High School Science - Limiting Reactants

Limiting Reactants

Objective Students will determine the limiting reactant from a given amount of reactants by applying the law of conservation of matter. Students will show attainment of the objective by answering a series of limiting reactant problems and creating their own limiting reactant problem.

California Content Standard 1) The conservation of atoms in chemical reactions leads to the principle of conservation of matter and the ability to calculate the mass of products and reactants. As a basis for understanding this concept: a) Students know how to describe chemical reactions by writing balanced equations e) Students know how to calculate the masses of reactants and products in a chemical reaction from the mass of one of the reactants or products and the relevant atomic masses.

Introduction The Port of Long Beach is one of the world's busiest seaports, a leading gateway for trade between the United States and Asia. It supports millions of jobs nationally and provides consumers and businesses with billions of dollars in goods each year. East Asian trade accounts for more than 90% of the shipments through the port. Some of the top imports include petroleum, electronics, plastics, furniture, and clothing. Some of the top exports include petroleum & petroleum coke, waste paper, chemicals, scrap metal, and plastic. In this lesson, students will learn how to use a balanced equation to determine the ratio between the quantity of reactants by using an analogy involving the exporting of chemicals through the Port of Long Beach.

Anticipatory Set Anytime a product is assembled from parts, one missing part will prevent a complete product from being assembled. This is true when building bicycles, making sandwiches, and performing chemical reactions.

Anticipatory Set (continued) Suppose you work in a sandwich shop and you received a rush order for 1000 turkey and cheese sandwiches. (It requires 2 slices of bread, 1 slice of cheese, and 3 slices of turkey to make 1 sandwich.) 2B + 1C + 3T



B2CT3

Anticipatory Set (continued) Can you fulfill this order if your sandwich shop is stocked with 2600 slices of bread, 1200 slices of cheese, and 2700 slices of turkey?

How many sandwiches can you make?

Anticipatory Set (continued) Quantities are not always given in number of items. Suppose your deli is stocked with: • 150 loaves of bread (1 loaf has 20 slices) • 20 pounds of cheese (1 lbs. has 40 slices) • 60 lbs. of turkey (1 lbs. has 20 slices).

Anticipatory Set (continued) • How many whole sandwiches can you make? • Which ingredient ran out? • How many slices of excess ingredients remain?

Input When a chemical reaction is performed in the laboratory, chemists often supply the necessary amount of reactants so that one reactant will not run out before the other. When chemicals are mixed together in these quantities, the mixture is said to be stoichiometric. However, stoichiometric mixtures are not always practical or desired for every reaction.

Port Analogy Suppose the Port of Long Beach exports two containers holding chemicals (A and B) to be assembled oversees. The chemical to be assembled has a molecular formula of AB2, that is it takes one A and two B’s to produce one AB2

Skeleton Equation Chemists use what is called a skeleton equation to illustrate the bare bones of a chemical reaction. The skeleton equation for the this reaction is: A

+

B



AB2

Balanced Equation In a chemical reaction, atoms are neither created nor destroyed. In a balanced chemical equation there must be the same number of each type of atom on the reactant and product sides of the arrow. The balanced equation for this reaction is: A

+

2B



AB2

Using a Balanced Equation If the port exports 1000 ‘A’ containers and 1000 ‘B’ containers, how many ‘AB2’ containers can be assembled? As you probably realized, the amount of products that could be assembled is based on the amount of reactants supplied. Since the product requires 2 ‘B’s for every 1 ‘A’, 1000 ‘A’ containers would require 2000 ‘B’ containers. In this case ‘B’ would run out before ‘A’. The reactant that runs out first in a chemical reaction is called the limiting reactant.

Use of the Limiting Reactant What does this information tell us? It tells us how much product can be made. If one of the reactants runs out, no more product can be made. Calculating the amount of product produced must be done with the limiting reactant. Therefore, 500 products can be made from this reaction.

1A 1000 B × = 500 AB2 2B The reactant that remains is called the excess reactant

How to Identify the Limiting Reactant Identify the limiting reactant when 500 ‘A’ containers reacts with 1200 ‘B’ containers. The solution to this problem can be found in many ways. Here is an example of one of those ways. • Choose a reactant (either A or B). • Use the quantity of that reactant to find out how much of the other reactant is needed.

500 A

2B × 1A

= 1000 B containers required

Since you have 1200 ‘B’ containers and only need 1000 ‘B’ containers, this is the excess reactant. Container ‘A’ must be the limiting reactant.

Or

1200 B

1A × 2B

=

600 A containers required

Since you have 500 ‘A’ containers and need a total of 600 ‘A’ containers, this is the limiting reactant. Container ‘B’ must be in excess.

As you can see, the limiting reactant can be identified from either reactant and the ratio between the reactants. The ratios used in this exercise come from the coefficients used to balance the equation. A + 2B  AB2

1A 2B

2B 1A

and

Guided Practice Question 1 For the reaction shown, find the limiting reactant when 0.90 mol of Cr reacts with 0.60 mol of O2. 4Cr

+

3O2



2Cr2O3

0.90 mol Cr x 3 mol O2 = 0.68 mol O2 required 4 mol Cr You have only 0.60 mol O2, therefore O2 is the limiting reactant.

Guided Practice Question 2 Find the limiting reactant when 5.0 mol Mg reacts with 4.0 mol O2. 2Mg

+

O2



2MgO

Guided Practice Question 3 Calculate the amount of moles of Al needed to react completely with 5.0 mol CuCl2. (The equation below is not balanced.) Al 2Al

+

CuCl2 

Cu + AlCl3

+ 3CuCl2  3Cu + 2AlCl3

5.0 mol CuCl2 x 2 mol Al 3 mol CuCl2

=

3.3 mol Al

Guided Practice Question 4 Identify the limiting reactant and calculate how much excess reactant will remain when 2.0 mol Na reacts with 1.0 mol O2? (The equation below is not balanced.) 4Na

+

O2



2Na2O

Guided Practice Question 5 Write the mole ratio that you would use to calculate the amount of oxygen needed to react with methane. (The equation below is not balanced.) CH4

+

O2



CO2

+

H2O

Closing Activity Reread the anticipatory set and write a scenario that involves a limiting reactant similar to the sandwich analogy. Write your own scenario involving a limiting reactant and exchange your problem with another classmate. Answer their question, exchange again and grade their response.