Physical Science Lesson 16 Chemical Reactions

Chemical Reactions Chemical Reactions

Chemical Reactions - Calculating the Percent Yield in a Chemical Reaction Chemical Reactions - Calculating the Percent Yield in a Chemical Reaction

Objective

Objective

 At the end of the lesson, you shoesson, you should be able to caluld be able to calculate the perculate the percent yield (% yieldcent yield (% yield) of a reaction.) of a reaction. Imagine that you have 100

Imagine that you have 100 chances to shoot a ball inside chances to shoot a ball inside the ring. Theoretically, you can mae all thethe ring. Theoretically, you can mae all the 100 shots. !o"ever, in actuality, you may not

100 shots. !o"ever, in actuality, you may not mae all them. There "ould be mae all them. There "ould be a difference bet"eena difference bet"een  your theoret

 your theoretical and actual ical and actual outcomes.outcomes.

In chemical reactions, there are also theoretical and actual outcomes. In this lesson, you "ill learn In chemical reactions, there are also theoretical and actual outcomes. In this lesson, you "ill learn that there is a theoretical amount of product that "ill be formed in a reaction. !o"ever, it is often that there is a theoretical amount of product that "ill be formed in a reaction. !o"ever, it is often different from the actual amount that you get from your e#periment.

different from the actual amount that you get from your e#periment.

How can you express the difference between theoretical and actual product yields? How can you express the difference between theoretical and actual product yields?

$ast pdated& 0'.0.1 $ast pdated& 0'.0.1

Learn about it!

Learn about it!

In the previous lesson, you have

In the previous lesson, you have calculated the amount of product that "ill be calculated the amount of product that "ill be formed from a givenformed from a given

amount of reactant. This amount of product is

amount of reactant. This amount of product is the theoretical yield of the the theoretical yield of the reaction. Thereaction. Thetheoreticaltheoretical

 yield

 yield is the ma#imum amount of product that can be formed from a given amount of reactant. is the ma#imum amount of product that can be formed from a given amount of reactant.

*onsider the reaction belo".

*onsider the reaction belo".

 What is

 What is the theothe theoretical yielretical yield (in gramd (in grams) of s) of that wilthat will be forml be formed when 4 ed when 4 moles omoles of f is usedis used

in the reaction?

Learn about it!

In reality, ho"ever, reactions are not al"ays 100% successful. This means that the actual amount of products formed from an e#periment, or the actual yield, may be lo"er than the theoretical yield. The actual yield is also called experimental yield .

 After performing an e#periment, you can calculate ho" efficient your reaction is in producing the desired product. Percent yield is one "ay of e#pressing the efficiency of a chemical reaction. It is the ratio of the actual yield to the theoretical yield e#pressed as percent.

In using the formula, it is important that the actual and theoretical yields have the same units (either in moles or mass) so you can cancel their units.

*onsider again the reaction bet"een sodium hydro#ide and sulfuric acid. Suppose you use 4 g and produced !" g # What is the $ yield of ? se the follo"ing molar masses& and .

Try it!

*onsider the synthesis of from and . The reaction is represented by the balanced chemical e+uation  belo".

 When !# g of is used% the reaction produced !&#" grams # What is the $ yield of the reaction?

se the follo"ing molar masses& and.  ou may follo" the steps belo".

Step !' *onvert mass of reactant to moles.

Step &' -etermine the mole ratio bet"een the reactant and the product. Step ' $et moles of product formed from the given mass of reactant. Step 4' et/up the proportion.

Step "' earrange the e+uation to solve for . Step ' olve for .

Step *' *onvert moles of product to mass. This is your theoretical yield. Step +' Tae note of the actual yield.

What do you think?

Photosynthesis is the process in "hich plants produce their food glucose, , from , , and sunlight. It is represented by the e+uation belo".

ugarcane is only '% efficient in doing photosynthesis. !o" many moles of "ill be needed by sugarcane if it needs to produce 10 moles of glucose

Key Points

 The theoretical yield is the ma#imum amount of product that can be formed by a chemical

reaction from the given amount of reactant.

 The actual yield, also called the e#perimental yield, is the real amount of product formed

from the reaction "hen the e#periment is performed.

 The ratio of actual yield to theoretical yield multiplied by 100 is the percent yield (% yield).

 *hemical reactions do not al"ays go into completion (not 100% successful). Therefore, percent

4

Question 1

If the reaction is only 20% efficient, "hat is the percent yield

Incorrect!

2

The percent yield of the reaction is 20%. It reflects the efficiency or the e#tent of completion of the reaction. 4e#t +uestion

1

3

4

Question 2

The yield is the ma#imum amount of product that can be formed from the given amount of reactant.

Incorrect!

2

The theoretical yield is calculated "ith the assumption that the reaction is 100% c omplete. 4e#t +uestion