Prof Gilbert LECTURE 10 CHEM 1211 Fall 10 Announcements Homework 3 due Sunday First mid term Ch 1 4 is 10 7 To register for Smartwork http wwnorton com students Last time Chapter 4 Chemical Reactions in Solution Dilutions Acids bases and neutralization reactions Solubility rules and precipitation reactions Zip Lock Bag Reaction CaCl2 aq 2 NaHCO3 aq This time Writing a net ionic equation Focus on the species in solution assume ionic solutes are separated into their ions Ca2 aq 2Cl aq 2Na aq 2HCO3 aq CaCO3 s 2Na aq 2Cl aq CO2 g H2O l Ca2 aq 2HCO3 aq CaCO3 s CO2 g H2O l Oxidation and reduction in solution Consider what happens when a piece of Zn is immersed in a solution of copper sulfate CuSO4 Let s write a balanced chemical equation describing what we see CuSO4 aq Zn s Analyzing the reaction Which element is oxidized and which is reduced How many electrons are transferred What are the oxidation numbers O N s on Cu and Zn DEFINITIONS In the reaction of zinc with Cu2 zinc is oxidized and Cu2 ions are reduced You might say that the zinc caused the Cu2 ions to be reduced making zinc a reducing agent Similarly the Cu2 ions caused Zn to be oxidized making them oxidizing agents In any redox reaction the substance oxidized is a reducing agent and the substance reduced in an oxidizing agent Inquiry Which of the following are examples of redox reactions CH4 g 2O2 g CO2 g 2H2O l HCl aq NaOH aq NaCl aq H2O l NaCl aq AgNO3 aq AgCl s NaNO3 aq Writing redox reactions by combining half reactions Ever notice the brownish black scummy coating inside toilet tanks It s caused by the oxidation of Fe2 and Mn2 in the water by dissolved O2 The products include Fe OH 3 s and MnO2 s Inquiry Write the chemical equation for the oxidation of Mn2 to MnO2 s by dissolved oxygen O2 Start with the appropriate reduction half reactions in Appendix 6 p A26 27 which include this one for the reduction of O2 O2 4H 4e 2H2O 1 And this one in which Mn2 and MnO2 are ingredients is MnO2 4H 2e Mn2 2H2O 2 First let s turn the second one around to make it an oxidation half reaction Mn2 2H2O MnO2 4H 2e 3 Now to balance the gain and loss of electrons multiply Equation 3 by 2 2Mn2 4H2O 2MnO2 8H 4e 4 And combine half reactions 1 and 4 canceling out terms when possible to get an overall balanced chemical equation Adding the physical state symbols O2 2 Mn2 2H2O 2 MnO2 4H Note that the charges on both sides 4 are also balanced in this net ionic equation O2 g 2 Mn2 aq 2H2O l 2 MnO2 g 4H aq Titrations are a highly accurate way to determine the concentration of a reactive species called analytes in solution Method 1 Pipet a known volume VA of a sample containing the analyte into a flask 2 Add a few drops of a reagent called an indicator that will sense the completion of the reaction that is its equivalence point 3 As the sample is stirred slowly add a solution of known concentration MS the standard solution that contains a solute that reacts with the analyte 4 Record the volume VS of the standard solution required to reach the equivalence point 5 From the stoichiometry of the titration reaction e g aA aq sS aq and the values of VA VS MS calculate the concentration of the analyte in the sample MA Ion exchange reactions when ions associated with oppositely charged binding sites on a solid are replaced by ions in the solution surrounding the solid The above cartoon depicts the exchange of Na ions for Ca2 ions in a water softener
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