Problem 76 - Chemistry The Molecular Nature of Matter

Question

Oxalic acid, \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4},\) is the poison in rhubarb leaves. Name its potassium salt.

Step-by-Step Solution

Answer

The potassium salt of oxalic acid is called dipotassium oxalate, with the chemical formula \(\mathrm{K}_2\mathrm{C}_2\mathrm{O}_4\).

Step 1: Determine the ionizable hydrogen atoms in oxalic acid

Oxalic acid, \(\mathrm{H}_2\mathrm{C}_2\mathrm{O}_4\), is a diprotic acid as it contains two hydrogen atoms that can ionize to form ions.

Step 2: Write down the formula for the potassium cation

The chemical symbol for potassium is K and it forms a cation with a charge of +1, denoted as K+.

Step 3: Replace the ionizable hydrogen ions of oxalic acid with potassium cations

Each ionizable hydrogen ion (H+) from oxalic acid is replaced by a potassium ion (K+) to form the salt. The potassium salt of oxalic acid is potassium oxalate, and since there are two hydrogen ions replaced, it's called dipotassium oxalate with the chemical formula \(\mathrm{K}_2\mathrm{C}_2\mathrm{O}_4\).

Step 4: Name the potassium salt

The name of oxalic acid's potassium salt is dipotassium oxalate, reflecting the two potassium ions that have replaced the two ionizable hydrogen atoms in oxalic acid.

Key Concepts

Oxalic Acid

Oxalic acid, chemically represented as \( \mathrm{H}_2\mathrm{C}_2\mathrm{O}_4 \), is an organic compound widely recognized for its presence in rhubarb leaves, where it acts as a natural toxin. Despite its notoriety, oxalic acid is also found in a variety of vegetables and plants, often at harmless levels. It's a colorless, crystalline substance that has the ability to donate two protons, or hydrogen ions, in solution, which leads us to classify it as a diprotic acid.

Oxalic acid's capacity to form stable salts and esters, collectively known as oxalates, is a crucial aspect of its chemistry, especially when it reacts with metals such as potassium. The formation of these compounds can be a double-edged sword; they can be useful in certain industrial and medical applications, but can also lead to the formation of kidney stones if accumulated in the body.

Diprotic Acid

A diprotic acid is an acid that has the potential to donate two protons or hydrogen ions per molecule to a base in an acid-base reaction. This type of acid undergoes dissociation in two stages: the first ionization releases one proton, resulting in a monovalent anion, whereas the second ionization releases another proton, leading to a divalent anion.

In the context of oxalic acid, the two ionizable hydrogen atoms, which we represent in chemical notation as \( \mathrm{H}^+ \), sequentially liberate from the molecule when in aqueous solution. The first ionization occurs and a hydrogen cation (\( \mathrm{H}^+ \)) is released, leaving behind the monovalent anion, hydrogen oxalate (\( \mathrm{HC}_2\mathrm{O}_4^- \)). Upon further ionization, the second \( \mathrm{H}^+ \) is released, resulting in the divalent oxalate anion (\( \mathrm{C}_2\mathrm{O}_4^{2-} \)).
Understanding the stepwise ionization of diprotic acids is essential for predicting their reactivity and interactions, particularly in the formation of salts with cations like potassium.

Potassium Cation

The potassium cation, denoted as \( \mathrm{K}^+ \), is a positively charged ion derived from the element potassium. Potassium is an alkali metal, located in the first group of the periodic table, and is vital for several biological processes, including nerve impulse transmission and muscle function.

In chemical reactions, potassium readily loses its single valence electron to achieve a stable electronic configuration, similar to the noble gases, thus forming a \( \mathrm{K}^+ \) ion. Due to its positive charge, the potassium cation has an affinity towards negatively charged anions. When reacting with oxalic acid, potassium displaces the hydrogen ions from the acid to form a salt. In the case of oxalic acid, with two replaceable hydrogen atoms, two potassium cations are necessary to completely neutralize the acid, resulting in dipotassium oxalate (\( \mathrm{K}_2\mathrm{C}_2\mathrm{O}_4 \)).
The importance of the potassium cation extends beyond its biological roles; it also plays a significant part in agriculture as a fertilizer component and is used extensively in various industrial processes.