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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.
The OpenStax book is available for free at http://cnx.org/content/col11760/1.9
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.
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