Experiment #5 DETERMINATION OF AN EQUILIBRIUM CONSTANT

There are two aspects to chemical reactions which though, interrelated, are dealt with as two separate topics. The first of these is the study of the reaction from it initiation to a point where the system seems to undergo no further change, called chemical kinetics. The deals with the system after all “apparent change” have stopped, and is called chemical equilibrium.

Reversible reactions A + 3B 2C+ 2D When reactants are mixed in stoichiometric quantities, they are not completely converted to products. Reactions that do not go to completion and that can occur in either direction are called reversible reactions. aA + bB cC+ dD forward backward

What is chemical equilibrium? Chemical equilibrium exists when two opposing reactions occur simultaneously at the same rate Kinetics Equilibrium Apparent change in concentration of reactants and products No apparent 2Hl(g) H2(g)+I2(g)

Equilibrium, an analogy Amount of water being pumped in = Amount of water being pumped out

Equilibrium Constant nAA + nbB nCC+ nDD At equilibrium At equilibrium Kforward nAA + nbB nCC+ nDD kbackward At equilibrium At equilibrium

Upper case Lower case

What is equilibrium concentration? Concentration of a reactant or a product left after the equilibrium has been established. Mathematically expressed:

A comparison of rate law and equilibrium constant Rate law Equilibrium constant kforward k nAA + nbB nCC+ nDD nAA + nbB nCC+ nDD kbackward unidirectional bidirectional k is called the rate constant and has units that depend on order. Keq is unitless m and n are the order of the reaction due to A and B respectively. They have to be determined experimentally nAA, nb, nCC and nD are stoichiometric coefficients of A,B,C and D respectively. obtained from the balanced chemical reaction

Experiment Determination of equilibrium constant of the following reaction Fe(H2O)63+ + SCN- FeSCN(H2O)52+ + H2O

For H2O [H2O]reacted is much smaller compared to the total amount of H2O in the solution. So we can assume that [H2O] essentially remains constant for this equilibrium.

How do we follow this equilibrium? Fe(H2O)63+ + SCN- FeSCN(H2O)52+ + H2O Fe3+ + SCN- FeSCN2+ Yellow Colorless Red color FeSCN2+ absorbs maximum amount of light at 447 nm 447 nm corresponds to blue light region in the visible light spectrum. FeSCN2+ absorbs all the blue light but lets all the red light pass through. Hence the red color.

How do we determine the equilibrium Constant (Kc) of the reaction? Fe3+ + SCN- FeSCN2+ Yellow Colorless Red color

What does the subscripts initial and equilibrium refer to? Initial concentration refers to the concentration of the species at a time, when the reactants have been mixed but the reaction has not begun. equilibrium concentration refers to the concentration of the species left at a time, when the reactants have reacted and an equilibrium has been achieved.

How do we determine [FeSCN2+]eqb? Fe3+ + SCN- FeSCN2+ Yellow Colorless Red color Mix specific amounts of Fe3+ solution to SCN- solution. The resulting FeSCN2+will make the solution look red and absorbs 447 nm light. From the absorbance of the solution we can figure out the concentration of FeSCN2+ using Beer’s law.

But the [FeSCN2+]formed will still be unknown? Firstly, we need to mix Fe3+ and SCN- such that we know the concentration of FeSCN2+ in the resulting solution For the known concentrations of FeSCN2+, we can measure the absorbance. Plot a graph of Absorbance Vs. Concentration. Obtain the relationship between concentration and absorbance from the equation of the best-fit line.

But the [FeSCN2+]formed will still be unknown? y Best-fit line Equation of the best-fit line: cn x c5,A5 n c4,A4 Y=mX + z Concentration m c3,A3 m = slope = c2,A2 cm x c1,A1 x Am An intercept Absorbance C= mA + Z concentration Absorbance

But the [FeSCN2+]formed will still be unknown? Plug in the absorbance of solution whose concentration of FeSCN2+ is unknown in the equation of best-fit line to obtain the value of concentration.

How do I make up a solution where [FeSCN2+] is known? Fe3+ + SCN- FeSCN2+ 1 ion 1 ion 1 ion 1 mole 1 mole 1 mole 100 moles 1 mole 1 mole Limiting reagent If we make one of the reactants rate limiting, the [FeSCN2+] formed will be equal to the concentration of the limiting reagent.

How is this done in the experiment? Fe3+ + SCN- FeSCN2+ 0.2 M 0.002 M 25 mL 0 mL 25 mL 2 mL 25 mL 4 mL 25 mL 6 mL 25 mL 8 mL 2 4 6 8 Limiting reagent [SCN-]known = [FeSCN2+]known 0.002 M Fe3+

C= mA + Z Equation of the best-fit line: [FeSCN2+], M Aknown intercept y Best-fit line Equation of the best-fit line: cn x c5,A5 n c4,A4 Y=mX + z [FeSCN2+], M m c3,A3 m = slope = c2,A2 cm x c1,A1 x Am An intercept Aknown C= mA + Z concentration Absorbance

How to find [FeSCN2+]unknown? Fe3+ + SCN- FeSCN2+ 0.002 M 0.002 M Solution "0.002 M" Fe(NO3)3 "0.002 M" KSCN 0.1 M HNO3 1 2.50 mL 0.50 mL 2.00 mL 2 2.50 1.00 1.50 3 2.50 1.50 1.00 4 2.50 2.00 0.50 5 2.50 2.50 0.0 Measure the absorbance of these solutions. C= mA + Z concentration Absorbance

How to find [FeSCN2+]eqb? [FeSCN2+]unknown = [FeSCN2+]formed= [FeSCN2+]eqb How to find [Fe3+]eqb and [SCN-]eqb? Known

How to find [Fe3+]reacted and [SCN-]reacted? Fe3+ + SCN- FeSCN2+ 1 ion 1 ion 1 ion 1 mole 1 mole 1 mole [FeSCN2+]eqb = [Fe3+]reacted= [SCN-]reacted

How do we determine the equilibrium Constant (Kc) of the reaction? Fe3+ + SCN- FeSCN2+ Yellow Colorless Red color [FeSCN2+]unknown = [FeSCN2+]formed= [FeSCN2+]eqb

When preparing 0.1 M HNO3 Wear SAFETY GOGGLES AND GLOVES Use graduated cylinder to dispense the acid from the bottle 3. Please have about 100 mL of water in 500 mL volumetric flask, before adding acid in to it. 4. Add acid to the flask slowly in small aliquots.