Prof Gilbert LECTURE 12 CHEM 1211 Fall 10 Announcements Reminders Mid Term Thursday Review session Tuesday from 5 00 6 30 in 10 BK Last time Chapter 4 Chemical Reactions in Solution Oxidation and reduction in solution Oxidation numbers Writing redox reactions by combining half reactions This time 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 1 The kinetic energy of particles increases with increasing temperature and so is also called thermal energy 2 Potential energy among ions is called electrostatic potential energy Eel 2 31 10 19 J nm where the Qs are the charges on a pair of ions and d is the distance between their centers There is a similar profile for the formation of the bonds in molecules such as H2 The energy change that occurs in a chemical reaction is the difference between the chemical potential energies in the ions or molecules of reactants and products For example the combination of H2 and O2 The Universe The System Its Surroundings from a thermodynamics point of view An isolated system does not exchange matter and energy with its surroundings A closed system exchanges energy but not matter with its surroundings An open system exchanges both energy and matter with its surroundings Inquiry can you think of some examples of all 3 Units of energy one calorie cal is the quantity of heat required to raise the temperature of 1 g of water from 14 5 to 15 5 C The SI unit of energy is the joule J 1 cal 4 184 J One dietary Calorie Cal with a capital C kilocalorie kcal 1000 cal Internal Energy E of a System the sum of kinetic and potential energies of the substances in the system Hard to quantify Easier to measure the change in internal energy Esys There must be a balancing change in the energy of the system s surroundings so that This is a way of expressing the First Law of Thermodynamics energy is not created or destroyed only changes from one form to another Calculating Esys two ways to increase the internal energy of a system Esurr Esys 1 2 Add heat q to it Do work w on it Esys q w Doing work on a gas could mean compressing it into a smaller volume So why is there a sign on the P V term Esys q P V Inquiry This balloon the first to fly around the world nonstop contained 550 000 cubic feet of He How much work was done to inflate it at 1 00 atm of pressure Given 1 L atm 101 34 J Change in Enthalpy Hsys heat flow into or out of the system at constant pressure qp Most reactions including the biochemical reactions in the body occur at constant P For these reactions Hsys Hreact Hreact 0 the reaction is exothermic Hreact 0 the reaction is endothermic H of phase changes such as Hfus and Hvap have values The reverse processes liquid freezing or vapor condensing have H values 0 Heating Curve of water Heat flow during phase changes