# 5 6 name partner part b heat of solution trial 1

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5-6NAME: ________________________PARTNER: ________________________Part B: Heat of solutionTrial 1Trial 2Mass of waterMass of solidInitial temperatureFinal temperatureTHeat change of solutionHeat change of calorimeterHeat change of the reactionHrxnper gram of solidAverageShow calculations for trial 1:TO HAND IN
6-1EXPERIMENT 6:SPECTROPHOTOMETRIC DETERMINATION OFTHE EQUILIBRIUM CONSTANT KcIntroductionAt any given temperature, a chemical reaction carried out in a closed vessel will reachequilibrium. When the reactants are mixed together, they will form products until a state is reachedwhere the amount of reactants and products no longer change. Equilibrium is adynamicprocess inwhich the concentration of the reactants and products remain constant over time. At this point thereaction has reached equilibrium and will remain so until the system is altered in some way. For agiven reaction, at a given temperature, the equilibrium state can be characterized by an equilibriumconstant K. For the general reaction:aA +bBcC +dD(1)The equilibrium expression is written:beqaeqdeqceqcBADCK][][][][The square brackets indicate the concentrations (in moles per liter) of the speciesat equilibrium.The magnitude of K expresses the position of equilibrium for the reaction. For example, asmall value of K indicates that the equilibrium lies far to the left (a reactants-favoured reaction)whereas a large value for K indicates that reaction lies far to the right (a products-favouredreaction).BackgroundThe purpose of this experiment is to determine KCfor the following reaction:Fe3+(aq)+SCN(aq)FeSCN2+(aq)(3)Iron (III) ionthiocyanate ionthiocyanoiron ionIn order to calculate Kcfor this reaction, the concentrations of all ions present atequilibrium ([FeSCN2+]eq, [SCN]eqand [Fe3+]eq) need to be determined, according to equation(2).FeSCN2+ions in solution produce a red-brown coloured solution. This coloured solutionabsorbs blue light very well. Aspectrophotometeris an instrument capable of measuring thisabsorption (see the section onspectrophotometryseveral pages below). This device is mostcommonly used to determine the concentration of a known solute in a given solution by theapplication of the Beer-Lambert Law.According to the Beer-Lambert lawthe concentration of a solute that absorbs light isproportional to that absorbance(see Spectrophotometry Overview on the following pages for(2)
6-2more details). The relationship can be expressed asA = εlcwhereAis the absorbance,εis themolar extinction coefficient (which depends on the nature of the chemical and the wavelength ofthe light used),lis the length of the path light must travel through the solution in centimeters, andcis the concentration of the given solution. For this experiment,εandlwill be kept constant, so theBeer-Lambert law can be simplified as follows, where k is a proportionality constant equal toεxl:A = kc(4)The Beer-Lambert relationship must be solved for a given solute using standard solutions.

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pH, Kc