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Engage: Sandwich Chemistry

Let’s say we want to make a sandwich with bread, cheese, and ham. We could write an equation like this:

bread + cheese + ham = sandwich

What if we made up a chemical formula for the sandwich? Let’s say the formula for a sandwich is . That means there are 2 slices of bread, 1 slice of cheese and 3 slices of ham in every sandwich. So, the equation for a sandwich would look like this:

2 Bread + Cheese + 3 Ham  

What do you notice about the number of bread, cheese, and ham slices before and after the sandwich is made?

Now, what if we wanted to make more than one sandwich? Click and drag the quantities to fill in the chart showing how many of each piece is needed for the sandwiches.

Explore 1: Conservation Clips

View the following short video clip demonstrating the law of conservation of mass. In your notebook, write a short explanation of what you saw. Why do you think this happened? Record your thoughts.

Next, view the video clip showing how the law of conservation of mass works with atoms. In your notebook, describe what you saw in the video.

What do you think would happen if there were twice as many zinc and sulfur atoms on the reactant (left) side of the reaction equation? Why? Record your thoughts.

Explain 1: Counting It Out

As you just saw in the videos, the number of each type of atom is the same before and after a reaction. If there are 5 sulfur atoms before a reaction, there will also be 5 sulfur atoms after the reaction. Atoms can’t just disappear! They might be in a different arrangement within new molecules that have formed, but there is the same number overall.

So, how do you know how many atoms are present in a chemical? The rule is to take the coefficient (the number in front of the chemical formula) multiplied by the subscript (the number to the lower right of the element symbols) for each atom in the formula. If there is no number, it represents a 1. For example:

   has 2 atoms of hydrogen and 1 atom of oxygen.

 has 1 atom of calcium and 2 atoms of chlorine.

 has 18 atoms of carbon, 36 atoms of hydrogen, and 18 atoms of oxygen. (Remember to multiply the coefficient 3 by each of the subscripts to get the total per atom.)

When an equation is written so that each type of atom has the same number of atoms in the reactants and products, we say the equation is balanced. For example:

Now, you try a few. Complete this short checkpoint activity to make sure you’ve got it. Sort each of the equations into the correct category—balanced or unbalanced—according to the law of conservation of mass.

Because the number of each type of atom on each side of the equation stays equal, we know that the total mass on each side is also equal. For example:

Just like you saw in the first video clip in the previous section, the mass of the flask before the reaction was the same as the mass of the flask after the reaction. This is the law of conservation of mass.

Be sure you have included this example and explanation in your notes!

Explore 2: Balancing Act

Now that you understand what the law of conservation of mass means and how to tell if an equation is balanced, let’s explore how to balance an unbalanced equation.

Go to the Balancing Equations Simulation (//phet.colorado.edu/en/simulation/balancing-chemical-equations). You will find a simulator that shows unbalanced equations and uses pictures of atoms to represent each chemical. (You may also use the Click to Run link in the following section to access the simulation.)

  • Once the simulation is running, make sure you’re on the introduction tab.
  • Click “Make Ammonia” at the top and “Balance Scales” at the bottom.
  • Click on the arrows next to each chemical formula to increase or decrease the number of each atom or molecule.
  • Experiment with this until the equation is balanced.
  • Record observations in your notebook about what you had to do to balance the equation.

Explain 2: Upholding the Law

OK, now it’s time to learn how to balance equations. In the simulation you just used, you saw that having multiples of a molecule is the way to increase the number of atoms needed for the equation to be balanced.

Watch the following video to see a few examples and to learn the steps for balancing equations. This method will help you balance without having pictures of atoms to look at. Be sure to take notes in your notebook, and write down the examples so you have something to look at later!

Loading Video...

Now, it is time for you to check for understanding. If you struggle with a concept or process, go back into the resource to help clarify any misunderstanding.

Elaborate: Life Abides By the Law

     

One example is in the process of photosynthesis. The chemical equation for this process is

 Light energy 

Imagine if all of the coefficients were removed. What effect would that have on the equation? Would it still be balanced? What would that change about the interaction of the atoms? If the coefficient for carbon dioxide was an 8 instead of a 6, how would that affect the products in the reaction? In light of what you now know about balancing equations, explain your answers to these questions in your notebook.

Teacher Notes

Teacher Notes
This resource is a compilation of text, videos, and other elements to create a scaffolded 5E learning experience for students. This is meant for Tier I instruction under the Response to Intervention (RtI) model for grades 10–12 and student expectation Chemistry (8)(D).

Be sure to review the entire resource and the related items before assigning it to, or working through it with, your students to check for prerequisite knowledge and skills as well as differentiation needs.

This resource can be used for instruction in a variety of ways.
• Use with a single computer and projector. (This can be delivered in a traditional classroom.)
• Use with a combination of teacher computer/projector and individual student computers (in a computer lab or other 1:1 environment).
• Assign to students as work to do outside of school as part of a flipped classroom to allow for application, practice, and additional support during the school day.
• Use with students as tutorials.
• Share with parents to inform them about what their child is learning in school.
• Use with students who are unable to participate in the traditional classroom environment.

Engage

The sandwich illustration and interactive activity are intended to give students a real-life example of balancing to which they can relate. The idea is for students to see that the number and types of ingredients do not change during the “reaction” of making sandwiches; they only rearrange. A common misconception is that atoms of one type of element change into a different element during a reaction rather than rearranging to form new or different molecules. It is common sense to students that cheese does not turn into bread, and therefore, the engage activity should lay the foundation for addressing this misconception. Students could be asked for other examples from everyday life in addition to the sandwich example in order to help them visualize this concept. Some ideas are making s'mores, building a bike, or making a car (using tires, a steering wheel, doors, and a frame).


Explore 1
The purpose of these video clips is to allow students to explore the concept of the law of conservation of mass and see what is happening in a sample reaction. They should notice from the mass shown on the balance that the mass did not change. The second clip is intended to begin to lay the foundation for addressing the concept that atoms do not change types but rearrange to form new or different molecules. This exploration could also be demonstrated or simulated in the classroom for kinesthetic learners who need to experience things in order to better interact with the concepts. The teacher could conduct a similar demo in person and have students discuss or write about their observations.

Explain 1
Students should gain an understanding of the law of conservation of mass and how to count atoms. It is vital that students master the concepts of subscripts and coefficients when counting atoms. They must be able to recognize if an equation is balanced or unbalanced before they can learn to complete the process of balancing for themselves. Another student misconception is that mass can be gained or lost during a chemical reaction. The section dealing with the masses of reactants and products is intended to address this misconception. Combining this information and the skill of counting atoms should help students bridge the gap from mass to atoms and see that both remain equal in a balanced chemical reaction.


Explore 2
Students are to experiment with balancing an equation through a simulation that allows them to see the atoms. This is helpful because it allows students to visualize what is taking place during a reaction. You do not want students to depend on a visual in order to balance equations, but it is key to helping them visualize the process. Because the simulation can be done by trial and error without understanding the process of balancing, it encourages students to explore the concept and make their own discoveries about how to balance if they do not remember this skill from middle school (8(5)(F)). In addition to the simulation, you also could demonstrate or act out the process of balancing using different colored circles of paper or balls to represent the different atoms.


Explain 2
The tutorial videos walk students through the process of balancing equations by showing an example and giving a step-by-step process. It is key that students master the steps in the process in order to be able to apply it to different equations. Once students have watched the video and have taken notes, they can attempt to practice the sample equations. A video explaining how to balance each of these equations is available in the related materials, and students should be encouraged to watch these videos in order to check their thought processes. It can also be beneficial to students to work through a few practice problems with a partner, particularly if a confident student is paired with a struggling student. One idea is to write an equation on a large piece of paper and have the students work out the balancing process on the poster. Each pair of students should have a different equation. When all groups have finished, students can do a gallery walk and check each other's work. If they find something they do not agree with on another group's poster, they can make the correction off to the side so that the owners of the poster can rethink their answer. It is strongly recommended that students be given more practice problems in order to fine tune their skills and until they reach mastery of this skill. The issue of balancing equations with the same element in more than one reactant or more than one product must also be addressed, as the total count of atoms must be done carefully. Finally, the Lavoisier video provides students with an understanding of the history behind the concept of the law of conservation of mass.


Elaborate
Students are challenged to look at balancing equations in everyday life and apply the law of conservation of mass to the example of photosynthesis. This allows students to see the importance of the concept outside the context of a chemistry lab or pharmaceutical company. They are also asked to present an explanation for a possible change to the equation and justify their thoughts. The questions encourage students to make predictions in terms of cause and effect. Upon completion of this section of the lesson, students would benefit from an opportunity to share their thoughts with others and discuss their ideas and rationale.