

Q5. Resonance structures can be drawn for:

a.) O3

b.) NH3

c.) CH3

d.) H2O

Correct Answer– (a.) O3

Q6. Write the resonance structure of CH3COO

Answer. The resonance structure of CH3COO is as follows-

Resonance Structures Questions 3 1
Q7. What is the difference between isomers and resonance structures?

Answer. Isomers do not exist for resonance structures. Isomers have different atom and electron arrangements. The only difference between resonance forms is the arrangement of electrons.

Since they clearly show bonding in molecules, resonance structures are a better representation of a Lewis dot structure.

Q8. Draw all resonance structure for the following-

Resonance Structures Questions 4Answer.

Resonance Structures Questions 5 1 1

Q9. State True or False.

All resonating structures must have same number of valence electrons.

Answer. True.

All resonating structures must have same number of valence electrons.

Electrons cannot be create or destroyed themselves. The structures you create must have the same number of electrons as the starting structure.

Q10. What are minor resonance structures?

Answer. All resonance contributors with higher energy than the lowest-energy contributor are considered minor resonance structures.

For example, in formamide HCONH2, we can identify three potential contributors.

We must determine which of these has the lowest energy. That will be the most significant contributor. All of the others will be minor contributors.

Some rules that help to decide, in order of importance:

  • Follow the octet rule
  • Minimise charge separation.
  • Charge the more electronegative atom with a negative charge.

Structures III and IV both follow the octet rule, but Structure III lacks charge separation. Structure V does not provide an octet to the C atom.

III is the most important contributor. As a result, IV and V are minor contributors.

IV is a greater contributor than V because it follows the octet rule.

Thus, we can say that IV is a minor contributor, while V is a very minor contributor.

Resonance Structures Questions 6

Q11. Write the resonance structure of CH2 = CH – CHO. Indicate the relative stability of the contributing structure.

Answer. The resonance structure of CH2 = CH – CHO is as follows-

Resonance Structures Questions 7 1

Stability is I > II > III

Structure I is most stable because each carbon and oxygen atom has an octet and no charge separation is present on any of the carbon or oxygen atoms. In structure II, the negative charge is applied to more electronegative atoms, and the positive charge is applied to more electropositive atoms. Structure III does not contribute because oxygen has a positive charge and carbon has a negative charge, making it the least stable.

Q12. Draw the resonance structures for the following compounds

(a) C6H5OH

(b) C6H5C+H2

Answer.

The resonance structure for the compounds are as follows-

(a)

Resonance Structures Questions 8

(b)

Resonance structure of C6H5C+H2

Q13. Show how hyper conjugation occurs in propene molecule.

Answer. Hyper conjugation occurs in propene molecules due to a partial overlap of the sp 3-s sigma bond orbital and the empty p-orbital of an adjacent carbon atom. This causes pi-electrons to delocalize and increases molecule stability.

The stabilising interaction that results from the interaction of electrons in a -bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a π-orbital to give an extended molecular orbital that increases the system’s stability is known as hyperconjugation.

Resonance Structures Questions 9

Q14. Draw the orbital diagram showing hyperconjugation in ethyl cation.

Answer.

i. A positively charged carbon atom is attached to a methyl group to form ethyl cation CH3C+H2.

ii. The positively charged carbon atom has six electrons, is sp2 hybridised, and has an empty p orbital for hyperconjugation.

iii. One of the methyl group’s C – H bonds can align with the plane of the empty p orbital. The C – H bond’s sigma electrons can be delocalized into this empty p orbital.

iv. As a result of the partial overlap of a C-H bond with an adjacent positively charged carbon atom’s empty p orbital, hyperconjugation occurs. Hyperconjugation is thus a σ-π conjugation.

v. In ethyl carbocation, hyperconjugation structures can be represented as:

Resonance Structures Questions 10

vi. There is no covalent bond shown between the carbon and one of the α-hydrogens in the contributing structures. As a result, hyperconjugation is also known as ‘no bond resonance.’

vii. Because the electron density from the adjacent a bond helps disperse the positive charge, this type of overlap stabilises the cation.

Q15. Explain why is (CH3)3 C+ more stable than CH3CH2+ and CH3+ is the least stable cation.

Answer. (CH3)3C+, i.e. 3° carbocation, is a more stable carbocation than 1° carbocation, i.e. CH3+ or CH3CH2+, because 3° carbocation has more hyper conjunction effect, i.e. it has 9 alpha hydrogen, which is more stable, whereas CH3+ has 0 alpha hydrogen, which is why it is the least stable carbocation, and CH3CH2+ has 3 alpha hydrogen which

The more alkyl groups attached to a positively charged carbon atom, the stronger the hyperconjugation interaction and +I effect of the methyl group, and the more stable the cation.

CH3CH2+ is thus more stable than CH3CH2+ and CH3+.

Practise Questions on Resonance Structures

Q1. Which of the following has the least stable resonating structure?

Resonance Structures Questions 11

Q2. Which compound will have equally stable resonating structures?

a.) PhO

b.) PhCOO

c.) CO32–

d.) CH2 = CH -CHO

Q3. Write the resonance structure of C6H5NH2.

Q4. Draw all resonance structures for the following-

Resonance Structures Questions 12

Q5. Fill in the blank.

Resonance structures of a molecule should not have ____.

Click the PDF to check the answers for Practice Questions.
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