1 / 33

# Outline - PowerPoint PPT Presentation

Outline. Final Comments on Titrations/Equilibria Titration of Base with a strong acid End-point detection Choice of indicators Titration Curve method Start Chapter 18 Spectroscopy and Quantitative Analysis. Weak Base titrated with strong acid.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'Outline' - teegan-tran

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

• Titration of Base with a strong acid

• End-point detection

• Choice of indicators

• Titration Curve method

• Start Chapter 18

• Spectroscopy and Quantitative Analysis

• Consider a 100 ml of a 0.0100 M base with 0.0500 M HCl

• Kb = 1 x 10-5

Buffer Region

pH after equivalence

Dominated by remaining

[H+]

pH @ equivalence

### Electronic SpectroscopyUltraviolet and visible

• c=ln

• Where

• c= speed of light = 3.00 x 108 m/s

• l= wavelength in meters

• n = frequency in sec-1

• E=hn

• or E=hc/l

• h=Planks Constant = 6.62606 x 1034 J.s

### Beer-Lambert Law

AKA - Beer’s Law

• Transmittance:

T = P/P0

P0

(power in)

P

(power out)

• Absorbance:

A = -log10 T = log10 P0/P

How do “we” select the

wavelength

to measure the absorbance?

b(path through sample)

• The Beer-Lambert Law (a.k.a. Beer’s Law):

A =ebc

Where the absorbance A has no units, since A = log10 P0 / P

e is the molar absorbtivity with units of L mol-1 cm-1

b is the path length of the sample in cm

c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)

Why?

• Maximum Response for a given concentration

• Small changes in Wavelength, result in small errors in Absorbance

A

380

400

420

460

440

Wavelength, nm

“Fundamental”

“Experimental”

• Not Using Peak wavelength

• Colorimetric Reagent is limiting

• Concentration/Molecular Interactions

• Changes in Refractive Index

### Interaction of Light and Matter

Finish with Molecules

Very simple view of Energy states

Assuming subshells have equivalent energies

n=6

n=5

Energy

n=4

A

n=3

n=2

Wavelength, nm

n=1

### Molecular Spectroscopy

• With molecules, many energy levels.

Interactions between other molecules and with the solvent result in an increase in the width of the spectra.

maxwith certain extinction 

UV

Visible

Electronic Spectrum

Make solution of concentration low enough that A≤ 1

(Helps to Ensure Linear Beer’s law behavior)

UV bands are much broader than the photonic transition event. This is because vibration levels are superimposed.

1.0

Absorbance

0.0

200

400

800

Wavelength, , generally in nanometers (nm)

### UV/Vis and Molecular Structure

•   * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis.

• n  * transitions: non-bonding electrons (lone pairs), wavelength (max) in the 150-250 nm region.

• n  * and   * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm.

Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.

Example:   * transitions responsible for ethylene UV absorption at ~170 nm calculated with semi-empirical excited-states methods (Gaussian 03W):

h 170nm photon

 antibonding molecular orbital

 bonding molecular orbital

Napthalene

Absorbs in the UV

• What compounds show UV spectra?

• Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers.

• The NIST databases have UV spectra for many compoundsYou will find molar absorbtivities  in L•cm/mol, tabulated.

• Transition metal complexes, inorganics

• Qualitatively

• Not too useful

• Quantitatively

• Quite Useful

• Beer’s Law is obeyed through long range of concentrations

• Thousands of methods

• Most commonly used

• Detection Limits ~ 10-4 – 10-6 M

• Quant (cont’d)

• Cheap, inexpensive, can be relatively fast

• Reasonably selective

• Can find colorimetric method or use color of solution

• Good accuracy ~1-5%

• Open Excel

• Find data sheet

• Input data table

Where:

x = determined concentration

k = number of samples

m = slope

n = number of Standards (data points)

D = ??