Lab 4: Types of Reactions Objectives: Observe changes in chemical and physical properties during a variety of chemical reactions Write balanced chemical equations for all reactions carried out Identify type of reaction carried out (some may be more than one type) Background: A chemical reaction consists of reactants and products. The reactants are what you start with (also called starting materials) and the products are the new compounds that form during the reaction. From section 6.4 of your text chemical reactions can be classified into four general categories: (1) Combination (2) Decomposition (3) Displacement (single and double) (4) Combustion. These reaction types can be represented by generic chemical equations as follows: Combination: A + B AB Decomposition: AB A + B Single-displacement: A + BC AC + B Double-displacement: AB + CD AD + CB Combustion: A +O2 AO2 or AH4 + O2 AO2 + H2O Within the above categories, reactions can be further categorized as precipitation or re-dox reactions (see Chapt 6 of text and check for charge changes). During a precipitation reaction, an insoluble solid (precipitate) is formed as a product. A re-dox reaction occurs when electrons are exchanged between the reactants. In addition, reactions can be classified as exothermic (releasing heat) or endothermic (consuming heat). In this lab you will carry out various reactions and make careful observations about what you see. In the data report you will express them as balanced chemical equations and determine what type of reaction has taken place. Pre-Lab: 1. What type of reaction do you expect each to be? 2. Compare reaction 7 to reaction 8. Are they the same reaction in different directions? How can you tell? Safety Notes: Wear your safety goggles at all times Handle all chemicals carefully (rinse them off your skin immediately) Take only the amount of chemicals that you need and never put unused chemicals back into the reagent bottle Discard all used or leftover chemicals in the waste containers provided (do not pour any chemicals down the sink and do not put your rinse water in the waste) Clean up any spills immediately and clean up everything when finished Procedures: Carry out the following reactions. For each reaction, carefully write down all observations (color change, gas evolution, formation of a precipitate, or temperature change) below the appropriate reaction on the pages you created for the pre-lab. Your observations will be turned in and graded along with the data sheet. Use clean test tubes for the reactions. Reaction #1 Mg + O2 MgO Procedure: Clean a Mg strip with glass or steel wool. Using tongs to hold the Mg strip, burn the strip. Dispose the product in the solid waste container. 1
Reaction #2! NH4OH(aq) NH3(g) + H2O Procedure: Open the bottle of conc. NH4OH. Test the ph of the fumes using a wet piece of ph paper. Do not let the paper touch the sides of the container. Reaction #3 Mg (s) + HCl (aq)!h2 (gas) + Mg 2+ (aq) + Cl - (aq). Procedure: Place a strip of Mg in 1 ml of 0.1M HCl. Reaction #4 AgNO3(aq) + HCl (aq)! AgCl(s) + HNO3 (aq) Procedure: Mix 0.1M HCl with 0.1M AgNO3 in equal volumes. Reaction #5 HCl(aq) + NaOH(aq) H2O + NaCl(aq) Procedure: Measure the ph of 0.1M HCl and 0.1M NaOH. Mix the two in equal volume and stir well. Measure the ph of this mixture. Did the reaction generate heat? Reaction #6 CO3 2- (aq) + H + (aq) H2CO3 (aq) H2O (l) + CO2 (g) Procedure: Place CaCO3 (s) in 2 ml of 0.1M HCl. Reaction #7 Cu (s) + ZnSO4(aq)!CuSO4(aq) + Zn (s) Procedure: Place a small piece of Cu (s) in 1 ml of 0.1 M ZnSO4. Reaction #8 Zn (s) + CuSO4(aq)! Cu (s) + ZnSO4(aq) Procedure: Place a small piece of Zn (s) in 1 ml of 0.1M CuSO4. 2
Data Sheet (To be attached to your lab report. Recopy if messy): For each reaction write the balanced chemical equation. List all types of reaction that apply: Combination, Decomposition, Replacement (single/double), Combustion and Redox. Record any observations such as formation of heat, generation of bubbles or precipitation of a solid etc. Reaction 1:!!!!!!! Reaction 2:!!!!!!! Reaction 3:!!! Reaction 4:! Reaction 5:!!!!!!! 3
Reaction 6:! Reaction 7:! Reaction 8:! 4
Lab Report Guide: 1. Results Data sheets neatly filled out with data Proper significant figures Legible sample calculations 2. Post Lab Questions Typed answers to the Post Lab questions. Note that single sentence answers will not suffice. State the answer to the question followed by a brief description of the evidence supporting that answer. Post Lab Questions: (Typed answers to be submitted with lab report) 1. In reaction #4, what was the solid that formed in the reaction? In reaction #3, what gas caused the bubbling in the solution? 2. The law of conservation of mass states that the mass of the reactants should be exactly equal to the mass of the products. When a piece of firewood burns the mass of the ashes is much lower than the mass of the wood. Where did the remaining mass go? 3. Where you surprised by the results of reactions 7 and 8? Why do these results make sense? Hint reactions generally proceed from high energy to low energy like a boulder freely rolling down a hill. 4. When iron or steel nails rust, their mass increases. Does this break the law of conservation of mass? 5