Identifying & Testing Gas-Producing, Soap-Making, & Catalyzed Chemical Reactions, Lecture notes of Chemistry

Download Identifying & Testing Gas-Producing, Soap-Making, & Catalyzed Chemical Reactions and more Lecture notes Chemistry in PDF only on Docsity! 41 Experiment 8 - Chemical Changes When a chemical change occurs, the chemicals that you start with are changed into different chemicals. We know when this happens because the new chemicals have different properties from the old ones. Thus, if bubbles appear in a liquid (and the liquid is not simply boiling), we know that a gas has been produced by some chemical change (a reaction). If an odor appears where there had been none, that also tells us that a new substance has been made. Appearance of a new color is also a sign of a chemical change. Many chemical reactions produce enough heat to be easily noticed, and that can alert us that a chemical change has occurred. (Other processes besides chemical reactions also involve heat, such as phase changes. Therefore, a temperature change does not necessarily mean that a chemical reaction has occurred, just that a chemical reaction might be what caused the temperature change.) Safety Precautions: • Wear your safety goggles at all times. • Sodium metal, sodium hydroxide, and hydrochloric acid are dangerous and should not be touched with your bare skin. If you skin or clothes accidentally come into contact with these chemicals, rinse them off immediately with lots of water. Waste Disposal: • Place the liquid waste in the inorganic waste container (which has a blue label) in one of the fume hoods. • Solids such as used litmus paper and wood splints should be placed in the waste basket. Part 1 – A Gas-Producing Reaction The chemical reaction of sodium metal with water produces hydrogen gas and a solution of sodium hydroxide. The instructor will demonstrate this. The process also produces heat. Any process that produces heat is said to be “exothermic.” The information about the process that you observe can be summarized by the following “word equation”: Sodium + water à hydrogen gas + sodium hydroxide (aq) + heat [(aq) means “dissolved in water”. It stands for “aqueous”.] 42 If we boil off the water, we will be able to see the sodium hydroxide crystals. The sodium hydroxide is one of the two new substances produced by this chemical change. The other is hydrogen gas. The instructor might demonstrate how we test for that gas. Part 2 – Turning Fat Into Soap Sodium hydroxide is the active ingredient in the granulated type of drain cleaner, and can also be used to make soap. The following reaction occurs in both cases: Sodium hydroxide (aq) + fat à glycerol (aq) + soap (aq) The glycerol and the soap both mix with the water, in contrast with the fat, which does not mix with water. This is one of the chemical reactions that occur when “Drano” cleans out the drain. Procedure – Part 2 1. Put a tiny, pea sized piece of margarine or other food fat in the bottom of a test tube, to represent fat lodged in a sink drain. (In a real case of plugging, other wastes would by mixed with the fat.) Add 2 or 3 mL of pure water to the fat, then poke and stir it with a glass stirring rod. Notice that the fat does not dissolve. 2. Put 2 or 3 pellets of sodium hydroxide into the tube, handling the pellets with tweezers, NOT WITH YOUR FINGERS. This substance is highly corrosive: it burns skin, hair, and eyes. Make sure you are wearing your safety goggles. Poke and stir the mixture. Feel the outside of the tube. If it is hot, this means that heat is being produced. After several minutes you should see the fat has disappeared. You may be able to produce a little bit of suds in the tube, showing that some soap has been make. (To make usable soap in larger amounts, a more complex procedure is necessary.) 3. Dump the liquid out of the tube into the sink, and wash out the tube with tap water while holding the tube with a test tube holder, so that none of its contents get on your skin. Then give the tube a good wash with liquid soap and water, using a test tube brush. Part 3 – The Effect of Catalysts on the Rate of a Reaction Hydrogen peroxide is another common household chemical. It is an “oxidizing agent”. This means that it works to supply oxygen atoms for chemical reactions, such as those involved in bleaching hair or in cleansing a wound. The hydrogen peroxide turns into water and oxygen gas when it is used: