Question

Oxalic acid \(\left(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\right)\) has the following structure: O=C(O)C(O)C(=O)O An oxalic acid solution contains the following species in varying concentrations: \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}, \mathrm{HC}_{2} \mathrm{O}_{4}^{-}, \mathrm{C}_{2} \mathrm{O}_{4}^{2-},\) and \(\mathrm{H}_{3} \mathrm{O}^{+} .\) (a) Draw Lewis structures of \(\mathrm{HC}_{2} \mathrm{O}_{4}^{-}\) and \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\).(b) Which of the four species can act only as acids, which can act only as bases, and which can act as both acids and bases?

Short answer

(a) Draw Lewis structures for both ions. (b) \(\text{H}_2\text{C}_2\text{O}_4\) is acid-only, \(\text{C}_2\text{O}_4^{2-}\) is base-only, \(\text{HC}_2\text{O}_4^-\) is both acid and base.

Step-by-step solution

Lewis Structure of HC2O4-

The HC2O4- ion, also known as the hydrogen oxalate ion, is formed when a proton is dissociated from one of the hydroxyl groups of oxalic acid. Start with the basic structure of oxalic acid and remove one hydrogen from the hydroxyl group to introduce a negative charge: \[ \text{O=C(O)C(O)O}^- \]. Draw it:- C=O double bonds remain in place.- One hydroxyl oxygen holds a single bond to a hydrogen and a negative charge (O-H).- The negative charge is present due to the removed hydrogen ion.

Lewis Structure of C2O4^2−

The C2O4^2− ion, or oxalate ion, is formed by dissociating both protons from the hydroxyl groups of oxalic acid:- Each oxygen that was previously attached to hydrogen in the hydroxyl groups will now have a negative charge.- The structure is \[ \text{O=C(O^-)C(O^-)O} \], with two singly bonded oxygen atoms each carrying a -1 charge.- Two C=O double bonds are in place, and these structural changes reflect the overall -2 charge of the ion.

Identifying Acid-Base Behavior

To determine the acid-base behavior of each species:- **Acid-Only:** \(\text{H}_2\text{C}_2\text{O}_4\) can only donate protons as it has no negative charge to accept them.- **Base-Only:** \(\text{C}_2\text{O}_4^{2-}\) acts as a base since it does not release protons; rather, it would accept protons to become \(\text{HC}_2\text{O}_4^-\) again.- **Amphoteric:** \(\text{HC}_2\text{O}_4^-\) can act as both an acid and a base, donating a proton to become \(\text{C}_2\text{O}_4^{2-}\) or accepting a proton to revert to \(\text{H}_2\text{C}_2\text{O}_4\).

Key concepts

Lewis structures

Lewis structures are a way to represent molecules that showcase the placement of electrons among the atoms. In the case of oxalic acid and its related ions, these diagrams help us understand the bonding and electron pairs involved.

For the hydrogen oxalate ion \( ext{HC}_2 ext{O}_4^-\), we start with oxalic acid's basic structure. By removing a hydrogen atom, primarily from one of the hydroxyl groups (\(-OH)\), it leaves that oxygen with a negative charge. Thus, in its Lewis structure:

• Two C=O double bonds remain unchanged.
• One oxygen holds onto its hydrogen and has an additional pair of electrons, showing the extra negative charge.

When both hydrogen ions are removed from oxalic acid, we have the oxalate ion \( ext{C}_2 ext{O}_4^{2-}\). Both oxygen atoms originally attached to hydroxyl groups bear negative charges. The Lewis structure here consists of:

• Two doubly bonded C=O groups.
• Two negatively charged oxygens.

Acid-Base Behavior

Understanding the acid-base behavior of oxalic acid and its ions involves assessing their ability to donate or accept protons. This behavior is crucial in predicting chemical reactions and interactions.

Acid-Only Species: \( ext{H}_2 ext{C}_2 ext{O}_4\) acts solely as an acid. It can donate hydrogen ions (protons) but lacks the capability to accept them because it has no negative charges.

Base-Only Species: \( ext{C}_2 ext{O}_4^{2-}\) functions purely as a base. It cannot provide protons but can accept them, reverting to its hydrogenated forms.

Amphoteric Species: The hydrogen oxalate ion \( ext{HC}_2 ext{O}_4^-\) is interesting because it can act both ways. It can donate a proton, further ionizing to \( ext{C}_2 ext{O}_4^{2-}\), or accept a proton to return to \( ext{H}_2 ext{C}_2 ext{O}_4\).

Hydrogen Oxalate Ion

The hydrogen oxalate ion, \( ext{HC}_2 ext{O}_4^-\), is formed from oxalic acid by losing a proton. This transformation illustrates the ion’s mixed character in acid-base reactions.

This ion retains:

• A single hydrogen bound to one of the hydroxyl oxygens.
• A negative charge, indicating its potential to either gain a proton (becoming neutral) or lose another, forming the oxalate ion.

Its structural flexibility, shown in its Lewis diagram, makes the hydrogen oxalate ion adaptable in various chemical environments, both as a mild acid or moderate base.

Oxalate Ion

The oxalate ion \( ext{C}_2 ext{O}_4^{2-}\) is a result of oxalic acid losing both its acidic hydrogens. This ion demonstrates base-only characteristics due to its strong negative charge.

In its fully deprotonated form:

• Each oxygen involved originally in a hydroxyl group carries a negative charge.
• The ion's stability and symmetry are pronounced due to its resonance forms, distributing the negative charge across the molecule.

This makes it effective in binding positively charged ions (cations) and playing roles in various chemical processes, including metal complexation.

Proton Donation and Acceptance

The concept of proton donation and acceptance is pivotal in understanding acids and bases. Oxalic acid and its derivatives present clear examples of these processes.

Proton Donation:

• Occurs when a species like \( ext{H}_2 ext{C}_2 ext{O}_4\) loses a hydrogen ion.
• This release forms ions like \( ext{HC}_2 ext{O}_4^-\) or \( ext{C}_2 ext{O}_4^{2-}\).

Proton Acceptance:

• Involves gaining a proton, as seen when \( ext{HC}_2 ext{O}_4^-\) reverts to \( ext{H}_2 ext{C}_2 ext{O}_4\).
• This flexibility is less in \( ext{C}_2 ext{O}_4^{2-}\), which primarily acts to accept protons.

Understanding these concepts aids in predicting the behavior of these substances in different chemical environments.