Limits and Excess

Apr 17 2011

Source: coffeeholicat

Summary Information

Keywords:

oxidation reduction, water, copper chloride dihydrate, aluminum, chemical reaction, solution, limiting reactants, excess reactants, stoichiometry

Topic:

Chemistry

Target Grade (Ages):

Grade 7 (Ages 12-13)

Grade 8 (Ages 13-14)

Estimated Time Required:

less than 1 hour

Diversity Indicators:

This is a high school lesson and should be used for higher level learners.

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LESSON

Goals and Prerequisites

Goals:

To use algebra and mole-mole ratios to theoretically discover the limiting reactant and excess of the chemical reaction.
To successfully conduct an experiment to discover the limiting reactant and excess of the chemical reaction.

Prerequisites:

H.S. Algebra

Instructional Objective(s)

Balance equations given the formulas for reactants and products
Interpret balanced chemical equations in terms of conservation of matter and energy
Create and use models of particles to demonstrate balanced equations
Calculate simple mole-mole stoichiometry problems, given a balanced equation
Determine the number of moles of a given substance, given its mass
Determine the mass of a given number of moles of a substance

Instructional Resources

Background:

Stoichiometry is the branch of chemistry that deals with the calculation of the quantities of reactantsand products involved in a chemical reaction.

A mole is the quantity of a given substance that contains as many molecules or formula units as the number of atoms in exactly 12 grams of C-12.

The limiting reactant is the reactant that is entirely consumed when a reaction goes to completion. The excess reactant is the reactant that is not completely consumed, and is left over. The moles of the product are always determined by the starting moles of the limiting reactant.

Materials:

Limits and Excess Lesson PPT
250 mL beaker
100 mL graduated cylinder
Spoon or stirring rod
Watch glass
Copper (II) chloride
Aluminum foil
Water

Limits and Excess Lesson (ppt)

Procedure:

Facilitate the following procedures for students. Circulate around the laboratory to ensure safety of all students, as well as to make sure they are conducting the experiment without tremendous error.

Pre-Lab (homework from the night before)

Ask students to consider the following unbalanced chemical reaction:
- aluminum + copper (II) chloride dihydrate àaluminum chloride + copper + water
- Aluminum metal reacts with aqueous copper (II) chloride dihydrate to form copper metal, aqueous aluminum chloride, and water. Write the correct balanced equation for this reaction.
What does a solution look like when it contains dissolved copper (II) ions?
For the balanced reaction, determine the mass of aluminum that would remain after a 10.00-g Al sample and 1.00 g of CuCl₂·2H₂O fully reacted. Show your work fully.

Lab

Tell students that they must answer the following question: What are the limiting and excess reactants of our lab experiment?
Students should compare pre-lab homework answers with their lab group. Discuss among group members if there are discrepancies.
Gather all the materials needed for the lab, and begin your experiments!

Note: Usual lab groups should have picked out a number from a hat during the previous day. The number indicates the amount of aluminum and copper (II) chloride dihydrate (in grams) they will be working with in the lab.

Using the chart below, select your group number and the mass of aluminum metal and copper (II) chloride dihydrate that you will be using in this activity.

Group	Mass of Al (g)	Mass of CuCl₂·2H₂O (g)
1	1.00	2.00
2	0.50	2.00
3	0.50	4.00
4	0.50	6.00
5	0.25	5.00
6	0.25	4.00

Weigh out the copper (II) chloride dihydrate in a 250 mL beaker.
Using a graduated cylinder, measure out exactly 100 mL of water (tap water is fine) and add it to the beaker containing the copper (II) chloride dihydrate. Stir until the salt dissolves completely. Note the color of the solution in your lab notebook.
Weigh out the appropriate mass of aluminum foil, crumple up into a few small pieces and add it to the copper (II) chloride dihydrate solution. Stir the solution gently and then allow it to sit undisturbed for 5 – 10 minutes. Write observations in your lab notebook.
At the end of the period, label your beaker and cover it with a watch glass. Your teacher will designate an area to store your beakers overnight. In the following class time, you will be able to confirm the identity of the limiting reactant.

For Teachers

See Limits and Excess Lesson PPT for suggestions on how to present the lesson and manage class time.
Please be aware that this lab is not an original lab. It is commonly used in AP Chemistry classes as well as Honors Chemistry classes.

Safety Precautions

Wear safety goggles during this experiment. Avoid skin contact with copper (II) chloride dihydrate. Wash your hands before leaving the laboratory.
Check all the students before starting the experiment.

Limits and Excess Lesson (ppt)

Extensions

Lab Report

Include a brief introduction, data, the balanced reaction equation, and calculated results (including work). Determine the relative size of any error between your predicted post-reaction mass of the Al and its actual mass. Discuss the possible causes for errors; be certain that the causes you discuss would result in the error you experienced, i.e. the actual post-reaction aluminum mass being larger or smaller than predicted.

Remember

Percent Error = abs value (accepted value - experimental value)/(accepted value) * 100

Assessments and Rubrics

Post-Lab

Show all work in your lab notebook. Note: when calculating the molar mass of the copper (II) chloride dihydrate, don’t forget to add the mass of 2 moles of water!

Write your group number.
What is the limiting reactant in this reaction?
What is the theoretical yield (in grams) of copper metal?
What is the theoretical yield (in grams) of aluminum chloride?
What mass of the excess reactant will remain when the limiting reactant is consumed?

External Resources

There are no External Resources for this lesson.

Credits

http://www.arps.org/users/hs/thompsom/honors/unit_03/Limiting_Reactants_Lab_Activity.pdf
http://dhs.dist113.org/faculty/HintonM/website/apchem/lab-01-2.pdf
General Chemistry 8^th Edition by Ebbing and Gammon

RESOURCES

Original Resources

Limits and Excess Lesson (ppt)

Contributed Resources

Contribute a Resource

Limiting Reactant Recipe Project

Through this project, students will explore the concept of limiting reactants with something they are familiar with - recipes!

Limiting Reactant Recipe Project Student Work Examples

Presentation showing examples of work from students for Limiting Reactant Recipe Project

STANDARDS

Click here to add Standards

NYS Standards

Standard Type: State New York

3.3cBalance equations given the formulas for reactants and products (3.3 i)

Interpret balanced chemical equations in terms of conservation of matter and energy (3.3 ii)

Create and use models of particles to demonstrate balanced equations (3.3 iii)

Calculate simple mole-mole stoichiometry problems, given a balanced equation (3.3 iv)

3.3fDetermine the number of moles of a given substance, given its mass (3.3 ix)

Determine the mass of a given number of moles of a substance (3.3 vi)

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Comments

Jill Fonda

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May 19, 2012 - 1:24pm

Recipe Project

The recipe project connects what is always a very challenging concept to something real and relatable. I imagine that my students will find this assessment very engaging, and I like that it allows their personal preferences and cultures to shine through. Thank you!

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