Chem. 31 – 3/30 Lecture
Announcements I
• Additional Problem due Wed (along with quiz)
• No Lab Tomorrow (Campus closed for Cesar
Chavez holliday)
• Exam 2
– April 13th (2 weeks from today)
– Will cover Ch. 6 (parts since Exam 1), Ch. 7, Ch.
17, and part or all (of pages assigned) of Ch. 22
• Homework Set 2
– Set 2.2 problem solutions posted
– AP2.2 – needed some minor corrections (see
website again for changes)
Announcements II
• Today’s Lecture
– Chapter 7 “Advanced Equilibrium Theory” - The
Systematic Method
• More Examples
• General Comments on the Systematic Method
– Chapter 17 Spectroscopy (skipping ahead to keep
up with lab)
• Introduction
• Nature of Light
• Absorption of Light/Regions of the Electromagnetic
Spectrum
The Systematic Method
2nd Example
• An aqueous mixture of CdCl2 and NaSCN is
made
– Initial concentrations are [CdCl2] = 0.0080 M and
[NaSCN] = 0.0040 M
– Cd2+ reacts with SCN- to form CdSCN+ K = 95
– Got through step 3 last time?? (maybe best to start
from step 1 again)
The Systematic Method
3rd Example
•
A student prepares a solution that
contains 0.050 mol of AgNO3 and 0.0040
mol NH3 in water with a total volume of
1.00 L. The AgNO3 is totally soluble, NH3
is a weak base, and Ag+ reacts with NH3
to form Ag(NH3)2+. Assume the Ag+
does not react with water or OH-. Go
through the first 5 steps of the
systematic method.
The Systematic Method
Stong Acid/Strong Base Problems
• When do we need to use the systematic
approach?
– when more than 1 coupled reaction occur
(unless coupling is insignificant)
– examples: 4.0 x 10-3 M HCl. 7.2 x 10-3 M
NaOH
– Key point is the charge balance equation:
- for strong acid HX, [H+] = [X-] + [OH-]
- If [X-] >> [OH-], then [H+] = [X-]
– for strong base NaOH, [H+] + [Na+] = [OH-]
The Systematic Method
General Comments
• Effects of secondary reactions
– e.g. MgCO3 dissolution
– Additional reactions increase solubility
– Secondary reactions also can affect pH (CO32+ H2O will produce OH- while Mg2+ + H2O will
produce H+)
• Software is also available to solve these
types of problems (but still need to know
steps 1 → 5 to get problems solved)
Chapter 17 - Spectroscopy
A. Introduction
1. One of the main branches of analytical chemistry
2. The interaction of light and matter (for purposes of
quantitative and qualitative analysis)
3. Topics covered:
- Properties of Light
- Absorption of Light
- Electromagnetic Spectrum
- Beer’s Law
- Spectrometers
Spectroscopy
B.
Fundamental Properties
of Light
1.
Wave-like properties:
λ
λ = wavelength = distance
between wave crests
n = frequency = # wave crests/s
n = wave number = # wave crests/length measure
c = speed of light (in vacuum) = 3.00 x 108 m/s
Relationships: c = λ·n and n = 1/λ
note: speed of light depends on medium (slower
in water than in vacuum) – not considered here
Spectroscopy
Fundamental Properties of Light
1. Other wave-like properties
- diffraction, interference
2. Particle-like properties
a) Idea of photons (individual entities of
light)
b) Energy of photons
E = hn = hc/l
Spectroscopy
Absorption vs. Emission
1. Absorption
- Associated with a
transition of matter
from lower energy to
higher energy
A + hn → A*
hn = photon
A* → A + hn
Energy
2. Emission
Excited State
Photon
out
- Associated with a
transition from high
energy to low energy
Ground State
Photon in
Spectroscopy
Regions of the Electromagnetic Spectrum
Many regions are defined as much by the types of
transitions occurring (e.g. outer shell electron) as by the
frequency or energy of the transitions
Short
wavelengths
Gamma
rays
High
Energies
Outer shell
electrons
X-rays
UV +
visible
Nuclear
Inner shell
transitions electrons
Bond
vibration
Infrared
Nuclear
spin
Microwaves
Molecular
rotations
Long
wavelengths
Radio waves
Electron spin
Low Energies
Spectroscopy
Some Example Questions
1.
2.
3.
A nuclear magnetic resonance (NMR) spectrometer absorbs light
at a frequency of 750 MHz. This is in the radio frequency and Hz
= s-1. What is the wavelength of this light?
An infrared absorption band occurs at a wavenumber of 812 cm-1.
What is the wavelength (in mm) and energy (J/photon) of that
light?
What type of light involves transitions of inner shell electrons?