Exp. 26 Objectives

1
Exp. 26 Objectives

• To use a spectrophotometer to determine the
  equilibrium constant of a chemical system.
• To use graphing techniques and data analysis to
  evaluate data.

2
Exp. 26 Introduction

• Spectrophotometric analyses involve the
  interaction of electromagnetic (EM) radiation
  with matter.
• The most common regions of the EM spectrum used
  in analyses are the
• UV
• Visible 400-700nm
• IR

3
Exp. 26 Introduction

• Every chemical substance produces its own unique
  set of electronic, vibrational, and rotational
  energy states. When EM radiation falls incident
  upon an atom or molecule, the radiation (light)
  absorbed is an energy equal to the difference
  between two energy states in the atom or
  molecule, placing the atom or molecule in an
  excited state. The remainder of the EM radiation
  (which is not absorbed) passes through the sample
  and can be detected by a detector.

4
Exp. 26 Introduction

• The higher the concentration of the absorbing
  species in the sample, the more EM radiation is
  absorbed and the more intense is the
  complementary transmitted radiation.
• Several factors control the amount of EM
  radiation that a sample absorbs
• Molar absorptivity coefficient of the substance
• Thickness of the sample (cuvet)
• Concentration of the absorbing substance.

5
Exp. 26 Introduction

• Absorbance
• A (a)(b)(c) log Io/It log 100/T
• Graph A vs. . Fig. 26.4
• Standards
• Unknowns

6
Exp. 26 Introduction

• For the chemical system
• Fe3 SCN- ? FeNCS2
• Kc FeNCS2
• Fe3SCN-

7
Exp. 26 Introduction

• Kc is constant for a chemical system at a given
  temperature.
• The value (magnitude) of the equilibrium constant
  (Kc) indicates the principal species (products
  and reactants) that exist in the system at
  equilibrium.
• A large Kc tells us that the equilibrium lies to
  the right, with a high concentration of products.

8
Exp. 26 Procedural Notes

• Part A read through all the procedure before
  proceeding
• A.1. Prepare a set of standards as shown in
  Table 26.1. Mix well!
• A.2. A.3. Prepare a blank and zero the
  spectrophotometer.
• A.4. Analyze the standards, recording the
  absorbances (omit transmittance)
• A.5. Graph the data

9
Exp. 26 Procedural Notes

• Part B
• B.1. Prepare the test solutions in your 150 mm
  test tubes according to Table 26.2. Concentrate
  on what youre doing! Dont mix up the pipets.
• B.2. Recalibrate (zero) the instrument
• B.3. Mix the test solutions for approx. 30
  seconds
• B.4. Read the absorbance of the test solutions,
  and, using the curve prepared in A.5, find the
  concentration of FeNCS2 in the solutions.
• B.5. Complete the calculations on the report
  sheet.

10
Exp. 26 Report Sheet

• Questions 2, 3, 6
• This is a formal.