Performance Expectation (PE)/Disciplinary Core Idea (DCI)
This lesson is aligned with HS-PS1-7, the uses of mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction; and DCI-PS1.B, the fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. Student were introduced the previous day to the 5 different chemical reactions that show that conservation of mass can occur several of ways during a chemical reaction.
Science and Engineering Practices (SEP)
This lesson does not directly tie into any of the SEP.
Crosscutting Concepts (XC)
During this lesson students will recognize that patterns occur in science. Students are expected to see that patterns exist in each of the 5 types of reactions they and be able to recognize each reaction based on these patterns.
The previous day students finished the lesson by categorizing 10 different reaction into the five different reactions types: synthesis, decomposition, single replacement, double replacement and combustion. To start the class, I have each student write down on a single sheet of paper one example of each. They are told to use their notes to aid them in this. This activity will help them recall what was learned from the previous day. After a five minutes I have them turn it, so that I can look at it after class. When they are done they take out their notebooks.
The two previous days consisted (day 1 and day 2 links) of students participating in the "Explore" stage of inquiry. During this process they were building a personal understanding of reaction types that might not necessarily be 100% accurate, but can be used as a scaffold to create a deeper understanding through formal explanation (lecture). This "Explain" portion of inquiry is critical because it helps students correct any misconceptions they may have attained during the Concept Attainment portion of the lesson.
I find the best way to help student take notes is to make them as interactive as possible. The Powerpoint I use was made by a coworker of mine and uses a relationship theme as analogy to represent each type of chemical reaction. I start by showing each type of reaction using a letter representation, such as A + B -->AB, to show the process of reactants changing into products. This is followed by the name of the reaction, an example of the reaction, finally followed with a relationship analogy. For example, man + woman --> married couple. The analogy provides another way for students to scaffold the information into a deeper understanding. After explaining all the information on the slide, I perform a demo that illustrates the reaction type. For example Mg + O2 --> MgO. Each slide and demo takes about 7 minutes.
Slide Explanation and Demo
All Demos should be done wearing goggles and an apron.
Slide 1-Combination reaction demo
For the synthesis reaction I fill 3 balloons with a small amount of different gas. The first one contains helium, the second contains a small amount (cantaloupe size balloon) of hydrogen gas and the third is filled with a mixture of 75% hydrogen and 25% oxygen (a little smaller than the previous balloon). see video
Behind a blast shield I first attach a helium balloon to a ring stand and ignite it using a wooden splint attached to a meter stick. This makes a little pop. Most kids are unimpressed. Next I do the same with the hydrogen filled balloon. Before igniting it, I tell them that when I throw parties I use special balloons and then ignite it. This balloon makes a fairly loud boom and will startle some students. I follow this up with the smallest of the balloons the hydrogen/oxygen filled balloon. I than tell them we are going to try this again, but this time cover your ears. This balloon make a significantly louder boom, so it’s important to warn the students.
After completing all three balloons I tell them what was in each one. I follow this by writing the synthesis equation, H2 + O2 à H2O on the board and explain that when hydrogen and oxygen are combined they make water and release a lot of energy. I ask them to image what the Hindenburg looked like as it was going down.
If these gasses are not present an alternative synthesis reaction that I use sometimes is the burning of Mg. This is simple and an effective way to illustrate this type of reaction.
Slide 2-decomposition reaction demo
Slide 3-single replacement
Slide 4-Double Replacement
Do not dump the solid down the drain. Use filter paper to separate the solid from the liquid. Let dry and dispose of in the garbage when dry. You can use smaller amount of each to produce less waste.
Slide 5 and 6-Combustion reaction
Generally the kids love this demo because it sounds like a jet engine. If you have difficulty igniting it the first time, add a little more alcohol, swirl and try again.
The part that is most difficult for students is predicting products. This is not the main focus of the unit but it does need to be covered. Typically they forget that some elements form diatomic molecules and also don't correctly write the chemical formulas. The only product they have to predict for the unit test is CO2 in a combustion which is memorization. The next day's lab activity will give them more practice in predicting products.
After everyone has written the notes and seen the last demo I hand the Types of Chemical Reaction Worksheet.doc homework out (answer key). The purpose of the homework assignment is to provide students with a final chance to categorize a variety of chemical reactions, then balance them and predict some products. This provides reinforcement for the lab the following period.
Most students had no difficulty completing the types of reaction and balancing portion of the worksheet. Some students had some difficulty predicting the products because they have only had limited exposure to the concept. The predicting portion of the lesson was not the main focus of the assignment; however, they will have to recognize the products present in a combustion reaction and single replacement reactions. Two thirds of the students that completed the assignment were able to see patterns in the reactions and were able to predict the products in problems #9, 10, 14 & 17.