The equilibrium constant experiment explores the relationship between the concentration of reactants and products in a chemical system. It focuses on the formation of the thiocyanatoiron(III) complex, where iron(III) ions react with thiocyanate ions. This experiment is designed for chemistry students studying equilibrium concepts and spectrophotometric analysis. Students will determine the equilibrium constant, Kc, by measuring absorbance at 447 nm and constructing a calibration curve. The experiment emphasizes the importance of concentration and light absorption in equilibrium systems.

Key Points

  • Explores the relationship between reactants and products in equilibrium systems.
  • Focuses on the formation of the thiocyanatoiron(III) complex in aqueous solutions.
  • Designed for chemistry students studying equilibrium and spectrophotometry.
  • Students determine the equilibrium constant, Kc, using absorbance measurements at 447 nm.
  • Emphasizes the significance of concentration and light absorption in chemical equilibria.
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exp 34 - an equilibrium constant
Study online at https://quizlet.com/_8z0oxl
1. The nearly colorless iron(III) ion forms an intensely col-
ored complex in the presence of the __________________
ion
thiocyanate
2. objectives (3) 1. To use a spectropho-
tometer to determine the
equilibrium constant of a
chemical system
2. To use graphing tech-
niques and data analysis
to evaluate data
3. To determine the equi-
librium constant for a sol-
uble equilibrium
3. Spectrophotometer? a laboratory instru-
ment that measures the
amount of light transmit-
ted through a sample
4. A spectrophotometric method of analysis involves
the interaction of ________________ ___________ with
__________.
What are the three most common regions of the EM
spectrum used for analyses?
We are most familiar with the visible region of the
spectrum, in which wavelengths range from ______ to
_____ nm
electromagnetic (EM) ra-
diation ; matter
ultraviolet, visible, and the
infrared regions
400 - 700
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exp 34 - an equilibrium constant
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5. The visible spectra of ions and molecules in solution
arise from electron ___________ within their respective
structures.
The greater the concentration of the absorbing
ions/molecules in solution, the greater is the absorp-
tion of the ____________ EM radiation (and the greater
the transmittance of the complementary radiation).
The degree of absorbed radiation (or the intensity of
the transmitted radiation) is measured using an in-
strument called a _________________________, which mea-
sures transmitted light intensities with a photosen-
sitive detector at specific (but variable) visible wave-
lengths. The wavelength where the absorbing ions or
molecules has a maximum absorption of visible radia-
tion is determined and set on the spectrophotometer
for the analysis.
transitions
visible
spectrophotometer
6. what 3 factors control the amount of EM radiation
(light energy) that absorbs?
1. concentration of the ab-
sorbing substance
2. Thickness of the sample
containing the absorbing
substance (determined by
the width of the cuvet)
3. Probability of light ab-
sorption by the absorbing
substance (called the mo-
lar absorptivity coefficient
or extinction coefficient)
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exp 34 - an equilibrium constant
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7. cuvet? a special piece of glass-
ware to hold solutions for
measurement in the spec-
trophotometer
8. The ratio of the intensity of the transmitted light, It ,
to that of the incident light, I0, is called the transmit-
tance, T, of the EM radiation by the sample. This ratio,
expressed as percent, is.....
%T = It (intensity of the
transmitted light) / I0 (in-
cident light) *100
9. Most spectrophotometers have a %T (percent trans-
mittance of light) scale. Because it is ___________, the %T
scale is easy to read and interpolate.
However, chemists often perform calculations based
on the amount of light __________________ by the sample
rather than the amount of light transmitted because
absorption is directly proportional to the ....
The absorbance, A, of the substance is related to the
intensity of the incident and transmitted light (and the
percent transmittance) by the equations....
linear
absorbed ; concentration
of the absorbing sub-
stance
A = log (I0 / It) = log (1 / T)
= log (100 / %T ) = a • b •
c
10. The molar absorptivity coefficient, a, is a ______________
at any given wavelength for a particular absorbing
substance
b is the ____________ of the absorbing substance in cen-
timeters
c is the molar concentration of the ...
constant
thickness
absorbing substance6
11.
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FAQs

What is the purpose of the equilibrium constant experiment?
The purpose of the equilibrium constant experiment is to investigate the relationship between the concentrations of reactants and products in a chemical equilibrium. Specifically, it examines the formation of the thiocyanatoiron(III) complex from iron(III) ions and thiocyanate ions. By measuring the absorbance of the resulting solution, students can calculate the equilibrium constant, Kc, which quantifies the extent of the reaction at equilibrium. This experiment helps students understand key concepts in chemical equilibrium and the application of spectrophotometry.
How do students measure the equilibrium constant in this experiment?
Students measure the equilibrium constant, Kc, by preparing a series of standard solutions of the thiocyanatoiron(III) complex and measuring their absorbance at a wavelength of 447 nm. A calibration curve is created from these absorbance values plotted against known concentrations. In Part B of the experiment, students mix known concentrations of iron(III) and thiocyanate ions, allowing the system to reach equilibrium. By measuring the absorbance of the mixed solution and using the calibration curve, students can determine the equilibrium concentration of the thiocyanatoiron(III) complex.
What role does the spectrophotometer play in this experiment?
The spectrophotometer is a critical instrument in this experiment, as it measures the intensity of light transmitted through the solution containing the thiocyanatoiron(III) complex. By quantifying the absorbance at 447 nm, students can determine how much light is absorbed by the complex, which is directly related to its concentration. This data is essential for constructing the calibration curve and calculating the equilibrium constant, Kc. The spectrophotometric method allows for precise and accurate measurements, enhancing the understanding of chemical equilibria.
What is the significance of the calibration curve in determining Kc?
The calibration curve is significant in determining the equilibrium constant, Kc, as it provides a visual representation of the relationship between absorbance and concentration of the thiocyanatoiron(III) complex. By plotting known concentrations against their corresponding absorbance values, students can establish a linear relationship that allows for the interpolation of unknown concentrations. This curve is used in Part B of the experiment to find the equilibrium concentration of the complex in mixed solutions. The accuracy of the calibration curve directly impacts the reliability of the calculated equilibrium constant.

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