Question

Would you expect \(\mathrm{Fe}^{3+}\) or \(\mathrm{Fe}^{2+}\) to be the stronger Lewis acid? Explain.

Short answer

We can conclude that \(\mathrm{Fe^{3+}}\) is the stronger Lewis acid compared to \(\mathrm{Fe^{2+}}\). This is because both ions have a similar size, but \(\mathrm{Fe^{3+}}\) has a higher positive charge (+3) than \(\mathrm{Fe^{2+}}\) (+2). As a result, \(\mathrm{Fe^{3+}}\) is more attracted to negatively charged electron pairs, making it a better electron-pair acceptor and thus a stronger Lewis acid.

Step-by-step solution

Understand the concept of a Lewis acid

A Lewis acid is a substance that can accept an electron pair to form a covalent bond. In other words, a Lewis acid is an electron-pair acceptor.

Analyze the charge of both ions

In this case, we are comparing the acid strength of Fe³⁺ and Fe²⁺. The Fe³⁺ ion has a charge of +3, while the Fe²⁺ ion has a charge of +2. The higher the positive charge, the more the ion is attracted to negatively charged electron pairs. This means that the higher the positive charge of an ion, the stronger the Lewis acid.

Analyze the size of both ions

The size of the ion is another factor that affects the Lewis acid strength. When comparing ions with the same charge, smaller ions are more attracted to electron pairs than larger ions, making them stronger Lewis acids. In this case, both ions have the same element (iron) and are therefore similar in size.

Draw a conclusion based on charge and size

Since both Fe³⁺ and Fe²⁺ ions have a similar size, and Fe³⁺ has a higher positive charge than Fe²⁺, we can conclude that Fe³⁺ is the stronger Lewis acid. The higher positive charge results in a stronger attraction to electron pairs, making it a better electron-pair acceptor.