4.4 Reaction Yields

• This reaction is very rapid, generating gaseous nitrogen that can deploy and fully inflate a typical airbag in a fraction of a second.
• The amount of sodium azide used must be appropriate for generating enough nitrogen gas to fully inflate the air bag.
• 50 L of N2 can be generated by a small mass of NaN3.

• Passengers are less likely to be injured in an impact.

• You will be able to explain the concepts of theoretical yield and limiting reactants/reagents by the end of this section.

• The exercises of the preceding module involved amounts of reactants.
• When calculating the amount of product generated from a given amount of reactant, it was assumed that any other reactants required were available in larger amounts.
• More realistic situations are considered in which reactants are not present.

• A 2:1 ratio of bread and cheese slices is what this recipe calls for.
• If you have 28 slices of bread and 11 slices of cheese, you can make 11 sandwiches using all of the cheese and six slices of bread left over.

• The concepts of limiting and excess reactants can be seen in sandwich making.

• The amount of product that can be generated is limited if one of the reactants is provided in any other amount.
• Identifying the limiting and excess reactants for a given situation requires computing the molar amounts of each reactant provided and comparing them to the balanced chemical equation.
• Imagine combining 3 moles of H2 and 2 moles of Cl2.
• The 3:2 ratio of hydrogen to chlorine present for reaction is greater than the ratio of1:1.
• The limit of the reactant is chlorine.
• The 2 mol of chlorine will consume 2 mol of the 3 mol of hydrogen, leaving 1 mol of hydrogen unreacted.

• An alternative approach to identifying the limiting reactant is to compare the amount of product expected for the complete reaction of each reactant.
• Each reactant amount is used to calculate the amount of product that would be formed.
• The limit reactant is the lesser amount of product.

• Once 4 moles of HCl have been produced, the chlorine will be completely consumed.
• There will be unreacted hydrogen left once this reaction is complete.

• One of the reactants will limit the amount of HCl that can be produced.
• There is a reaction in which hydrogen is present in excess and chlorine is the limiting reactant.

• This interactive shows the concepts of limiting and excess reactants.

• A component of turbine blades in jet engines is made of Silicon nitride.

• To identify the limiting reactant, compare the provided molar amounts of reactants to the balanced equation.

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• The amount of product expected for complete reaction of each of the provided reactants can be calculated.

• It is the limiting reactant since it yields less product.

• The partial reactions of weak acids and bases are discussed in this chapter.
• Less than perfect recovery will reduce the actual yield because some products are difficult to collect without some loss.
• If the product is a gas, the theoretical and actual yields can be expressed as molar amounts.
• When percent yield is calculated, the units will cancel if both yields are expressed using the same units.

• Green chemistry is a philosophy that is being applied to many areas of science and technology, and its practice is summarized by guidelines known as the "Twelve Principles of Green Chemistry".
• One of the principles is designed to maximize the efficiency of processes for making chemical products.

• The percent yield of a given chemical process is used to evaluate the efficiency of a process by comparing the yield of product actually obtained to the maximum yield predicted.

• The success of a green chemistry approach has been demonstrated by the synthesis of the common nonprescription pain medication, Ibuprofen.
• An alternative process that requires only three steps and has an atom economy of 80% was developed in the 1990s.
• Significant cost-savings to the manufacturer and the consumer can be achieved by using the BHC process.
• The company received an award from the EPA in 1997 for the positive environmental impact of the BHC process.