Percentage Yield Calculations. However, not all of the reactants may actually be converted into products. As we have seen, some reactions have 100% atom.

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Presentation transcript:

Percentage Yield Calculations. However, not all of the reactants may actually be converted into products. As we have seen, some reactions have 100% atom economy i.e. all the atoms of the reacting molecules are converted into useful products. C2H4C2H4 H2OH2OC2H6OC2H6O + e.g. There are several reasons for this : a. The reaction may be reversible. Such reactions never go to completion and some of the reactants always remain. N H 2 2 NH 3 e.g. The Haber Process What happens to the un reacted nitrogen and hydrogen gases ? Unreacted gases are recycled

b. An un wanted ‘side reaction’ may occur reducing the yield of the desired product. This often occurs in Drug manufacture. Unwanted Impurity Wanted Product

c. The product may be impure and require purification before use. This purification process always causes mass to be ‘lost’. e.g. pharmaceutical products

We can assess how efficient the production process will be by calculating the percentage yield. Percentage Yield = Actual Yield of Product Maximum possible yield x 100 For example:A pharmaceutical process is being used to produce aspirin. The process could produce a maximum possible yield of 350g. The actual yield of aspirin after purification is only 270g. Calculate the Percentage Yield. Answer Percentage Yield = Actual Yield of Product Maximum possible yield x 100 = 270 x = 77.1 %

e.g. 1The Haber Process. Nitrogen reacts with Hydrogen to form Ammonia according to the following equation: N H 2 2 NH kg of nitrogen are used to produce the ammonia kg of ammonia are produced. (i.) Calculate the maximum possible yield of ammonia. (ii.) Calculate the actual percentage yield. Answer(i.) Step 1.Write the information under the equation. 200 kgYield 48.6 kg Step 2.Calculate the Mr values for each molecule. Mr (N 2 ) = 2 x 14 = 28Mr (NH 3 ) = x 1 = 17 Mr (N 2 ) = 28Mr (NH 3 ) = 17 Atom Economy 100 %?????

Step 3Use the equation 1 N 2 produces 2 NH 3 Step 4Convert to masses. 1 x 28 kg of N 2 produces 2 x 17 kg of NH 3 28 kg of N 2 produces 34 kg of NH 3 1 kg of N 2 produces 34 / 28 kg of NH kg of N 2 produces 34 / 28 x 200 kg of NH 3 = 243 kg of NH 3 Maximum Possible Yield N H 2 2 NH 3 Mr (N 2 ) = 28 Mr (NH 3 ) = 17 Percentage Yield = Actual Yield of Product Maximum possible yield x 100 Answer(ii.) = 48.6 / 243 x 100 = 20 % Atom Economy 100% Although the reaction has a 100% atom economy, only 20% of the reactants are actually converted into products.