Question

STANDARD ENTHALPY AND ENTROPY CHANGES OF A REACTION INTRODUCTION (1) Part 3, Mes Consider the following equilibrium system sscc Fe(a)SCNag) FeSCNP(o) The equilibrium constant, K of this system is (2) resCN KTeSCN Finding K for this system using a colouurimeter and absorhance values has already been described in the "Determination of an Equilibrium Constant" experiment. A similar method is used in this experiment, but the temperature is varied so give values of the equilibrium constant at different temperatures. From the values of K at different temperatures, the standard enthalpy changpe AlP and standard entropy change AS for the reaction will be determined in this experiment The standard free energy change AG for an equilibrium resction is expressed as: (3) A--RT In K where R is the gas constant, Tis the absolute temperature and Ke is the equilibrium constant. AG can also be expressed as: AG-AHP -TAS (4) Combining oquations (3) and (4) and solving for In Keg gives the following: AH AS In K (5) RT In this experiment, a dilute solution (solution 4) containing Fe, SCN and [Fe(SCN)] at equilibrium will be prepared. The Keg values at different temperatures will be determined for this solution and In Ke plotted against 1/7 The standard enthalpy change and the standard entropy change of the equilibrium reaction will R be determined based on the graph. Since absorbance is directly proportional to concentration The equilibrium concentration of FeSCN can be calculated as follows: Cample Candand X (6) Where Cample is the equilibrium concentration of [Fe(SCN) ] * in solution 4; Candard is the concentration of [Fe(SCN) in solution 2; Asmple and Astandand are absorbance of solution 4 and solution 2, respectively. TS Cstandord te eguititrium concent ratran o fIEescntt 48

Answer 1

As per the given data,

C_standard is the equilibrium concentration of [FeSCN]⁺²

However this is the already known equilibrium concentration or the equilibrium concentration of [FeSCN]⁺² which is already found.

In the uploaded image, it has been given that C_standard is the equilibrium concentration of [FeSCN]⁺² in solution - 2.

Now by using this standard concentration, the equilibrium concentration of [FeSCN]⁺² in unknown sample can easily be calculated and with the help of absorbance values using the formula

C_sample = C_standard x A_sample / A_standard

C_standard is already known from the previous experiment

Absorbance value A_sample and A_standard are taken experimentally from solution absorbances.

Then C_sample can be calculated from the available data.