Problem 75 - Chemistry Matter and Change

Question

Polyunsaturated Fats How many moles of hydrogen gas are required for complete hydrogenation of 1 mol of linolenic acid, whose structure is shown below? Write a balanced equation for the hydrogenation reaction. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}=\mathrm{CH}\) \(\left(\mathrm{CH}_{2}\right)_{7} \mathrm{COOH}\)

Step-by-Step Solution

Answer

3 moles of \(H_2\) are required. Balanced equation: \( \mathrm{C}_{18}\mathrm{H}_{30}\mathrm{O}_2 + 3 \mathrm{H}_{2} \rightarrow \mathrm{C}_{18}\mathrm{H}_{36}\mathrm{O}_2 \).

Step 1: Understand the Structure

Linolenic acid's structure, as given, shows three double bonds: \( CH=CHCH_2CH=CHCH_2CH=CH \). These are the parts of the molecule that can react with hydrogen gas during hydrogenation.

Step 2: Determine the Stoichiometry of Hydrogenation

Each double bond in a molecule can react with 1 mole of hydrogen (\( H_2 \)) to form a single bond. Therefore, for every double bond in linolenic acid, we need 1 mole of hydrogen gas.

Step 3: Calculate the Total Moles of Hydrogen Gas Needed

Given that linolenic acid has 3 double bonds, we need 3 moles of hydrogen gas to fully hydrogenate 1 mole of linolenic acid. This completes the conversion of all double bonds to single bonds.

Step 4: Write the Balanced Chemical Equation

The balanced chemical equation for the hydrogenation of linolenic acid is as follows: \[ \mathrm{C}_{18}\mathrm{H}_{30}\mathrm{O}_2 + 3 \mathrm{H}_{2} \rightarrow \mathrm{C}_{18}\mathrm{H}_{36}\mathrm{O}_2 \] The left side represents the initial components (linolenic acid and hydrogen gas), and the right side represents the hydrogenated product.

Key Concepts

Understanding Hydrogenation

Hydrogenation is a chemical reaction where hydrogen is added to molecules, typically organic compounds that contain double bonds. This process is often used in the food industry to convert liquid vegetable oils into solid or semi-solid forms. During hydrogenation, each carbon-carbon double bond (\( C=C \)) in a molecule reacts with hydrogen gas (\( H_2 \)) to form a carbon-carbon single bond (\( C-C \)).

• It generally involves the use of a catalyst, such as nickel, to speed up the reaction.
• Hydrogenation not only changes the physical properties of a substance but can also impact its chemical stability and nutritional content.

Understanding hydrogenation is crucial for many applications, from industrial processes to academic research, as it affects the texture and shelf-life of products.

The Role of Linolenic Acid

Linolenic acid is a type of polyunsaturated fatty acid and falls into the Omega-3 category of fatty acids. It is crucial for health benefits, such as reducing inflammation and supporting brain function.

• This acid contains three double bonds in its hydrocarbon chain, where hydrogenation can occur.
• In its unmodified state, linolenic acid is a liquid at room temperature because of these double bonds that create kinks in the molecule, preventing tight packing.
• Upon hydrogenation, these double bonds are converted to single bonds, turning the previously unsaturated fat into a saturated fat, which is generally solid or semi-solid.

The chemical manipulation of linolenic acid through hydrogenation alters its properties and potential uses in food and industrial products.

Discovering Stoichiometry

Stoichiometry is the area of chemistry that deals with the relative quantities of reactants and products in chemical reactions. It's like a recipe, ensuring the right amounts of each ingredient are used to achieve the desired outcome.

• When applying stoichiometry to hydrogenation, the key is understanding how many moles of each substance are needed.
• In the case of linolenic acid, stoichiometry tells us that 3 moles of hydrogen gas are required to completely hydrogenate 1 mole of the acid due to the presence of three double bonds.
• This balanced approach ensures that no excess reactants are left over, optimizing the efficiency of the reaction process.

Appreciating the principles of stoichiometry can illuminate the underlying mechanics of chemical reactions and their practical applications.

Creating a Balanced Chemical Equation

A balanced chemical equation is a vital tool that depicts the transformation of reactants into products in a chemical reaction. It ensures that the law of conservation of mass is adhered to, with equal amounts of each element on both sides of the equation.

• In the context of the hydrogenation of linolenic acid, the equation is: \[\mathrm{C}_{18}\mathrm{H}_{30}\mathrm{O}_2 + 3 \mathrm{H}_{2} \rightarrow \mathrm{C}_{18}\mathrm{H}_{36}\mathrm{O}_2\]
• The equation shows that the long-chain molecule of linolenic acid reacts with three molecules of hydrogen, transforming every double bond into a single bond.
• This balanced representation allows chemists to predict the behavior of the substances involved under specific conditions.

Being able to write and interpret balanced chemical equations is fundamental for anyone studying chemistry, as it lays the groundwork for deeper chemical understanding.