EXPERIMENT 5: ENTHALPY OF NEUTRAIISATION POTENTIAL HAZARDS Safetyglassesmustbewornotalltimesinthelaborotory.T.0MsotutionsofNooH,HCl,and CH3CooHareusedinthtisexperiment.Avoidcontoctwithyourskin,Washtheaffectedoreo thoroughly with cold water if spilloge occurs' INTRODUCTION The enthalpy change accompanying a chemical.reaction in solution can be determined by carrying out the particular reaction in a caloiimeter. ln this process there is.a transfer of energy between the reacting substances and a definite mass of liquid - usually water' The enthalpy can then be found by measuring the change iniemperature of the tiquia. we also need to determine the heat capacity of the calorimeter System, since it also undergoes the temperature change during the reaction. ln this experiment, the heat capacity of a calorimeter is determined using the known heat of solution ofpotassiumnitrate.Theheatofneutralisationofsodiumhydroxidewithbothhydrochloricand acetic acids is then determined using the same calorimeter system' The reactions in this experiment are carried out at a constant (atmospheric) pressure in a simple Vacuumflaskwhichisanexcellentthermalinsulator.Hencetheprocessiscarriedoutunder Reactants (Tr) Reactants (Tr) lsothermal Process , Products (Tr) Adiabatic Process (Calorimeter) adiabatic(qo=AHo=0)ratherthanisothermal(Tconstant)conditions.Enthalpiesofreactionare usuallytabulatedforisotl.rermalprocesses(e'g.at298K)butinthiscase,thereactantsandproducts are at different temperatures. The following diagiam shows the relationship between the enthalpy change accompanying the process in the aJabati. .utotir"t"t (AH.), and the one accompanying the isoth-ermal process (AHr), which we wish to determine' 38 We can see that the adiabatic process is thermodynamically equivalent to an isothermaI process plus a term in which the reactants have their temperature changed from Tr to Tz I2 aHc = AHrz + JrPoo, (1) But, for our adiabatic process 9p = 0, i'e' AH. = g' Hence' .T" 6gr, = -J_'CrdT (2) lfthetemperaturechangeissmall,Cowillbeindependentoftemperaturesothat AH1 = -cp(Tz - T1) (3) Let's see how we can use equation (3) in this experiment' Heat CapacitY of Calorimeter Here we are using a chemical reaction with known AHr' KNos(s) + 250Hzo(l) ) ruo, in 250Hzo; AH1= +34'6 kJ mol-1 (4) ir' Frpm (3), kfrowlng AHr, Cp can be found. The heat capacity iri thls case ls thts sum of the hpat caPacitY of the calorimeter:vesselt ,na of tnu solutldn in the calorlmeter' lf weassume thet solutlon has the ;;;;;"r; .;p..'rv f if ;, *ili;.ilt*ri g'1, t'u'-'' the heat capacltv af the solutioh ls simplv equal to the mars 1my oiwater plus KNOr (ln g)times the heat capaclty s (ln J K'* g'')' ltii .. - '":' -,':1t' ' r"'j Hbnce, equation (3) becomes -' i .. , AH1 = -(m.s + CXTz - l) (s) The heat capacity of the calorimeter, C, can therefore be found' Enthalpy of Neutralisation ln this experiment a solution of acid of mass ma, at an initial temperature T" is mixed with a solution of base of mass mb at temperature T6, the base having been equilibrated in the calorimeter before mixing. Therefore, for the purposes of calculation, it is necessary to treat the system as if it consisted of two separate quantities of solution each having undergone a different temperature change' say AT"=(Tt-T.)fortheacidand4T6=11.Tu)forthebase.Sincethecalorimeterisatthesame temperatureasthebase,itundergoesatemperaturechangeequaltoATu' From equation (3), the heat of neutralisation is then given by s + c)aro]