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Questions and Answers
The nearly colorless iron(III) ion forms an intensely colored complex in the presence of the __________________ ion.
The nearly colorless iron(III) ion forms an intensely colored complex in the presence of the __________________ ion.
thiocyanate
What are the objectives of using a spectrophotometer to determine the equilibrium constant?
What are the objectives of using a spectrophotometer to determine the equilibrium constant?
- To use a spectrophotometer to determine the equilibrium constant of a chemical system. 2. To use graphing techniques and data analysis to evaluate data. 3. To determine the equilibrium constant for a soluble equilibrium.
What is a spectrophotometer?
What is a spectrophotometer?
A laboratory instrument that measures the amount of light transmitted through a sample.
A spectrophotometric method of analysis involves the interaction of ________________ ___________ with __________.
A spectrophotometric method of analysis involves the interaction of ________________ ___________ with __________.
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What are the three most common regions of the EM spectrum used for analyses?
What are the three most common regions of the EM spectrum used for analyses?
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Wavelengths range from ______ to _____ nm for the visible region of the spectrum.
Wavelengths range from ______ to _____ nm for the visible region of the spectrum.
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The visible spectra of ions and molecules in solution arise from electron ___________ within their respective structures.
The visible spectra of ions and molecules in solution arise from electron ___________ within their respective structures.
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The greater the concentration of the absorbing ions/molecules in solution, the greater is the absorption of the ____________ EM radiation.
The greater the concentration of the absorbing ions/molecules in solution, the greater is the absorption of the ____________ EM radiation.
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What instrument measures transmitted light intensities at specific wavelengths?
What instrument measures transmitted light intensities at specific wavelengths?
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The ratio of the intensity of the transmitted light, It, to that of the incident light, I0, is called the transmittance, T, given by ______.
The ratio of the intensity of the transmitted light, It, to that of the incident light, I0, is called the transmittance, T, given by ______.
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The %T scale on most spectrophotometers is nonlinear.
The %T scale on most spectrophotometers is nonlinear.
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The absorbance, A, of the substance is related to the intensity of the incident and transmitted light by the equations A = log(I0 / It) = log(1 / T) = log(100 / %T ).
The absorbance, A, of the substance is related to the intensity of the incident and transmitted light by the equations A = log(I0 / It) = log(1 / T) = log(100 / %T ).
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The molar absorptivity coefficient, a, is a ______________ at any given wavelength for a particular absorbing substance.
The molar absorptivity coefficient, a, is a ______________ at any given wavelength for a particular absorbing substance.
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B is the ____________ of the absorbing substance in centimeters.
B is the ____________ of the absorbing substance in centimeters.
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The absorbance value is directly proportional to the ______ if the same cuvet and a set wavelength are used for all measurements.
The absorbance value is directly proportional to the ______ if the same cuvet and a set wavelength are used for all measurements.
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What does the magnitude of an equilibrium constant, Kc, express?
What does the magnitude of an equilibrium constant, Kc, express?
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For the reaction, aA + bB xX + yY, the mass action expression equals the ___________ _______________ at equilibrium.
For the reaction, aA + bB xX + yY, the mass action expression equals the ___________ _______________ at equilibrium.
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What does a large equilibrium constant indicate about the principal species in the chemical system?
What does a large equilibrium constant indicate about the principal species in the chemical system?
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The value of Kc is ____________ for a chemical system at a given temperature.
The value of Kc is ____________ for a chemical system at a given temperature.
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What does Beer’s Law state?
What does Beer’s Law state?
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This experiment determines Kc for a chemical system in which all species are _____________.
This experiment determines Kc for a chemical system in which all species are _____________.
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The chemical system involves the equilibrium between iron(III) ion, Fe3, thiocyanate ion, SCN-, and thiocyanatoiron(III) ion, FeNCS^2+: (give the formula)
The chemical system involves the equilibrium between iron(III) ion, Fe3, thiocyanate ion, SCN-, and thiocyanatoiron(III) ion, FeNCS^2+: (give the formula)
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The 'free' thiocyanate ion is commonly written as SCN-; however, its bond to the ferric ion is through the ________________ atom.
The 'free' thiocyanate ion is commonly written as SCN-; however, its bond to the ferric ion is through the ________________ atom.
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Because the concentration of water is essentially constant in dilute aqueous solutions, we simplify the equation to what?
Because the concentration of water is essentially constant in dilute aqueous solutions, we simplify the equation to what?
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In Part A you will prepare a set of five standard solutions of the ____________ ion.
In Part A you will prepare a set of five standard solutions of the ____________ ion.
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As FeNCS2 is a deep, blood-red complex, its absorption maximum occurs at about _____ nm.
As FeNCS2 is a deep, blood-red complex, its absorption maximum occurs at about _____ nm.
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The absorbance at 447 nm for each solution is plotted versus the molar concentration of FeNCS2; this establishes a _________________ __________.
The absorbance at 447 nm for each solution is plotted versus the molar concentration of FeNCS2; this establishes a _________________ __________.
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What is a standard solution?
What is a standard solution?
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What is a calibration curve?
What is a calibration curve?
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What does it mean to be complexed?
What does it mean to be complexed?
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In preparing the standard solutions of FeNCS^2+, the Fe3 concentration is set to far exceed the ___________ concentration.
In preparing the standard solutions of FeNCS^2+, the Fe3 concentration is set to far exceed the ___________ concentration.
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This huge excess of Fe3+ pushes the equilibrium far to the _________, consuming nearly all of the SCN- placed in the system.
This huge excess of Fe3+ pushes the equilibrium far to the _________, consuming nearly all of the SCN- placed in the system.
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As a result, the FeNCS2 concentration at equilibrium approximates the original SCN- concentration, forming ____________.
As a result, the FeNCS2 concentration at equilibrium approximates the original SCN- concentration, forming ____________.
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In Part B, the concentrations of the Fe3+ and SCN- ions in the various test solutions are nearly the _________.
In Part B, the concentrations of the Fe3+ and SCN- ions in the various test solutions are nearly the _________.
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In Part B, precise volumes of known molar concentrations of _____ and ______ are mixed.
In Part B, precise volumes of known molar concentrations of _____ and ______ are mixed.
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How is the equilibrium molar concentration of FeNCS^2+ determined?
How is the equilibrium molar concentration of FeNCS^2+ determined?
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For every mole of FeNCS^2+ that exists at equilibrium, an equal number of moles of...have reacted to reach equilibrium.
For every mole of FeNCS^2+ that exists at equilibrium, an equal number of moles of...have reacted to reach equilibrium.
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The equilibrium molar concentrations of Fe3+ and SCN- (their equilibrium concentrations) can be calculated, providing the value of the...
The equilibrium molar concentrations of Fe3+ and SCN- (their equilibrium concentrations) can be calculated, providing the value of the...
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What is done with one set of solutions having known molar concentrations of FeNCS^2+?
What is done with one set of solutions having known molar concentrations of FeNCS^2+?
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A second set of equilibrium solutions is prepared and mixed to determine the respective equilibrium.... of FeNCS^2+.
A second set of equilibrium solutions is prepared and mixed to determine the respective equilibrium.... of FeNCS^2+.
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Study Notes
Iron(III) Complexation
- Iron(III) ions form complexes with thiocyanate ions, creating a distinctively colored solution that allows for analysis.
- Complex: [Fe(H2O)6]^3+ + SCN- ↔ [Fe(H2O)5NCS]^2+ + H2O.
Objectives of the Experiment
- Determine the equilibrium constant (Kc) for a soluble equilibrium system using spectrophotometry.
- Utilize graphing and data analysis to evaluate equilibrium data.
- Analyze the relationship between reactants and products in a chemical system.
Spectrophotometry Basics
- Spectrophotometer: Measures light transmitted through a sample to determine concentrations of absorbing substances.
- Analysis involves the interaction of electromagnetic radiation with matter across ultraviolet, visible, and infrared regions (400-700 nm).
Absorption and Transmittance
- Absorption results from electron transitions; higher concentrations lead to increased absorption and reduced transmittance of visible light.
- Transmittance (T) is calculated as %T = (It/I0) * 100, where It is transmitted light and I0 is incident light.
- Absorbance (A) is preferred in calculations: A = -log(%T) or A = log(I0/It).
Factors Affecting Light Absorption
- Concentration, sample thickness (cuvet width), and molar absorptivity coefficient influence light absorption.
- Cuvet holds sample solutions in spectrophotometers.
Equilibrium Constant (Kc)
- Kc expresses equilibrium position: a large Kc suggests product dominance at equilibrium.
- For the general reaction aA + bB ↔ xX + yY, Kc is derived from the mass action expression.
Beer's Law
- Followed relationship: A = abc, where a is molar absorptivity, b is path length, and c is concentration.
Standard Solutions and Calibration Curves
- Standard solutions of FeNCS^2+ are prepared to create a calibration curve by plotting absorbance vs. concentration at 447 nm.
- Ensure excess Fe3+ concentration drives the equilibrium toward product formation, complexing all SCN- resulting in nearly constant across various tests.
Analyzing Equilibrium Concentrations
- In Part B, mix known concentrations of Fe3+ and SCN- to establish equilibrium.
- Measure absorbance of FeNCS^2+ to determine its concentration and subsequently calculate equilibrium concentrations of all species involved.
- The equilibrium molar concentrations allow for Kc calculation using the derived concentrations in the mass action expression.
Ensuring Experimental Accuracy
- Maintain consistent cuvet type and specific wavelengths during measurements to uphold data reliability.
- Precise volume measurements facilitate accurate concentration calculations at equilibrium.
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Description
This quiz focuses on the complexation of Iron(III) ions with thiocyanate ions and the analysis of the resulting colored solutions. It covers how to determine the equilibrium constant using spectrophotometry and emphasizes the relationship between reactants and products in chemical systems.
