Investigation (Nathan Macy)

Title: Le Chatelier’s Principle, and Chemical Equilibrium

Standards:

Chemistry

Chemical Equilibrium

9. Chemical equilibrium is a dynamic process at the molecular level. As a basis for understanding this concept:

a. Students know how to use Le Chatelier’s principle to predict the effect of changes in concentration, temperature, and pressure.

Materials:

· 2 plastic cups for each student or pair of students

· Hydrogen peroxide

· Vinegar

· Manganese Dioxide

· Baking soda

· Wooden splints

· lighters

Procedure:

1. Pour several ounces of the peroxide into 1 of the cups being used by the student. Then pour some vinegar into the other cup. (this quantity can be less than the Peroxide, but make sure that there is at least half the cup empty)

2. Once each student or pair has a set of cups distribute the splints and the lighters. Instruct the students to light the wood and stick it into the empty portion of their cups. Blow out the splints and note that nothing happened.

3. Have the students add a small amount of the Manganese Dioxide to their Peroxide, (this is a catalyst, so only a little is needed), then add a sizable amount of baking soda to the vinegar, (this is a titration, so you will need to add much more than the Manganese)

4. Instruct the students to light their splints and then stick them into the EMPTY portion of their Vinegar cup. Tell them to quickly move the smoldering splints to the empty section of the peroxide cup and see what happens

Student prior knowledge: types of reactions, gases, reaction rates*, and combustion. (*familiar with the term; doesn’t have to know)

Explanation:

When we discussed reactions in class, it is very possible that this class may have gotten the idea that a reaction happens and that is the end. However, that is not always the case. Some reactions go in reverse as well. But wait! What do I mean by reverse reaction? Don’t all reactions just go forward? Nope. All reactions have the potential to go in reverse, and many of them do. In fact, many reactions have both forward and reverse reactions happening at the same time.

As we learned before, Chemical reactions proceeds at different rates. Generally speaking these rates are dependent on many different factors such as activation energy, (which is the amount of energy required to start or activate the reaction), temperature, concentration, and particle size. One of the ways that we can calculate the reaction rates is to let the reaction proceed to the very “end”, and use the quantities of the reactants and products in a rather complicated equation that we will learn in the near future.

In the case of a reversible reaction, this “end” of the reaction is called equilibrium, and is what happens when the forward reaction rate is equal to the reverse reaction rate. Remember how I said that soon we would have to learn an equation to calculate the rates? Well now is not that time when we have to learn it, but just remember what I said about the quantities of the reactants and the products. They are used to calculate the rates. So, therefore, as such, and etc, if the rates are calculated from the products and reactants, then as the products and reactants change in quantity than the rates change as well. When the rates stop changing in a reversible reaction we have equilibrium. You can of equilibrium as the reaction’s happy place because it tries to get there as soon as possible. Now riddle me this, if Equilibrium this state that makes the reaction “happy”, (why else would the reaction always proceed there?), then what will happen if you change something that affects the reaction? This is a question that Henri-Louis Le Chatelier asked. And he came up with this principle. If stress is applied to the system in a dynamic equilibrium, the system changes to relive the stress.

Think about it this way. If we all went on a trip to Shadow Lake in Mammoth Lakes so that we could go swimming, we would have to do a lot of hiking to get there, and probably brave some mosquitoes as well. Soon we would reach Thousand Island Lake, which is really pretty, but really cold because there is still snow there even in June! We don’t want to swim in the snow, so we bandage our blistering feet and continue hiking to Shadow Lake. When we get there, we discover that it too has snow, (we really should have realized that lake up in the mountains have higher elevations that are often cold even in the summer), but we are hot and tired and our feet hurt so we go swimming anyways. Donny thought that it would be a good idea to bring a boat, and he and Andrew lugged it all the way up to Shadow Lake. So we all hop in the lake and swim out to the middle, Donny however paddles the boat with Amanda Andrew and Avni. When we reach the middle of the lake we realize that it really is cold, and everyone wants to be in the boat. But the boat only holds 4 people, so if Taylor decides to get in, somebody else must go for a swim.

This is an equilibrium, with 2 reactions happening. Reaction 1 is people getting cold in the water, (kind of like an exothermic reaction), when they get too cold, they climb in the boat and let the sun heat them up, (kind of like an endothermic reaction). However, this is an equilibrium, so when we get too many people in the boat, it drives somebody else out.

Equalibrium: After doing the experiment:

So what happened in what we just saw? How would you describe it to someone? Stepping back, what was taking place in the various cups? What was being formed? Why did we only add a little manganese dioxide to the hydrogen peroxide?

We formed gas in both cases. In the first reaction, H₂O₂ degrades to H₂O and O₂. In the second, vinegar, or acetic acid (CH₃CO₂H) to give us CO₂ Among other things.

So these are our chemical reactions:

2H₂O₂(aq) → O₂(g) + 2H₂O(l)

2CH₃COOH + CaCO₃ -----> Ca(CH₃COO)₂ + H₂O + CO₂

What do we know about Oxygen and Carbon Dioxide? Well one is a reactant in a combustion reaction, and the other is a product. According to Le Chatelier, we know that if we add products or reactants to a system, it stresses the system into changing the rate.

Questions & Answers:

Question. What drove Le Chatelier to discover his principle?

Answer. Le Chateleir had a curiosity about why reaction happened differently. He then took and experimented to see what would happen if he added different stimuli to various reactions.

Question. What things in science are you curious about and how might you go about discovering the solutions?

Question: In the applications below, we discuss hemoglobin and oxygen. In high altitudes this could cause a problem with breathing. what might the body do to "stress" the interaction between oxygen and hemoglobin to compensate and allow for the transfer of oxygen.

Answer: the body will start producing higher concentrations of 2,3-Bisphosphoglyceric acid, which help regulate hemoglogin's affinity for oxygen.

Applications to Everyday Life: There are several applications to everyday life:

1. Robert Grubb won a Nobel Prize in chemistry for his work with a Ruthenium based catalyst that can make certain forms of epoxies, including ones that can heal small cracks. during his research.

2. Any time we add stress to a situation, the situation changes, this includes our relationships, which can change for better or worse depending on what we add to the mix.

3. Hemoglobin works to carry oxygen from the lungs to other parts of the body. Oxygen only binds to hemoglobin if relatively high concentrations are present.

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Videos: