Draw out the Lewis Structure of H3PO4, and consider its major resonance forms. Box the major resonance contributor to the hybrid. List the intermolecular forces the compound would exhibit.
The Correct Answer and Explanation is:
Lewis Structure of H₃PO₄ (Phosphoric Acid):
To draw the Lewis structure of phosphoric acid, follow these steps:
Total Valence Electrons Calculation:
- Hydrogen (H): 3 × 1 = 3 electrons
- Phosphorus (P): 1 × 5 = 5 electrons
- Oxygen (O): 4 × 6 = 24 electrons
Total = 3 + 5 + 24 = 32 valence electrons
Structure Drawing:
Phosphoric acid has the following structure:
- Phosphorus is the central atom.
- Three hydroxyl groups (OH) are bonded to phosphorus via oxygen atoms.
- One double-bonded oxygen connects directly to phosphorus.
Lewis Structure:
mathematicaCopyEdit O
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H - O - P - O - H
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O - H
- The double-bonded oxygen satisfies octet requirements for phosphorus and oxygen.
- Each hydroxyl group (O-H) has two lone pairs on oxygen.
- Phosphorus has five bonds in total, but this is allowed for elements in period 3 or higher like phosphorus.
Resonance Forms:
- The double bond between phosphorus and oxygen can shift to different oxygen atoms.
- Major resonance contributors include structures where the double bond alternates between each of the three oxygen atoms bonded to phosphorus.
However, the structure where the double bond is on the oxygen not bonded to hydrogen (the terminal oxygen) is the major resonance contributor. This is because the hydroxyl groups (O-H) maintain their single bonds, and formal charges are minimized.
Boxed Major Resonance Contributor:
mathematicaCopyEdit O
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H - O - P - O - H
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O - H
The double bond is with the terminal oxygen, and no formal charges appear on atoms in this form.
Intermolecular Forces in H₃PO₄:
Phosphoric acid exhibits several types of intermolecular forces:
- Hydrogen Bonding
Due to the presence of O-H bonds, strong hydrogen bonds form between H₃PO₄ molecules. - Dipole-Dipole Interactions
The molecule is polar because of the asymmetrical arrangement of polar bonds, contributing to dipole-dipole interactions. - London Dispersion Forces
These weak forces exist in all molecules, including H₃PO₄, but they are not the dominant force here.
Thus, hydrogen bonding is the strongest and most significant intermolecular force in phosphoric acid.
