Draw out the Lewis structures for H2CO3, HCO3-, and CO32-

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Draw out the Lewis structures for H2CO3, HCO3-, and CO32-. Then determine the maximum number of equivalent resonance structures for each species. Note that carbon is the central atom in all three cases. If the molecule contains hydrogen atoms they are attached to oxygen atoms. Only count the best structures. For example, A structure with really bad formal charges should not be counted.

The molecule H2CO3 has____ equivalent Lewis structures.

The molecule HCO3- has_____ equivalent Lewis structures.

The molecule CO32- has_____ equivalent Lewis structures.

The Correct Answer and Explanation is :

Lewis Structures:

  1. H₂CO₃ (Carbonic Acid):
    • The central atom is carbon (C), which will form single bonds with two hydroxyl groups (OH) and one double bond with an oxygen atom (O).
    • Oxygen atoms (in the OH groups) will have single bonds with hydrogen atoms.
    • The central carbon atom will have two single bonds with oxygen atoms and one double bond with an oxygen atom.
    • The structure of H₂CO₃: H O | || H-C–O–H | O
    In this structure, the carbon atom has one double bond with oxygen and two single bonds with the other oxygens (each attached to a hydrogen atom). The maximum number of equivalent Lewis structures for H₂CO₃ is 1. There are no resonance possibilities because the structure is already optimized in terms of bond type and formal charge.
  2. HCO₃⁻ (Bicarbonate Ion):
    • The central carbon atom forms one single bond with a hydroxyl group (OH) and a double bond with one oxygen atom.
    • The third oxygen has a negative charge and forms a single bond with the carbon atom.
    • In the best structure, the negative charge is placed on one of the oxygens, and the formal charges are minimized.
    • The structure of HCO₃⁻ is: O || H-C–O⁻ | O
    The maximum number of equivalent Lewis structures for HCO₃⁻ is 2, corresponding to two possible placements of the negative charge (on either of the two oxygens not bonded to hydrogen).
  3. CO₃²⁻ (Carbonate Ion):
    • The central carbon atom forms one double bond with one oxygen atom and two single bonds with two other oxygens.
    • Each of the singly bonded oxygen atoms carries a negative charge.
    • The structure of CO₃²⁻: O⁻ || O–C–O⁻
    The maximum number of equivalent Lewis structures for CO₃²⁻ is 3, corresponding to the three different ways the negative charges can be distributed among the three oxygen atoms.

Summary:

  • H₂CO₃ (Carbonic Acid): 1 equivalent Lewis structure.
  • HCO₃⁻ (Bicarbonate Ion): 2 equivalent Lewis structures.
  • CO₃²⁻ (Carbonate Ion): 3 equivalent Lewis structures.

Explanation:

Each of these species has a central carbon atom that is bonded to oxygen atoms. The number of equivalent resonance structures depends on the ability to distribute the formal charges while satisfying the octet rule. In H₂CO₃, there is no resonance because the bonds are already stable. In HCO₃⁻ and CO₃²⁻, resonance allows for multiple equivalent structures, as the formal charge can be redistributed between the oxygens. The best resonance structures minimize formal charges and satisfy the octet rule for each atom.

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