Exp. 20: Spectrophotometric Analysis: Determination of the Equilibrium Constant for a Reaction

Transcription

1 Exp video (time: 41:13 minutes) Exp. 20: Spectrophotometric Analysis: Determination of the Equilibrium Constant for a Reaction

2 Chemical Equilibrium Previously we have assumed that chemical reactions results in complete conversion of reactants to products: A + B C + D No A or B remaining or possibly an excess of A or B but not both and eventually reaction stops. Many chemical reactions do not completely convert reactants to products. Stop somewhere between no rxn and complete rxn. A + B some left C + D some formed A + B C + D reversible (both directions) A + B C + D 2 exchange, constant conc., Rate f = Rate r uilibrium

3 Chemical Equilibrium Therefore, many reactions do not go to completion but rather form a mixture of products and unreacted reactants, in a dynamic uilibrium. A dynamic uilibrium consists of a forward reaction, in which substances react to give products, and a reverse reaction, in which products react to give the original reactants. Chemical uilibrium is the state reached by a reaction mixture when the rates of the forward and reverse reactions have become ual. 4

4 Graphically we can represent this A + B C + D The concentrations and reaction rate (less collisions, less component) of A and B decreases over time as the concentrations and reaction rate of C and D increases (more collisions, more component) over time until the rates are ual and the concentrations of each components reaches a constant. This occurs at what we call uilibrium -- R f = R r. If the rates are ual, then there must be a relationship to show this. 5

5 Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, H 2 + CO CH 4 + H 2 O R f =R r 3H 2 + CO CH 4 + H 2 O rate decreases over time CH 4 + H 2 O 3H 2 + CO rate increases over time

6 If we assume these reactions are elementary rxns (based on collisions), we can write the rate laws directly from the reaction: A + B C + D R f = k f [A][B] For the reverse reaction we have, C + D A + B R r = k r [C][D] We know at uil that R f = R r ;therefore, we can set these two expressions as ual k f [A][B] = k r [C][D] Rearrange to put constants on one side we get 7 K k k f r [ C][ D] [ A][ B]

7 Constant divided by constant just call a new constant K. This ratio is given a special name and symbol called uilibrium constant K relating to the uilibrium condition at a certain temperature (temp dependent) for a particular reaction relating conc of each component. This is basically a comparison between forward and reverse reaction rates. At uilibrium, the ratio of conc of species must satisfy K. K k k f r [ C][ D] [ A][ B] 8

8 The Equilibrium Constant Every reversible system has its own position of uilibrium - K- under any given set of conditions. The ratio of products produced to unreacted reactants for any given reversible reaction remains constant under constant conditions of pressure and temperature. If the system is disturbed, the system will shift and all the concentrations of the components will change until uilibrium is re-established which occurs when the ratio of the new concentrations uals "K". Different constant conc but ratio same as before. 9 The numerical value of this ratio is called the uilibrium constant for the given reaction, K.

9 10 The Equilibrium Constant The uilibrium-constant expression for a reaction is obtained by multiplying the uil concentrations ( or partial pressures) of products, dividing by the uil concentrations (or partial pressures) of reactants, and raising each concentration to a power ual to its coefficient in the balanced chemical uation. aa K c bb [ C] [ A] c a [ D] [ B] d b cc K p ( P dd C ( P A ) c a ( P D ( P The molar concentration of a substance is denoted by writing its formula in square brackets for aq solutions. For gases can put P a - atm. As long as use M or atm, K is unitless; liquids and solids = 1; setup same for all K s (K a, K b, etc.) Temp dependent; any changes, ratio will still ual K when uil established ) B ) ) d b

10 In our experiment, we will determine the K for the following reaction: Fe 3+ + SCN - FeSCN 2+ K? K 2 [ FeSCN ] 3 [ Fe ] [ SCN ] Fe 3+ + SCN - FeSCN 2+ Starting, [ ] o M Fe 3+ M SCN - 0 Change, [ ] -x -x +x Equilibrium, M Fe 3+ - x M SCN - - x x [ ]

11 K 2 [ FeSCN ] 3 [ Fe ] [ SCN ] [M Fe 3 - [x] x][m SCN - x] If we can determine x, we can solve for K for this reaction. First part of experiment involves making a calibration curve for [FeSCN 2+ ]

12 Calibration Curve We basically force the [FeSCN 2+ ] to ual the initial [SCN - ] by using a 1:1000 ratio of SCN - to Fe 3+ pg 139: ml of 2.00 x 10-4 M KSCN ml of M Fe(NO 3 ) 3 Therefore, [SCN - ] o diluted within rxn = [FeSCN 2+ ] for calibration only not actual K experiment

13 Calibration Curve [SCN - ] o = [FeSCN 2+ ] within rxn (meaning taking into account dilution) No.1 Standard (pg 139) ml of 2.00 x 10-4 M KSCN ml of M Fe(NO 3 ) 3 CV = CV (2.00 x 10-4 M SCN - ) (20.00 ml) = C SCN- diluted No. 1 (40.00 ml) [C SCN-diluted No.1 ] = 1.00 x 10-4 M = [FeSCN 2+ ] No. 1

14 No. 3 is done same way and then measure the absorbance at 460 nm (watch filter on spec 20) of each for calibration curve. Tip for plotting: make the three conc the same power of 10 and label axis conc x 10-5 M. No. 2 standard 10.0 x x x x 10-5 blank: 0.00 M 0.00 (1.00 x 10-4 M SCN - ) (10.00 ml) = C SCN- diluted No. 2 (20.00 ml) [C SCN-diluted No. 2 ] = 5.00 x 10-5 M = [FeSCN 2+ ] No. 2

15 pg 139: Preparation of solutions for K experiment Fe 3+ solution CV = CV Make solutions No. 1-3 from stock M; skip No

16 pg. 140: Preparation of solutions for K experiment Initial [SCN - ] o same dilution across (5 ml SCN - and 5 ml Fe 3+ ) [SCN - ] o = 1.00 x 10-4 M (skip No.4) Initial [Fe 3+ ] o same dilution but different stock of Fe 3+ (5 ml/5 ml) [Fe 3+ ] o = ½ [Fe 3+ solution pg 139] (skip No. 4) M M M [SCN - ] o 1.00 x x x 10-4 [Fe 3+ ] o M M M

17 Determination of K -Obtain calibration curve from first three solutions (top of pg 139); this plot will allow you to determine the [FeSCN 2+ ] for each of the three experimental mixtures. Mix solutions for exp 1 (pg 140), obtain absorbance from spec 20 at 460 nm, read absorbance off calibration curve for [FeSCN 2+ ] ; technically the x in K expression

18 Bottom table pg 140 Determination of K [Fe 3+ ] = [Fe 3+ ] o x = [Fe 3+ ] o [FeSCN 2+ ] [SCN - ] = [SCN - ] o x = [SCN - ] o [FeSCN 2+ ] -Plug values into K expression and solve for K -Repeat for exp 2 and 3 exp1 exp 2 exp 3 spec 20 spec 20 spec 20 K graph = x [Fe 3+ ] o [FeSCN 2+ ] = graph exp1 [SCN - ] o [FeSCN 2+ ] = 1.00 x 10-4 graph exp1 2 [ FeSCN ] 3 [ Fe ] [ SCN ] graph graph graph exp graph exp x 10-4 graph exp x 10-4 graph exp3 2 [ FeSCN ] 3 [ Fe ] [ SCN ] 2 [ FeSCN ] 3 [ Fe ] [ SCN ]

19 Goal: Average K for reaction and std dev. Only computer plot ruired but make sure change axis to some small increment. Chemicals: Take only 45 ml of each component.