• Hydrogen (HHydrogen (H22) and iodine (I) and iodine (I22) react to produce hydroiodic ) react to produce hydroiodic

acid (HI).acid (HI).Forward ReactionForward Reaction: H: H2(g)2(g) + I + I2(g)2(g) → 2HI → 2HI(g)(g)

• However, as soon as some HI is produced it begins to However, as soon as some HI is produced it begins to dissociate back to Hdissociate back to H22 and I and I22..

Reverse ReactionReverse Reaction: 2HI: 2HI(g)(g) → H → H2(g)2(g) + I + I2(g)2(g)

• These 2 opposing reactions, 1 forward and 1 reverse, go on These 2 opposing reactions, 1 forward and 1 reverse, go on simultaneously. Ultimately, the rate of the forward reaction simultaneously. Ultimately, the rate of the forward reaction will equal the rate of the reverse reaction and the system is will equal the rate of the reverse reaction and the system is in in dynamic equilibriumdynamic equilibrium..

HH2(g)2(g) + I + I2(g)2(g) = 2HI = 2HI(g)(g)

CH 104: DETERMINATION OF AN EQUILIBRIUM CONSTANTCH 104: DETERMINATION OF AN EQUILIBRIUM CONSTANT

EQUILIBRIUMEQUILIBRIUM

• How many moles of HHow many moles of H2(g)2(g), I, I2(g)2(g), and HI, and HI(g)(g) are initially used in experiment 1? are initially used in experiment 1?• HH2(g)2(g) = 0.0015 moles, I = 0.0015 moles, I2(g)2(g) = 0.0015 moles, and HI = 0.0015 moles, and HI(g)(g) = 0 moles. = 0 moles.• How many moles of HHow many moles of H2(g)2(g), I, I2(g)2(g), and HI, and HI(g)(g) are initially used in experiment 2? are initially used in experiment 2?• HH2(g)2(g) = 0 moles, I = 0 moles, I2(g)2(g) = 0 moles, and HI = 0 moles, and HI(g)(g) = 0.0015 moles. = 0.0015 moles.• How many moles of HHow many moles of H2(g)2(g), I, I2(g)2(g), and HI, and HI(g)(g) are initially used in experiment 3? are initially used in experiment 3?• HH2(g)2(g) = 0.0015 moles, I = 0.0015 moles, I2(g)2(g) = 0.0015 moles, and HI = 0.0015 moles, and HI(g)(g) = 0.0015 moles. = 0.0015 moles.• In summary, all 3 experiments reach a dynamic equilibrium regardless of In summary, all 3 experiments reach a dynamic equilibrium regardless of

the initial concentrations of Hthe initial concentrations of H2(g)2(g), I, I2(g)2(g), and HI, and HI(g)(g)..

EQUILIBRIUMEQUILIBRIUM

HH2(g)2(g) + I + I2(g)2(g) = 2HI = 2HI(g)(g)

• At equilibrium the rate of the forward reaction equals the At equilibrium the rate of the forward reaction equals the rate of the reverse reaction. Hrate of the reverse reaction. H2(g)2(g) and I and I2(g)2(g) are still producing are still producing

HIHI(g)(g). And HI. And HI(g)(g) is still producing H is still producing H2(g)2(g) and I and I2(g)2(g). However, the . However, the

final concentrations of Hfinal concentrations of H2(g)2(g), I, I2(g)2(g), and HI, and HI(g)(g) are constant. are constant.

EQUILIBRIUMEQUILIBRIUM

• If the rate of people going to the first If the rate of people going to the first floor equals the rate of people going to floor equals the rate of people going to the second floor, the number of people the second floor, the number of people on each floor remains constant and the on each floor remains constant and the 2 groups are at dynamic equilibrium.2 groups are at dynamic equilibrium.

EQUILIBRIUMEQUILIBRIUM

• For the general reaction:For the general reaction:

aaA + A + bbB = B = ccC + C + ddDD

• Equilibrium Constant = Equilibrium Constant = KKcc = =

• WhereWhere• [A], [B], [C], and [D] are the concentrations of reactants [A], [B], [C], and [D] are the concentrations of reactants

and products in moles per liter.and products in moles per liter.• a, b, c, and d are the stoichiometric coefficients from the a, b, c, and d are the stoichiometric coefficients from the

balance reaction.balance reaction.• KKcc is the equilibrium constant. is the equilibrium constant.

• The value of The value of KKcc depends on the particular reaction and on depends on the particular reaction and on

the temperature.the temperature.

EQUILIBRIUMEQUILIBRIUM

[C][C]cc[D][D]dd ProductsProducts

[A][A]aa[B][B]bb ReactantsReactants

• In today’s experiment you will measure the equilibrium In today’s experiment you will measure the equilibrium constant (constant (KKcc) for the following reaction.) for the following reaction.

FeFe3+3+ + SCN + SCN–– = FeSCN = FeSCN2+2+

KKcc = =

• Solutions of known iron(III) nitrate (Fe(NOSolutions of known iron(III) nitrate (Fe(NO33))33) and potassium ) and potassium

thiocyanate (KSCN) will be mixed in prescribed proportions. thiocyanate (KSCN) will be mixed in prescribed proportions. The concentration of FeSCN The concentration of FeSCN2+2+ at equilibrium will be at equilibrium will be measured spectrophotometrically. These data will be used measured spectrophotometrically. These data will be used to calculate to calculate KKcc..

MEASURING AN EQUILIBRIUM CONSTANTMEASURING AN EQUILIBRIUM CONSTANT

[FeSCN[FeSCN2+2+]]

[Fe[Fe3+3+][SCN][SCN––]]

• If 10.00 mL of 0.00200 M Fe(NOIf 10.00 mL of 0.00200 M Fe(NO33))33, 5.00 mL of 0.00200 M , 5.00 mL of 0.00200 M

KSCN, and 5.00 mL of distilled water are mixed together, KSCN, and 5.00 mL of distilled water are mixed together, what is the initial concentration of Fewhat is the initial concentration of Fe3+3+??

• [Fe[Fe3+3+]]initialinitial = 0.00100 M = 0.00100 M• What is the initial concentration of SCNWhat is the initial concentration of SCN––??• [SCN[SCN––]]initialinitial = 0.000500 M = 0.000500 M• What is the initial concentration of FeSCNWhat is the initial concentration of FeSCN2+2+??• [FeSCN[FeSCN2+2+]]initialinitial = 0 M = 0 M• If the concentration of FeSCNIf the concentration of FeSCN2+2+ at equilibrium is 0.000120 M, at equilibrium is 0.000120 M,

what is the equilibrium concentration of Fewhat is the equilibrium concentration of Fe3+3+??FeFe3+3+ + SCN + SCN–– = FeSCN = FeSCN2+2+

• [Fe[Fe3+3+]]equilibriumequilibrium = 0.00100 M – 0.000120 M = 0.00088 M = 0.00100 M – 0.000120 M = 0.00088 M• What is the equilibrium concentration of SCNWhat is the equilibrium concentration of SCN––??• [SCN[SCN––]]equilibriumequilibrium = 0.000500 M – 0.000120 M = 0.000380 M = 0.000500 M – 0.000120 M = 0.000380 M

MEASURING AN EQUILIBRIUM CONSTANTMEASURING AN EQUILIBRIUM CONSTANT

• An An ICEICE ( (InitialInitial, , ChangeChange, , EquilibriumEquilibrium) table is used to solve ) table is used to solve these types of problems.these types of problems.

• What is the equilibrium constant?What is the equilibrium constant?

KKcc = =

KKcc = = 3.6x10 = = 3.6x1022

MEASURING AN EQUILIBRIUM CONSTANTMEASURING AN EQUILIBRIUM CONSTANT

Reaction: Reaction: FeFe3+3+ + + SCNSCN–– = = FeSCNFeSCN2+2+

Initial Initial

concentrations:concentrations:

0.00100 M0.00100 M 0.000500 M0.000500 M 0 M0 M

Change:Change: – – 0.000120 M0.000120 M – – 0.000120 M0.000120 M + 0.000120 M+ 0.000120 M

Equilibrium Equilibrium

concentrations:concentrations:

0.00088 M0.00088 M 0.000380 M0.000380 M 0.000120 M0.000120 M

[FeSCN[FeSCN2+2+]]

[Fe[Fe3+3+][SCN][SCN––]]

[0.000120][0.000120]

[0.00088][0.000380][0.00088][0.000380]

SCHEMATIC OF A SPECTROPHOTOMETERSCHEMATIC OF A SPECTROPHOTOMETER

• The most common The most common light sourcelight source for the visible region for the visible region spectrophotometry is a tungsten filament incandescent lamp. A spectrophotometry is a tungsten filament incandescent lamp. A tungsten lamp emits useful light from approximately 325 nm to 3,000 tungsten lamp emits useful light from approximately 325 nm to 3,000 nm.nm.

• A A monochromatormonochromator uses a prism or a diffraction grating to separate uses a prism or a diffraction grating to separate polychromatic (many wavelengths) light into monochromatic (single polychromatic (many wavelengths) light into monochromatic (single wavelength) light.wavelength) light.

• A A cellcell or or cuvettecuvette is used to hold the sample during analysis. is used to hold the sample during analysis.• The The detectordetector uses a phototube or a photomultiplier tube to convert light uses a phototube or a photomultiplier tube to convert light

into an electrical signal that is sent to a recorder or computer.into an electrical signal that is sent to a recorder or computer.

THE SPECTRONIC 20D SPECTROPHOTOMETERTHE SPECTRONIC 20D SPECTROPHOTOMETER

The controls.The controls. Loading a sample.Loading a sample.THE HACH DR2010 SPECTROPHOTOMETERTHE HACH DR2010 SPECTROPHOTOMETER

• In today’s experiment you will use the Beer-Bouguer-Lambert law, more In today’s experiment you will use the Beer-Bouguer-Lambert law, more commonly called commonly called Beer’s lawBeer’s law, to measure the equilibrium concentration , to measure the equilibrium concentration of FeSCNof FeSCN2+2+..

A sample in a cell or cuvette during spectrophotometric analysis.A sample in a cell or cuvette during spectrophotometric analysis.

• PPoo = the power of monochromatic = the power of monochromatic light entering the sample.light entering the sample.• PP = the power of monochromatic = the power of monochromatic light leaving the sample.light leaving the sample.• aa = the absorptivity constant, which depends on the wavelength and the = the absorptivity constant, which depends on the wavelength and the

nature of the absorbing compound.nature of the absorbing compound.• bb = the path length through the absorbing compound. = the path length through the absorbing compound.• cc = the concentration of absorbing compound in the cuvette. = the concentration of absorbing compound in the cuvette.

ABSORBANCE AND CONCENTRATIONABSORBANCE AND CONCENTRATION

SOURCESSOURCES

• Beck, J. 2006. Unit 3 Spectrophotometry. Available: Beck, J. 2006. Unit 3 Spectrophotometry. Available: http://iws.ccccd.edu/jbeck/Spectrophotometryweb/Page.html [accessed 2 October 2006].[accessed 2 October 2006].

• Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc.York, NY: John Wiley & Sons, Inc.

• Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company.New York, NY: W.H. Freeman Company.

• McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall.Saddle River, NJ: Prentice Hall.

• Petrucci, R.H. 1985. General Chemistry Principles and Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.Publishing Company.