PE: MS-PS1-5 - Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.
DCI: PS1.B: Chemical Reactions – The total number of each type of atom is conserved, and thus the mass does not change.
Scienceand Engineering Practice (SP) 2: Developing and Using Models
CCC: (5) Energy and Matter
Chemical reactions are mysterious things that happen at a molecular level that may be hard for students to grasp. This demonstration has students works with models then observe the exact chemical reaction they modeled (MS-PS1-5). Your students will observe two chemical reactions that demonstrate that atoms are never created of destroyed they simple change forms (PS1.B). Before the reaction takes place I have students model the reaction using chemical reaction model to get a hands-on understanding of what is occurring (S&E Prac: Developing and using models). This demonstration can be connected to other learning by tying it to Energy and Matter (CCC – Energy and Matter).
Materials: Wal-Mart toilet bowl cleaner or HCl, Mg ribbon, flask, balloon, meter stick, coffee stir-stick, matches.
Pour approximately 150mL of Wal-Mart toilet bowl cleaner or HCl in a flask.
Tip: I use Wal-Mart toilet bowl cleaner (look for HCl as its main ingredient) because it is cheaper and just as effect as HCl. It has a blueish tint to it, which I explain is blue food coloring that I add to highlight the chemical reaction. I don’t tell them that it is toilet bowl cleaner because this is a potentially dangerous and explosive reaction that I don’t want them to recreate at home.
Wind approximately 10 cm of Mg ribbon around a pencil and insert into the balloon.
Stretch the opening of the balloon over the mouth of the flask. When you are ready for the reaction to begin, lift up the balloon and the Mg ribbon drops into the HCl (toilet bowl cleaner) solution.
I place the following two chemical reactions on the board:
1) 2Mg + 2HCl -> 2MgCL + H2
magnesium + hydrochloric acid produces magnesium chloride and hydrogen gas
2) H2 + O2 -> H2O
hydrogen gas + oxygen gas produce water
1) The magnesium ribbon reacts the hydrochloric acid in what is called a single replacement reaction. Magnesium replaces hydrogen and bonds with the chlorine, allowing the hydrogen to become a gas and float out of the solution. The balloon traps the hydrogen and inflates to a small balloon. This reaction is exothermic as it releases energy.
2) The hydrogen gas reacts with the oxygen in the air, but needs a spark to initiate the reaction, which is known as a combustion reaction. A small amount of water is produced on the inside skin of the balloon. Since this reaction releases heat it is an exothermic reaction.
Demonstrate proper laboratory safety, wear eye protection, gloves, and lab coat.
I first start out this demonstration by explaining to my students that they will be observing an exothermic, single-replacement reaction when mass is neither created nor destroyed -- just rearranged. This will follow a second reaction that will be an exothermic, combustion reaction that will once again conserve all the mass in the reaction.
I refer to reaction one on the board.
1) 2Mg + 2HCl -> 2MgCL + H2
I explain that 2 parts of magnesium (holding up the sample) will combine with two parts of hydrochloric acid (point to sample) to produce (arrow means produce) magnesium chloride and hydrogen gas. At no point will any atoms be destroyed -- they will simply be rearranged into different molecules. I also point out that ingredients (2Mg + 2HCl) are referred to as reactants and the resulting chemicals (2MgCL + H2) are known as products. I let them know that the magnesium is replacing the hydrogen to bond with the chlorine, kicking the hydrogen out (like cutting in on a slow dancing couple). The free hydrogen will bond with another free hydrogen to form a gas and exit the solution and inflate the attached balloon. The reaction will release heat and be defined as an exothermic reaction.
I then tilt up the reaction and let it commence. After the balloon has filled I carefully pry off the balloon and tape it to a stand. Wearing gloves I walk the flask around and allow the kids to carefully touch the flask confirming it is hot (check it yourself first), thus confirming it is an exothermic reaction.
I have a class set of chemical reaction model that the students work with in order to simulate this reaction.
Referring now to the second equation on the board.
2) H2 + O2 -> H2O
I explain that now I will ignite the hydrogen balloon and the class will see a small explosion causing an exothermic combustion reaction. The hydrogen will react with oxygen in the air, but needs activation from an outside spark to trigger this reaction. Combustion reactions require three things (fuel, oxygen, and ignition). I ask if any of my students are interested in firefighting and explain that these are the same ingredients in the fire triangle taught in fire-science courses.
Using a meter stick with a coffee stirrer taped to the end (I fray the end of the coffee stirrer to help with lighting) I light the coffee stirrer with a match and turn off the lights. Carefully hold the flame directly next to the balloon and a pop with a small explosion will occur. Immediately after the explosion, I carefully grab the balloon, turn it inside-out and show the class the moist inside of the balloon. Explaining that some hydrogen combined with some oxygen, I share that this hydrogen-oxygen bonding created from new molecules of water that have never been part of the water-cycle here on Earth.
Capture the energy and interest, and show the chemical reaction models to show students a simulation of what is happening.
I find it very helpful to students that they know something about the people behind the topics they are covering (History of Science). As we are covering the Law of Conservation of Mass I talk about Antoine Lavoisier, who discovered that matter is never created or destroyed, it just changes form. My students get a kick out of science trivia -- he married a 13 year-old girl who became an essential lab assistant.
I have created three PowerPoints that provide background information for this demonstration: