Pre-Lab Read the entire laboratory assignment. Answer all pre-lab questions before beginning the lab.

Transcription

1 Name: Date: Pd: Lab Partner: Lab # 13: Types of Reactions, Predicting Products of Chemical Reactions Lab Accelerated Chemistry 1 Introduction: If you examine your bicycle after it has been left out in the rain a number of times you will find that it has begun to rust. Rust is a slow chemical reaction of the iron in the bicycle with oxygen. If a piece of sodium is put in water, a much more rapid chemical reaction occurs. Sodium reacts with water to produce sodium hydroxide and hydrogen gas. During this reaction, enough heat is liberated to ignite the hydrogen gas, causing it to explode. Chemists observe what is happening in a chemical reaction and try to describe it in language that is clear and simple. A chemical equation uses formulas and symbols to describe the substances involved in a reaction, the physical state of the substance, the use of a catalyst, and relative proportions. The general form of an equation is: Reactants --> Products In this investigation, you will perform a series of reactions and make careful observations of the changes that occur. Using simple tests and your knowledge of chemistry, you will determine the identity of the products. With this information, you will write and balance chemical equations to describe the reactions. Materials - Goggles - Bunsen burner - Crucible and crucible tongs - plastic U tube - Power source and battery clips - Carbon electrodes - Water and universal indicator - Watch glass - Methanol - 4 test tubes - Wooden splints - Matches - Copper powder - Test-tube holders - Beral pipettes - MnO 2 catalyst - Hydrogen peroxide (H 2 O 2 ) 10% solution - Potassium iodide - Silver nitrate Safety Warnings 1. Wear your goggles at all times during this investigation. 2. Tie back loose hair. 3. Hydrochloric acid is corrosive. Avoid direct contact with it. If you get some on your skin, immediately wash it off with copious amounts of water. 4. Lead compounds are poisonous so be sure to avoid contact with skin. If contact occurs immediately wash it off with copious amounts of water and inform your teacher. Pre-Lab Read the entire laboratory assignment. Answer all pre-lab questions before beginning the lab. 1. What constitutes a positive test for each of the following gases? Think about your previous experiences. a. oxygen b. hydrogen c. carbon dioxide d. water vapor AccelLab13 TypesRxns 1

2 2. What is the role of a catalyst in a reaction? How can you tell when a substance serves as a catalyst? Procedures: Put on your goggles and wear them throughout the lab. For each of the reactions, record your observations in the data table. Observations should include any evidence that a chemical reaction has taken place; the results of any test performed on any gases produced; the appearance of the products; and any other relevant data. Reaction # 1 1. Obtain a battery-u-tube apparatus and place it in a 250-mL beaker. Fill the U-tube with water containing universal indicator to within 0.5 cm from the top of each end of the tube. 2. Obtain a battery cap with two wired clips attached. Carefully attach the cap to the top of the battery. Carefully connect two graphite electrodes to the clips. Place the graphite electrodes into the opposite ends of the glass tube. 3. Observe and record all the changes that take place as the current flows through the solution. Draw a diagram of the cell. Be specific as to the changes that take place at the electrodes attached to the positive and negative terminals of the battery. 4. After five minutes disconnect the battery cap and electrodes. Pour the water down the drain and use a clean pipete filled with tap water to rinse the tubing thoroughly. Reaction # 2 5. Read steps 5 and 6 completely before starting step 5. Place a medium test tube in a test-tube rack. Have a second test tube of a smaller size ready in a test-tube holder. Add 4 ml of 3.0 M hydrochloric acid to the medium test tube using a Beral pipete. Drop a 2-cm piece of Mg into the acid. (The length of the Mg is approximate. The exact size is not important.) CAUTION: Hydrochloric acid is corrosive. Avoid spills and splashes. If you do spill the acid immediately rinse the area with plenty of water and notify your teacher. 6. Invert the smaller test tube into the mouth of the first test tube. When the reaction appears to have ended, remove test tube # 2 from the top of test tube #1 and cover its mouth. Light a wooden splint and quickly test the collected gas for flammability by holding the test tube and inserting the burning splint into the mouth of test tube #2. Record observations in Data Table 1. Reaction # 3 7. Place a clean, dry crucible on an electronic balance. Record its mass in Data Table 1 to the nearest 0.01 g. Press the tare button. Add approximately 1.00 g of copper powder to the crucible. Record the mass of the copper in Data Table 1, to the nearest 0.01 g. 8. Set up the crucible, clay triangle, and Bunsen burner as shown by your teacher. Light the burner, and adjust it so that the flame tip just touches the bottom of the crucible. Heat the crucible and its contents for two minutes. Remove the crucible from the flame using crucible tongs. AccelLab13 TypesRxns 2

3 9. Use a spatula to break up the solid in the crucible so that as much of the remaining copper metal is exposed as possible. DO NOT HIT THE CRUCIBLE TOO HARD. IT WILL BREAK AND YOU WILL HAVE TO START OVER AGAIN. Caution: DO NOT TOUCH THE CRUCIBLE WITH YOUR BARE HANDS! 10. Continue heating for about 10 minutes more, removing the flame and breaking up the solid every two or three minutes. When you have finished heating, extinguish the flame and allow the crucible and its contents to cool to room temperature. Answer questions 1 and 2 while you are waiting. DO NOT PUT THE CRUCIBLE ON THE BALANCE WHEN THE CRUCIBLE IS STILL HOT!! 11. After the crucible and its contents have cooled, find their masses by placing the crucible on the electronic balance. Use this value, and the mass of the empty crucible previously recorded to calculate the mass of the copper sample. 12. Scrape the bottom of the crucible to loosen the copper sample and throw the contents away in the trash can. Reaction # Using a Beral pipete squirt 3 ml of methanol onto a watch glass. Using a match, light the methanol. Observe the reaction and record observations in Data Table 1. Include observations such as the color and height of the flame. Caution: Move all of your belongings and papers away from the watch glass before you light the methanol. Tie long hair back. Do not touch the watch glass with your bare hands after the reaction is finished. Reaction # Place approximately 4 ml of 10% hydrogen peroxide into a medium test tube using a Beral pipete. Add a small amount of (about the tip of a spatula or wooden splint) of MnO 2 to the hydrogen peroxide; then wait 15 seconds. Light a wooden split and allow it to burn for a few seconds. Blow out the flame and quickly place the glowing embers halfway into the test tube. Do not stick the splint in the liquid. Reaction # Place two drops of sodium iodide solution into a small test tube. Add two drops of silver nitrate solution. Record your observations in Data Table 1. AccelLab13 TypesRxns 3

4 Data and Observations: Data Table 1: Reaction #1 1. Observations at the positive electrode: 2. Observations at the negative electrode: 2. Drawing of apparatus: (Save until class discussion) 3. What product is being produced at the positive electrode? 4. What product is being produced at the negative electrode? Reaction # 2 1. Observations of reaction: 2. What gas is produced? How do you know? Reaction # 3 1. Mass of empty crucible: 2. Mass of Cu before reaction: 3. Mass of Cu after reaction: 4. Observations of reaction: 5. Why did the mass of the Cu sample increase? AccelLab13 TypesRxns 4

5 Reaction # 4 1. Observation of reaction: 2. What are signs that a chemical change was taking place in this reaction? 3. Compare the color of this flame to a candle flame. What is different? Reaction # 5 1. Observations of reaction: 2. What is the role of the MnO 2 in this reaction? (Hint: MnO 2 does not react!) Reaction # 6 1. Observations of reaction: Analysis and Synthesis 1. Write a balanced equation for each reaction performed. Include the physical state of each substance. Rxn # 1 Rxn # 2 Rxn # 3 Rxn # 4 Rxn # 5 Rxn #6 AccelLab13 TypesRxns 5

6 2. Classify each of the reactions as synthesis, decomposition, single replacement, double replacement, or combustion In Reaction #1, changes in the solution color were observed at both the positive and negative electrodes. Hypothesize what you think was occurring to cause the color change. (Hint: In what type of chemical reactions are indicators used?) 4. For each of the following situations, determine the identity of the gas produced from the information provided, and write a balanced chemical equation that represents the reaction. a. When potassium bromate (KBrO 3 ) is heated it decomposes into potassium bromide and a gas that reignites a glowing splint. b. Sodium metal reacts violently with water to produce sodium hydroxide and a gas that pops in the presence of a burning splint. 5. Which reaction(s) required a catalyst? Does the catalyst appear in the balanced equation? Explain. 6. Why was the flame produced in Reaction # 4 not a white flame? AccelLab13 TypesRxns 6