Question

Potassium stearate, \(\mathrm{KC}_{18} \mathrm{H}_{36} \mathrm{O}_{2},\) is an effective soap for washing your hands or clothes. What acid is reacted with \(\mathrm{KOH}\) to make this compound?

Short answer

Stearic acid (C17H35COOH) reacts with KOH to make potassium stearate.

Step-by-step solution

Identify the soap formation reaction

Soap is typically formed by the reaction of a fatty acid with a strong base in a process called saponification. In this case, potassium stearate is the soap, which means a fatty acid has reacted with potassium hydroxide (KOH).

Determine the fatty acid structure

The general formula for a fatty acid is \( RCOOH \), where \( R \) represents the hydrocarbon chain. Potassium stearate has the formula \( \mathrm{KC}_{18}\mathrm{H}_{35}\mathrm{O}_2 \), indicating that the fatty acid must have an \( C_{17}H_{35} \) hydrocarbon chain attached to a carboxyl group (\( COOH \)).

Write the reaction equation

Stearic acid \( (C_{17}H_{35}COOH) \) reacts with potassium hydroxide (KOH) to produce potassium stearate \( (KC_{18}H_{35}O_2) \) and water (H2O).

Key concepts

Understanding Fatty Acids

Fatty acids play a central role in the creation of soaps through the saponification process. These organic compounds typically consist of a long hydrocarbon chain and a carboxyl group (\( -COOH \)) at one end. This structure is denoted as \( RCOOH \). In the case of the potassium stearate soap, the 'R' corresponds to a \( C_{17}H_{35} \) hydrocarbon chain, indicating that stearic acid is the fatty acid involved.

Fatty acids are crucial to this process because their amphipathic nature allows soap to interact with both water and oils. Their hydrophilic (water-attracting) carboxyl group and hydrophobic (water-repelling) hydrocarbon tail enable soaps to remove oils and dirt from surfaces when used with water. Understanding the structure and properties of fatty acids is essential because it helps in predicting the behavior and effectiveness of the soap produced in the saponification process.

The Role of Potassium Hydroxide in Soap Making

Potassium hydroxide (\( \text{KOH} \) is a strong base used in the saponification process to create soap. When a fatty acid, such as stearic acid, reacts with potassium hydroxide, a chemical reaction occurs. This reaction results in the formation of a salt of the fatty acid, which, in this context, is a type of soap. For example, potassium stearate is formed when potassium hydroxide and stearic acid react.

The selection of potassium hydroxide specifically, as opposed to other bases such as sodium hydroxide, impacts the characteristics of the soap produced. Potassium soaps tend to be softer and more soluble in water than their sodium counterparts, which is why potassium hydroxide is often chosen to make liquid soaps and softer soap bars.

Importance of KOH Concentration

The concentration of potassium hydroxide can also affect the reaction, with higher concentrations leading to a higher soap yield. Pure potassium hydroxide is preferred in commercial settings to ensure consistency and efficiency in soap production. By understanding the properties of potassium hydroxide and its role in saponification, individuals can manipulate the process to yield soaps with desired properties.

Soap Formation through Saponification

Soap formation is the essence of saponification, a process that transforms fatty acids and a strong base into soap and glycerol. This chemical reaction takes a triglyceride (fat/oil) and reacts it with a base such as potassium hydroxide. This specifically produces a 'potassium soap' which, in our example, is potassium stearate.

Understanding the Reaction Mechanism

During saponification, the fatty acid reacts with the hydroxide ions from potassium hydroxide to form glycerol and the salt of the fatty acid – the soap. The beauty of this reaction lies in its simplicity and the way it turns common fats and oils into cleaning powerhouses that can emulsify dirt and grease, making them washable with water.

The process can be represented by a simple chemical equation where stearic acid (\( C_{17}H_{35}COOH \) reacts with potassium hydroxide (\( \text{KOH} \) to produce potassium stearate (\( \text{KC}_{18}\text{H}_{35}O_{2} \) and water (\( H_2O \) as a byproduct. The practical application of saponification in daily life is vast, ranging from personal hygiene to industrial cleaning agents. By understanding the fundamentals of soap formation, one can appreciate the chemical elegance behind the many cleaning products that are essential in our everyday lives.