Identify the Lewis acid and Lewis base from among the reactants in the equation.
Fe3+ (aq) + 6H2O (l) = Fe(H2O)63+ (aq)
The correct answer and explanation is :
Identifying the Lewis Acid and Lewis Base
In the given chemical reaction:
[
\text{Fe}^{3+} (aq) + 6H_2O (l) \rightarrow \text{Fe(H}_2O)_6^{3+} (aq)
]
- Lewis Acid: Fe³⁺ (aq)
- Lewis Base: H₂O (l)
Explanation
Understanding Lewis Acid and Lewis Base
The Lewis acid-base theory, proposed by Gilbert N. Lewis, defines:
- A Lewis acid as a species that can accept an electron pair.
- A Lewis base as a species that donates an electron pair.
This theory focuses on electron pair transfer rather than proton exchange (as in the Brønsted-Lowry definition).
Why Fe³⁺ is a Lewis Acid?
- The iron(III) ion (Fe³⁺) is highly electron-deficient due to its high positive charge.
- It has empty d orbitals that can accept electron pairs.
- It readily interacts with species that can donate electron pairs to form coordinate bonds.
Thus, Fe³⁺ acts as a Lewis acid because it accepts electron pairs from water molecules.
Why H₂O is a Lewis Base?
- Each water molecule (H₂O) has two lone pairs on the oxygen atom.
- Water can donate these electron pairs to Fe³⁺ to form coordinate covalent bonds.
- Since it donates electron pairs, water acts as a Lewis base.
Formation of the Complex Ion [Fe(H₂O)₆]³⁺
When Fe³⁺ is added to water, it gets surrounded by six water molecules, forming the hexaaquairon(III) complex ([Fe(H_2O)_6]^{3+}). Each water molecule donates an electron pair to Fe³⁺, creating Fe–O coordinate covalent bonds.
Conclusion
In this reaction:
- Fe³⁺ acts as the Lewis acid because it accepts electron pairs.
- H₂O acts as the Lewis base because it donates electron pairs.
This illustrates a classic example of Lewis acid-base interaction leading to the formation of a complex ion.