Percentage Yield

3 Key excerpts on "Percentage Yield"

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  eBook - PDF

  Basic Concepts of Chemistry

  • Leo J. Malone, Theodore O. Dolter(Authors)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  SYNTHESIS A third type of problem involving percent yield is to inquire how much reactant is necessary to produce a given amount of product, assuming a percent yield less than 100%. The trick here is to perform the percent yield calcu- lation first, to calculate theoretical yield, and then use that amount in the mass-to-mass stoichiometry calculation to determine the amount of starting material. For example, if a reaction had a 50% yield, this means that you’d have to use double the amount of starting material you’d have calculated for a straightforward stoichiometry problem with 100% yield. For an example, see Exercise 7-3d. EXERCISE 7-3(a) LEVEL 1: If a chemical reaction theoretically should produce 32.0 g of product, but the actual amount is 24.0 g, what is the percent yield for the reaction? EXERCISE 7-3(b) LEVEL 1: A chemical reaction has a percent yield of 75%. If you desired an actual yield of 1.8 kg, what mass of product should you attempt to make? EXERCISE 7-3(c) LEVEL 2: For the reaction Zn(s) + 2AgNO 3 (aq) ¡ Zn(NO 3 ) 2 (aq) + 2Ag(s) If 10.0 g of Zn produces 25.0 g of Ag, what’s the percent yield of the reaction? EXERCISE 7-3(d) LEVEL 3: For the reaction 4KNO 3 (s) ¡ 2K 2 O(s) + 2N 2 (g) + 5O 2 (g) What mass of KNO 3 is needed to produce 20.0 g K 2 O, assuming the reaction pro- duces an 84.0% yield? EXERCISE 7-3(e) LEVEL 3: Anyone with experience in a teaching lab knows that students frequently report percent yields in excess of 100%. Could this be accurate? What are some potential explanations for yields this high? For additional practice, work chapter problems 7-32, 7-35, and 7-37. C C ASSESSING THE OBJECTIVE FOR SECTION 7-3 PART A SUMMARY KEY TERMS 7-1.1 Stoichiometry calculations involve conversions among amounts of reactants and products. p. 214 7-1.2 Mole ratios from balanced equations are the keys to solving stoichiometry problems. p. 215 7-2.1 The limiting reactant is completely consumed in a reaction.

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  eBook - ePub

  Chemistry for Sustainable Technologies

  A Foundation

  • Neil Winterton(Author)
  • 2015(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  2 who has developed some of these metrics and unified them into powerful and useful tools for analysing reaction efficiency in ways relevant to fine chemical processing. See the Bibliography for texts that review the most recent work including the wider (and more complex) topics of metrics for sustainable development.

  7.1 REACTION YIELD

  A question for chemists to ask (and to answer) is whether the metric long used in assessing the effectiveness or otherwise of a chemical transformation, the reaction yield , is useful in assessing its efficiency and the extent of associated waste produced. Process chemists or chemical engineers may also be familiar with some related terms such as mass balance (Section 7.2), conversion (Section 7.3) and selectivity (Section 7.4), the importance and relevance of which will become evident in Chapter 9.

  Let us first have a look at reaction yield for estimating reaction efficiency in the context of sustainable technologies. We will consider a very simple transformation (eqn 7.1 ). While this chemistry is apparently simple, it is also extremely important technologically, being operated on a very large scale. The conversion of methanol to chloromethane provides raw materials for the manufacture of silicones, amongst other products.

  (7.1)

  Say, we begin with 32 g (1 mole) of methanol and react it with sufficient hydrogen chloride, producing 20 g chloromethane. Water is a co-product .i The yield is given in eqn (7.2)

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  eBook - PDF

  Principles of Chemical Reactor Analysis and Design

  New Tools for Industrial Chemical Reactor Operations

  • Uzi Mann(Author)
  • 2009(Publication Date)
  • Wiley-Interscience

    (Publisher)

  2.6 CHARACTERIZATION OF REACTOR PERFORMANCE 57 2.6.2 Product Yield and Selectivity When several simultaneous chemical reactions take place producing both desired and undesired products, it is convenient to define parameters that indicate what portion of the reactant was converted to valuable products. Below, we define and discuss two quantities that are commonly used: yield and selectivity of the desirable product. Yield is a measure of the portion of a reactant converted to the desired product by the desirable chemical reaction. It indicates the amount of the desirable product, species V, produced relative to the amount of V that could have been produced if only the desirable reaction took place. The yield is defined such that its value is between zero and one. For batch reactors, the yield of product V at time t is h V (t ) ; Stoichiometric factor   Moles of product V formed in time t Moles of reactant A initially in the reactor and in mathematical terms h V (t ) ;  s A s V   des N V (t )  N V (0) N A (0) (2:6:12) where s A and s V are, respectively, the stoichiometric coefficients of A and V in the desirable chemical reaction. Using stoichiometric relations (Eq. 2.3.3), the yield relates to the extents of the independent chemical reactions by h V (t ) ¼ s A s V   des 1 N A (0) X n I m (s V ) m X m (t ) " # (2:6:13) For flow reactors, the yield of product V at the reactor outlet is h V out ; Stoichiometric factor   Rate product V is formed in the reactor Rate of reactant A is fed into the reactor and in mathematical terms h V out ;  s A s V   des F V out  F V in F A in (2:6:14) Using Eq. 2.3.11, the yield relates to the extents of the independent chemical reac- tions by h V out ¼ s A s V   des 1 F A in X n I m (s V ) m _ X m " # (2:6:15) 58 STOICHIOMETRY

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