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
Top Related