Steady State Slides REACTION DYNAMICS
Steady State
Visualizing Dynamic Equilibrium
Chemistry Unit 9
The One-Way Street
Most reactions we study go to completion.
Reactants turn into Products.
The reaction stops when reactants run out.
Symbol: A + B → C + D
🔥
Example: Combustion
Once the wood is ash, it doesn't spontaneously turn back into a log!
The Two-Way Street
Reversible Reactions
Some reactions can go backwards. Products can collide and reform the reactants.
REACTANTS
↔
Reversible Arrow
PRODUCTS
Forward Rate
Reactants → Products
Reverse Rate
Products → Reactants
Dynamic Equilibrium
A state where the Forward Rate exactly matches the Reverse Rate.
CONTINUOUS
The reaction has NOT stopped! Both directions still happen.
BALANCED
Concentrations stay constant, but particles keep moving.
NO CHANGE
To our eyes, nothing looks like it is changing.
🧪
The Phenolphthalein System
Chemical Indicator Equilibrium:
\[ \text{InH}_2 \rightleftharpoons \text{In}^{2-} + 2\text{H}^+ \]
Colorless Bright Pink
Add Base: Reaction shifts right to make more products (Pink!).
Add Acid: Reaction shifts left to restore reactants (Clear!).
Acidic
↔
Basic
Modeling the Motion
Think: The Escalator
If you walk up a descending escalator at the exact same speed it moves down, what happens?
1 You are still moving (Dynamic).
2 The escalator is still moving (Dynamic).
3 Your position stays constant (Equilibrium).
Rate vs Time Graph
Rate
Time
Equilibrium Reached
Forward
Reverse
The Particle Level
Initial
Reactants only. Forward rate is at its highest.
Reacting
Products form. Reverse rate starts to pick up speed.
Equilibrium
Rates are equal. No net change in the number of particles.
Exit Ticket Prep
Critical Thinking
If you look at a beaker in equilibrium, does it look like it is "doing" anything? Why or why not? Use the word dynamic in your explanation.
Hint: Consider macroscopic vs microscopic levels.
Phenol Flip Activity Phenol Flip Activity
Dynamic Equilibrium & Reversibility
Name:
Date:
TEKS: C.9A, C.13A
Focus: Reversible Reactions & Particle Modeling
1
The Reversible System
In a reversible reaction , reactants turn into products, but those products can also collide and reform the original reactants. Today, we investigate phenolphthalein, which shifts between colorless and pink forms.
InH2 (Colorless) ⇌ In2- (Pink) + 2H+
Chemical Logic: Adding Base (OH-) removes the H+ ions through neutralization. This "stresses" the system, causing the forward reaction to speed up to replace the missing H+, which turns the solution pink .
Adding Acid (H+) adds more product. This causes the reverse reaction to speed up to use up the extra H+, turning it colorless .
2
Macroscopic Observations
Action Taken Observation (Color & Clarity) 1. Add indicator to 10mL water 2. Add 2 drops of NaOH (Base) 3. Add 5 drops of HCl (Acid) 4. Add 10 drops of NaOH (Base)
3
Microscopic Particle Modeling
Draw a particle model for each stage. Use Circles (○) for Colorless (InH2) and Stars (★) for Pink (In2-). Use arrows to show the rate (size/number of arrows = speed).
State A: Just Base Added
Pink form is low. Forward rate is fast.
State B: Reacting
Both forms present. Rates approaching equality.
State C: Equilibrium
Forward Rate = Reverse Rate
State D: Added Acid
Shift left to reform colorless indicator.
4
Analysis & Synthesis
1. At State C (Dynamic Equilibrium), did the particles stop reacting? Use evidence from your model to justify your answer.
The particles (did / did not) stop reacting because...
2. Based on Step 4 of the lab (adding NaOH after HCl), how does the color return prove the reaction is still moving in both directions?
This observation shows the reaction is reversible because...
Connect to Kinetics
If you increase the concentration of Reactants, the forward collision rate increases. How does the system return to a
Dynamic Balance CER Dynamic Balance CER
Lab Assessment Task
Name:
Date:
Complete this after the Phenol Flip lab activity.
The Investigation Question
Explain the phenolphthalein "two-way" reaction as a dynamic process. Justify why the reaction reaches a state of balance but never stops.
Claim
(A one-sentence statement that answers the investigation question)
The phenolphthalein reaction is a dynamic process because...
Evidence
(Data from your lab observations and details from your particle models)
According to my lab observations, I saw...
In my particle model, I represented the rates by...
Reasoning
(How does the principle of "Equal Rates" connect your evidence to your claim? Use the word "Dynamic".)
These observations prove the reaction is dynamic because...
When equilibrium is reached, the forward and reverse rates...
CER Scoring Rubric
Criteria Exceptional (4) Proficient (3) Developing (1-2) Scientific Claim Claim clearly states that equilibrium is a dynamic process where rates are equal. Claim identifies the reaction as reversible or balanced. Claim is missing or inaccurate (e.g., states reaction stops). Evidence Selection References specific color shifts and particle arrow/count details from the models. References either macroscopic or microscopic evidence, but not both clearly. Evidence is vague or unrelated to the phenolphthalein lab. Reasoning & Logic Connects the lack of visible change to equal rates of forward/reverse reactions. Explains that the reaction is moving in both directions. Restates the claim without explaining the "why" behind equal rates.
Steady State Key Steady State Key
Teacher Reference & Solution Guide
1. Phenolphthalein Observation Guide
Action Expected Result Chemical Shift Initial in Water Colorless / Clear System starts with reactant form dominant (InH2). Add NaOH (Base) Vibrant Magenta / Pink Base reacts with H+, removing it. Shift Right (→). Add HCl (Acid) Colorless / Clear Adds H+. System shifts Left (←) to restore balance. Add More NaOH Vibrant Magenta / Pink Proves the reaction is still active and reversible.
2. Particle Model Look-Fors
Initial (Just Base)
Mostly circles (○).
1-2 stars (★) just beginning to form.
Long forward arrows; short reverse arrows.
Equilibrium
Mix of circles and stars (ratio depends on pH).
CRITICAL: Forward and reverse arrows must be IDENTICAL in length/size.
Model CER Response
Claim
Chemical equilibrium is a dynamic process where the forward and reverse reaction rates are equal, meaning the reaction never stops even when visible changes cease.
Evidence
In the lab, we saw the solution change from clear to pink, then back to clear when acid was added, and back to pink again with more base. This shows the reaction can move in both directions. In my particle model for equilibrium, I drew the forward and reverse arrows as equal lengths to show the rates matched, even though both circles and stars were still present in the container.
Reasoning
The reaction is "dynamic" because particles continue to collide and react at all times. When equilibrium is reached, the number of pink molecules turning colorless is exactly balanced by the number of colorless molecules turning pink. Because these rates are equal, the macroscopic appearance (color) stays constant. The reaction hasn't stopped; it has simply reached a steady state where the "one-way" net change is zero.
Common Misconceptions
"Equilibrium means equal concentrations": Students often think there must be 50% reactant and 50% product. Emphasize that it's the rates that are equal, not the amounts."The reaction stopped": Because the color stops changing, students assume the particles have stopped moving. Use the "Escalator" analogy from the slides to correct this. Vocab Vitals Activity Vocab Vitals
Equilibrium & Reversibility
Name:
Date:
1
Term Matching
1. Reversible Reaction
2. Dynamic Equilibrium
3. Forward Rate
4. Reverse Rate
5. Concentration
A. The speed at which products reform the original reactants.
B. A chemical system where products can collide to reform reactants.
C. A state where forward and reverse rates are equal and constant.
D. The amount of a substance present in a specific volume.
E. The speed at which reactants turn into products.
2
The Particle Model
Draw a particle model for a reaction in Dynamic Equilibrium . Use two different colors or symbols for reactants and products.
Sketch Area
Checklist for your drawing:
Both reactants and products are present.
Arrows show the reaction moving in both directions.
The length/size of the arrows is identical .
3
Concept Spotlight: Dynamic Equilibrium
Definition in your own words
Real-world Analogy (e.g. escalator)
Misconception (What it is NOT)
Key Symbol or Graph
In your own words, why is the word "dynamic" used to describe equilibrium?
Vocab Vitals Key Vocab Vitals Key
Teacher Solution Guide
1. Term Matching Solutions
B
1. Reversible Reaction
C
2. Dynamic Equilibrium
E
3. Forward Rate
A
4. Reverse Rate
D
5. Concentration
2. Visual Vocab Guidance
Look-fors in student sketches:
Presence: Both reactants and products are inside the same container.Motion: One arrow pointing from Reactant → Product AND one arrow from Product → Reactant.Rate: The two arrows must be the same length, showing the speeds are identical.
3. Concept Spotlight Solutions
Definition
A state in a reversible reaction where the forward speed equals the reverse speed, so total amounts stop changing.
Analogy
A person walking up a descending escalator at the exact same speed the escalator moves down. They move, but stay in one place.
Misconception
Equilibrium does NOT mean the reaction stopped. It also does NOT mean there are equal amounts of reactants and products (50/50).
Symbol/Graph
Use of double arrows \(\rightleftharpoons\) or a graph where Rate lines meet and become flat/horizontal.
Summary Answer:
"Dynamic" means active or moving. We use this word because the individual molecules are still constantly colliding and reacting in both directions, even though we can't see a change in color or amount with our eyes.
Reaction Roleplay Teacher Guide The Royal Chemical Theater Presents
Steady State
Role-Playing Reversible Reactions
The Cast
10-20 StudentsParticles
TeacherDirector
1 StudentScorekeeper
The Stage
Divide the room into Left (Reactants) and Right (Products) using floor tape.
The Script (Instructions)
1
All students start on the Reactant Side.
2
THE MOVEMENT RULES:
Forward Rate: 50% of Reactants move Right.
Reverse Rate: 25% of Products move Left.
*Rounding Rule: Round movement UP to the nearest whole person.
3
On each cue, populations move. Record counts after everyone has swapped.
Director's Debrief Cues
Key Look-Fors
Act 1: Rapid forward shift. Reactants pool drops significantly.Act 2: Forward rate slows as reactant concentration drops; Reverse rate increases as product pool grows.Act 3 (Equilibrium): Counts stabilize. Crucial: People are still moving across the line!
Discussion Prompts
"At equilibrium, did the reaction stop ? Point to the students still crossing the line!"
"Why did the forward rate change from Round 1 to Round 5? Connect this to collisions ."
"Using our performance as evidence, define dynamic balance ."
Reaction Roleplay Log The Performance Log
Role-Playing Chemical Balance
Observer:
Populations & Rates
Round Reactant Count Forward Rate (# moving) Product Count Reverse Rate (# moving) START Total Count N/A 0 N/A 1 2 3 4 5 6 7 8
Stage Directions: Watch the population shift! In the first few acts, the Forward Rate will be much higher than the Reverse Rate. As products accumulate, the Reverse Rate will "pick up speed" until they match.
Post-Performance Analysis
1. Critical Timing: At what round did the population on each side stop changing drastically? Compare the Forward and Reverse rates at this point.
2. The Big Misconception: Based on your data, does equilibrium mean you have the SAME NUMBER of people on both sides? Why or why not?
3. Scene Analysis: Even after the counts stabilized, were people still crossing the line? How does this connect to the term dynamic balance?
Reaction Roadmap Study Guide Reaction Roadmap
Unit Study Guide: Kinetics & Equilibrium
Name: __________________________
Date: ___________________________
1. The Spark: Collision Theory
For a reaction to occur, particles must collide with:
1 Frequency: Enough total collisions.
2 Orientation: Correct alignment for bond-breaking.
3 Energy: Minimum Activation Energy (Ea).
Rate Accelerators
Factor Effect on Rate Temperature Increases frequency & energy of collisions. Concentration Increases frequency by crowding. Surface Area Exposes more particles to collide. Catalyst Lowers Ea via alternative pathway.
Energy Diagram Checklist
• Reactants: Initial energy.
• Products: Final energy.
• Transition State: Peak.
• Ea: Reactants to Peak.
• ΔH: Reactants to Products.
• Exo (-): Products lower.
2. The Balance: Equilibrium
"Rateforward = Ratereverse"
Dynamic: The reaction is still moving!Macro: Properties (color) are constant.Closed: No matter enters or leaves.
3. The Shift: Le Chatelier's
Concentration
Add : Away Remove : Toward
Temperature
Heat Up: Endo
Cool Down: Exo
Pressure (Gas only)
High P: Fewer Moles
Low P: More Moles
Exam Strategist
Catalysts and inert gases (like Argon) do NOT shift equilibrium! They change how fast you get there, but not where "there" is. Always look for (g) states before shifting for pressure!
Kinetics = Mechanics
•
Equilibrium = Landscape
Dynamic Drive MCQs Dynamic Drive MCQs
Unit Mastery: Kinetics & Equilibrium
Name: ____________________
Date: _____________________
1. According to collision theory, what is required for a chemical reaction to occur?
A) Different electrical charges.
B) Sufficient energy and orientation.
C) Same phase of matter.
D) Same ambient temperature.
2. How does a catalyst increase the rate of a chemical reaction?
A) Increasing kinetic energy.
B) Increasing total particles.
C) Lowering activation energy.
D) Shifting equilibrium right.
3. In an exothermic reaction, which of the following is true?
A) Potential energy of products is higher than reactants.
B) Energy is absorbed from the surroundings.
C) The enthalpy change (ΔH) for the reaction is negative.
D) The activation energy for the reaction is zero.
4. At dynamic equilibrium, what is true about the forward and reverse rates?
A) Forward rate > Reverse rate.
B) Forward rate = Reverse rate.
C) Both rates are zero.
D) Reverse rate > Forward rate.
5. A reversible endothermic reaction is heated. In which direction will it shift?
A) Toward products (right) to absorb heat.
B) Toward reactants (left) to release heat.
C) No shift occurs.
D) Toward fewer gas moles.
Scratch Area / Reasoning
Which change shifts the system left?
N2(g) + 3H2(g) ⇌ 2NH3(g) + Heat
A) Increase [H2].
B) Decrease [NH3].
C) Increase Temp.
D) Increase Pressure.
7. Why does increasing surface area increase the reaction rate?
A) Increases particle temperature.
B) Exposes more particles to collide.
C) Lowers activation energy.
D) Increases particle speed.
8. Activation energy (Ea) is the energy difference between:
A) Reactants and Products.
B) Peak and Products.
C) Reactants and Peak.
D) Delta H and Enthalpy.
9. Higher reactant concentration increases rate because:
A) Effective collision frequency increases.
B) Activation energy increases.
C) Reaction becomes more exothermic.
D) Orientation requirements decrease.
10. For a gaseous reaction, decreasing container volume will:
A) Decrease rate by slowing particles.
Dynamic Drive Key Dynamic Drive Key
Teacher Answer Key & Rationales
Unit: Reaction Dynamics
# Ans Key Conceptual Connection 1 B Collision Theory: Particles must hit with enough energy (Ea) and proper orientation. 2 C Catalysts: They change the mechanism to a path with lower activation energy. 3 C Exothermic: Energy is released, so final enthalpy is lower (ΔH < 0). 4 B Equilibrium Definition: Rates are equal; concentrations are constant but not necessarily equal. 5 A LCP (Temp): Increasing T shifts toward the endothermic direction (consumes heat). 6 C LCP (Temp): Heat is a product (exo); adding heat shifts the system left. 7 B Kinetics: More surface area = more collision opportunities per second. 8 C Energy Diagrams: Ea is the "hill" between reactants and transition state. 9 A Kinetics: Crowding particles results in more frequent effective collisions. 10 B LCP (Pressure): Decreased volume increases P, shifting to side with fewer gas moles.
Common Pitfalls
Q4: Clarify that "equilibrium" refers to rate stability, not amount equality.Q6: Emphasize identifying if energy is a reactant or product before applying LCP.Q8: Distinguish ΔH (start to end) from Ea (start to peak).
Leveling Up
Challenge mastery students to sketch a path for a catalyzed reaction on an energy diagram, specifically lowering the peak (Ea) but leaving start and end points identical.
Reaction Dynamics Instructional Resources © 2026
Reaction Pathways Slides Reaction Pathways
Collision Theory, Energy, and Rates
Energy
Rates
Collisions
Collision Theory
For a chemical reaction to occur, reactant particles must collide. But not every collision results in a reaction!
The Two Requirements:
Proper Orientation: Particles must hit each other in the correct position.Sufficient Energy: Particles must hit with enough force to overcome the barrier.
Activation Energy (Ea)
The minimum amount of energy required to initiate a chemical reaction.
Think of it as the "hill" that reactants must climb before they can turn into products.
High Ea = Slow Reaction
Low Ea = Fast Reaction
R
Ea
P
Enthalpy (ΔH)
Enthalpy is the "heat content" of a system. The change in enthalpy (ΔH) tells us if energy was absorbed or released.
ΔH = Hproducts - Hreactants
Exothermic (-ΔH)
Energy is released. Surrounding gets hot.
Endothermic (+ΔH)
Energy is absorbed. Surrounding gets cold.
Exothermic Pathway
Potential Energy
Reaction Progress
Reactants Products Ea ΔH (-) Activated Complex
Reactants > Products
Energy is "given off" to the surroundings.
Cool Fact:
This is why burning wood feels hot!
Speeding Things Up
The Rate of Reaction depends on the number of successful collisions per second.
Temp
Particles move faster.
Conc
More frequent hits.
Surface
More area to hit.
Catalyst
Lower energy hill.
Catalysts: The Lower Path
Uncatalyzed Catalyzed
A Common Trap!
Adding a catalyst does not change the ΔH. It only makes the reaction go faster by lowering the Ea.
"Lowering the barrier, not changing the start or finish."
Quick Check
1. In an endothermic reaction, which has more energy?
A) Reactants
B) Products
2. A catalyst changes the...
A) Activation Energy
Energy Map Worksheet ENERGY MAP
Reaction Pathway Analysis
Subject: Chemistry
Unit: Kinetics
Name:
Date:
Blueprint References
Enthalpy Change (ΔH): Eproducts - Ereactants
Activation Energy (Ea): Eactivated complex - Ereactants
Exothermic: Energy released (-ΔH)
Endothermic: Energy absorbed (+ΔH)
Problem 1: Combustion Analysis
PE (kJ) 200 150 100
1a. Reactant Energy:
1b. Product Energy:
1c. Calculate ΔH (Show work):
1d. Calculate Ea:
Reaction is:
Exothermic
Endothermic
Energy Map Worksheet - Page 2 Verify all values and calculations.
Problem 2: Synthesis Pathway
400 200
2a. Ea (forward):
2b. Ea (reverse):
2c. Calculate ΔH (Show work):
Problem 3: Multiple Catalysts
Uncatalyzed Catalyzed A B C
3a. What does point "A" represent?
3b. How did catalyst change Ea?
3c. How did catalyst change ΔH?
Part 2: Summary Checklist
Scenario Effect on Ea Effect on ΔH Adding a Catalyst Increasing Temperature
Energy Peaks Quiz Energy Peaks Quiz
Potential Energy Diagrams & Reaction Rates
Name:
Date:
Part I: Multiple Choice
1. If the potential energy of the products is lower than the potential energy of the reactants, the reaction is:
Endothermic
Exothermic
Catalytic
Spontaneous
2. Which of the following is the primary function of a catalyst?
Increasing the total enthalpy (ΔH) of the reaction.
Lowering the activation energy (Ea) by providing an alternate pathway.
3. The term "transition state" is also known as the:
Intermediate
Activated complex
4. In a multi-step reaction diagram, which step determines the overall rate of the reaction?
The step with the lowest activation energy.
The step with the highest transition state peak (highest Ea).
5. Biological catalysts that speed up reactions in living systems are called:
Reactants
Enzymes
6. If the forward activation energy is 300 kJ and the reverse activation energy is 500 kJ, the enthalpy (ΔH) is:
+200 kJ
−200 kJ
7. In an endothermic potential energy diagram, the products have:
Lower energy than the reactants.
Higher energy than the reactants.
Part II: Structured Response
Reference Scenario Values:
Reactants
300 kJ
Transition State
600 kJ
Products
100 kJ
8. Calculate the activation energy (Ea) for the forward reaction and show your work below.
Answer: ________ kJ
9. Calculate the enthalpy change (ΔH) for this reaction. Is it endothermic or exothermic? Explain.
Answer (ΔH): ________ kJ
10. Sketch a simple two-step potential energy diagram where the first step is endothermic, the second step is exothermic, and the overall reaction is exothermic. Label TS1 and TS2.
Potential Energy (kJ)
Reaction Coordinate
Energy Map Key ANSWER KEY
Energy Map Analysis
Teacher Resource
Problem 1: Combustion Analysis
1a. Reactant Energy:
100 kJ
1b. Product Energy:
25 kJ
1c. Calculate ΔH:
ΔH = 25 - 100 = -75 kJ
1d. Calculate Ea:
Ea = 200 - 100 = 100 kJ
Type:
Exothermic
Energy Map Key - Page 2
Problem 2: Synthesis Pathway
2a. Ea (forward):
400 - 100 = 300 kJ
2b. Ea (reverse):
400 - 200 = 200 kJ
2c. Calculate ΔH:
ΔH = 200 - 100 = +100 kJ
Problem 3: Multiple Catalysts
3a. Point "A":
Activated Complex (Uncatalyzed)
3b. Catalyst change on Ea:
Lowered the activation energy (C to B vs C to A).
3c. Catalyst change on ΔH:
No change. Reactants and products stayed same.
Part 2: Summary Key
Scenario Effect on Ea Effect on ΔH Adding a Catalyst Decreases No Change Increasing Temperature No Change No Change
Energy Peaks Key Energy Peaks Key
Teacher Reference Guide
Answer Key
Part I: Multiple Choice
Exothermic
Lowering activation energy (Ea).
Activated complex
Highest transition state peak (highest Ea).
Enzymes
−200 kJ
Higher energy than reactants.
Part II: Structured Response
8. Activation Energy (Ea) Calculation:
600 kJ (TS) − 300 kJ (Reactants) = 300 kJ
9. Enthalpy Change (ΔH) Calculation:
100 kJ (Products) − 300 kJ (Reactants) = −200 kJ
Conclusion: Exothermic reaction.
10. Two-Step Diagram Sketch Details:
TS1 TS2 Reactants Products
TS1 must be at highest point for slow step.
End point lower than start (Overall Exothermic).
Pedagogical Insights
Common Calculation Error:
ΔH = Forward Ea − Reverse Ea.
Visual Check Tip:
Draw horizontal dashed lines from reactants and products to clearly see the "step" height (ΔH).
Reaction Blueprint Notes Reaction Blueprint
Guided Notes & Practice
Name:
Date:
1. Collision Theory
For a reaction to occur, particles must ____________________ with each other. A "successful" collision requires two things:
A. Proper ____________________
The particles must hit in the right spot to break existing bonds.
B. Sufficient ____________________
The particles must hit with enough force to overcome the energy barrier.
2. Energy Definitions
Activation Energy (Ea):
Enthalpy (ΔH):
Activated Complex:
3. Diagram Blueprinting
Exothermic Diagram
Label: Reactants, Products, Ea, and ΔH
Endothermic Diagram
Label: Reactants, Products, Ea, and ΔH
4. Practice Problems
Problem 1: Calculation Challenge
PE Reactants: 40 kJ
PE Products: 10 kJ
PE Complex: 100 kJ
Activation Energy (Ea): ______ kJ
Enthalpy (ΔH): ______ kJ
Reaction Type? __________
Problem 2: Catalysts & Rates
A reaction has an initial Ea of 85 kJ. A catalyst is added, lowering it to 55 kJ. The enthalpy (ΔH) of the reaction is +25 kJ.
1. What is the ΔH with the catalyst added?
2. How does the catalyst change the rate?
Rate Factors Recall:
List 3 ways to speed up this reaction:
Rate Chasers Lab Guide Rate Chasers Lab
Investigation: Factors Affecting Reaction Rates
Lab Guide
Objective
Investigate how temperature and surface area influence the rate of reaction between Alka-Seltzer tablets and water.
Materials
Alka-Seltzer tablets (4)
Beakers (250 mL)
Stopwatch & Thermometer
Hot/Cold Water
Mortar & Pestle
Procedure
Part A: Temperature Trial
Fill beakers with 150 mL of ice water and hot water (approx. 50°C).
Record temperatures in the data table below.
Drop a whole tablet into each beaker and time the reaction until bubbling stops.
Table A: Temperature Results
Condition Temp (°C) Time (s) Ice Water Hot Water
Part B: Surface Area Trial
Fill two beakers with 150 mL of room temperature water.
Time the dissolution of one whole tablet .
Crush a second tablet into powder and time its reaction.
Table B: Surface Area Results
Form Time (s) Relative Rate (Fast/Slow) Whole Tablet Crushed Powder
Blueprint Analysis
1. Using Collision Theory, explain why the hot water reaction occurred faster than the ice water reaction.
2. Why did crushing the tablet change the rate? Use the term "particle collisions" in your answer.
3. Sketch an energy diagram for this reaction. Label the "hill" for both a catalyzed and uncatalyzed path.
Reaction Practice Key Teacher Solution Key
Reaction Blueprint & Calculations
1. Collision Theory Key
Blank: Particles must collide with each other.
Requirement A: Proper Orientation
Requirement B: Sufficient Energy
Problem 1: Calculation Blueprint
R (40) P (10) C (100)
Activation Energy (Ea)
100 - 40 = 60 kJ
Enthalpy (ΔH)
10 - 40 = -30 kJ
Exothermic
Problem 2: Catalysts & Rates
Q: What is the ΔH with the catalyst?
Still +25 kJ. Catalysts DO NOT change ΔH.
Q: Why does the rate increase?
It provides a pathway with lower Ea, so more particles can successfully react.
Rate Factors (Any 3):
Increase Temperature Increase Concentration Increase Surface Area
Rate Chasers Rubric Lab Rubric
Rate Chasers Investigation
Grading Criteria
Criteria Excels (5) Developing (3) Needs Support (1) Data Accuracy All data points recorded clearly with units. Specific temps. Most data recorded. Some units or details missing. Missing multiple data points or illogical values. Collision Theory Expertly links results to particle collisions and energy. Identifies relationship but lacks detail on particles. Failed to mention collisions or particles. Diagramming Accurately labeled with reactants, products, and Ea hill. Diagram present but missing labels or catalyst hill. Diagram is messy, incorrect, or missing. Lab Conduct Followed safety protocols. Station cleaned after use. Followed instructions but required reminder. Unsafe behavior or station left messy.
Instructor Feedback
Final Score: ______ / 20
Weight: 15% of Unit Grade
Tea Time Lab Guide Tea Time Lab
Household Equilibrium Investigation
Name:
Date:
Objective
Determine how changing concentrations "stresses" an equilibrium system found in your kitchen and predict resulting color shifts using Le Chatelier's Principle.
The Tea System
Tea-H (Light Yellow/Orange) ⇌ Tea− (Dark Brown) + H+
This system is sensitive to Hydrogen ions (H+).
Materials
Strong Black Tea (concentrated)
Lemon Juice or Vinegar (Acid / H+ Source)
Baking Soda solution (Base / H+ Remover)
4 clear cups or small jars
Pipettes or small spoons
Procedure
Prepare 50mL of very strong black tea. It should be deep brown.
Divide the tea equally into 4 clear cups.
Cup 1: Do nothing (Control).Cup 2: Add 20 drops of Lemon Juice.Cup 3: Add 20 drops of Baking Soda solution.Cup 4: Add Baking Soda (to turn it dark), then add Lemon Juice until it changes again.
Observations
Action Taken Stress Type Resulting Color Shift Direction 1. Control None Deep Brown -- 2. Add Lemon Juice Add H+ 3. Add Baking Soda Remove H+ 4. Add Base, then Acid Reversibility Check
?
Final Synthesis (CER)
The Lab Prompt:
Explain how adding lemon juice shifts the tea equilibrium. Is this change permanent or reversible? Justify your answer using Le Chatelier's Principle.
Scientific Claim
Adding lemon juice causes the system to shift...
Evidence from Lab
In my investigation, I observed that when lemon juice was added...
Furthermore, when I added base then acid in Step 4, the color...
Scientific Reasoning
According to Le Chatelier's Principle, adding a product (H+) creates a stress that...
This shows the reaction is dynamic and reversible because...
Shift Predictor Worksheet Shift Predictor Worksheet
Topic: Le Chatelier's Principle & Thermal Effects
Name:
Date:
Learning Objectives
C.9A - Use Le Chatelier’s principle to predict shifts in equilibrium.
Part 1: The Principle of Balance
Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will shift its position to counteract the disturbance. Complete the table below.
Stress Applied Equilibrium Response (Shift) Increase [Reactant] Decrease [Product] Increase Temp (Exothermic) Decrease Temp (Endothermic)
Part 2: Scenario Analysis
Analyze the Haber Process reaction system for the production of ammonia:
N2(g) + 3H2(g) ↔ 2NH3(g) + 92 kJ
1. Stress: Nitrogen (N2) gas is added to the container.
Shift Direction:
Toward Products
Toward Reactants
No Shift
Explanation (Collision Theory):
2. Stress: The reaction vessel is cooled in an ice bath.
Shift Direction:
Toward Products
Toward Reactants
No Shift
Explanation (Energy Effects):
Part 3: The Carbonic Acid System
Maintaining blood pH requires a precise balance. Analyze the reaction sequence:
CO2(aq) + H2O(l) ↔ H2CO3(aq) ↔ H+(aq) + HCO3−(aq)
3. During hyperventilation, CO2 is rapidly removed from the blood. Predict the shift and the effect on H+ concentration.
Will blood pH increase or decrease? Explain.
4. If a patient is given an enzyme that acts as a catalyst, how will the equilibrium position shift?
Toward Products
Toward Reactants
No Shift
Reasoning:
Part 4: Graphical Representation
At time t1 , more reactant A is added to the system A ↔ B . Sketch the concentration changes for both substances.
Time
Concentration
t1
Sketch your graph here...
Thinking Prompt: Connect the shift to Collision Theory. How does adding more A affect the initial collision frequency between particles?
Stress Response Posters Stress Response Posters
Reaction Dynamics: Le Chatelier's Principle
Name:
Date:
The Objective
Design a high-impact vocabulary poster that predicts and explains how a chemical system at equilibrium reacts to a specific "stressor." Your poster will serve as a visual guide for your peers.
Poster Requirements
1
Term & Definition
Clearly display your assigned stressor and a definition of how it affects a system.
2
Visual Representation
Create a "Before and After" diagram showing the system at balance, the stress, and the shift.
3
Stress-Response Rule
Justify the shift: "The system shifts to [oppose/consume/replace] the stress of ______."
4
Equilibrium Language
Use terms like: Reversible, Rate of Reaction, Forward/Reverse, Favored.
Pro Tip
Use high-contrast colors (like red for stress and blue for balance) to make your poster readable from across the classroom!
Assigned Topics
Circle or check your assigned topic:
Concentration Increase
Concentration Decrease
Temperature Up (Exo)
Temperature Down (Exo)
Temperature Up (Endo)
Pressure Increase (Gas)
Pressure Decrease (Gas)
Catalyst (No Shift)
Poster Blueprint
Draft your content here before creating your final poster.
1. Technical Definition
2. Example Chemical Equation
Sketch your equation here:
e.g., \( A + B \rightleftharpoons C + \Delta H \)
3. Visual Layout Sketch
Sketch your "Before, Stress, After" diagrams here
4. Final Justification (The Script)
Write the exact explanation you will put on your poster. Use the Stress-Response rule to justify your prediction.
The Prediction:
When stress is applied, the equilibrium will shift... ______________________
The Scientific Reason (Justification):
Grading Checklist (Teacher Use Only)
Prediction
Correct shift direction identified.
Justification
Uses Stress-Response rule correctly.
Visuals
Before/After/Stress clearly labeled.
Clarity
Professional, bold, and readable.
Shift Shortcut Sheet SHIFT SHORTCUT
Le Chatelier's Quick Guide
Reference Sheet
Concentration
Changes to amount of reactant or product.
Add Substance
Shift AWAY from the added side.
Remove Substance
Shift TOWARD the removed side.
Temperature
Treat "Heat" as a chemical substance!
Increase Temp
Shift AWAY from heat (Endothermic).
Decrease Temp
Shift TOWARD heat (Exothermic).
Pressure & Volume
Affects systems with gaseous species.
Pressure ↑ (Volume ↓)
Shift to side with FEWER moles of gas.
Pressure ↓ (Volume ↑)
Shift to side with MORE moles of gas.
Special Cases
Common "Trick" questions.
Add Catalyst
NO SHIFT.
Increases rate of BOTH reactions equally.
Add Inert Gas (Constant V)
NO SHIFT.
Partial pressures of reactants do not change.
The Golden Rule
"If a system at equilibrium is subjected to a stress, the system will shift in a direction that opposes the stress."
Quick Tip: Counting Moles
When evaluating Pressure or Volume shifts, only count coefficients for gaseous (g) species. Solids, liquids, and aqueous solutes are not affected by pressure changes in the same way.
Shift Happens Slides REACTION DYNAMICS
Shift Happens
Le Chatelier's Principle & Stress
Le Chatelier's Principle
"If a system at equilibrium is subjected to a stress, the system will shift its equilibrium position to counteract the effect of the stress."
STRESS
A change in concentration, temperature, or pressure.
SHIFT
Reaction speeds up in one direction (Forward or Reverse).
RESULT
A new equilibrium is established.
Stress: Concentration
Add a Species
Shift away from the added substance.
Remove a Species
Shift toward the removed substance.
The Logic
Adding: More particles = higher collision frequency in that direction. The reaction "speeds up" to consume the excess.
Removing: Fewer particles = lower collision frequency. The opposite direction "wins" until balance returns.
Stress: Temperature
Heat is a Reactant or Product
Exothermic
Reactants → Products + Heat
• Add Heat: Shift Left
• Remove Heat: Shift Right
Endothermic
Reactants + Heat → Products
• Add Heat: Shift Right
• Remove Heat: Shift Left
☕
Lab Spotlight: Black Tea
Tea-H (Light) ⇌ Tea− (Dark) + H+
High [H+]
Lighter Yellow
Neutral State
Deep Brown
Low [H+]
Black / Opaque
Writing Your CER
C
Claim
Adding lemon juice causes the system to shift...
E
Evidence
According to my lab observations, I saw...
R
Reasoning
According to Le Chatelier's Principle, adding a product creates a stress that...
Shift Predictor Key Shift Predictor Key
Teacher Resource & Answer Key
Corrected Copy
Part 1: The Principle of Balance
Stress Applied Equilibrium Response (Shift) Increase Concentration of a Reactant Shift Toward Products (Right) Decrease Concentration of a Product Shift Toward Products (Right) Increase Temperature (Exothermic) Shift Toward Reactants (Left) Decrease Temperature (Endothermic) Shift Toward Reactants (Left)
Part 2: Scenario Analysis (Haber Process)
N2(g) + 3H2(g) ↔ 2NH3(g) + 92 kJ
1. Stress: Nitrogen (N2) gas is added.
Shift Direction: Toward Products
Explanation (Collision Theory): Adding N2 increases the frequency of effective collisions between N2 and H2 particles. This increases the forward reaction rate initially, shifting the system right until a new equilibrium is reached.
2. Stress: The reaction vessel is cooled.
Shift Direction: Toward Products
Explanation (Energy Effects): Since the forward reaction is exothermic (heat is a product), cooling removes energy from the system. The equilibrium shifts right to replace the lost heat energy.
Part 3: The Carbonic Acid System
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3−
3. Hyperventilation (removing CO2):
Shift: Toward Reactants (Left)
Effect on H+: Concentration decreases.
pH Effect: Blood pH will increase (become more basic) because H+ ions are consumed as the system shifts left to replace lost CO2.
4. Adding a Catalyst:
Shift: No Shift
Reasoning: Catalysts lower the activation energy for both the forward and reverse reactions equally. This speeds up the rate to reach equilibrium but does not change the concentrations at equilibrium.
Part 4: Graphical Representation
[A] [B] t1
Key Visual Elements:
Flat lines indicate dynamic equilibrium before the stress.Vertical spike in [A] shows the instantaneous addition of reactant.Smooth curves show the gradual shift as reaction rates adjust.Final plateaus are at different levels than the start. Shift Check Slides Shift Check
Le Chatelier's Principle Video Quiz
Video Embed Needed
Please share your YouTube link to complete this quiz!
Watch carefully, then answer the comprehension checks!
1
Concentration Stress
If additional reactant is added to a system at equilibrium, what happens?
A
The forward reaction speeds up to use the extra reactant.
B
The reverse reaction speeds up to produce more reactant.
C
The system stops completely until reactant is removed.
2
Temperature Stress
Reactants ↔ Products + Heat
For this Exothermic reaction, what happens if you increase the temperature?
A
Shift right to create more heat.
B
Shift left to use up the excess heat.
C
The equilibrium does not shift.
3
Pressure Stress
If the volume of a gaseous system is Decreased (increasing pressure), the system shifts to the side with...
A
The more particles side.
B
The fewer particles side.
4
Apply Your Knowledge
X2(g) ↔ 2X(g)
If you Increase the volume (lowering pressure), which direction is favored?
A
Shift Right (Forward)
B
Shift Left (Reverse)
Stress Relieved!
Le Chatelier's Principle in one sentence:
"The system will shift to relieve the stress and regain balance."
Shift Master Worksheet Shift Master Worksheet
Equilibrium Dynamics Practice
Name:
Date:
Mission Instructions: For each reaction system, predict the equilibrium shift direction (Left, Right, or No Shift) following the stress. Provide a technical justification for your prediction.
1 Iron-Thiocyanate System (Concentration)
Fe3+(aq) + SCN-(aq) ⇌ [FeSCN]2+(aq)
Stress: Add Fe3+ ions
Direction of Shift:
Justification:
Stress: Remove SCN- ions
Direction of Shift:
Justification:
2 Haber Process (Temperature)
N2(g) + 3H2(g) ⇌ 2NH3(g) + Heat
Stress: Heat the system
Direction of Shift:
Justification:
Stress: Cool the system
Direction of Shift:
Justification:
3 Nitrogen Dioxide Equilibrium (Pressure)
N2O4(g) ⇌ 2NO2(g)
Stress: Compression (Volume ↓)
Direction of Shift:
Justification:
Stress: Expansion (Volume ↑)
Direction of Shift:
Justification:
Industrial Engineering Synthesis
You are an engineer tasked with increasing the efficiency of ammonia production (Scenario 2). Beyond just temperature, explain how you could manipulate the pressure of the reaction vessel to maximize the amount of NH3 produced. Justify your answer using molar ratios from the balanced equation.
Color Shift Lab Key Tea Time Lab Key
Teacher Solutions & Household Lab Notes
1. Expected Observations
Action Resulting Color Shift Direction Reasoning 2. Add Lemon Juice Lighter Yellow/Orange Left (←) Added H+ (product); system shifts left to consume excess. 3. Add Baking Soda Dark Brown / Black Right (→) Base removes H+; system shifts right to replace it. 4. Add Base, then Acid Lighter Yellow/Orange Left (←) Restores H+ concentration; proves reversibility.
2. Analysis Solutions
Question 1: Lemon Juice Stress
Lemon juice provides Hydrogen ions (H+), which are a product in the tea equilibrium equation. According to Le Chatelier's Principle, adding more product creates a stress. The equilibrium shifts LEFT to consume that excess reactant, converting the dark Tea− form into the lighter Tea-H form.
Question 2: Baking Soda Neutralization
Baking soda is a base that reacts with H+ ions, effectively removing them from the system. This is a removal stress. To counteract this, the system shifts RIGHT to produce more H+, creating the dark brown Tea− form in the process.
Question 3: Equilibrium Proof
The observation in Step 4 proves this is an equilibrium system because the reaction is reversible . We were able to push the reaction to the product side (dark) and then pull it back to the reactant side (light) by changing the concentration of Hydrogen ions.
Instructional Tips
Tea Preparation: Use 3 tea bags in 1/2 cup of water to get a very concentrated "stock" solution. The shift is most dramatic when starting from a very deep brown.Baking Soda Solution: It is better to provide a liquid baking soda solution (baking soda dissolved in water) to avoid cloudiness from unreacted powder.Chemistry Note: Black tea contains polyphenols (tannins) like theaflavins. These molecules have phenolic hydroxyl groups that lose protons (H+) as pH increases (adding base), changing their electronic structure and color from yellow to dark brown.Reversibility: Emphasize that Step 4 is the "smoking gun" for equilibrium. If it wasn't a reversible system, adding acid wouldn't "undo" the darkening caused by the base. Shift Check Quiz Shift Check Quiz
Video Comprehension: Le Chatelier's Principle
Name:
Date:
Video Instructions
Watch the "Professor Dave Explains" video on Le Chatelier's Principle. For each question, select the best answer and justify your choice using the "stress-response" rule.
1
If more reactant is added to a system at equilibrium, the system will shift right. What does "shifting right" mean in this context?
A) The forward reaction speeds up to use the extra reactant and produce more product.
B) The reverse reaction speeds up to produce even more reactant.
C) The reaction stops completely because there is too much substance to move.
Justification:
2
In an exothermic reaction (ΔH is negative), how does the system react if the temperature is increased?
A) It shifts toward the products to release more energy.
B) It shifts toward the reactants to use up the excess heat.
C) It has no effect because exothermic reactions only depend on concentration.
Justification:
3
When the volume of a gaseous system is decreased (increasing pressure), how does the equilibrium shift to restore balance?
A) It shifts toward the side with more particles to increase pressure further.
B) It shifts toward the side with fewer particles to lower the total pressure.
C) It shifts to the side with larger molecules, regardless of the particle count.
Justification:
4
Consider the system: X2(g) ↔ 2X(g). If the volume of the container is increased, what will happen?
A) The equilibrium shifts right (forward) toward the side with more particles.
B) The equilibrium shifts left (reverse) toward the side with fewer particles.
C) No shift occurs because the number of atoms remains conserved.
Justification:
Shift Master Key Shift Master Key
Instructor Technical Reference
Answer Key
1. Iron-Thiocyanate System
Fe3+(aq) + SCN-(aq) ⇌ [FeSCN]2+(aq)
Stress: Add Fe3+
Direction of Shift:
RIGHT (Forward)
Justification:
Addition of a reactant increases collisions on the reactant side. The system shifts away from the stress to consume the excess Fe3+.
Stress: Remove SCN-
Direction of Shift:
LEFT (Reverse)
Justification:
Removing a reactant creates a "hole." The system shifts toward the removed species to replace the lost SCN-.
2. Haber Process
N2(g) + 3H2(g) ⇌ 2NH3(g) + Heat
Stress: Heat the system
Direction of Shift:
LEFT (Reverse)
Justification:
The reaction is exothermic. Adding heat favors the endothermic direction (reverse) to absorb the excess energy.
Stress: Cool the system
Direction of Shift:
RIGHT (Forward)
Justification:
Cooling removes heat (a product). The system shifts right to produce more heat and restore thermal balance.
3. Nitrogen Dioxide Equilibrium
N2O4(g) ⇌ 2NO2(g)
Stress: Compression (Volume ↓)
Direction of Shift:
LEFT (Toward N2O4)
Justification:
Increased pressure shifts the system to the side with fewer gas moles (1 mole of N2O4 vs 2 moles of NO2).
Stress: Expansion (Volume ↑)
Direction of Shift:
RIGHT (Toward NO2)
Justification:
Decreased pressure shifts the system to the side with more moles of gas to increase internal pressure.
Engineering Synthesis Answer
To maximize NH3 yield, the engineer should choose high pressure.
The balanced equation shows 4 total moles of reactant gas (1 mole N2 + 3 moles H2) and only 2 moles of product gas (2 NH3). Increasing pressure forces the system to shift toward the side with fewer gas molecules to reduce the stress of the pressure increase. Thus, high pressure shifts the equilibrium to the right, increasing the ammonia output.
Shift Check Key Shift Check Key
Teacher Guide & Answer Key
1
Concentration Shift
Answer: A
Expected Justification:
According to Le Chatelier's Principle, adding a reactant creates a stress. The system responds by shifting forward (right) to consume the excess reactant and produce more products, restoring equilibrium.
2
Temperature Shift (Exothermic)
Answer: B
Expected Justification:
In an exothermic reaction, heat is released and acts as a product . Increasing the temperature adds heat energy (the stress). To relieve this, the system shifts left (reverse) toward the reactants to absorb or consume the extra heat.
3
Volume & Pressure Relationship
Answer: B
Expected Justification:
Decreasing volume increases the pressure of the system. To alleviate this pressure stress, the equilibrium shifts to the side with fewer particles (moles of gas). Fewer particles hitting the container walls results in a lower overall pressure.
4
Scenario: X2(g) ↔ 2X(g)
Answer: A
Expected Justification:
Increasing the volume lowers the pressure. The system reacts by shifting toward the side with more particles to regain some of the lost pressure. Since the right side has 2 moles of gas and the left has only 1, the equilibrium shifts right .
Facilitation Tips
Pause the video: Stop the video at the end of each segment (Concentration, Temperature, Pressure) to allow students time to write their justifications.Focus on Language: Ensure students use the terms "stress," "relief," and "shift" in their justifications.Visual Aids: Use the accompanying slides to review the answers collectively after the video ends. Yield Wars Worksheet Yield Wars
Ammonia Optimization CER
Engineer:
Project Date:
The Industrial Goal
Ammonia (NH3) production is a delicate balance. Your mission is to optimize the reaction yield without sacrificing speed.
N2(g) + 3H2(g) ⇌ 2NH3(g) + Heat
1
Construct: Maximizing Yield
Based on Le Chatelier's Principle , identify the "theoretical" best temperature for maximum yield. Complete the argument.
Technical Claim
To achieve the highest possible yield of ammonia, we should set the temperature to...
Evidence & Reasoning
Since the forward reaction is (circle one): [ Exothermic / Endothermic ], heat acts as a product. According to Le Chatelier's Principle, by (circle one): [ Increasing / Decreasing ] the temperature, the system will shift...
This shift occurs because...
The Kinetic Conflict
Wait! Collision Theory states that lowering temperature slows down molecules. If you follow Section 1's advice too strictly, the reaction will happen so slowly it will never finish.
2
Critique: The "Extreme Pressure" Solution
A Student Claims:
"We should run the factory at 5,000 atm of pressure. This will force nearly 100% yield without needing to lower the temperature and slow down the rate."
Engineering Critique
Scientifically, the student is (circle one): [ Correct / Incorrect ] because high pressure favors the side with...
However, the engineer should reject this claim because using 5,000 atm of pressure would...
A more realistic industrial solution would be to...
The Engineer's Final Choice
The most effective tool to increase the rate of reaction without hurting the yield is to add a...
Yield Wars Key Yield Wars Key
Facilitation & Optimization Reference
Answer Key
Haber-Bosch Reference
N2(g) + 3H2(g) ⇌ 2NH3(g) + Heat
Standard Enthalpy of Reaction: ΔH = -92 kJ/mol
1. Maximizing Yield (Equilibrium Claim)
Technical Claim
...set the temperature to as low as possible / a very low temperature.
Evidence & Reasoning
Reaction is Exothermic. By Decreasing the temperature, system shifts Right (toward the NH3 product side).
This occurs because heat is a product of the reaction. Removing heat energy (by cooling) causes the system to shift in the exothermic direction to try and replace the lost thermal energy, favoring the products.
2. Critique: The "Extreme Pressure" Solution
Engineering Critique
Scientifically Correct because high pressure favors the side with fewer moles of gas (2 moles product vs 4 moles reactants).
Reject because 5,000 atm would require extremely thick, specialized steel vessels that are too expensive and pose a massive explosion risk for a factory environment.
More realistic solution: Use a moderate pressure (200-300 atm) which provides a significantly better yield than standard pressure without the extreme cost/risk.
The Engineer's Final Choice
Add a Catalyst (typically an iron-based catalyst).
Teacher Note: A catalyst provides an alternative pathway with a lower activation energy. This increases the rate of reaction at lower temperatures, allowing the high yield of equilibrium to be reached much faster.
