Chemical Reactions Lesson

Last updated on March 6, 2008 at 5:00AM Chemical and Physical Reactions Lesson

Part 1: Theory on Physical and Chemical Reactions What is the difference between a physical and a chemical reaction? Try to move through part one pretty quickly, especially the groups with shorter sessions. There’s a lot to be done when the kids get to do their own experiments. 1. Ask the students to define chemical and physical reactions. Write their suggestions on the board and then write the following definitions: a. Physical Reactions: A physical reaction is a change in shape, size or temperature of something where after the change, the material still has the same properties. b. Chemical Reactions: Chemical reactions are changes that involve breaking up reactant molecules to make new, product molecules. Make sure to use these terms, in your explanation. Spend some time talking about how reactants and products are what go into and come out of chemical reactions. 2. Ask for examples of physical and chemical reactions and start a list on the board of their thoughts • Prompt them with ideas such as: Physical Reactions Ice/wax melting Crushing a can with your foot •

•

Chemical Reactions Paper burning Bread rising

For physical reactions: Have the students look at their examples to help them think of definitions. What are the properties of a can before and after it is crushed? Is it still aluminum? Were the aluminum boats they made last week the same material in any shape? They look different, but they’re still made up of the same thing. For chemical reactions: Note to the students that in this sort of reaction the resulting product is not the same substance as the starting product. This can often be seen in a significant change in appearance between the reactant and product (like paper and burned paper) in a chemical reaction, although in some cases the product of a chemical reaction can look like the reactant even though it is now some other substance.

3. Next, drive the distinction between physical and chemical reactions home by having the students try to explain why melting and burning are different

Last updated on March 6, 2008 at 5:00AM a. Burning involves changing the material’s structure. May want to mention here that burning it is a reaction with oxygen since we will talk about that later on in one of the demonstrations. You can also mention that burning results in the creation of carbon dioxide and water. The students should be familiar with both of these products. b. Melting simply changes the shape the material is in. May want to remind the students of the physical states (solid, liquid, gas, and plasma which is more difficult to explain) which they will have likely leaned in class, since it is part of the California science standards for elementary school students (plasma is not part of the standards, but you can explain it by telling the students that plasma is a gas that doesn’t behave like normal gases, and that the sun and stars for example, are made up of plasma). 4. Now tell the students that you will conduct 2 experiments in the front of class to demonstrate chemical and physical reactions. Tell them one of the reactions you will show them is physical and one is chemical, and ask the students after you do each one what reaction, chemical or physical, they guess each to be. Don’t reveal the correct answers until you have performed both experiments. Experiment 1: Supplies: 1-filter flask (or Erlenmeyer or beaker, or any completely transparent container), dry ice, paper towel to cover the lid Place the dry ice into the transparent container and cover the top. Ask the students to describe what is happening. Take off the paper towel to watch the subliming gas rush out of the opening. Ask the students: Was this a physical or chemical reaction? What is happening here? What state if the dry ice in? What state is the fog in? Let the students take a guess and record the results on the board. Ask a few students to explain their hypothesis as to whether the reaction with the dry ice is physical or chemical. Experiment 2: Supplies: 1-large Erlenmeyer flask (500 mL) with tight fitting rubber stopper, 250 mL tap water, 2.5 g glucose, 2.5 g NaOH, 1 mL of 0.1% methylene blue (percentage by weight or volume; it doesn’t matter since the concentration is so low) 1. 2. 3. 4.

Add 250 mL of tap water to Erlenmeyer flask. Dissolve 2.5 g of glucose in the flask Dissolve 2.5 g of sodium hydroxide (NaOH) in flask Add ~1 ml of 0.1% methylene blue to the flask.

Last updated on March 6, 2008 at 5:00AM 5. Stopper the flask and shake to dissolve the dye. The resulting solution will be blue. 6. Set the flasks aside (this is a good time to explain the chemistry of the demonstration at a high level since the students are likely not familiar with molecules and bonds). The liquid will gradually become colorless as glucose is oxidized by the dissolved oxygen (the students will be amazed, by this colorful change). A thin blue boundary can be expected to remain at the solution-air interface, since oxygen remains available via diffusion so don’t worry if you see blue at the liquid-air boundary, since the students in the class likely won’t be able to see it from their desks (if they do see it you can discuss why they see it in terms of the oxygen which reinforces the learning goal for this part of the lesson). 7. The blue color of the solutions can be restored by swirling or shaking the contents of the flask, which you should do (the students will once again be amazed by this colorful change). 8. The reaction can be repeated several times. Demonstrate the reaction. You should have the solution ready with the glucose and sodium hydroxide dissolved in the water. Then for the demonstration you can add the methylene blue to the flask, stopper, and swirl. The students will see the color change. Then have them continue to watch to see as the color fades. Was this a physical or chemical reaction? Let the students guess, and record the votes on the board. Ask the students what they think happened in the flask. Why did it turn blue? Why did it return to clear? What is in the flask? What happens when I shake it? Why do you think it changes color when I shake it up? Shake the flask again and watch as it turns blue once more and then fades.

Explanation of the chemistry involved for the tutors:

In this reaction glucose is slowly oxidized by O2 to form gluconic acid. The gluconic acid is then converted to gluconate in the presence of NaOH. Methylene blue speeds up this reaction by acting as an oxygen transfer agent. By oxidizing glucose, methylene blue is itself reduced (forming leucomethylene blue), and becomes colorless. If there is sufficient available oxygen (from air), leucomethylene blue is reoxidized and the blue color of solution can be restored. Upon standing, glucose reduces the methylene blue dye and the color of the solution disappears.

Last updated on March 6, 2008 at 5:00AM Experimental Review and Wrap-up Reveal Experiment 1 to be a Physical Reaction, because it is simply the sublimation of frozen carbon dioxide. Explain that sublimation is a change of state between a solid and a gas directly. In essence there is no change in the molecular structure of CO2 just a change in its state/temperature. Then, ask the students to identify the terms for other changes in state. For example, have the students identify boiling, condensing, freezing, melting, evaporating. Reveal Experiment 2 to be a Chemical Reaction, because the molecules present in the solution actually change. This reaction in particular involved the reaction with oxygen, the more oxygen there is to react, the more brightly the blue shows up. As a transition to the next part of the lesson, tell the students that they will now be doing 2 hands-on chemical experiments in groups of 3 or 4. The experiments the students will be doing are an acid-base reaction and a reaction to demonstrate the difference between endothermic and exothermic reactions. Note to the students that these reactions (and many others) happen in our bodies every day.

Part 2: Hands on Chemical Experiments Divide the class into groups of 3 or 4 for each of the following experiments: Acid-Base Reaction with Vinegar and Baking Soda: Safety note: Vinegar and baking soda are both fairly harmless materials. However, don’t let the students get either in their eyes.

1. Provide an overall explanation of the lesson to the students, and while the explanation is being given, supplies will be passed out by other tutors. 2. Each group gets one plastic bowl with a small amount of baking soda in the bottom. 3. Have the students fill out their hypothesis about what is going to happen on worksheets for the chemical reactions lesson. 4. Have the students copy down the experimental procedure as you tell them on their worksheets. You should also write this procedure on the board. Tell the students make sure that they fill out the rest of the sheet as the experiment continues. This worksheet goes through the canonical scientific method for the experiment the students are doing, and as a broad model for how science is done (and as a California learning standard for elementary school science education), the students should learn and understand this model. 5. A tutor will then walk around to each group and put a small amount of vinegar on the baking soda, and allow the students to stir the resulting mixture if the reaction does not take place immediately (it should take place immediately, but it may be possible to get more out of the reaction if the students have a chance to stir the

Last updated on March 6, 2008 at 5:00AM reaction). The students should record observations about the reaction on their worksheet. 6. After the student groups complete their reactions, the tutors can do a similar reaction on a larger scale with a flask (or Erlenmeyer, or any transparent container with a narrow neck that can be closed off with a balloon) in front of the class. a. Put about an inch of water in a flask with a small amount of baking soda. b. Add some vinegar, and seal the reaction with a balloon on top. The balloon should inflate, if only mildly. This should lead to some discussion about carbon dioxide as a product in the reaction performed. The students should know what carbon dioxide is, since they will have likely learned about the greenhouse effect. Explanation of the Chemistry Involved for the Tutors: This is an acid-base reaction. Baking soda (aka sodium bicarbonate; NaHCO3) reacts with vinegar, which contains acetic acid (C2H4O2). The acetic acid reacts with the aqueous sodium bicarbonate to produce sodium acetate (NaC2H3O2) and carbonic acid (H2CO3) as follows: NaHCO3 (aq) + C2H4O2 (aq) → NaC2H3O2 (aq) + H2CO3 (aq) - The carbonic acid then breaks down further into carbon dioxide (CO2) and water (H2O). The carbon dioxide gets released as the gas.

(Someone should be cleaning up while the others continue with more reactions.) Endo and Exothermic Reactions Via Warming Pad: Start by introducing Endo and Exothermic Reactions and what they mean. For those of you who need to brush up on your chemistry, here is a basic explanation of the differences and results in Endo and Exothermic Reactions: Heat is a very important part of chemical and physical reactions. Some reactions require heat in order to proceed. These are called endothermic reactions. Endothermic reactions take the heat they need from their surroundings – the air, the vessel, etc. Think of endothermic reactions as heat thieves, taking whatever heat they see around them. On the other hand, some reactions give off (or release) heat. These are called exothermic reactions. Exothermic reactions are like the Santa Claus of heat, giving heat away to everything around them. If a reaction releases a lot of heat, it is called highly exothermic, just as a reaction that requires a lot of heat be added is called highly endothermic. If a reaction is highly endothermic or exothermic, we can actually feel the change in heat as the reaction progresses by touching the objects surrounding the chemicals (not by touching the chemicals themselves!).

Last updated on March 6, 2008 at 5:00AM 1. You will introduce to each group a small cut out of an off the shelf instant heating pad. 2. Have them all feel it and see what their response is about its temperature. 3. Have them remove the sticky backing to the pad and place it on someone’s hand 4. Monitor the temperature change (Possibly use the boxed mini lecture above to give the small pads time to heat up.) 5. Ask them questions about what they think is going on (examples are listed below.) a. Does this mean the reaction is exothermic or endothermic? Be careful! Remember that a reaction is endothermic if the chemicals in the reaction take heat from the surroundings and exothermic if the chemical give extra heat away to the surroundings. b. Draw a diagram showing the flow of heat for this reaction. Where does the heat go? c. Is this a chemical reaction or a physical reaction? d. Can you think of any exothermic or endothermic reactions that you see in everyday life?