Chemistry Discussion Questions

Chemistry Discussion Question and classmates response:

DQ2 must be 2 paragraphs and classmates response must be 1 paragraph. Must be you own work.

DQ2

Define the terms theoretical, actual, and percentage yield. Why is the percentage yield always less than the theoretical yield? Give a specific example. You need to respond to my questions and to at least three classmates substantially over three days in a manner that furthers the discussion.  Some ideas for responses:             1. Add information that your classmate didn’t include in his/her post.             2. Respond to a classmate’s example.             3. Make up a similar example using a classmate’s information.

Classmate’s responses:

1. The theoretical yield of product is “the maximum amount of product that can be obtained by a reaction from given amounts of reactant” (Ebbing & Gammon, 2012, p.94). Actual yield of the amount actually produced of a product, and percentage yield is “the actual yield expressed as a percentage of the theoretical yield” (Ebbing & Gammon, 2012, p. 95).

There are several reasons why the actual yield is always less than the theoretical yield and here are some reasons, impure reactants and competing side reactions, products can be lost during processes, miscalculations in measurement, and reactions do not always go through completion. An example, is nitrobenzene (C6H5NO2), start with 15.6g of benzene (c6H6), in excess of nitric acid (HNO3). However, is all C6H6 was converted to product, in theory we should obtain 24.6g of product, which would be 100% yield. If 18.0g was isolated, we could then calculate the percent yield: 18.0g/24.6g*100 = 73.2%. So this would make the percentage yield less than the theoretical yield.

2. Theoretical yield is the maximum quantity of a product that can be formed in a chemical reaction if all the limiting reactant reacted to form products. Actual yield is an amount of a specified pure product obtained from a given reaction. Percentage yield is getting the efficiency of a reaction. There are a few reasons the percentage yield is always that the theoretical yield. One reason is part of the reactants may not react, or they may react in a way different from that desired (side reactions). In addition, it is not always possible to recover all of the reaction product from the reaction mixture.

An example of this is the reaction of rust to iron. The theoretical yield is 699.5 grams of iron; my actual yield is 525.0 grams. Therefore, the precent yield is

%yield= 525.0 g/699.5 g x100= 75.05%

A percent yield of about 75 percent isn’t too bad, but chemist and chemical engineers would rather see 90+ percent.

3. The theoretical yield: is the max amount of product that can be obtained in a chemical reaction. This is calculated from the limiting reagent. This can be obtained only under perfect conditions if no product is lost in the process.The actual yield :is the amount of product actually obtained from a chemical reaction. This must be experimentally determined; it can not be calculated. This amount is never more than the theoretical yield. And also can never be 100% due to limitations. Percent yield: is defined as the actual yield/ theoretical yield * 100%Example: if 16.0g of CaCO3 are decomposed and 7.54g of CaO are obtained, what is the % yield of CaO obtained? Equation: CaCO3 > CaO + CO2 (balanced equation)

step 1) make sure that you have a balanced equation

step 2) calculate theoretical yield: (must convert in moles)

16g CoCO3* (1 mol CaCO3/100.1g CaCO3(molar mass)) * (1mol CaO/1mol CaCO3) * (56.1g CaO (molar mass)/1 mol    CaO=8.97g of CaO (theoretical yield)

step 3) calculate percentage yield: 7.54g (actual yield)/8.97g(theoretical yield)* 100%=84.1%

Actual yield is less than the theoretical yield for a few reasons: there may be competing reactions which hinder the process under consideration; external conditions may not be perfectly maintained; reactants may not be perfect