Next Generation Science Standards

Performance Expectation

Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

• Clarification Statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.
• Assessment Boundary: Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.

In this performance expectation, students will discover that chemical reactions involve more bond-making and bond-breaking than a season of The Bachelor. Their goal will be to create a model that shows how bond energy changes during a chemical reaction and to decide whether energy is released or absorbed as a result. And if their model is really something special, it might even earn a rose.

It's time to make it or break it, bond-style, with these activity ideas:

• Assign pairs of students to different chemical reactions. Have them create a poster to show which bonds are broken and which are created, as well as how energy is transferred between the components of the reactions and the surroundings.
• Have students work in small groups to create a short children's book detailing how energy changes through the course of a reaction. Their story should include endothermic and exothermic reactions and should be kid-friendly, while still being scientifically accurate.
• Divide students into small groups to create a claymation video showing how the bond energy changes during a reaction and whether energy is absorbed or released.
• Set up stations around the room with information describing common exothermic and endothermic reactions (like fireworks or ice packs). Have small teams of students visit each station and discuss how changes in bond energy resulted in an endothermic or exothermic reaction. After they have finished, review their responses together as a class.

Disciplinary Core Ideas

PS1.A – Structure and Properties of Matter: A stable molecule has less energy than the same set of atoms separated; one must provide at least this energy in order to take the molecule apart.

Imagine we have a molecule of carbon dioxide. It's sort of like a coffee table from Ikea. It's got parts and when they're together, they're stable enough to support a book with pictures of cats in baskets, or hang out in the atmosphere with a bunch of other molecules and not cause any problems.

Students should know that atoms like to snuggle, and when they're lonely they like to use their energy to find buddies to hang out with. When they form bonds with those buddy atoms, they release their energy and the resulting molecule therefore has a lower energy than the atoms did by themselves.

If we try to break that molecule apart into its original atoms, we're going to need to put in at least as much energy as those atoms had before they formed the molecule. Just like if we want to take apart our coffee table, we're going to need to put in some elbow grease (and maybe some colorful language).

Students might get weirded out by the fact that a big, blobby molecule is actually more stable than a lone atom. Just remind them that most atoms don't have the confidence of a complete shell of eight valence electrons, so they need other atoms to complete them. If they still don't get it, ask them if they'd rather arrive at a school dance alone or surrounded by their friends.

The fact that they're creating a model should give them a better command of this concept. Allow them to work in small groups and encourage a little peer teaching. Also, stick to relative energies of reactants and products and don't torture them with bond energy calculations.

PS1.B – Chemical Reactions: Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy.

Atoms are party animals. Sometimes that party involves a sit-down dinner and, well, sometimes it's a mosh pit. Either way those atomic party animals are constantly colliding and rearranging themselves to form new molecules.

Students should know that all of the different aspects of a chemical reaction, like how quickly it occurs or whether it is endothermic or exothermic, depend on how often the atoms or molecules involved collide with one another.

If we increase the temperature or the concentration, it's sort of like turning that dinner party into a mosh pit—we're increasing the chances that atoms will collide with one another and a chemical reaction will occur, and thereby increasing the rate of the reaction. Cooling things down or decreasing the concentration makes it more difficult for particles to bump into each other, which reduces the rate of the reaction.

Students also need to understand that there is some kinetic energy, or motion energy, involved in all of this partying. Basically the kinetic energy present in an atom before a collision is going to be converted into bond energy after that atom collides to form a molecule. What's a party without kinetic energy anyways?

You'll be tackling some pretty heavy-duty concepts here, so don't be alarmed if your students start playing possum in hopes you'll stop talking. Take it slow, encourage them to use their models, and don't worry about making them calculate the total bond energy changes. There's also nothing like a good lab partner to bond over bonding with.

Science and Engineering Practices

Developing and Using Models: Develop a model based on evidence to illustrate the relationships between systems or between components of a system.

Making and breaking bonds is really just a slightly more complicated version of Red Rover. In order for your students to really understand how bond energy impacts whether a reaction releases or absorbs energy, they're going to need to create a model. This model can be a molecular level drawing, a diagram of a reaction, a graph, or any other creative representation that helps them visualize how making and breaking bonds releases or absorbs energy.

Oh yeah, the model should be based on evidence, so let them collect calorimetric data on their own, do a class demonstration, or find a good demonstration online that they can cull data from.

Crosscutting Concepts

Energy and Matter: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system.

Students will become energy bloodhounds as they track changes in bond energy and the energy released or absorbed during a chemical reaction. In order to be effective energy trackers, they should understand that the reaction and its surroundings make up a system.

Energy shuffles through that system and can even flow into or out of the system to the surroundings. For example, the chemicals in hand warmers undergo a reaction and some of the resulting energy that is released is lost to your very lucky hands that just happen to be a part of the surroundings.