Limiting Reactants

CHEM XXX

Learning Objectives

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• Define the concept of a limiting reactant and differential it from an excess reactant
• Show how the limiting reactant determines the amount of a product formed in a chemical reaction
• Utilize the stoichiometry of the chemical equation
• Calculate the theoretical yield based upon the limiting reactant
• Determine the amount of the excess reactant remaining

A Recipe

Our balanced chemical equation is much like a recipe. The reaction will be controlled by the amounts of the starting ingredients.

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Let’s make Soda Bread

Our Recipe:

1 Can of Club Soda

3 Tbsp Sugar

3 Cups flour

We can write like a chemical equation

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1 Can + 3 Tbsp Sugar + 3 Cups Flour

Yields 1 Soda bread

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How many soda breads can we make?

Our balanced equation -

1 Can + 3 Tbsp Sugar + 3 Cups Flour Yields 1 Soda bread

Which ingredient limits the number of soda breads?

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Activity - Let’s Make Sandwiches - pHET Simulation

Simulation Instructions

Step 1 – Click on the Link to run the simulation game (CLICK ON THE GAME ICON) (https://phet.colorado.edu/sims/html/reactants-products-and-leftovers/latest/reactants-products-and-leftovers_all.html) Note a link to the game is provided in Blackboard as well.

Step 2 – Play all levels of the Game with “nothing” hidden and record your scores. (Share your high scores with your colleagues in class!) Play a few practice rounds to learn the game! While you are playing, take notes about aspects of the game that are helping you understand the relationship between reactants and products.

Step 3 – Play all levels of the Game with “molecules” hidden and record your scores. (Again, share your high scores with your colleagues in the class!) While you are playing, take notes about aspects of the game that are helping you understand the relationship between reactants and products.

The goals for the activity are:

1. Predict the amounts of products and leftovers after reaction using the concept of limiting reactant

2. Predict the initial amounts of reactants given the amount of products and leftovers using the concept of limiting reactant

3. Translate from symbolic (chemical formula) to molecular (pictorial) representations of matter

4. Explain how subscripts and coefficients are used to solve limiting reactant problems.

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Activity - Let’s Build Rovers - Card Game

• Cards (enough for 4-6 players):
• Body (4 cards)
• Brain (computer) (4 cards)
• Directional Sensor (16 cards) - 4 sets of 4 (North, South, East, West)
• Wheel & Gear Set (16 cards) - 4 sets of 4

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The winner is the first person to complete a rover. A complete rover will have 1 Body, 1 Brain, 4 Sensors, and 4 Wheels.

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To Play

1. Preparation: Shuffle all cards and deal 5 cards face down to each player. Place the remaining deck face down in the center as a drawing pile.
2. Turns: On their turn, a player takes two actions:
• Draw: Draw two cards from the deck.
• Play/Discard:
• Play a set of four matching cards (Body, Brain, Directional Sensor set, or Wheel & Gear Set) to the discard pile. This indicates they have built that component of their rover.
• Discard any unwanted cards to the discard pile.

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Understand Limiting Reactants using a card game to build a Mars Rover.

Now that you have some experience

Na₂CO₃ + 2 AgNO₃ → Ag₂CO₃ + 2 NaNO₃

Let’s do it with a chemical reaction

We are given 5.00 grams of both sodium carbonate and silver nitrate. What how many grams of silver carbonate will be produced? What is the limiting reactant?

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Steps to solve

1st - Verify that you have a balanced equation.

2nd - Determine the number of moles of the starting materials.

3rd - Compare the available moles to the required moles.

4th - Identify the limiting reactant.

5th - Determine the theoretical yield based upon the reaction.

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Steps to solve

1st - Verify that you have a balanced equation.

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Na₂CO₃ + 2 AgNO₃ → Ag₂CO₃ + 2 NaNO₃

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2nd - Determine the number of moles of the starting materials.

Calculate the mw of the individual components.

Na₂CO₃ 105.99 grams/mole

AgNO₃ 169.91 grams/mole

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Steps to solve

1st - Verify that you have a balanced equation.

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Na₂CO₃ + 2 AgNO₃ → Ag₂CO₃ + 2 NaNO₃

2nd - Determine the number of moles of the starting materials.

Calculate the mw of the individual components.

Na₂CO₃ 105.99 grams/mole

AgNO₃ 169.91 grams/mole

Calculate the moles of the individual components.

Na₂CO₃ 0.0472 moles

AgNO₃ 0.0294 moles

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Steps to solve

Determine the number of moles of the starting materials.

Na₂CO₃ 0.0472 moles

AgNO₃ 0.0294 moles

3rd - Compare the available moles to the required moles.

Na₂CO₃ + 2 AgNO₃ → Ag₂CO₃ + 2 NaNO₃

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We need 2 moles AgNO₃of for every 1 mole of Na₂CO₃

But we have 1.6 times more Na₂CO₃ than AgNO₃ - not correct ratio.

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Steps to solve

4th - Identify the limiting reactant.

We need 2 moles AgNO₃of for every 1 mole of Na₂CO₃

We have:

Na₂CO₃ 0.0472 moles

AgNO₃ 0.0294 moles

We have 1.6 times more Na₂CO₃ than AgNO₃ - not correct ratio.

And we need more of the AgNO₃ which means that AgNO₃ is the limiting reactant.

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Steps to solve

5th - Determine the theoretical yield based upon the reaction.

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Na₂CO₃ + 2 AgNO₃ → Ag₂CO₃ + 2 NaNO₃

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For every 2 moles of AgNO₃ we product 1 mole of Ag₂CO₃.

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We have 0.0294 moles of AgNO₃ thus, can only produce 0.0147 moles of Ag₂CO₃ or 4.05 grams of Ag₂CO₃. This is the theoretical yield.

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