Percentage Yield

3 Key excerpts on "Percentage Yield"

• 

  eBook - ePub

  General Chemistry for Engineers

  • Jeffrey Gaffney, Nancy Marley(Authors)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  actual yield , the mass of products actually obtained from a chemical reaction in a laboratory or industrial process, is almost always less than the theoretical yield. There are several reasons for this. The reaction may not go completely to the products, but may stop before completion leaving unconsumed reactants or the products may decompose back to reactants. Unexpected reactions may sometimes occur which give products other than those expected. Also, the processes of separation and purification of the products almost always leads to product loss.

  The difference between the theoretical yield and the actual yield can be expressed as a percent yield . The percent yield specifies how much of the theoretical yield was actually obtained. It is calculated as the ratio of the actual yield to the theoretical yield multiplied by 100.

  Percent yield =

  Actual yield Theoretical yield

  × 100

    (6)

  In industrial processes, it is important to achieve the highest percent yield possible in order to maximize the desired product and minimize the amount of reactants consumed. This in turn reduces the cost of the process. It also reduces the amount of undesired, and sometimes environmentally hazardous, products. Many times, industrial processes require several successive reactions followed by several purification steps to achieve a desired product. Determining the percent yield at each step can help to identify inefficiencies and increase the overall product yield giving a higher profit margin.

  Case Study: Percent Yield and Atom Economy in the Steel Industry

  The industrial production of steel begins with the production of iron from iron ore. The most common iron ore used is hematite (iron(III) oxide). The ore is converted to the pure metal in a reaction with carbon at high temperatures. The reaction is typically conducted in a blast furnace shown in Fig. 4.3

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  eBook - ePub

  Unit Operations in Environmental Engineering

  • Louis Theodore, R. Ryan Dupont, Kumar Ganesan(Authors)
  • 2017(Publication Date)
  • Wiley-Scrivener

    (Publisher)

  A at some later time (or position). Also note that all of the conversion variables can be based on mass, but this is rarely employed in practice. The conversion variable of choice is almost always X.

  The yield of a reaction is defined as a measure of how much of the desired product is formed relative to how much would have been formed if only the desired reactions occurred, and if that reaction went to completion. Alternatively, selectivity is a measure of how a desired reaction is completed relative to one of the side reactions.

  To drive a chemical reaction to completion it is common practice to add an excess of one of the reactants, especially an inexpensive one. In most reaction mixtures, there is a reactant that is present in the lowest number of chemical equivalents. This is the limiting reactant since it sets the absolute limit upon the extent of chemical change and the quantities of products that can form. The degree of completion of a reaction is the percentage of the limiting reactant that undergoes the reaction in question. The degree of completion is also called the extent of reaction. Once having identified the limiting reactant, it is possible to express in a precise way the amount to which any other reactant is present in excess. The percentage excess for any reactant is the total amount added less the theoretical amount, divided by the amount theoretically required for complete reaction (stoichiometric requirement) with the limiting reactant, multiplied by 100 (Equation 6.9 ).

  (6.9)

  If half again as much as that theoretically required is added, the percentage excess is 50 percent; if a double quantity is added, the excess is 100 percent, etc. The addition of a triple quantity often introduces a semantic “booby trap” that the reader should be aware of, i.e., a 200 percent excess means that a three-fold quantity of the reactant is being used, not a double quantity.

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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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