Percent Yield and Limiting Reactants

Advanced Chemistry

Ms. Grobsky

Percent Yield In theory, when a teacher gives an exam

to the class, every student should get a grade of 100%

Your exam grade, expressed as a percent, is a quantity that shows how well you did on the exam compared with how well you could have done if you had answered all questions correctly

Percent Yield

You might have assumed that if we use stoichiometry to calculate that our reaction will produce 5.2 g of product, that we will actually recover 5.2 g of product in the lab

This assumption is as faulty as assuming that all students will score 100% on an exam

Percent Yield

When an equation is used to calculate the amount of product that is possible during a reaction, a value representing the theoretical yield is obtained

The theoretical yield is the maximum amount of product that could be formed from given amounts of reactants

In contrast, the amount of product that forms when the reactionn is carried out in the lab is called the actual yield

The actual yield is often less than the theoretical yield

A. Percent Yield

100yield ltheoretica

yield actualyield %

calculated on paper

measured in lab

The percent yield is the ratio of the actual yield to the theoretical yield as a percentIt measures the measures the

efficiency of the reaction What causes a percent yield to be

less than 100%?

Why The Chemical World Isn’t Perfect

Reactions don’t always go to completionWhen this occurs, less than the expected

amount of product is formedImpure reactants and competing side

reactions may cause unwanted products to form

Actual yield can also be lower than the theoretical yield due to a loss of product during filtration or transferring between containers

If a wet precipitate is recovered, it might mass heavy due to incomplete drying, etc.

Limiting Reactants

What Is a Limiting Reagent?

Many cooks follow a recipe when making a new dish

When a cook prepares to cook he/she needs to know that sufficient amounts of all the ingredients are available

Think back to page 126 Were there stipulations to how many MacChem

sandwiches you could make? Why?

Limiting Reactants

We can’t make any more MacChem sandwiches than the number of HambChem sandwiches we haveThe number of HambChems limits the

number of MacChems we can make If one of our ingredients gets used up during

our preparation it is called the limiting reactant (LR)

The LR limits the amount of product we can form; in this case, MacChems

It is equally impossible for a chemist to make a certain amount of a desired compound if there isn’t enough of one of the reactants

Limiting Reactants

As we’ve been learning, a balanced chemical reaction is a chemist’s recipe.This allows the chemist to predict

the amount of product formed from the amounts of ingredients available

Let’s look at the reaction equation for the formation of ammonia:

N2(g) + 3H2(g) 2NH3(g)

• When 1 mole of N2 reacts with 3 moles of H2, 2 moles of NH3 are produced

• How much NH3 could be made if 2 moles of N2 were reacted with 3 moles of H2?

Limiting Reactant

The amount of H2 limits the amount of NH3 that can be madeFrom the amount of N2 available we

can make 4 moles of NH3From the amount of H2 available we

can only make 2 moles of NH3H2 is our limiting reactant here.

It runs out before the N2 is used upTherefore, at the end of the reaction

there should be N2 left overWhen there is reactant left over it is

said to be in excess

Limiting Reactant

How much N2 will be left over after the reaction?In our reaction, it takes 1 mol of N2 to react

all of 3 mols of H2, so there must be 1 mol of N2 that remains unreacted

We can use our new stoich calculation skills to determine 3 possible types of LR type calculations.

1. Determine which of the reactants will run out first (limiting reactant)

2. Determine amount of product 3. Determine how much excess reactant is

wasted

Steps to Calculate Limiting Reactants

Limiting Reactant Steps

1 Step oneo Write and balance the equation for the reaction.

2 Step twoo Convert known masses to grams of product.

4 Step threeo Determine limiting reactant and amount that can

be made.5 Step four

o Determine the grams of excess from limiting reactant.

Limiting Reactants Using Moles

When 4.00 mol H2 is mixed with 2.00 mol Cl2,how many moles of HCl can form?

H2(g) + Cl(g) 2HCl (g)

4.00 mol 2.00 mol ??? mol

• Calculate the moles of product from each reactant, H2 and Cl2.

• The limiting reactant is the one that produces the smaller amount of product.

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Limiting Reactants Using Moles

HCl from H2

4.00 mol H2 x 2 mol HCl = 8.00 mol HCl

1 mol H2 (not possible)

HCl from Cl2

2.00 mol Cl2 x 2 mol HCl = 4.00 mol HCl

1 mol Cl2 (smaller number)

The limiting reactant is Cl2 because it is used up first. Thus Cl2 produces the smaller number of moles of HCl.

Checking Calculations

Initially H2

4.00 mol

Cl2 2.00 mol

2HCl 0 mol

Reacted/Formed

-2.00 mol -2.00 mol +4.00 mol

Left after reaction

2.00 mol Excess

0 mol Limiting

4.00 mol

Limiting Reactants 79.1 g of zinc react with 0.90 L of 2.5M HCl.

Identify the limiting and excess reactants. How many liters of hydrogen are formed at STP?

Zn + 2HCl ZnCl2 + H2

79.1 g ? L0.90 L2.5M

Limiting Reactants

79.1g Zn

1 molZn

65.39g Zn

= 27.1 L H2

1 molH2

1 molZn

22.4 LH2

1 molH2

Zn + 2HCl ZnCl2 + H2 79.1 g ? L0.90 L

2.5M

Limiting Reactants

22.4L H2

1 molH2

0.90L

2.5 molHCl

1 L= 25 L

H2

1 molH2

2 molHCl

Zn + 2HCl ZnCl2 + H2 79.1 g ? L0.90 L

2.5M

Limiting Reactants

Zn: 27.1 L H2

Limiting reactant: HCl

Excess reactant: Zn

Product Formed: 25 L H2

Left over zinc

HCl: 25 L H2

Steps in Calculating Amount of Excess Reactant Left Over

Step One Calculate mass of non-limiting reactant used up

from ACTUAL moles of product produced Stoichiometric calculation of:

Moles A →Moles B →Mass B• Step Two

• Subtract mass from step one from initial mass