Next Generation Science Standards

Performance Expectation

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 Chatelier’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.

Have you ever made a batch of cupcakes and wished you'd made more? Happens to us every time. If only there was a way to prevent such a travesty…

In this performance expectation, students will put their greedy pants on and figure out a way to make more product when a chemical reaction is at equilibrium. They'll use their expertise in equilibrium and understanding of what is happening at the molecular level to refine the design of an existing system so they can end up with more product. And if that product happens to be cupcakes, well, that wouldn't be the worst thing.

Be a glutton for product with these activity ideas:

• Provide small groups of students with different chemical system scenarios. Ask them to discuss the reactions involved and how they would change the components of that reaction to produce more product when the reaction is at equilibrium. Share their ideas as a class and address any misconceptions about reactions at equilibrium.
• Have students study the chemical reaction of cellular respiration. In small groups, have them discuss the reaction and potential ways they could change the reaction to generate more ATP.
• Divide students into small groups. Provide each group with two large beakers of differently colored water (food coloring is fine, no need to gather questionable pond water). Explain that if they were to take a pipette and transfer water back and forth from each beaker, they will eventually see a color change in both beakers and this represents equilibrium. Then assign students to set up a system that allows them to produce a greater amount of the new color of water. They will test their new system and make any necessary changes until they can produce more product.
• Provide pairs of students with a bunch of LEGO pieces in two colors. Have them put two of the pieces together to form a molecule, then make five total molecules. Then explain that one student will be the forward reaction, building the molecule while the other is the reverse reaction, taking the molecule apart. For this portion, they must always maintain five intact molecules. Start some music and have them begin working on their task. After a minute or so, stop and discuss what they needed to do individually to keep five intact molecules at all times. Then have them discuss how they can alter their system to end up with ten intact molecules. Share different ideas as a class, then turn on the music and see if their plan works.

Disciplinary Core Ideas

PS1.A – Structure and Properties of Matter: The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms. (Secondary to HS-PS1-6)

To get started here, students need to understand the whole concept of a system at equilibrium. Have them imagine that Mama Shmoop has made a stack of ten pancakes. As they remove pancakes from the stack and promptly put them in their belly, Mama Shmoop continues to add them at an equal rate so the stack is constantly at ten.

Now let's say they realize they're running late and start to eat pancakes at a much faster rate. What happens to the stack of pancakes? It decreases. What does Mama Shmoop need to do to maintain equilibrium? Cook faster!

The same is true in reactions at equilibrium. They occur at steady rates with steady concentrations of products and reactants. Then a stressor comes along. We're not talking about taking your behind-the-wheel driving test or sitting outside the principal's office kinds of stressors. Reactions at equilibrium get stressed out by changes in concentration, temperature, or pressure (but only for gases on that last one). It's these atomic interactions that determine how the reaction will proceed and the final equilibrium it will reach.

PS1.B – Chemical Reactions: In many situations, a dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present.

Okay, we've got a reaction running, and we just changed the concentration, temperature, or pressure. What happens next?

This is where Le Chatelier's Principle comes in. Hint: it's not a fancy restaurant or the best place to go alpine skiing. Students should know that Le Chatelier's Principle tells us that when a system gets stressed, it will readjust itself and create a new equilibrium.

Depending on the stressor and whether the forward or reverse reaction is stressed, we will end up with increased or decreased amounts of our products and reactants.

For example, if we increase the concentration of one of the reactants in the forward reaction, we can expect to see a decrease in the other reactant and an increase in products. This concept will come in handy as students work to increase the amount of products at equilibrium.

Students will tend to think that equilibrium means equal products and reactants, which is not usually the case. Since equilibrium can be tricky for students to visualize, use models, diagrams, and simulations wherever you can.

You also don't need to teach them how to calculate equilibrium constants and concentrations, but sometimes seeing a math example with real numbers instead of just variables can actually help some students.

Also, stick to changing only one variable at a time. There's no need to stress about what will happen if we increase the pancakes and the syrup at the same time (we don't even want to go there).

ETS1.C – Optimizing the Design Solution: Criteria may need to be broken down into simpler ones that can be approached systematically, and decisions about the priority of certain criteria over others (trade-offs) may be needed. (Secondary to HS-PS1-6)

Well look here, we've had a visit from the engineering fairy. Beware: her wand is infinitely more effective and energy efficient than your typical fairy wand. It appears she has left some sage advice for your student chemists as they venture on their quest for increased products.

In attempting to refine a design, students are going to inevitably make some changes for the better, and some for the worse. You can't win them all, right? As frustrating as this may be, students will have to find a way to prioritize the criteria for their solution.

This process may involve breaking criteria down into chunks that are easier to deal with. It may also involve having to pick and choose what criteria can be met and which will be abandoned for the overall good of the solution.

Students will need to make some tough decisions and some students (you know who they are) may really struggle with the fact that they won't be able to meet all of the criteria. Have them work in teams to bounce ideas off of each other and discuss the pros and cons of each set of criteria so they can reach a conclusion that works for everyone.

Science and Engineering Practices

Constructing and Designing Solutions: Refine a solution to a complex real-world problem, based on scientific knowledge, student-generated sources of evidence, prioritized criteria, and tradeoff considerations.

Your problem-solving students are going to have their hands full with this performance expectation. They will need to revise a design of a chemical system to generate more product when the system is at equilibrium. They'll have to deal with a real-world problem, which can be something as benign as how to make fluffier pancakes, or something really problematic like oxygen or carbon monoxide levels in the bloodstream.

In doing so, they'll need an arsenal of tools that rivals Batman's Bat Cave. They will need their knowledge of atomic structure and chemical reaction basics, as well as any evidence they've collected during their experiments. They'll also need to create a list of prioritized criteria, so in the event they can't meet all of the criteria, they know where to make tradeoffs. Alfred better get crackin'.

Crosscutting Concepts

Stability and Change: Much of science deals with constructing explanations of how things change and how they remain stable.

If you wanted to put this performance expectation in a nutshell, this statement accomplishes that pretty nicely. Basically your chemistry commanders need to be able to explain how chemical systems can remain stable, or at equilibrium.

They'll also need to be able to explain what happens when that equilibrium is disrupted, or changed. Then they're going to use that information to manipulate the system and make a whole bunch more product than would have been made initially. Muahahaha!