Questions and Answers
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?
- 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?
A laboratory instrument that measures the amount of light transmitted through a sample.
A spectrophotometric method of analysis involves the interaction of ________________ ___________ with __________.
Signup and view all the answers
What are the three most common regions of the EM spectrum used for analyses?
Signup and view all the answers
Wavelengths range from ______ to _____ nm for the visible region of the spectrum.
Signup and view all the answers
The visible spectra of ions and molecules in solution arise from electron ___________ within their respective structures.
Signup and view all the answers
The greater the concentration of the absorbing ions/molecules in solution, the greater is the absorption of the ____________ EM radiation.
Signup and view all the answers
What instrument measures transmitted light intensities at specific wavelengths?
Signup and view all the answers
The ratio of the intensity of the transmitted light, It, to that of the incident light, I0, is called the transmittance, T, given by ______.
Signup and view all the answers
The %T scale on most spectrophotometers is nonlinear.
Signup and view all the answers
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 ).
Signup and view all the answers
The molar absorptivity coefficient, a, is a ______________ at any given wavelength for a particular absorbing substance.
Signup and view all the answers
B is the ____________ of the absorbing substance in centimeters.
Signup and view all the answers
The absorbance value is directly proportional to the ______ if the same cuvet and a set wavelength are used for all measurements.
Signup and view all the answers
What does the magnitude of an equilibrium constant, Kc, express?
Signup and view all the answers
For the reaction, aA + bB xX + yY, the mass action expression equals the ___________ _______________ at equilibrium.
Signup and view all the answers
What does a large equilibrium constant indicate about the principal species in the chemical system?
Signup and view all the answers
The value of Kc is ____________ for a chemical system at a given temperature.
Signup and view all the answers
What does Beer’s Law state?
Signup and view all the answers
This experiment determines Kc for a chemical system in which all species are _____________.
Signup and view all the answers
The chemical system involves the equilibrium between iron(III) ion, Fe3, thiocyanate ion, SCN-, and thiocyanatoiron(III) ion, FeNCS^2+: (give the formula)
Signup and view all the answers
The 'free' thiocyanate ion is commonly written as SCN-; however, its bond to the ferric ion is through the ________________ atom.
Signup and view all the answers
Because the concentration of water is essentially constant in dilute aqueous solutions, we simplify the equation to what?
Signup and view all the answers
In Part A you will prepare a set of five standard solutions of the ____________ ion.
Signup and view all the answers
As FeNCS2 is a deep, blood-red complex, its absorption maximum occurs at about _____ nm.
Signup and view all the answers
The absorbance at 447 nm for each solution is plotted versus the molar concentration of FeNCS2; this establishes a _________________ __________.
Signup and view all the answers
What is a standard solution?
Signup and view all the answers
What is a calibration curve?
Signup and view all the answers
What does it mean to be complexed?
Signup and view all the answers
In preparing the standard solutions of FeNCS^2+, the Fe3 concentration is set to far exceed the ___________ concentration.
Signup and view all the answers
This huge excess of Fe3+ pushes the equilibrium far to the _________, consuming nearly all of the SCN- placed in the system.
Signup and view all the answers
As a result, the FeNCS2 concentration at equilibrium approximates the original SCN- concentration, forming ____________.
Signup and view all the answers
In Part B, the concentrations of the Fe3+ and SCN- ions in the various test solutions are nearly the _________.
Signup and view all the answers
In Part B, precise volumes of known molar concentrations of _____ and ______ are mixed.
Signup and view all the answers
How is the equilibrium molar concentration of FeNCS^2+ determined?
Signup and view all the answers
For every mole of FeNCS^2+ that exists at equilibrium, an equal number of moles of...have reacted to reach equilibrium.
Signup and view all the answers
The equilibrium molar concentrations of Fe3+ and SCN- (their equilibrium concentrations) can be calculated, providing the value of the...
Signup and view all the answers
What is done with one set of solutions having known molar concentrations of FeNCS^2+?
Signup and view all the answers
A second set of equilibrium solutions is prepared and mixed to determine the respective equilibrium.... of FeNCS^2+.
Signup and view all the answers
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.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
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.
