EXPERIMENT 9 ENTHALPY OF REACTION HESS S LAW INTRODUCTION Chemical changes are generally accompanied by energy changes energy is absorbed or evolved usually as heat Breaking chemical bonds in reactants requires energy and energy is released as new bonds form in products Whether the combination of these steps absorbs or releases energy depends on the relative sizes of the energies associated with breaking and forming bonds The amount of heat involved in a reaction depends not only on what the reaction is but also on the temperature at which the reaction occurs and whether the reaction occurs under conditions of constant pressure or constant volume In the laboratory many reactions are conveniently carried out at constant pressure in beakers or flasks that are open to the atmosphere The amount of heat absorbed or released under this condition is the enthalpy change H for the reaction where H Hproducts Hreactants 9 1 Enthalpy H can be thought of as the heat content of a substance this heat is stored as potential energy in the form of bond and other energies When atoms rearrange during a reaction the heat content of the products is usually different from the heat content of the reactants This difference in heat content appears as heat absorbed or released This heat is generally indicated in Joules for the reaction as written For example in this experiment you will examine an acid base neutralization in aqueous solution H3O aq OH aq 2 H2O l H 55 8kJ 9 2 The enthalpy change for this reaction could be given in J mol of H3O or in J mol of OH or in J mol of H2O To avoid confusion it is customary to report H for the reaction with the numbers of moles of reactants and products simply as written Thus for reaction 9 2 in which 1 mole of H3O and 1 mole of OH combine to form 2 moles of H2O H 55 8 kJ as shown Note that the enthalpy change H Hproducts Hreactants is positive if heat is absorbed that is if Hproducts Hreactants the reaction is endothermic The enthalpy change is negative if heat is released If Hproducts Hreactants the reaction is exothermic This is illustrated in the following diagrams products reactants heat absorbed H heat evolved reactants products H 0 endothermic H 0 exothermic Experiment 9 9 1 H In an exothermic reaction the overall process causes the system to become more energetically stable excess energy is released as heat The system changes from a higher energy state to a lower energy state It seems plausible intuitively that reactions would have a natural tendency to proceed spontaneously to a state of lower energy in fact a negative enthalpy change is part of what causes some reactions to occur spontaneously Physical changes also have associated enthalpy changes For the melting of water for example H2O s H2O l H 6 1kJ 9 3 In this process no chemical bonds are broken or formed The energy input converts the potential energy from that characteristic of the rigid solid state organization of water molecules to that characteristic of the liquid state in which water molecules move over under and around one another but are still held together in a relatively small volume Similarly enthalpy changes accompany the dissolving of solids or the dilution of solutions For example when AlCl3 s dissolves in water the Al3 and Cl ions must be separated In the solid state they are packed together in a way that results in the greatest stability cations surrounded by anions and vice versa In liquid water the water molecules are extensively H bonded to one another They must be separated to create cavities for the Al3 and Cl ions to occupy Both of these processes cost energy i e they are endothermic In contrast energy is released when Al3 ions are hydrated becoming closely surrounded by the negative ends of six polar water molecules in solution Similar interaction of Cl ions with the positive ends of polar water molecules also releases energy The balance among the energies for all these processes determines whether the overall Hsolution for AlCl3 will be positive or negative It turns out to be negative Enthalpy is one member of an important class of thermodynamic functions known as state functions A state function has the unique characteristic that its value for a system does not depend upon the history of the system No matter how a system was formed the value for any state function depends only on the present state of the system This has the extremely useful practical consequence that the change in any state function for a process depends only on the initial and final states and thus is independent of the pathway by which the process takes the system from the initial state to the final state Another way of stating this for enthalpy is in the form of Hess s Law of Constant Heat Summation If a reaction or physical process is carried out in a series of steps H for the overall process is equal to the sum of the enthalpy changes for the individual steps It is a consequence of Hess s Law that as long as we begin with the reactants in one particular state given temperature etc and end with the products in another particular state the overall Hrxn is the same no matter what individual steps are carried out or in what order to convert the reactants to the products We will examine how well this principle holds for two different chemical reactions in this experiment TECHNIQUE Calorimetry is the measurement of the heat change for a reaction and the device used to measure heat changes is a calorimeter In this experiment the calorimeter consists of two nested Styrofoam cups with a MeasureNet temperature probe inserted into the cup to monitor the temperature If the reaction is exothermic the heat released which will ultimately be transferred to the surroundings is temporarily trapped by the insulating walls of the Styrofoam cups and causes the temperature of the calorimeter contents to rise For an endothermic reaction the temperature of the calorimeter contents falls The Styrofoam cups temporarily prevent heat from entering the system from the surroundings and the calorimeter contents must supply the heat absorbed in the process In either case the size of the temperature change is used to determine the quantity of heat released or absorbed Experiment 9 9 2 The amount of heat released or absorbed is related to the temperature change by the equation 9 4 where q amount of heat m mass of material undergoing the temperature change c specific heat of the material undergoing the temperature change T temperature