Synthesis of Aspirin Pre Lab Answers: A Comprehensive Guide

The synthesis of aspirin is a fundamental experiment in organic chemistry, involving the reaction of salicylic acid with acetic anhydride to produce acetylsalicylic acid (aspirin). Understanding the pre-lab questions is crucial as they prepare you for the experiment by covering key concepts such as the chemical reaction involved, the calculation of theoretical yield, and the methods for analyzing the purity of the product. This foundational knowledge ensures accurate execution and comprehension of the experiment’s objectives and outcomes.

Objective of the Experiment

Here are the main goals of the synthesis of aspirin pre-lab:

1. Identify the Chemical Reactions Involved:

   • Reaction: Salicylic acid reacts with acetic anhydride in the presence of an acid catalyst (like phosphoric acid) to form aspirin (acetylsalicylic acid) and acetic acid.
   • Equation:

     $$\text{C}_7\text{H}_6\text{O}_3 + \text{C}_4\text{H}_6\text{O}_3 \rightarrow \text{C}_9\text{H}_8\text{O}_4 + \text{C}_2\text{H}_4\text{O}_2$$C7​H6​O3​+C4​H6​O3​→C9​H8​O4​+C2​H4​O2​

2. Understand Theoretical Yield Calculations:

   • Theoretical Yield: Calculate the maximum amount of aspirin that can be produced from given amounts of salicylic acid and acetic anhydride.
   • Steps:
     1. Determine the moles of each reactant.
     2. Identify the limiting reactant (the one that will be completely consumed first).
     3. Use stoichiometry to calculate the moles of aspirin produced.
     4. Convert moles of aspirin to grams using its molar mass.

These steps help ensure you understand the chemical process and can predict the amount of product formed under ideal conditions.

Materials and Reagents

Here are the materials and reagents required for the synthesis of aspirin, along with their roles:

1. Salicylic Acid: Reactant that provides the phenol group.
2. Acetic Anhydride: Reactant that acetylates the phenol group of salicylic acid to form aspirin.
3. Sulfuric Acid (H₂SO₄): Catalyst that speeds up the reaction.
4. Water (H₂O): Used to quench the reaction and hydrolyze excess acetic anhydride.
5. Ethanol: Solvent for recrystallization.
6. Iron (III) Chloride (FeCl₃) Solution: Used to test the purity of the synthesized aspirin.
7. Phosphoric Acid (H₃PO₄): Alternative catalyst option.
8. Ice: Used to cool the reaction mixture and promote crystallization.
9. Filter Paper and Buchner Filtration Apparatus: For filtering and collecting the aspirin crystals.

These components are essential for the synthesis and purification of aspirin.

Procedure Overview

Here are the detailed steps for the synthesis of aspirin, highlighting critical points to ensure a successful experiment:

1. Weigh Salicylic Acid:

   • Measure out 2.0 grams of salicylic acid accurately.

2. Add Acetic Anhydride:

   • Add 5 mL of acetic anhydride to the salicylic acid. Be cautious as acetic anhydride is a strong irritant.

3. Add Catalyst:

   • Carefully add 5 drops of concentrated sulfuric acid (H₂SO₄) as a catalyst. Use a dropper and handle with care due to its highly corrosive nature.

4. Heat the Mixture:

   • Heat the mixture in a hot water bath (70-80°C) for about 10-15 minutes. Ensure the temperature is maintained to avoid decomposition of reactants.

5. Add Water:

   • After heating, add 20 mL of distilled water to the mixture to decompose any excess acetic anhydride. This step is exothermic, so add water slowly to control the reaction.

6. Cool the Mixture:

   • Allow the mixture to cool to room temperature. If necessary, place the flask in an ice bath to speed up the cooling process and promote crystallization.

7. Induce Crystallization:

   • If crystals do not form, scratch the sides of the flask with a glass rod to induce crystallization.

8. Filter the Crystals:

   • Use vacuum filtration to separate the crude aspirin crystals from the liquid. Ensure the filter paper is properly placed to avoid loss of product.

9. Wash the Crystals:

   • Wash the crude aspirin crystals with small amounts of cold distilled water to remove impurities.

10. Dry the Crystals:

    • Allow the purified aspirin crystals to dry completely, preferably in a desiccator.

11. Test Purity:

    • Test the purity of the aspirin using iron (III) chloride solution. A deep purple color indicates the presence of unreacted salicylic acid, suggesting impurities.

Critical Points to Note:

• Safety: Handle all chemicals, especially sulfuric acid and acetic anhydride, with care. Use appropriate personal protective equipment (PPE).
• Temperature Control: Maintain the correct temperature during heating to ensure proper reaction without decomposing reactants.
• Crystallization: Proper cooling and scratching are essential to induce crystallization if it does not occur naturally.
• Purity Testing: Ensure to test the final product for purity to confirm the success of the synthesis.

Following these steps carefully will help ensure a successful synthesis of aspirin.

Theoretical Yield Calculation

Here’s a step-by-step guide to calculating the theoretical yield in the synthesis of aspirin:

1. Balanced Chemical Equation:

   $$\text{C}_7\text{H}_6\text{O}_3 (\text{salicylic acid}) + \text{C}_4\text{H}_6\text{O}_3 (\text{acetic anhydride}) \rightarrow \text{C}_9\text{H}_8\text{O}_4 (\text{aspirin}) + \text{C}_2\text{H}_4\text{O}_2 (\text{acetic acid})$$C7​H6​O3​(salicylic acid)+C4​H6​O3​(acetic anhydride)→C9​H8​O4​(aspirin)+C2​H4​O2​(acetic acid)

2. Calculate Moles of Reactants:

   • Determine the moles of salicylic acid (C₇H₆O₃) using its molar mass.

     $$\text{Moles of salicylic acid} = \frac{\text{mass of salicylic acid (g)}}{\text{molar mass of salicylic acid (g/mol)}}$$Moles of salicylic acid=molar mass of salicylic acid (g/mol)mass of salicylic acid (g)​

   • Example: If you have 2.00 g of salicylic acid (molar mass = 138.12 g/mol):

     $$\text{Moles of salicylic acid} = \frac{2.00 \, \text{g}}{138.12 \, \text{g/mol}} = 0.0145 \, \text{mol}$$Moles of salicylic acid=138.12g/mol2.00g​=0.0145mol

3. Determine Limiting Reactant:

   • Compare the mole ratio of salicylic acid to acetic anhydride (usually 1:1 in this reaction).
   • If acetic anhydride is in excess, salicylic acid is the limiting reactant.

4. Calculate Theoretical Yield:

   • Use the moles of the limiting reactant to find the moles of aspirin produced (1:1 ratio).

     $$\text{Moles of aspirin} = \text{Moles of salicylic acid}$$Moles of aspirin=Moles of salicylic acid

   • Convert moles of aspirin to grams using its molar mass (180.16 g/mol).

     $$\text{Mass of aspirin (g)} = \text{Moles of aspirin} \times \text{molar mass of aspirin (g/mol)}$$Mass of aspirin (g)=Moles of aspirin×molar mass of aspirin (g/mol)

   • Example:

     $$\text{Mass of aspirin} = 0.0145 \, \text{mol} \times 180.16 \, \text{g/mol} = 2.61 \, \text{g}$$Mass of aspirin=0.0145mol×180.16g/mol=2.61g

So, the theoretical yield of aspirin in this example is 2.61 grams.

Safety Considerations

Here are the key safety precautions for the synthesis of aspirin:

Handling of Chemicals:

1. Wear Safety Goggles and Gloves: Always wear safety goggles and gloves to protect your eyes and skin from chemical splashes.
2. Use Fume Hood: Conduct the experiment under a fume hood to avoid inhaling harmful vapors.
3. Avoid Direct Contact: Do not touch chemicals directly. Use appropriate tools like spatulas and pipettes.
4. Handle Acids with Care: Sulfuric acid and acetic anhydride are highly corrosive. Avoid contact with skin, eyes, and clothing.

Handling of Equipment:

1. Proper Attire: Wear long pants, closed-toed shoes, and shirts with sleeves to minimize skin exposure.
2. Vacuum Filtration: Use vacuum filtration carefully to avoid spills and splashes.
3. Clean Spills Immediately: In case of spills, clean them immediately with plenty of water and notify the instructor.
4. Wash Hands: Always wash your hands thoroughly before leaving the lab.

Stay safe and follow these precautions closely!

Common Pre-Lab Questions

Here are some typical pre-lab questions related to the synthesis of aspirin, along with detailed explanations and answers:

1. Identify the reagents used in the synthesis of aspirin.

• Salicylic Acid: This is the starting material.
• Acetic Anhydride: This reacts with salicylic acid to form aspirin.
• Acid Catalyst (usually sulfuric or phosphoric acid): This speeds up the reaction.

2. Write the chemical equation for the synthesis of aspirin.

The reaction involves salicylic acid and acetic anhydride:

$$\text{C}_7\text{H}_6\text{O}_3 + \text{(CH}_3\text{CO)}_2\text{O} \rightarrow \text{C}_9\text{H}_8\text{O}_4 + \text{CH}_3\text{COOH}$$C7​H6​O3​+(CH3​CO)2​O→C9​H8​O4​+CH3​COOH

Salicylic acid reacts with acetic anhydride to form acetylsalicylic acid (aspirin) and acetic acid.

3. Calculate the theoretical yield of aspirin.

To calculate the theoretical yield, you need the molar masses of the reactants and products:

• Molar Mass of Salicylic Acid (C$_7$H$_6$O$_3$): 138.12 g/mol
• Molar Mass of Acetylsalicylic Acid (C$_9$H$_8$O$_4$): 180.16 g/mol

If you start with 2.01 g of salicylic acid:

1. Convert grams to moles:

   $$\frac{2.01 \text{ g}}{138.12 \text{ g/mol}} = 0.01455 \text{ mol}$$138.12 g/mol2.01 g​=0.01455 mol

2. Use the stoichiometry of the reaction (1:1 ratio):

   $$0.01455 \text{ mol} \times 180.16 \text{ g/mol} = 2.62 \text{ g}$$0.01455 mol×180.16 g/mol=2.62 g

So, the theoretical yield of aspirin is 2.62 g.

4. Identify safety concerns in the synthesis of aspirin.

• Acetic Anhydride: It is a strong irritant and can cause burns.
• Sulfuric/Phosphoric Acid: These are corrosive and can cause severe burns.
• Irritating Vapors: Produced during the reaction, especially when water is added.

5. How can you test for the presence of unreacted salicylic acid?

• Ferric Chloride Test: Add a few drops of 1% FeCl$_3$ solution to the aspirin product. If a purple color forms, it indicates the presence of phenol groups, suggesting unreacted salicylic acid.

6. What should you do if crystal growth does not start on its own?

• Scratch the Inside of the Flask: Gently scratch the inside wall of the flask with a stirring rod to induce crystallization.

7. What is the best method for purifying aspirin?

• Recrystallization: Dissolve the crude aspirin in a minimal amount of hot ethanol and then add water to precipitate the pure aspirin crystals. Filter and dry the crystals.

8. Calculate the percent yield of aspirin.

Percent yield is calculated using the formula:

$$\text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100\%$$Percent Yield=(Theoretical YieldActual Yield​)×100%

If the actual yield is 2.50 g:

$$\text{Percent Yield} = \left( \frac{2.50 \text{ g}}{2.62 \text{ g}} \right) \times 100\% = 95.4\%$$Percent Yield=(2.62 g2.50 g​)×100%=95.4%

These questions and answers cover the essential aspects of the synthesis of aspirin, ensuring a thorough understanding of the process and safety measures involved.

The Synthesis of Aspirin

The synthesis of aspirin involves careful preparation and execution to ensure accurate results.

• Understanding the chemical equation for the synthesis of aspirin, including the reactants and products involved.
• Recognizing the importance of proper laboratory safety measures, such as wearing protective gear and handling chemicals with care.
• Following a step-by-step procedure to synthesize aspirin, including measuring and mixing reagents, heating the mixture, and filtering and drying the crystals.
• Calculating the percent yield of aspirin using the formula: (Actual Yield / Theoretical Yield) x 100%.
• Interpreting results and understanding the significance of the actual yield in relation to the theoretical yield.

By thoroughly reviewing these key points, students can gain a deeper understanding of the synthesis of aspirin and develop essential laboratory skills.