Chemical Reactions

Chemical Reactions Lab For each table / group: 1 covered container of baking soda (labeled NaHCO3) 1 covered contained of calcium chloride (labeled CaCl2) **Be sure to get the “flake” CaCl2 (laboratory grade). *** CaCl2 is a hydrophilic compound and if left uncovered will quickly absord moisture from the air and become unuseable. An easy set up is to use clear plastic cups and cover them with half of a petri dish. 2 plastic spoons (labeled NaHCO3 and CaCl2) 2 small glass or plastic vials (empty pill bottles work) squeeze bottle of phenol red solution (labeled C14H12NaO5S) *Phenol red solution can be purchased at swimming pool supply companies or as a powder from any chemical supply company. To make the liquid, add a SMALL amount of the powder to distilled water to make a deep rose-red liquid) sandwich sized zip lock bags graduated cylinder goggles *There are several safety concerns with htis lab. The students must keep the goggles on as long as the tray of equipment is on the table. Be sure to point out to them the warnings on the bottle of CaCl2 which clearly state “Body tissue irritant”. Also, during the lab, occasionally the bag will rupture. As one of the products is a weak HCl solution, it could be a serious problem if it got into a students eyes. In part 1, students follow the directions to conduct the investigation. Begin by getting the students to generate ideas on some common chemical reactions (milk going sour, baking a cake, a banana ripening, digesting food, mixing baking soda and vinegar). Ask what are some of the signs that tell them a chemical change is taking place. 1. change in color, change in smell, change in temperature and production of a gas. There may also be a change of state or a change of texture, but those are not necessarily a sign of a chemical change. Then have them make observations of the reactants. Be sure they don’t forget the basics such as, solid, white, liquid, etc. To check for an odor, be sure to show how to CORRECTLY waft the fumes! Go over the lab. Demonstrate how to place the vial (with the phenol red in it) into the bag and hold it while someone else securly zips the bag shut. Once the reactions begins, tell them to have at least one person with their hand under the bag to constantly check for a change in temperature. Have one person in the group write down what they see going on. (color changes, fizzes and bubbles, produces a gas) One the reaction has stopped, remind them to carefully check for a change in small. (If they do not use the correct technique here, they will get a whiff of sulfur that will stay with them for a while! And the next time, they will be SURE to waft!) For clean up, stand by a sink with 2 basins of water. In one have a vinegar/water solution and just water in the other. As groups finish, put on a rubber glove and have one group member bring the bag to you and open it. Remove the vial, dump any contants and drop it into the vinegar basin. Eventually rinse it in the clean water. The bags are then ziped up and placed in the trash. Groups need to re-stock the tray (new bags, clean vials) before returning the trays to the materials area. As a group, discuss what happened and list the changes on the board. Change of color Change of smell Change of temperature (they should have noticed both an andothermic reaction AND an exothermic reaction) Production of a gas Now tell the students that you are going to focus on what caused the exothermic reaction (what made it get hot). Since there were several reactants in this lab, the next day each group will carry out another experiment, limiting the number of reactants. Get the students to geterate a list of possible ‘mini-combinations’. (Point out that the phenol red actually contains water, so water must be considered as a reactant). NaHCO3 and CaCl2 and C14H12NaO5S (and H20) was doen by everyone. So the reactions to be carried out are: NaHCO3 and CaCl2 and water NaHCO3 and water NaHCO3 and C14H12NaO5S CaCl2 and water CaCl2 and C14H12NaO5S Students are allowed to design their own experiment. I allow up to 4 spoonfulls of NaHCO3 but I limit the CaCl2 to 2 spoonfulls. The water and C14H12NaO5S can be up to 20 ml (they may need 2 vials). Before beginning, each group has to select on of the possible combinations. Usually there are not many who want to do the NaHCO3 and water, but I explain to them that once all of the reactions are complete, we will all share observations. Once they design their experiment and carry it out, everyone cleans up. We then hold a scientific conference, where students move around the room, discussing their findings, trying to answer the question: What caused the exothermic reaction? I tell the students that if they are through in collecting data, they will also be able to answer the following questions: 1) What caused the endothermic reaction? [NaHCO3 and water or C14H12NaO5S] 2) What reactants are necessary for a color change? [NaHCO3 and CaCl2 and C14H12NaO5S (and H20)] 3) What reactants are necessary for the smell? [NaHCO3 and CaCl2 and C14H12NaO5S (and H20)] 4) What reactants are necessary for the production of gas? [NaHCO3 and water or C14H12NaO5S] Additional information: If they are interested, the final products of the original reaction (Day 1) are salt (in solution), hydrochloric acid, some sulphur compounds (the bad smell) and calcium carbonate – or chalk! If you like, pour the materials left in the bags on day 1 into a funnel lined with filter paper. Rinse well (to get out the HCl) and mush it together and let it dry. You will get a piece of very soft but useable chalk!