Unit 2, Topic 5a:

An Introduction to Stoichiometry

Lesson Instructions

Review the learning goals and success criteria for Topic 5a: Stoichiometry.
- Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
- Complete the fill-in-the-blanks handouts that have been provided to you. They have been linked below.
- Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.

Learning Goals

I am learning to ...

- Predict the amount of reactants and products from a chemical reaction when the amount of a reactant is given.

Success Criteria

I am able to ...

- Predict the number of molecules and/or moles required to react with a given amount of reactant.
- Predict the number of molecules and/or moles expected as products from a given amount of reactant.
- Predict the mass of products resulting from a given amount of reactant in a chemical reaction.

Minds On

The Automotive Airbag - Stoichiometry in Action!

The safety air bag system in an automobile uses chemical reactions in order to function. When a collision occurs, a sensor indicates that the car’s momentum has changed and this triggers a series of chemical reactions that quickly produces an appropriate amount of gas in order to inflate the safety air bag. The safety air bag, in turn, works to minimize the harm to the car’s driver and passenger(s) during a collision.

(Retrieved from University of Ontario Institute of Technology, 2019).

Figure 1. Diagram showing the basic design of an automotive airbag.

The airbag apparatus contains a large inflatable cloth balloon and a precisely measured amount of chemicals, including sodium azide, NaN₃.
During impact, a sensor in the front of the car sends an electrical signal to the airbag, which causes this compound to decompose into nitrogen gas and sodium.
The Nitrogen gas that is released from this reaction rapidly inflates the airbag into the car occupants.
The airbag is made of porous fabric that allows it to deflate during its collision with the occupant.
All this has to happen in less time than it takes to blink.
The amount of sodium azide used is critical to saving the life of the occupant.
- - Too much sodium azide could burst the balloon or cause it to be too rigid.
  - Insufficient sodium azide could cause the balloon not to inflate fast enough to prevent serious injury.

Continue reading to find out how this very precise type of chemical balance is calculated using a technique called stoichiometry.

Action

What is Stoichiometry?

Stoichiometry is the study of quantitative relationships between the quantities of reactants used and products formed in a chemical reaction.

Recall, in a chemical reaction the reactants are on the left side of the reaction and the products are on the right side of the reaction, such that, Reactants → Products.

Example - Baking Cookies

- Stoichiometry relates to cookies because all of the ingredients are added/mixed together to make the cookies.
- The ingredients that are combined together are the reactants and the cookies are the product.

Take a look at the following chocolate-chip cookie recipe. Pay attention to the quantity of each ingredient required.

Figure 2. Chocolate-chip cookies are formed in a stoichiometric relationship, similar to chemical reactions.

Figure 3. The amount of ingredients used in a recipe will yield a specific quantity of products. In this recipe, 18 cookies can be made from the quantity of ingredients used.

This recipe yields 18 cookies.

Ask yourself?

- What if you wanted to make more cookies? For example, you needed to make exactly 36 cookies?
- What if you wanted to make less cookies? For example, you needed to make 9 cookies.

The ratio of ingredients must always stay the same.

Let's say Mr. Coleiro needs to bake 3 dozen cookies for his Grade 12 College Chemistry Class (they're hungry!). He has a recipe that yields 18 cookies. How will he adjust the quantity of the ingredients so that 36 cookies are made to share with his chemistry students?

You can change the amount of cookies or the quantity of products (cookies) being produced, while maintaining the ratios of the recipe.

Thus, the product always turns out to be exactly how you wanted and expected it to be.

The same is true regarding chemical reactions.

Figure 4. The original recipe is doubled. Each ingredient gets multipled by 2 in order to increase the yield from 18 cookies to 36 cookies.

The Importance of a Balanced Chemical Equation

The coefficients of the chemical equation provide the ratio of one chemical to another in the chemical equation. Being able to work with ratios is an essential skill in chemistry. It’s especially important when solving stoichiometric problems which we will examine throughout this lesson.

Molecular Models - Stoichiometric Relationships

We can examine the reaction between nitrogen gas and hydrogen gas to form ammonia gas using molecular models. One molecule of nitrogen gas reacts with three molecules of hydrogen gas to form 2 molecules of ammonia gas.

N_2(g)+ 3H_2(g)→ 2NH_3(g)

Here is a visual representation of the reaction using molecular models.

Figure 5. A visual of the molecular relationship that produces two molecules of ammonia gas.

Recall - Diatomic Elements

- Nitrogen is a diatomic element. Thus, one molecule of Nitrogen is found as N_2.This means that in one molecule of nitrogen, there are two nitrogen atoms bonded to one another.
- Hydrogen is also a diatomic element, H₂. Each hydrogen molecule consists of two hydrogen atoms bonded to one another.

"I work for an industrial chemical company and require exactly 6 moles of ammonia for a new cleaning product being developed. How many molecules of nitrogen are hydrogen needed to react to yield my desired product?"

We know that the reaction that produces ammonia from nitrogen and hydrogen gas is N_2(g)+ 3H_2(g)→ 2NH_3(g)_.

Using what we have just learned about mole ratios, by examining this reaction we can conclude that ...

- There is a 1:2 ratio between nitrogen gas and ammonia gas. 1 mole of nitrogen gas is required to produce 2 moles of ammonia gas.
- There is a 3:2 ratio between hydrogen gas and ammonia gas. 3 moles of hydrogen gas are required to produce 2 moles of ammonia gas.

Thus, if we want to produce 6 moles of ammonia gas for the new cleaning product being developed, we need to look at the relationship between our current yield and our desired yield.

- With our current reaction of N_2(g)+ 3H_2(g)→ 2NH_3(g)we can produce 2 moles of ammonia gas. However, we would like to produce 6 moles of ammonia gas.
- Therefore, we need triple the amount of ammonia gas. Where did I get this number? 6 (desired yield) ÷ 2 (our current yield) = 3 (the number we need to multiply each coefficient by).

So, we now know that in order to produce 6 moles of ammonia gas, we need 3 times the amount of each entity in the reaction. This means that every coefficient in the reaction needs to be multiplied by 3 in order to produce 6 moles of ammonia gas. Note, this will follow the law of conservation of mass.

- 3 x (N_2(g)+ 3H_2(g)→ 2NH_3(g)) = 3N_2(g)+ 9H_2(g)→ 6NH_3(g)

Therefore, in order to produce 6 moles of ammonia gas for the new cleaning product being developed, we would require 3 moles of nitrogen gas and 9 moles of hydrogen gas.

Could I have arrived at this answer a different way? Yes. I could have just multiplied the mole ratios that we developed above.

- 3 x (1 mole nitrogen gas : 2 moles ammonia gas) = 3 moles nitrogen gas : 6 moles ammonia gas.
- 3 x (3 moles hydrogen gas : 2 moles ammonia gas) = 9 moles hydrogen gas : 6 moles ammonia gas.

Calculations Using Stoichiometry

Example 1. How many grams of water are produced when 16.4 g of oxygen gas react?

Example 2. How many grams of water must react if you want to produce 45 g of PH₃?

Example 3. What mass of silver chloride, AgCl_(s), will precipitate when 36 mL of 0.25 mol/L magnesium chloride, MgCl_{2 (aq),}reacts?

Example 4. If 6.06 g of hydrogen gas react, how many molecules of water would be produced?

Consolidation

There are a few tasks to complete in order to consolidate your understanding of stoichiometry.

(1) Practice Questions

Using the textbook, complete the following textbook questions:

- Page 214 #2-3
- Page 219 #5-6, #8-9

(2) Making an Analogy

In this lesson I made the analogy between mole ratios in a balanced chemical equation to the ingredients in a chocolate chip cookie recipe.

On D2L, under the DISCUSSION page, you will create and share your own analogy for stoichiometry using a real life example. You will ...

- Briefly summarize an analogy that helps demonstrate your understanding of stoichiometry and mole ratios.
- Provide a detailed example. For example, if you talk about the recipe for a ham sandwich, make sure to list all the ingredients in the original recipe and how they would change in the altered recipe.
- You can pick more than just a recipe. You can examine anything that is quantity based (computer components, assembling a car, parts of a shoe, etc.). Be creative.
- This will be evaluated under the communication component of your final grade.

You must comment on one of your classmates analogies. Don't just say "good job" or "cool". Put some effort into your comments. Your comment should mention the usefulness or effectiveness of your classmates analogy.

(3) Summary Quiz

Complete the Unit 2, Topic 5a: Intro to Stoichiometry quiz on D2L.

This lesson is now complete. Return to D2L - Brightspace and complete the assigned tasks to consolidate your learning.

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