Consider the formate ion, HCO₂^-, which is the anion formed when formic acid loses an H⁺ ion. The H and the two O atoms are bonded to the central C atom. (c) Would you predict that the C—O bond lengths in the formate ion would be longer or shorter relative to those in CO₂?

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To predict the C—O bond lengths in the formate ion relative to those in CO2, we need to consider the type of bonding and resonance structures involved.

In CO2, the carbon atom forms double bonds with each oxygen atom. This results in a linear molecule with two equivalent C=O double bonds.

In the formate ion (HCO2-), the structure can be represented by resonance forms. The negative charge is delocalized between the two oxygen atoms, leading to a resonance hybrid where the C—O bonds are neither purely single nor purely double bonds.

The resonance in the formate ion results in partial double bond character for both C—O bonds, making them shorter than a typical C—O single bond but longer than a C=O double bond.

Therefore, the C—O bond lengths in the formate ion are expected to be shorter than a C—O single bond but longer than the C=O double bonds in CO2, due to the resonance stabilization and partial double bond character.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Bond Lengths and Strengths

Bond lengths are influenced by the type of bond and the atoms involved. Generally, shorter bonds are stronger due to greater overlap of atomic orbitals. In molecules like CO2, the double bonds between carbon and oxygen result in shorter bond lengths compared to single bonds. Understanding this relationship is crucial for predicting bond lengths in different compounds.

Resonance Structures

Resonance structures are different ways of drawing a molecule that represent the same arrangement of atoms but differ in the distribution of electrons. The formate ion has resonance between two structures, which can affect bond lengths and strengths. This delocalization of electrons can lead to bond lengths that are intermediate between single and double bonds, impacting predictions about bond lengths in comparison to CO2.

Electronegativity and Bond Character

Electronegativity refers to the tendency of an atom to attract electrons in a bond. In the case of the formate ion, the difference in electronegativity between carbon and oxygen affects the bond character. The C—O bonds in formate are influenced by the partial charges resulting from this difference, which can lead to variations in bond lengths compared to the purely double-bonded C=O in CO2.