Prof Gilbert LECTURE 11 CHEM 1211 Fall 10 Announcements Reminders Homework 3 due Sunday First mid term Ch 1 4 is 10 7 Review session from 5 00 6 30 in 10 BK Last time Chapter 4 Chemical Reactions in Solution Net ionic equations Oxidation and reduction in solution Oxidation numbers This time 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 O2 2 Mn2 2H2O 2 MnO2 4H Adding the physical state symbols 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 Ions associated with oppositely charged binding sites on a solid polymer called an ion exchange resin are replaced by ions in the solution surrounding the resin Water softeners This figure depicts the exchange of Na ions for Ca2 ions in a household water softener The resin contains immobilized carboxylic acid groups which have lost their H ions leaving behind COO called carboxylate groups Ion exchange also takes place in nature as for example when ions in ground water exchange with sites on the surfaces of soil particles Water purification In labs ion exchange is used to purify deionize water We need both a cation exchanger in the H form such as COO H and an anion exchanger in the OH form such as NH3 All the cations in the water are replaced with H ions all the anions are replaced with OH ions and then OH Chapter 5 Thermochemistry Energy as a Reactant or Product Basic Concepts Energy the capacity to transfer heat or do work H OH H2O Thermochemistry the study of heat flow in chemical reactions When heat flows out the reaction is exothermic when it flows in endothermic Work in physics application of a force F on an object to move it through a distance d w F d Work in another context expanding gases do work called P V work for example the energy released during combustion of gasoline in an automobile engine Units are equivalent to those of F d work because P F A F d2 and V d3 Potential Energy in physics the energy an object has based on its position the work to push an object up a hill is stored in it as potential energy PE PE is a state function meaning it does not depend on the path followed to get the object the hill only on where it is on the hill Kinetic Energy KE the energy of an object in motion KE mv2 Energy at the atomic level energy 1 The kinetic energy of particles increases with increasing temperature and so is also called thermal 2 Potential energy among ions is called electrostatic potential energy Eel where the Qs are the charges on a pair of ions and d is the distance between their centers