Study Buddy: Stoichiometry 1: Theoretical
Terms
Description
Relative quantities of reactants, products and
energy in a chemical reaction
Diagram/Example
Stoichiometry
http://en.wikipedia.org/wiki/Stoichiometry
Theoretical Yield
Reaction:
A + B  C
Theoretical:
5g + 5g  10g
In the lab, 5g of each reactant were
used and the reaction yielded 8g of
C. (Actual yield)
Calculated amount of reaction products
Amount of product determined by laboratory
experiment
Ratio of amount of product from a reaction
Percent Yield
compared to the calculated yield
Representations:
Actual Yield
1. For the reaction
a. Draw a particle diagram to represent the reaction
% Yield: 8g/10g x 100% = 80% yield
2HCl
+
+
Mg

H2

+
MgCl2
+
b. Circle the ratios below are invalid for this reaction, and explain why they are not correct.
2 moles HCl
1 atom Mg
1 mole Mg
1 molecule H
1 mole MgCl
1 molecule H
1) 1 mole H
2) 1 mole MgCl
3) 1 mole H 4) 1 atom Mg 2 5) 1 mole HCl 2 6) 1 mole MgCl2
2
2
2
#2 is invalid because one atom of Mg would be proportional to one formula unit of MgCl 2, not
a mole. The “scale” of the two units does not match.
#3 is invalid because the subscript on Hydrogen was left off.
#5 is invalid because the coefficient of HCl means each mole of MgCl2 would be proportional
to 2 moles of HCl.
#6 is invalid because the scale of the units does not match.
2. a. Rank the amounts of oxygen gas (O2) in increasing order: 1.2x1024 molecules, 32g, 1.5 moles
least amount: __32g__, ___1.5 moles___, __1.2x1024 molecules__: most amount
b. Rank the number of particles in 100g of: NaCl, CaCO3, FeCl3, MnO2
least amount: __FeCl3__, __CaCO3__, __MnO2__, __NaCl __: most amount
3. When potassium chlorate is heated it decomposes into potassium chloride and oxygen
__2_KClO3(s)  __2__KCl(s) + __3__O2(g)
2 moles KClO3= 245 g
2 moles KCl= 149 g
Students performed this reaction in class and obtained 85g of KCl.
a. How many grams of KCl should be produced from 165 grams of KClO3?
Note: Students can use ratios or dimensional analysis to solve this type of problem.
x g KCl
165 g KClO
= 245 g KClO3
x = 100 g KCl
149 g KCl
3
b. Circle the theoretical yield. How would a student know this is the theoretical yield?
The amount of expected yield from the reaction was calculated.
c. Place a box around the actual yield. How would a student know this is the actual yield?
The actual yield either comes from a lab or has to be given.
d. What is the percent yield of potassium chloride for this reaction?
Percent Yield=(
actual yield
theoretical yield
)(100)
Percent Yield=(
85 g KCl
100 g KCl
)(100) = 85% yield KCl
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Study Buddy: Stoichiometry 2: Applications
Terms
Description
Substance added in excess and partially
Excess Reactant consumed in a reaction
Diagram/Example
Excess =
Substance totally consumed in a reaction and Limiting =
Limiting Reactant limits the amount of products formed
http://stoichiometricequiv.blogspot.com/2013/04/limitingreactantreagent.html
Representations:
1.
_1_ CH4(g) + _2_O2(g)  _1_CO2(g) + _2_H2O(l) + 891 kJ
1 mol CH4 = 16 g
2 mol O2 = 64 g
a. When using natural gas (mostly methane) to cook, which reactant is limiting and why?
Normally methane is limiting because without oxygen humans cannot breathe and there are
many more oxygen particles in a kitchen than those with which the methane can react.
b. In a closed container, 100.g of methane and 100.g of oxygen gas are ignited by a spark.
Which reactant is limiting and how do you know?
Students could answer from calculations. However, from the balanced equation 64 g of O 2 is
equivalent to 2 moles, which reacts with 1 mole of CH4 at 16 g. Based on the reaction ratio of
16g CH4: 64g O2, the reaction will run out of oxygen before methane.
c. Draw particles to represent this reaction in the containers.
Key:
spark
Reactants Before Reaction
Carbon
Hydrogen
Oxygen
Products After Reaction
d. How many liters of carbon dioxide can be produced?
From the balanced equation: 1 mole CO2 = 22.4 L CO2
x L CO2
100. g O
= 64 g O 2
x = 35 L CO2
22.4 L CO
2
2
e. How many grams of the excess reactant will be used in the combustion?
x g CH4
100. g O
= 64 g O 2
x = 25 g CH4
16 g CH
4
2
2. What can cause the percent yield for a reaction to be less than 100%, other than human error?
Especially with gaseous products, some of the products may not be collected from a reaction.
3. Draw particle diagrams to represent this reaction:
_2_ KI (aq) + _1_ Pb(NO3)2 (aq)  _1_ PbI2 (s) + _2_ KNO3 (aq)
Key:
K
I
Pb
NO3
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Precipitate
of PbI2
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Units: Stoichiometry 1 & 2: Theoretical and Applications
Planning Considerations
Content:
Unit side-by-side (show spiraling)
Unit 1 (PROP)
Mole concept
Avogadro’s number
Significant figures, scientific notation, units
Particle diagrams for physical and chemical changes
Unit 2 (STATES)
Avogadro’s Law
Unit 5 (SUBMIX)
Molarity and calculations
Unit 9 (BOND)
Identify type(s) of bonding present
Lewis valence electron dot structures
Unit 10 (FORM)
Molar mass of a compound
Unit conversions for a compound
Laws of definite and multiple proportions
Unit 11 (ENRXN)
Chemical and physical changes
Writing chemical formulas
Unit 12 (CLASSRXN)
Conservation of mass through particle diagrams and balanced equations
Identify reactions as precipitation or redox
Precipitates and solubility rules
Possible flow – conceptual development flow rationale
Meaning of chemical equation coefficients  Quantities in a chemical reaction  Excess
and limiting reactants  Gas stoichiometry  Reactions in aqueous solution
This set of two units (STOICH1 and STOICH2) begins with student previous knowledge of
properties associated with chemical changes and the importance of balanced chemical
reactions. Particle diagrams and simulations are valuable for students to understand
interactions between particles and visualize chemical processes at the particulate level.
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This is also an opportunity to revisit student weaknesses in a new context for content from
many previous units if they have not mastered the content yet.
Students will revisit unit conversions for a single compound, then expand this to
conversions between compounds in a balanced chemical equation. Before performing
stoichiometry calculations, students should understand the relationships in shown in
chemical equations at the particulate level. Students should be able to translate between
particle diagrams, chemical equations and the physical reactions they represent and
explain the information contained within each. As much as possible, students should
perform reactions that are being discussed so they have a deeper understanding and can
connect the reactions they are performing with the relevance chemical equations and
particle diagrams. Once students understand conceptually, the additional layer of
computation can be added on top using ratios and/or dimensional analysis.
Students can approach stoichiometry calculations many different ways and should be free
to use logical methods for arriving at answers. Dimensional analysis and ratios are two
common methods used for calculations, but as long as students can justify and explain
their process and it is scientifically valid then the method of arriving at an answer is not of
key importance. For the calculations, use examples of reactions that go (or almost go) to
completion; the TEKS do not include the teaching of chemical equilibrium or reversible
reactions. Instruction and calculations should focus towards the understanding of the
relationships between quantities (mole to mole ratio, mass to mole ratio, etc.) within a
chemical reaction and use whole number multiples or quantities especially for L- level
students when performing stoichiometry calculations. Stoichiometry is a supporting TEKS
to the mole concept and law of conservation of mass readiness TEKS. Stoichiometry in
this way is viewed as an extension of the law of conservation of mass and the mole
concept to account for quantities in chemical reactions.
STOICH2 begins with the concepts of limiting and excess reactants because now that
students can convert between quantities of different chemicals in a reaction they can use
this ability to determine which reactant will limit formation of products. This should be
accomplished using particle diagrams and calculations to support each other. Once
students have mastered determining the limiting and excess reactants, this layer can be
added on top of stoichiometry calculations and students asked to determine from
amounts of the reactants the amount of products that can be made, amount of products
consumed, etc.
The remainder of STOICH2 concerns the application of stoichiometry principles to
reactions that produce gases and those that produce precipitates. As with the rest of the
units, student calculations should be grounded in reactions they have physically
performed and made measurements themselves. Use of gas law calculations should be
to support students’ conceptual understanding of relationships being observed in
reactions, especially if non-STP calculations need to be considered. Gas laws use and
calculations should emphasize real-world relevance.
Precipitation reactions provide another venue for students to apply their knowledge of
limiting and excess reactants and an opportunity to measure the percent yield of a solid
product from a laboratory experience.
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Math/algebra
Conversions between units for a single compound (g, mol, L, molecule, particles)
Conversions between units for different compounds (dimensional analysis and/or ratios)
Percent yield
Limiting and excess reactants
Molarity
Misconceptions:




Not distinguishing between grams and moles.
Not distinguishing between coefficients and subscripts and their physical meanings.
That all chemical reactions go to completion.
Not distinguishing between experimental and theoretical yield.
WEB resources:
Classroom Resources (Can be used in the classroom for instruction)





Limiting Reactant Animation
(http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/limitr15.swf)
PhET: Reactants Products and Leftovers (http://phet.colorado.edu/en/simulation/reactantsproducts-and-leftovers)
Stoichiometry Applet (http://chemcollective.org/activities/simulations/stoich)
Introduction to Limiting & Excess Reactant
(http://islcs.ncsa.illinois.edu/media/com_resource/docs/Intro_to_Limiting_Reactants.pdf)
Pearson
 Avagadro’s Cookies Untamed Science Video
Student Resources (Students can use for review at home)



Pearson
 Interpreting a Balanced Chemical Equation Interactivity
 Balanced Equation as a Recipe Student Tutorial
 Calculating the Mass of a Product Tutorial
 How Do You Fill an Air Bag? Pearson Flipped Video for Science
 Calculating Percent Yield Pearson Flipped Video for Science
YouTube
 Bozeman Science: Stoichiometry (https://www.youtube.com/watch?v=LQq203gyftA)
 Bozeman Science: Limiting Reactant and Percent Yield
(https://www.youtube.com/watch?v=LicEaaXhlEY)
 Crash Course Chemistry: Stoichiometry (https://www.youtube.com/watch?v=UL1jmJaUkaQ)
Khan Academy Stoichiometry Practice Problems
(https://www.khanacademy.org/science/chemistry/chemical-reactions-stoichiome/stoichiometryideal/e/ideal_stoichiometry)
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POGIL:
Flinn Scientific POGIL Activities for High School Chemistry (2012)
Relative Mass and the Mole; Page 161
Mole Ratios; Page 169
Limiting and Excess Reactants; Page 175
PD Support:
Share sessions, Team meetings
Released STAAR/EOC Questions: 9, 12, 18, 27, 35
8th Grade STAAR Questions: none
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