Chapter 4 4B: Indicators of Chemical Reactions A NATURAL APPROACH TO CHEMISTRY Key Question: How can we tell when a chemical reaction is taking place? In this Investigation, students perform five qualitative chemistry experiments, each showing a different indication that a chemical reaction is happening. The goal of this lab is not to perform any calculations or balance any chemical equations; it is simply to make them aware of what to look for when performing chemistry experiments. Specific labs have been chosen using simple chemicals that show definitive results, with explanations on what to look for. Reading Summary Questions and Goals Students read section 4.1 in NATC before the Investigation. Chemical change takes place when bonds are broken and reformed. Physical changes are reversible in the sense that they can return back to their original state. Chemical changes are dictated by the energy and reactivity of the ingredients in the chemical reaction. In a chemical change one substance is changed into another substance, as bonds are broken and reformed. Some chemical reactions are accompanied by a release or absorption of heat. Other types of chemical change produce a gas, or a color change. A precipitate reaction occurs when a solid is formed during the mixing of two aqueous solutions. A precipitate is a compound that is not soluble in water. Oxidation and reduction reactions are another important class of reactions. During this type of reaction an electron is transferred from one substance to another. Some of these reactions can cause color changes and the formation of new solid substances. Lastly, acid and base reactions can be identified by using an indicator such as phenolphthalein, and noting a color change. Main Questions Learning Goals Key Vocabulary How do we know when a chemical reaction is happening? What are the various ways by which we can tell that a chemical reaction is happening? Can we see a chemical change? What physical changes indicate the presence of a chemical change? By the end of the Investigation, students will be able to: Have descriptive knowledge of chemical change. Understand some characteristics of chemical change. Be able to identify some types of chemical change. Remember the different physical changes that indicate a chemical change. Recognize these changes in the lab during an actual experiment. Chemical change, physical change, gas evolution, precipitate, color change, acid, base, clock reaction, neutralization, hydration, temperature change, chemical bond, anhydrous, indicator, irreversible change, aqueous. 85 Chapter 1: The Science of Chemistry
Materials and Setup Details 4 B Students work in groups of four to six at lab tables. Each group should have: 20 ml vinegar 20 ml phenolphthalein 5 g of dry (anhydrous) copper sulfate powder 20 ml 1 M NaOH 40 ml of 3% hydrogen peroxide (H 2 O 2 ) 1 g iron powder 20 ml 1 M AgNO 3 solution 10 g of NaOH pellets 60 ml of water 30 ml of Vitamin C solution 30 ml of 3% iodine tincture 30 ml liquid starch solution Five 40 ml test tubes Glass stirring rod LabMaster with temperature probe Students will have to wash and dry their test tubes between experiments, as there won t be enough to perform all the experiments without washing them. The group size can be large, since it is more important for the students to see the reactions happen than perform them. Each student will ideally perform one for the others in the group. Preparation You will have to prepare solutions of Vitamin C, liquid starch and the other chemicals if they are not purchased at the correct concentrations from Lab-Aids. You will also have to procure the iodine tincture, vitamin C tablets, hydrogen peroxide and vinegar from the grocery store or pharmacy. Finally, make sure the LabMasters are charged before the students start their experiments. Teaching Time Preparation Assignments Misconceptions One class period Outline of the Investigation Prepare any aqueous solutions ahead of time. Allow the students to measure their own solutions and powders, and perform the experiments. Section 4.1 in the Student Text before the Investigation. Students may not realize that a precipitate is a solid, unless it is dried and filtered. In the acid-base reaction, because the addition of more vinegar makes the pink phenolphthalein color go away, students think this is a physical change because it appears to be reversible. Introducing the investigation 1 Chemical safety rules 2 Changes in color - hydration 3 4 Changes in color - acids and bases 5 6 Timed reactions 7 8 Precipitate formation reactions 9 10 Temperature change reactions 11 12 Gas creation 13 Indicators of Chemical Reactions 86
Investigation sections Part Ideas and Dialog Introducing the Investigation Review the types of reaction indicators, and explain to the students why they need to see each type rather than just read about them. Chemical safety rules Make sure the students understand proper safety procedures and equipment before starting. Changes in color - hydration Explain the difference between a hydrated and anhydrous salt, and that sometimes the color can change depending on the hydration. Get the students to think together about what they saw. You can also have them try to dehydrate and rehydrate the salt to show the principle of hydration. In your reading, you have learned about the different indications that a chemical reaction is happening. It is also important to see them as well as read about them, because we will be performing many more labs in the near future. It is important that you can recognize when a reaction is happening so you can perform the labs correctly and efficiently. This lab will set the stage for many more to come. This is the first time that the students will be exposed to a multitude of more dangerous chemicals than in previous labs, so they need to be able to quickly recognize when a reaction is happening. They also need to perform all safety procedures correctly, so it is more intuitive in labs to come. 1 This experiment will use some chemicals that should not be handled with bare hands. Wear protective gloves. Wear eye protection while doing these experiments Explain why gloves are necessary in these experiments. Even if the starting chemicals aren t corrosive, the things that are created may be! Some reactions, such as the gas creation one, can get violent depending on the strength of things. Explain the need to protect the eyes at all cost, and also mention to always point vessels away from all other people, just in case. Go around the classroom and make sure the students are all wearing gloves and goggles before proceeding. If you spill any chemicals, inform your instructor immediately. Wash any areas of skin or clothing with cold water. This should have been explained before, but be sure to mention it again. Students should never be afraid to seek help when they need it instead of trying to figure out how to clean up a spill. Also, this is the time to tell the students that if they spill dangerous chemicals (such as acids) on their clothing, that the clothing should be removed IMMEDIATELY. There is no time for modesty when it comes to safety. Make sure the students understand this well and are mature about it before starting. 2 Put 5 g of dry copper sulfate powder into a clean, dry test tube. Observe the color of the dry salt. The salt should be very white at this point. It may have started to turn blue on a humid day. It is best to dry it in an oven of sorts (NOT a microwave!!!) immediately before the experiment. Pour 20 ml of water into this test tube. Observe what happens. The students should notice the white salt turn blue, and then a blue solution as they continue adding water. The color change happens when the salt hydrates. Clean out your test tubes in the sink, then answer the questions in part 3. 3 How could you tell that a reaction happened? This is the first reaction that the students will observe, so make sure to emphasize that the color change is an indication of a reaction - a reaction may or may not have been happening before the color change. This is an example of a hydration reaction, where a dry (anhydrous) salt absorbs water. What does the word anhydrous mean? Anhydrous means without water - an means not, and hydrous means contains water. Go over the chemical formula for hydrated and anhydrous copper sulfate to illustrate. What do you think would happen to the salt if the water were taken back out (if the solution was dried)? If the salt were dried again, it should turn white again. It would then be dehydrated. It can be dehydrated and rehydrated as many times as you want. 87 Chapter 1: The Science of Chemistry
Investigation page Sample answers Example Answers 3.1. There was a change in color. The salt changed from white to blue when water was added. 3.2. Anhydrous means without water. It is the chemical term for a dry salt. 3.3. If the water were removed the salt would appear white again and become anhydrous. This is an example of a physical change. Teaching Tips This lab is intended to provide students with some clear and tangible examples of chemical change. The lab shows students what to look for when deciding whether or not a chemical reaction has taken place. Students really enjoy and remember the indicators much better if they experience them first hand. Students develop a descriptive sense of some basic types of chemical change. Before the lab it is helpful to discuss two polyatomic ions with them, such as OH -, and NO 3 -, because they are used in this investigation. Polyatomic ions can make things confusing for students, so it is good to address these at the start. Keep it simple and clear. These polyatomic ions act together as one unit (group). To illustrate hydration, you can have the students spray a water mist on a pile of dehydrated copper sulfate. The salt will begin to take on a blue tinge. You can then explain that the moisture in the water mist is hydrating the salt. the change in color is a result of the hydration process. To demonstrate dehydration, you can put the hydrated salt on a heater and watch it turn white. You can even put copper sulfate solution in a test tube or in the desiccator and use the LabMaster to heat it. Once it begins to dry out, the students will notice that white crystals will form where the solution dries out. 4 B: Indicators of Chemical Reactions 88
Investigation sections Part Ideas and Dialog Changes in color - acid and base reactions Have the students see that sometimes a reaction can happen without its own color change. However, indicators can be used to show the presence of a reaction. The students should think about the fact that an indicator gives a color change to indicate a certain type of reaction - in this case, a change in ph. A reaction might need time to happen Here the students will see another color change reaction that takes time to occur. This should show them that not all reactions are instantaneous, and that sometimes they have to wait for a reaction to happen. Get the students to think about why the color change happened, and why it took time to happen. Also, a color change indicates a reaction happened - it may have started before the color change! 4 Place approximately 2 ml (20 drops) of vinegar in a test tube. The acid in vinegar is called acetic acid (HC 2 H 3 O 2 ). Add 2-3 drops of phenolphthalein indicator to the vinegar. Swirl to mix. Indicators can be used in small amounts, as they create drastic color changes dependent on ph. Using a dropper bottle, slowly add drops of 1.0 M NaOH to the vinegar. Make sure the students do this one drop at a time. This way they will be able to see just how sensitive this sort of reaction is. The NaOH should be added while swirling. Once one group gets close to neutralization, have them perform it for the class to see how long they must mix to make the phenolphthalein stay pink. 5 How could you tell that a chemical reaction happened in this experiment? The change in color indicated that a chemical reaction was happening. HOWEVER - it only indicates that a certain ph has been reached. A chemical reaction was happening from the first drop of NaOH, but there was no color change until the ph hit about 7. What color formed in the test tube? What element could have caused that color? The solution should turn dark pink - the color of phenolphthalein in a base. The concentration of H + ions causes the phenolphthalein to change color, so hydrogen in the answer. It is not an element that the students see, but its effect on another chemical. Try this: add a few more drops of vinegar and record your observations. Where did the color go? The solution was re-acidified. Once the ph drops, the color goes away. The effect can be repeated as many times as desired. If the students have time, repeat it a couple of times to illustrate this effect. 6 Put 10 ml of water in a test tube. Add 4 ml of Vitamin C solution and 4 ml of iodine tincture to this test tube. The liquid should be clear. This first solution contains the iodine (which will stain the starch) and vitamin C, which is used as a safe alternative to a concentrated acid like sulfuric acid. Put 20 ml of water in a second test tube. Add 4 ml of hydrogen peroxide and 4 ml of liquid starch solution to this second test tube. This second solution contains starch, which will be stained once the acid in solution 1 runs out. Pour the contents of the second test tube into the first one. Watch the test tube for a couple minutes. After mixing the two solutions, nothing will happen at the outset. A reaction starts to happen immediately, but only once the acid is consumed will the starch be stained and the solution turn purple-black. Explain to the students that this is a clock reaction, or a reaction that changes color after a certain amount of time depending on the amounts and concentrations of solutions used. 7 How could you tell that a chemical reaction happened in these experiments? Once again, the color change indicates the presence of a chemical reaction. What color formed in the test tube? What element could have caused that color? The color in the test tube is purple-black because of iodine. The color of the tincture should be similar. The students should remember two things from this experiment. First, some reactions take time to happen, and patience is important when doing experiments. Second, the color change indicates a chemical change at a certain concentration. The actual reaction starts as soon as the solutions are mixed. 89 Chapter 1: The Science of Chemistry
Investigation page Sample answers Example Answers 5a. The color of the solution changed from clear to pink. 5b. The test tube formed a dark pink color. 5c. Phenolphthalein caused that color, but it is the lack of hydrogen (H + ions) that caused the color. 5d. Right after the solution turned pink, adding a few drops of vinegar made the color go away. This is because H + ions were added. 7a. A chemical reaction happened because the solution turned from clear to purple-black. 7b. A dark purple color formed in the test tube because of the iodine in the solution. Teaching Tips Have the students move back and forth on the indicator experiment to show that the indicator shows how much H + there is in the solution. The clock reaction happens because the acid blocks the iodine from staining the starch. Once it is all consumed, the iodine is free to stain the starch. This brings up an interesting point - one reaction happened with no color change, and the purple color indicates that the second reaction is happening. 4 B: Indicators of Chemical Reactions 90
Investigation sections Part Ideas and Dialog Precipitate formation reactions Here the students will see that a powder can form from the mixing of two liquids. This is a clear indication that a chemical change can produce a substance with different physical properties from the reactants. Students should be able to understand that a precipitate has physical properties different from the two aqueous solutions used to create it. Silver hydroxide is an insoluble powder, while both silver nitrate and sodium hydroxide are both clear, colorless and highly soluble in water. 8 Put 10 ml water into a test tube. Add 20 drops of Silver Nitrate (AgNO 3 ) into the test tube. (Safety Note: Use gloves. Do not get AgNO 3 on your skin.) Silver nitrate stains the skin. If a student inadvertently gets a drop of silver nitrate on his/her skin, it will likely turn black and stay that way for a few days. Wash the area with water for 15 minutes, and explain that the student should be more careful, for the silver nitrate freckle will be there for a few days. Add Sodium Hydroxide (NaOH) drop by drop and observe the results. Reactions similar to this one happen when film is processed. Black and white film contains silver-bearing chemicals that change on contact with light, and developers are often highly basic (like NaOH) and create a color change on the silver compounds exposed to light. 9 How could you tell that a chemical reaction had occurred? The precipitate should be the clear indicator here. Note that sometimes a precipitate will form that has the same color as its original solutions, but the important physical difference here is solubility in water, or the phase (solid versus aqueous). The precipitate formed is Silver Hydroxide (AgOH). Why do you think AgOH has a color to it while AgNO 3 does not? Many compounds change color depending on whether their constituent halves are in ionic or neutral form. In this example, silver nitrate dissolved in water has Ag + ions, which do not absorb any wavelengths of light. The silver in AgOH does absorb certain wavelengths of light, giving it its color. Reactions that result in temperature changes Even if no color change or new compound is created, a change in temperature signifies a change in the energy of the system in the test tube, and usually indicates that a reaction is taking place. 10 Put 20 ml of water in a test tube. Measure the temperature of the water and record the value. The solution should be at roughly room temperature. At this point, have the students set up the Lab- Master, because the temperature change may not be enough to notice in a student s gloved hand. Also, explain that basic safety would recommend that students not hold the test tube in their hand, in case it gets too hot. Add 4 pellets of Sodium Hydroxide (NaOH) using a gloves and tweezers. (Safety first: Do not touch the NaOH pellets with your bare hands). An interesting effect can be observed during the NaOH solution. Once the pellet is dropped into the test tube, it will begin to dissolve in the water around it. The solution may appear to contain swirls, or appear like the air above an open flame. This is due to the water directly around the pellet being heated up, and light refracting differently through water of differing temperature (and therefore density). Explain this effect to the students as it relates to the air above a flame, such as a match or a candle, and make sure they observe it in this experiment. Now measure the temperature of the water in the test tube. The temperature should increase due to the heat of dissolution. NaOH is in a more energetically favorable when dissolved, and it releases this extra energy once it dissolves in water. 91 Chapter 1: The Science of Chemistry
Investigation page Sample answers Example Answers 9a. A precipitate formed. A substance was formed that has different physical properties than the reactants. 9b. Many compounds change color depending on whether their constituent halves are in ionic or neutral form. In this example, silver nitrate dissolved in water has Ag + ions, which do not absorb any wavelengths of light. The silver in AgOH does absorb certain wavelengths of light, giving it its color. Teaching tips If you want to illustrate how silver nitrate stains the skin, you can use a piece of chicken skin if available. Make sure the students don t try to give each other silver nitrate freckles, as this is common in laboratory situations with high school students. Any student that puts a chemical on another should be dealt with as if that chemical were concentrated acid. Make sure the students don t hold the test tube in their bare hands. The temperature could be high. Even if it s not, this is a good chance to describe proper safety procedures. 4 B: Indicators of Chemical Reactions 92
Investigation sections Part Ideas and Dialog The students should internalize the following ideas from this lab - 1) Chemical reactions change the energy of the system, and this energy is absorbed into the system around it as a temperature change. 2) Because of this, solutions should be mixed slowly to avoid a dangerous temperature change. Gas creation Here students can see the creation of another substance with different physical properties than its reactants - in this case a gas. Gas creation is yet another example that the students should remember. Make sure that they know the difference between gas evolution and boiling - boiling is a physical change, while gas evolution is a chemical change. 11 12 13 By how much has the temperature changed? The temperature will likely change by a few degrees Celsius. Explain that the effect would have been more drastic had more chemicals been used. Also explain that this is why we mix solutions a bit at a time, to ensure the temperature doesn t change so much that it becomes unsafe to handle. Where did the energy for the temperature change come from? The energy came from the heat of dissolution, or the energy released when the ionic bonds in the NaOH were broken. Why did the temperature increase? The temperature increased because dissolved NaOH is at a lower energy state than when it is an ionic solid, and that difference in energy gets transferred to the water around it. Pour about 20 ml of 3% hydrogen peroxide solution into a clean test tube. Add a small amount of iron powder to the test tube. The iron should slowly liberate pure oxygen from the hydrogen peroxide. This reaction is often done more dramatically with manganese dioxide (MnO 2 ), but this chemical is harder to find. How could you tell that a chemical reaction happened? A gas was created without the solution reaching its boiling point. Iron acts as a catalyst in this reaction, meaning that it is not used up. What would happen if you added more hydrogen peroxide? What about if you added more iron? Because iron is a catalyst in this reaction, it is not consumed. That means that the iron stays, and adding more hydrogen peroxide would cause more oxygen to evolve, while adding more iron would not. The gas that is created in this reaction is pure oxygen. Usually storing oxygen requires a lot of pressure. Can you think of two places where it would be very useful to use the reaction from this experiment, because it can generate oxygen without compressing it? Some very useful places that this reaction could be used would be underwater (diving, submarines, caves) or in outer space (shuttle, space station), because no pressure would be needed to store oxygen. However, it would likely not be cost-effective to bring up 3% H 2 O 2. It may be useful to bring a higher purity H 2 O 2 (90% and above), especially because some rockets use this as rocket fuel. 93 Chapter 1: The Science of Chemistry
Investigation page Sample answers Example Answers 11a. The temperature changed by about 9 degrees Celsius. 11b. The energy from the temperature change came from the dissolution of the NaOH. 11c. The energy comes because NaOH is in a lower energy state when it is aqueous compared to an ionic solid. That extra energy gets turned into heat, which is absorbed by the solution around the NaOH. 13a. A gas was created from liquid and solid reactants - again, a substance was created that has different physical properties from its reactants. 13b. If you added more H 2 O 2, it would be broken apart by the iron, which is not used up. Adding iron wouldn t do anything. 13c. Having this system either under water or in outer space would be useful, especially if having pressurized gas would be dangerous. Teaching tips Make sure the students use a test tube holder in Part 10 to avoid getting burned. Mixing the solution well, either with a stirring rod or by squeezing a plastic pipet, will help equilibrate the temperature much faster and give better results. 4 B: Indicators of Chemical Reactions 94