HS-PS1-6: Refining Chemical Systems to Increase Products at Equilibrium
In plain English: HS-PS1-6 is the NGSS high school performance expectation that asks students to refine the design of a chemical system by choosing one change in conditions that gives more product at equilibrium. Students apply Le Chatelier's principle, for example adding a reactant or removing a product, and explain the shift with molecular collisions. It is usually taught in Chemistry.
Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
Clarification Statement: Emphasis is on the application of Le Chatlier's Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.
Assessment Boundary: Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.
Next Generation Science Standards · Disciplinary Core Idea: Matter and Its Interactions · Official standard
Students act as chemical engineers. They learn that a reversible reaction reaches a dynamic equilibrium, where the forward and reverse reactions keep going at equal rates, and that changing one condition upsets that balance. The system then shifts in the direction that partly undoes the change (Le Chatelier's principle). Students use this to specify changes that give more product: adding a reactant, removing a product, changing the temperature, or changing the pressure of a gas system. They connect each change they make at the macroscopic level to what happens to the collisions between molecules.
The NGSS Clarification Statement says: "Emphasis is on the application of Le Chatlier's Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products." (The official text spells the name "Chatlier"; the usual spelling is Le Chatelier.) The Assessment Boundary says: "Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations." Every design task on this page changes one variable at a time, and no item asks for an equilibrium constant or a calculated concentration. Data sets are invented. The student lab uses a dilute indicator and exhaled breath with goggles; toxic or strongly colored equilibrium systems appear only as teacher demonstrations or videos.
Learning Objectives
By the end of this lesson, students will be able to:
Describe dynamic equilibrium as a state where the forward and reverse reactions continue at equal rates
Specify one change in conditions (adding a reactant, removing a product, temperature or pressure) that increases the amount of product at equilibrium
Explain each change at the molecular level in terms of how it changes the forward and reverse collision rates
Use data from a model or an invented industrial data set to refine the design of a chemical system, changing one variable at a time
Prior Knowledge Required
Students should already be comfortable with:
The collision model of reaction rates, including the effects of temperature and concentration HS-PS1-5
Exothermic and endothermic reactions and energy diagrams HS-PS1-4
Balanced equations with state symbols (g), (aq) and (s)
Gas pressure as the result of particles colliding with the walls of a container
Show a photo or short video of a busy escalator pair in a mall: people ride up and down at the same time, and the number of people on each floor stays about the same.
Warm-Up Prompt
"People keep moving between floors, yet the number on each floor does not change. How can both be true? Now a bus drops 40 extra people on the ground floor. What happens to the flow of people over the next few minutes, and where do the numbers settle?"
Collect answers. Students usually see that more people ride up until the flows balance again. Tell them this is the key idea of the lesson: a chemical equilibrium is balanced motion, not a stop.
Direct Instruction15-20 minutes
Dynamic equilibrium and Le Chatelier's principle. In a reversible reaction, reactant molecules collide to form products while product molecules collide to re-form reactants. At equilibrium the two rates are equal, so the amounts stop changing even though both reactions continue. If one condition changes, the rates are no longer equal, and the system shifts until they are equal again. Le Chatelier's principle predicts the direction: the system shifts to partly counteract the change.
Add a reactant: more reactant collisions, so the forward rate rises and more product forms.
Remove a product: fewer product collisions, so the reverse rate falls and more product forms.
Raise the temperature: the endothermic direction is favored. Lower the temperature to favor the exothermic direction.
Raise the pressure of a gas system (by making the volume smaller): the side with fewer gas molecules is favored. If both sides have the same number of gas molecules, pressure has no effect.
Engineers change one variable at a time so they know which change caused the result.
Removing a product: ammonia plant
N2(g) + 3H2(g) ⇌ 2NH3(g). The plant cools the gas leaving the reactor so ammonia condenses to a liquid and is drawn off, and the unreacted gases go back to the reactor.
Result: Removing NH3 lowers the number of NH3 molecules colliding to break back apart, so the reverse rate drops below the forward rate and more NH3 forms.
Adding a reactant
H2(g) + I2(g) ⇌ 2HI(g) is at equilibrium in a sealed flask. Extra H2 is injected, with the temperature and volume unchanged.
Result: More H2 molecules mean more H2-I2 collisions each second, so the forward rate rises and the amount of HI increases until the rates match again. There are 2 gas molecules on each side, so changing the pressure would not help here.
Changing the temperature
N2O4(g) ⇌ 2NO2(g): colorless N2O4 forms brown NO2, and the forward reaction absorbs energy (+57.2 kJ). Sealed tubes are shown in a video, because NO2 is toxic.
Result: In hot water the tube turns darker brown: raising the temperature favors the endothermic forward direction, so more NO2 forms. In ice water it turns paler.
Changing the pressure
Back to the ammonia plant: N2(g) + 3H2(g) ⇌ 2NH3(g), with 4 gas molecules on the left and 2 on the right. The plant raises the pressure and keeps the temperature the same.
Result: Squeezing the gas into a smaller volume favors the side with fewer gas molecules, so more NH3 forms. At the molecular level, the crowded reactant molecules collide more often, so the forward rate gains more than the reverse rate.
Show Diagram 2, invented data for an ammonia plant, and ask students to find each bar that differs from the baseline in only one condition. Remind them that the page never asks for an equilibrium constant or a calculated concentration (beyond this standard).
Guided Practice15 minutes
Pairs refine the design of a hydrogen plant that uses steam and methane: CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g). The forward reaction absorbs energy, and there are 2 gas molecules on the left and 4 on the right. For each single change, pairs predict the shift and whether it gives more hydrogen, then explain one change at the molecular level.
Guided practice: one change at a time
Single change
Shift
More hydrogen?
Add more steam
Toward products
Yes
Remove H2 through a membrane as it forms
Toward products
Yes
Raise the temperature
Toward products (endothermic direction)
Yes
Raise the pressure
Toward reactants (fewer gas molecules)
No
Lower the temperature
Toward reactants
No
Give pairs the table with the last two columns blank, then reveal it. Ask: "Which change would you make first, and what is happening to the forward and reverse collision rates right after you make it?"
Independent Practice10-15 minutes
Each student writes a short design memo for a methanol plant: CO(g) + 2H2(g) ⇌ CH3OH(g). The forward reaction releases energy, and there are 3 gas molecules on the left and 1 on the right. The memo proposes three separate refinements, each changing one variable, and explains each one at the molecular level.
Expected refinements: raise the pressure (fewer gas molecules on the product side), add more CO or H2, remove methanol by condensing it, or lower the temperature (the exothermic direction).
Closure5 minutes
Exit ticket: "A reaction at equilibrium has 3 gas molecules on the left and 2 on the right. Name one change to the amounts of substances and one other change to the conditions that would each give more product, and explain one of them with collisions." Return to the escalator: which change in the chemical system is like the bus dropping off extra people?
Differentiation Strategies
For Struggling Students
Run the water-transfer model (Activity 2) before any chemical equations, so students see equal two-way transfer and the shift after adding water
Give a decision card with four rows (add reactant, remove product, temperature, pressure) and the question to ask for each, such as "Which side has fewer gas molecules?"
Mark each equation with the energy direction and the gas-molecule count on each side before asking for predictions
For Advanced Students
Ask students to explain why the amount of product rises after reactant is added but the system never uses up all of the added reactant
Have students compare two refinements that both give more product and argue which is better once cost, safety and reaction rate are considered
Ask students to design a model, like the water transfer, that shows the effect of temperature, and state its limits
Assessment Guidance
What to Look For
Check that students change only one variable in each design and state the direction of the shift. Strong answers connect the change to the molecular level: which collisions become more or less frequent and how that makes the forward and reverse rates unequal until a new equilibrium forms. Watch for the idea that the reaction stops at equilibrium, for adding a product to get more product, and for pressure predictions that ignore how many gas molecules are on each side.
02
Classroom Activities
3 Activities
1
Bromothymol Blue and Carbon Dioxide
20 minPairs
Carbon dioxide dissolved in water forms carbonic acid, which releases hydrogen ions: CO2 + H2O ⇌ H+ + HCO3- (shown as one overall equation). Bromothymol blue is blue or green when there are few hydrogen ions and yellow when there are more. Pairs change one condition at a time and use the color as evidence of which way the system shifts.
Procedure
Safety: wear goggles. Each student uses their own straw and only breathes out through it, never in. Do not drink anything in the lab, including the sparkling water. Wash hands afterward and pour solutions down the sink.
Put 100 mL of tap water in each of three clear cups and add 10 drops of bromothymol blue solution to each. Keep Cup 1 as the control.
Cup 2, add a reactant: breathe out gently through a straw into the water for 30-60 seconds. Record the color.
Cup 3, add a reactant another way: add 20 mL of sparkling water (which contains dissolved CO2) and record the color.
Remove a reactant: pour Cup 2 into a wide dish and bubble room air through it with an aquarium pump (or swirl it in the open) for 10 minutes. Record the color every 2 minutes.
Sample Observations (invented)
Cup 1 stays blue-green. Cup 2 turns yellow after about 40 seconds of breathing out. Cup 3 turns yellow-green at once. The dish from Cup 2 goes from yellow back to green over about 8 minutes.
Discussion Questions
Which cups show a shift toward the products? What is the evidence, and what single change caused it?
Explain the color change in Cup 2 at the molecular level: which collisions became more frequent?
Bubbling air through the dish lets CO2 escape. Which direction does the system shift, and why does the color change slowly?
Why do we keep Cup 1 as a control?
Modification for Distance Learning
Students can use red cabbage juice at home as a safe indicator (it turns more pink with more hydrogen ions) and breathe out through a straw into it, or analyze the sample observations and a teacher video.
2
Water-Transfer Model of Equilibrium
25 minGroups of 3-4
Cylinder A stands for the reactant and cylinder B for the product. In each round, one student moves water from A to B with a wide straw (the forward reaction) while another moves water from B to A with a narrow straw (the reverse reaction). A straw dipped to the bottom and capped with a finger picks up an amount of water that depends on the depth, so a fuller cylinder sends more water, just as a higher concentration gives a faster rate.
Procedure
Put 60 mL of water in cylinder A and none in B. Both students dip their straws to the bottom at the same time, cap them, move the water and release it. Record both volumes after every round.
After 12 rounds the volumes stop changing much. Then add 30 mL of water to cylinder A (adding reactant) and run 12 more rounds.
Plot both volumes against the round number, as in Diagram 1. Wipe up spills at once so no one slips.
Sample Data (invented, from a group whose wide straw moves about 1/5 and narrow straw about 1/10 of the water in a cylinder)
Round
0
1
2
3
6
12
13
18
24
A (mL)
60
48
40
34
25
21
44
32
30
B (mL)
0
12
20
26
35
39
46
58
60
The 30 mL was added just before round 13.
Discussion Questions
At round 12, both straws still move water. Show that the two transfers are about equal: roughly 4 mL each way. What does that tell you about equilibrium?
After water was added to A, how did the amount in B change, and why did the forward straw move more water at first?
In what ways is this a good model of a reaction at equilibrium, and in what ways is it not?
Challenge Variation
Groups predict, then test, what happens if 20 mL is poured out of cylinder B at equilibrium (removing product), and explain the result in terms of the two transfer rates.
3
Design Brief: Refining an Ammonia Plant
25 minGroups of 3-4
Groups act as a design team for an ammonia plant that makes fertilizer: N2(g) + 3H2(g) ⇌ 2NH3(g), with a forward reaction that releases energy. They get the invented data in Diagram 2 and must recommend one refinement to the baseline design, supported by evidence and a molecular-level explanation.
Tasks
For each bar in Diagram 2, name the one condition that differs from the baseline and whether the percent of ammonia went up or down.
Explain each result with Le Chatelier's principle and with collisions: gas molecules per side for pressure, and the energy direction for temperature.
Choose one refinement to recommend. Write a one-paragraph memo with a claim, the evidence from the chart and the reasoning, and list one cost or risk of your change.
Discussion Questions
Two bars show a higher percent of ammonia than the baseline. Which conditions changed in each?
Why is it not fair to compare the higher-pressure bar with the lower-temperature bar to decide whether pressure or temperature matters more?
The plant also removes ammonia by condensing it. Why does that help even though it is not shown in the chart?
Modification for Distance Learning
Groups meet in breakout rooms, fill in a shared memo template and present their recommendation in a two-minute video.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Water-Transfer Model Before and After Adding Reactant
Invented data from the water-transfer model, drawn to scale. Each round, the wide straw moves one fifth of the water in A to B and the narrow straw moves one tenth of the water in B to A. The volumes level off when the two transfers are equal. Adding 30 mL of reactant to A makes the forward transfer larger for a while, and B rises from about 40 mL to about 60 mL: more product at the new equilibrium.
Diagram 2: Ammonia at Equilibrium When One Condition Changes
Invented equilibrium data for N2 + 3H2 ⇌ 2NH3. Each bar changes one condition from the baseline (200 atm, 450 °C). Higher pressure favors the side with fewer gas molecules, and lower temperature favors the forward reaction, which releases energy.
04
Homework Assignment
~30 min
HS-PS1-6 Homework: Refining Chemical Systems
Directions: All data are invented for practice. In every design, change only one variable at a time, and explain what happens at the molecular level. You do not need to calculate equilibrium constants or concentrations.
Part 1: Predicting Shifts (Problems 1-2)
Sulfuric acid plants make sulfur trioxide: 2SO2(g) + O2(g) ⇌ 2SO3(g). The forward reaction releases energy. For each single change, say whether the amount of SO3 at equilibrium increases, decreases or stays the same: (a) add more O2; (b) raise the temperature; (c) raise the pressure; (d) remove SO3 as it forms. (e) Explain (a) at the molecular level.
A chemist makes ethyl acetate (a solvent used in nail polish remover) in solution: CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O. (a) The chemist adds a drying agent that removes water as it forms. Explain, using forward and reverse collisions, why more ethyl acetate forms. (b) Describe a different single change to the amounts of substances that would also give more ethyl acetate.
Part 2: Using Data (Problems 3-4)
Invented data for CO2(g) + H2(g) ⇌ CO(g) + H2O(g): at 800 °C, the equilibrium mixture has 30% CO at 1 atm and 30% CO at 5 atm. At 1 atm, it has 26% CO at 700 °C and 34% CO at 900 °C. (a) What do the pressure data show, and how does the equation explain it? (b) Is the forward reaction exothermic or endothermic? Use the temperature data as evidence. (c) Specify one change to the conditions that gives more CO.
Two students propose refinements for N2(g) + O2(g) ⇌ 2NO(g), whose forward reaction absorbs energy. Student A: "Lower the pressure so the molecules have more room to react." Student B: "Cool the reactor, because cooler gases pack together and make more NO." (a) Critique each proposal. (b) Specify one correct single change that gives more NO, and explain it at the molecular level.
Part 3: Models and Design (Problems 5-6)
A group builds the water-transfer model with two straws of the same width, so each straw moves the same fraction of the water in its cylinder. They start with 60 mL in A and none in B. (a) Predict the volumes when the levels stop changing. (b) At that point they pour 10 mL out of B and set it aside. Predict the new volumes when the levels stop changing. (c) Counting the water set aside, did the total amount of "product" made go up or down? Explain what this shows about removing a product.
A lime kiln makes quicklime by heating limestone: CaCO3(s) ⇌ CaO(s) + CO2(g). The forward reaction absorbs energy. (a) A closed kiln stops making quicklime after a while even though it stays hot. Explain why at the molecular level. (b) Specify two different single refinements (one at a time) that would give more CaO, and explain each. (c) Would raising the pressure help? Explain using the number of gas molecules.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Predicting the Shift
Correct direction for every single change, using the energy direction or gas-molecule count where needed
One or two incorrect predictions
Molecular-Level Explanation
Explains how the change alters the forward or reverse collision rate and how a new equilibrium forms
States the shift without a molecular explanation
Design and Evidence
Each refinement changes one variable, is supported by data or a model, and names a limitation
Refinement is correct but changes more than one variable or lacks evidence
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All data are invented. Each question changes one variable at a time, and no calculations of equilibrium constants are needed. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
Which statement describes a chemical system at equilibrium?
Answer: C
Equilibrium is dynamic: both reactions keep going, but at the same rate, so the amounts stay constant. Choice A is a frequent misconception. Choice D confuses equal rates with equal amounts.
Question 2 of 20 · Multiple Choice
A(aq) + B(aq) ⇌ C(aq) + D(aq) is at equilibrium. More A is added, and nothing else changes. What happens?
Answer: A
Extra A means more A-B collisions each second, so the forward rate rises and more C and D form until the rates are equal again. Choice C ignores that adding a reactant upsets the balance. Choice D is backward: B is used up as more product forms.
Question 3 of 20 · Multiple Choice
For 2NO(g) + O2(g) ⇌ 2NO2(g), the forward reaction releases energy. Which single change increases the amount of NO2 at equilibrium?
Answer: B
Lowering the temperature favors the exothermic direction, which is the forward reaction here. Choice A favors the reverse. Choice C favors the side with more gas molecules (3 on the left), and choice D removes a reactant.
Question 4 of 20 · Multiple Choice
For PCl5(g) ⇌ PCl3(g) + Cl2(g), the forward reaction absorbs energy. What happens when the temperature is raised?
Answer: D
Raising the temperature favors the endothermic direction, the forward reaction, so more PCl3 and Cl2 form. Choice A would be true if the forward reaction released energy. Choice C confuses a shift with stopping.
Question 5 of 20 · Multiple Choice
For which reaction would raising the pressure (by reducing the volume) NOT change the amounts at equilibrium?
Answer: B
H2 + Cl2 ⇌ 2HCl has 2 gas molecules on each side, so neither side is favored by pressure. The others have 1 and 2, 3 and 2, and 1 and 2 gas molecules, so pressure shifts them. Picking choice C may come from counting coefficients on one side only.
Question 6 of 20 · Multiple Choice
C(s) + H2O(g) ⇌ CO(g) + H2(g) makes hydrogen. Which single change gives more H2 at equilibrium?
Answer: C
Adding steam, a reactant, raises the forward collision rate, so more CO and H2 form. Choice A adds a product, which shifts the system back toward reactants. Choice B favors the side with fewer gas molecules: 1 gas molecule on the left against 2 on the right.
Question 7 of 20 · Multiple Choice
At the molecular level, why does removing a product increase the amount of product formed?
Answer: A
With fewer product molecules, reverse collisions are less frequent, so the forward reaction runs faster than the reverse until the rates balance again. Choice B describes a temperature change, not a change in amount.
Question 8 of 20 · Multiple Choice
A student refines a design by lowering the temperature and removing product at the same time, and gets more product. What is the main problem?
Answer: D
When two variables change together, the effect of each cannot be separated. The Assessment Boundary and good design practice both call for one change at a time. Choice B is the opposite of what removing a product does.
Question 9 of 20 · Multiple Choice
In a teacher demonstration with dilute solutions, Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq), and FeSCN2+ is deep red. A few drops of concentrated SCN- solution are added. What is observed and why?
Answer: C
Adding a reactant increases the forward collision rate, so more red FeSCN2+ forms. Choice A describes removing SCN-, the opposite change. Choice D names a variable that did not change.
Question 10 of 20 · Multiple Choice
Invented data for a gas reaction show 60% product at equilibrium at 300 °C and 40% at 400 °C, at the same pressure. What do the data show?
Answer: A
Less product at the higher temperature means heating favors the reverse direction, so the reverse is endothermic and the forward reaction releases energy. Choice B reverses this reasoning. Choice D confuses the amount at equilibrium with the rate: warmer reactions are usually faster.
Question 11 of 20 · Multiple Choice
For X(g) ⇌ 2Y(g), which change in pressure gives more Y at equilibrium?
Answer: D
There is 1 gas molecule on the left and 2 on the right. Lowering the pressure favors the side with more gas molecules, so more Y forms. Choice A favors the side with fewer molecules, which is X.
Question 12 of 20 · Multiple Choice
Which proposal is a valid single-variable refinement to get more product?
Answer: B
Only choice B changes one variable. Choices A and C change two at once. Choice D changes two variables, and both changes push the system toward reactants.
Question 13 of 20 · Multiple Choice
In the water-transfer model at equilibrium, the forward straw moves 6 mL of water each round. How much does the reverse straw move each round?
Answer: C
At equilibrium the volumes stop changing, which is only possible if both transfers are equal: 6 mL each way. Choice A reflects the idea that equilibrium means no motion. Choice D would make cylinder A fill up.
Question 14 of 20 · Multiple Choice
For H2(g) + Br2(g) ⇌ 2HBr(g), the forward reaction releases energy. Which single change increases the amount of HBr at equilibrium?
Answer: A
Lowering the temperature favors the exothermic forward direction. Choice C does nothing, because there are 2 gas molecules on each side. Choice D removes a reactant, which shifts the system toward H2 and Br2.
Question 15 of 20 · Short Answer
Ethanol is made from ethene and steam: C2H4(g) + H2O(g) ⇌ C2H5OH(g). The forward reaction releases energy. Specify two different single changes that would each give more ethanol, and explain one at the molecular level.
Any two of: raise the pressure (2 gas molecules on the left, 1 on the right), add more steam or ethene, remove ethanol as it forms, or lower the temperature (favors the exothermic forward reaction). For example, adding steam means more ethene-steam collisions each second, so the forward rate exceeds the reverse rate until the rates balance again with more ethanol present.
Question 16 of 20 · Short Answer
A + B ⇌ AB is at equilibrium. Extra B is added. Describe what happens to the forward and reverse rates right after the addition and over the next few minutes.
Right after the addition, the forward rate jumps up (more A-B collisions) while the reverse rate is unchanged. As more AB forms, the reverse rate rises, and as A and B are used up, the forward rate falls. When the two rates are equal again, a new equilibrium is reached with more AB than before.
Question 17 of 20 · Short Answer
Invented data for 2X(g) ⇌ Y(g) at constant temperature: at 1 atm the equilibrium mixture is 20% Y, and at 3 atm it is 35% Y. Explain the pattern.
Raising the pressure favors the side with fewer gas molecules. There are 2 molecules of X for every 1 of Y, so the higher pressure shifts the system toward Y, giving 35% instead of 20%. At the molecular level, crowding the molecules makes X-X collisions more frequent, so the forward rate gains more than the reverse rate.
Question 18 of 20 · Short Answer
A classmate says: "To make more product, add more of the product at the start, so the reaction has a head start." Correct the claim.
Adding product does the opposite: more product molecules collide to re-form reactants, so the reverse rate increases and the system shifts toward reactants. To get more product, add a reactant or remove a product.
Question 19 of 20 · Short Answer
Your teacher has a sealed vial of an equilibrium mixture in which only the product is colored and will run your plan as a demonstration. Plan how to find out whether the forward reaction absorbs or releases energy, changing one variable.
The teacher puts the vial in an ice-water bath and then in a warm-water bath (tap water, not boiling), and compares the color with an identical vial kept at room temperature. Change only the temperature. If the color gets darker when warm, heating favors the products, so the forward reaction absorbs energy; if it gets paler when warm, the forward reaction releases energy. The teacher keeps the vial sealed and handles it, because the gas inside can be toxic; everyone wears goggles.
Question 20 of 20 · Short Answer
A factory makes a product by an exothermic reversible reaction. Lowering the temperature would give more product at equilibrium, yet the factory runs at a moderate temperature. Explain the trade-off, and name one other single change it could make instead.
At a low temperature the molecules move slowly, so collisions are less frequent and fewer have enough energy: the reaction would take much longer to reach equilibrium. A moderate temperature gives a smaller percent of product but gets there fast enough to be useful. Instead it could add more reactant, remove the product as it forms, or, for a gas reaction with fewer molecules on the product side, raise the pressure.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does HS-PS1-6 mean?
HS-PS1-6 asks students to refine the design of a chemical system by choosing a change in conditions that gives more product at equilibrium. Students apply Le Chatelier's principle and explain the change at the molecular level. The code means High School, Physical Science, core idea 1 (Matter and Its Interactions), performance expectation 6.
Is HS-PS1-6 taught in chemistry?
Yes, it is usually taught in a high school Chemistry course, in a unit on reaction rates and equilibrium, often right after HS-PS1-5. Some integrated and physical science courses include a shorter version with the same ideas.
What is the assessment boundary for HS-PS1-6?
Assessment is limited to changing only one variable at a time, and it does not include calculating equilibrium constants or concentrations. Students predict and explain shifts; K expressions and ICE tables are beyond this standard, although many chemistry courses add them later.
What is Le Chatelier's principle?
Le Chatelier's principle says that when a condition of a system at equilibrium changes, the system shifts in the direction that partly counteracts the change. The official NGSS Clarification Statement spells the name "Le Chatlier"; the usual spelling is Le Chatelier, after the French chemist Henry Le Chatelier.
What does dynamic equilibrium mean?
It means the forward and reverse reactions are both still happening, at the same rate, so the amounts of reactants and products stay constant. Nothing looks like it is changing, but molecules keep reacting in both directions.
Why should students change only one variable at a time?
If two conditions change together, there is no way to know which one caused the result. The Assessment Boundary limits assessment to one variable at a time, and it is also how engineers test and refine designs.
Does a catalyst increase the amount of product at equilibrium?
No. A catalyst speeds up the forward and reverse reactions equally, so the system reaches equilibrium sooner but with the same amounts. Catalysts are not one of the changes this standard asks about.
What safe labs show equilibrium shifts in class?
A good student lab is bromothymol blue with carbon dioxide from exhaled breath or sparkling water, with goggles and one straw per student. A water-transfer model with two cylinders and two straws needs only water. Systems with toxic or hazardous chemicals, such as the brown NO2 tubes, are best shown as a teacher demonstration or a video.
How does HS-PS1-6 connect to HS-PS1-5?
HS-PS1-5 explains reaction rates with collisions. HS-PS1-6 uses the same idea twice: equilibrium is reached when the forward and reverse rates are equal, and a change in conditions shifts the system because it changes one rate more than the other.
What comes after HS-PS1-6?
The engineering design standards, such as HS-ETS1-2 and HS-ETS1-3, ask students to break down and evaluate real design problems with trade-offs, which fits the refinement work here. In later chemistry courses students may study equilibrium constants, which are beyond this standard.
07
Related Standards
5 standards
These standards connect to HS-PS1-6: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
HS-PS1-5Prerequisite
Explain how temperature and concentration affect the rate of a reaction
NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.
