The purpose of this lab is to synthesise acetylsalicylic acid (aspirin) by creating a reaction between acetic anhydride and salicylic acid. This was be accomplished through the use of recrystallization. Acetic anhydride and salicylic acid are mixed together, and then acidified by the addition of a few drops of concentrated sulfuric acid, which catalyzed the reaction. The percent yield is calculated to determine the effectiveness of the reaction in preparing the desired product (aspirin).
The limiting reactant of the equation was salicylic acid. After the limiting reactant was determined, the theoretical yield of aspirin was calculated at approximately 1.97g. The actual yield was only around 0.67g, producing a percent yield of 34.3%. These results show that the methods used were only partially successful at achieving the goal of the experiment (synthesising aspirin). The findings showed that acetylsalicylic acid can be produced through a reaction between salicylic acid and acetic anhydride, but that a much lower yield will be produced. A higher yield could surely be achieved if several sources of error were to be eliminated.
Acetylsalicylic acid is commonly used to alleviate minor aches and pains (Wikipedia, Aspirin, 2013). The active metabolite ingredient in acetylsalicylic acid (aspirin) is salicylic acid (Wikipedia, Salicylic acid, 2013), which was first discovered by Edward Stone in 1763 (Wikipedia, Aspirin, 2013).
Salicylic acid is toxic in large quantities but in small doses can be useful for food preservatives and as an antiseptic. Other than being used in the production of aspirin, acetic anhydride is used to convert cellulose to cellulose acetate, a key component in photographic film and other coated materials (Wikipedia, Acetic anhydride, 2013). Sulfuric acid has many applications, such as pigments, explosives, lubricants, batteries, antifreeze, and detergents. In the synthesis of aspirin, sulfuric acid is also used as a catalyst to speed up the reaction (Wikipedia, Sulfuric acid, 2013).
Limiting reactants are important in chemical reactions because a reaction cannot proceed without all of the reactants. That is to say, a reaction can only occur until one reactant is used up (Kirk, 2013). Percent yields are related to limiting reactants because the formula to solve for percent yield includes theoretical and actual yield.
The theoretical yield is the amount of a product formed when the limiting reactant in completely consumed, and is the maximum amount that can be produced from the amount of reactants used in the reaction. The theoretical yield is rarely obtained because of sources of error, side reactions, or other complications. The percent yield is the actual yield of a product given as a percentage of the theoretical yield (Kirk, 2013).
How can one prepare aspirin through a reaction between salicylic acid and acetic anhydride?
When the salicylic acid and acetic anhydride were mixed, a white, powdery solution formed. When the sulfuric acid was added, a clear solution formed that produced heat. After heating, then cooling and scratching the solution, a white precipitate formed. The moisture in the precipitate was filtered overnight and what was left over was the desired product, aspirin.
The boiling chips (calcium carbonate) were white, opaque crystals. The acetic anhydride was a clear solution with a vinegar-like odour. The salicylic acid was a find, white solid powder. The ethanol was a clear solution with an odour similar to strong alcohol. The sulfuric acid was a clear solution with a strong odour when heated. The aspirin (acetylsalicylic acid) was a white, solid powder.
When the acetic anhydride and salicylic acid were mixed, they produced a white, powdery solution. When the sulfuric acid was added to this solution, it turned clear and was warm. Upon heating the solution and adding water, puffs of smoke were produced. When the solution cooled and the “scratching” method was used, a white precipitate formed.
Density (Table #1)
Volume (Table #2)
Molar weight (Table #3)
Substance/ Molar weight
Percent Yield (Table #5)
Determine whether the limiting reactant is the salicylic acid or the acetic anhydride. First, convert both masses to moles. 1 mole is equal to the molar mass of a substance; therefore, the grams of acetic anhydride and grams of salicylic acid must be divided by their molar masses respectively.
Since both acetic anhydride and salicylic acid have a coefficient of 1, the smaller number is the limiting reactant. Therefore, salicylic acid is the limiting reactant of the reaction. Next, calculate the theoretical yield of aspirin by converting the mass of the limiting reactant to grams of aspirin. 1.51g C7H6O3 x 1 mole C7H6O3 x 1 mole C9H8O4 x 180.154 g C9H8O4 138.118 g C7H6O3 1 mol C7H6O31 mol C9H8O4
= 1.969566168g Aspirin
Using the theoretical yield and the actual yield (from the data table), calculate the percent yield.
Percent yield = Actual yield x 100 = l 0.676g l x 100 = 34.3%
Sources of error that were possibly present in the synthesis of aspirin lab are an improperly calibrated balance, an inaccurate hot plate, contamination of the substances, age of the substances, or contamination of the glassware. Inaccurate calibrations of the hot plate or balance could have shown higher or lower quantitative data which would have affected the results by producing either a higher or lower percent yield. A contamination of the substances used or the glassware could have caused the substances to react differently with each other, again causing the percent yield to change depending on how the contamination affected the substances by producing a higher or lower percent yield.
Another source of error could have been due to the transport of the aspirin from the Erlenmeyer flask after cooling to the funnel to be filtered. Some of the aspirin may have been lost or left behind and that would have showed a lower percent yield. The accuracy of this lab could be improved with more precise equipment, allowing the experimenter to be more confident in the accuracy of the measurements obtained. While the aspirin was left overnight to allow time for the moisture to be filtered out, the air could have been a contaminant.
A way to get rid of the source of error would be to keep the samples in a more isolated area where there is a smaller chance of contamination. Aspirin has many real-life applications. It is a pain reliever and a non-steroidal anti-inflammatory (NSAID). It has become very common as an antiplatelet to prevent blood clot formation and is used to prevent heart attacks and strokes. It has been present for over one hundred years and is one of the most widely used medications in the world.
The main objective of the synthesis of aspirin lab was so produce aspirin (acetylsalicylic acid) through the reaction of salicylic acid and acetic anhydride. The methods used included recrystallization and scratching to produce a precipitate, which was then filtered to remove any excess moisture. The results displayed a percent yield of 34.3%, from a theoretical yield of about 1.97g of aspirin and an actual yield of approximately 0.68g of aspirin. Upon completion of the lab, analysis, and calculations, it is evident that the synthesis of aspirin is possible using these methods but that the yield will be relatively low.
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