Problem 9 - Chemistry An Atoms First Approach

Question

Oxalic acid is often used to remove rust stains. What properties of oxalic acid allow it to do this?

Step-by-Step Solution

Answer

Oxalic acid's ability to remove rust stains is due to its dicarboxylic acid structure, solubility in water, and its ability to form water-soluble complexes with iron ions in rust. Its chemical reaction with rust is: \( 2 \text{FeO(OH)}(s) + C_{2}H_{2}O_{4}(aq) \rightarrow \text{Fe}_{2}(C_{2}O_{4})_{3}(aq) + 4 \text{H}_{2}\text{O}(l) \). The presence of two carboxylic acid groups allows it to effectively chelate iron ions, forming soluble complexes that can be washed away easily.

Step 1: Understand the structure and formula of oxalic acid

Oxalic acid is a dicarboxylic acid with the chemical formula C2H2O4. Its structure consists of two carboxylic acid groups (-COOH) connected to a central carbon atom.

Step 2: Determine the chemical reaction of oxalic acid with rust

When oxalic acid comes into contact with rust (iron(III) oxide-hydroxide), it forms a water-soluble complex by chelating the iron ions. The chemical reaction is: \( 2 \text{FeO(OH)}(s) + C_{2}H_{2}O_{4}(aq) \rightarrow \text{Fe}_{2}(C_{2}O_{4})_{3}(aq) + 4 \text{H}_{2}\text{O}(l) \)

Step 3: Explain the chelation in the reaction

Chelation is the formation of a complex between a metal ion and a molecule. In this reaction, oxalic acid (C2H2O4) forms a stable complex with the iron(III) ions (Fe3+) in rust, which has a higher solubility in water. This process makes it easier to remove the rust by washing it away with water.

Step 4: Describe the properties of oxalic acid that enable rust removal

The properties of oxalic acid that allow it to remove rust stains are: 1. Dicarboxylic acid structure: The presence of two carboxylic acid groups in oxalic acid allows it to chelate metal ions, such as iron, effectively. 2. Solubility in water: Oxalic acid is soluble in water, which allows it to dissolve and react with rust. 3. Formation of water-soluble complexes: By chelating with iron ions in rust, oxalic acid forms water-soluble complexes that can be easily washed away. In conclusion, oxalic acid can effectively remove rust stains due to its dicarboxylic acid structure, solubility in water, and its ability to form water-soluble complexes with iron ions in rust.