Determination of the Equilibrium Constant

Theory

• Beer’s Law: Concentration is proportional to Absorbance

• The reaction: Fe+3 + SCN- [Fe(SCN)]+2

Kc = ]][[

])([3

2

SCNFe

SCNFe

• Beer’s Law: Concentration is proportional to Absorbance

• The reaction: Fe+3 + SCN- [Fe(SCN)]+2

Kc = ]][[

])([3

2

SCNFe

SCNFe

• Fe(SCN)+2 absorbs light in the blue-green region

– Set up standard curve with absorbance readings for solutions of known concentration

– Beer’s Law plot

Math

• Determine moles of each reactant species• Determine M for each based on total volume of

solution• ICE

• Keq =

• Lab,P. 298

]][[

])([3

2

SCNFe

SCNFe

Spectrophotometer• 3 clean cuvettes, wipes

• Empty chamber– Light source on– Power knob: 0%T

• Water blank– 447 nm– Clean cuvette with distilled water– Light source on– T knob: 100% T

• HNO3 blank

– Set absorbance = to 0

• Test solutions

• Zero the machine frequently

• http://www.chm.davidson.edu/java/spec/spec.html