Lecture 3: Energy of Electromagnetic
Waves and Spectra, Rate of Chemical
Reaction and Energy
Gabor SAMJESKÉ/Quan PHUNG/Yoshiaki SHUKU

Week 11; 30th June: Experiment 7
Determination of concentration by absorption photometry (Lambert– Beer law)
Week 12; 7th July: Experiment 8
Absorption spectra of bromothymol blue (BTB) at diﬀerent pH
Week 13; 14th July: Experiment 9
Chemical oscillation reactions (online only)2Chapter 5: Energy of Electromagnetic
Radiation And Spectra3Spectra of Various Light Sources (skip)
Main theme: In this experiment, you will study the relation between color,wavelength and energy of light.

https://en.wikipedia.org/wiki/Electromagnetic_spectrum

4c: the rate of travel of all electromagnetic energy in a vacuum and is aconstant exact value—speed of light.
c = 2.997 924 58 × 108 m s−1, usually rounded oﬀ to 3.00 × 108 m s−15

A beam of light behaves as if it were a stream of small particles, called
photons, whose energy (E) is related to their frequency, 𝜈 (or wavelength, 𝜆)h = 6.626 × 10−34 J sPlanck’s constantUnit:
J s × s−1 = J
Sometimes in kJ mol−1(Energy of 1 photon)(Energy of NA photons)

6Line Spectrum: A series of discrete lines on an otherwise dark
background as a result of light emitted by an excited atomLine Spectra and the Bohr Model. (2021, November 8).
https://chem.libretexts.org/@go/page/21730

7

410 nm 434 nm

H-δ H-γ

486 nm

656 nmH-βH-α

The “visible” H emission spectrum lines in the Balmer series.
The Hydrogen Atomic Spectrum. (2021, October 3).
https://chem.libretexts.org/@go/page/210778

8

Rydberg formula (1888)

Rydberg
constant forhydrogen109677 cm−1

principal
quantum
number9

Emission spectrum of atomic sodium

each element has its
own unique spectral“signature”

The Bohr Atom. (2015, September 24).
https://chem.libretexts.org/@go/page/37017
10

Absorption
spectrum

Emission
spectrum

Absorption vs. emission spectra
http://faculty.sdmiramar.edu/fgarces/labmatters/instruments/aa/AAS_Theory/AASTheory.htm
11

Absorption spectra and structure of molecules
Main theme: In this experiment, you will
● measure molecular absorption spectrum.
● learn how absorption spectrum relates to molecular structure.

12Light absorption
When a molecule interacts with light and energy is absorbed → themolecule is excited.

13

Molecular orbital (MO) picture: promotion of an electron from an occupied
MO to an unoccupied MO.

h𝜈Ground state

Excitedstate
14ColorOur perception of color is determined by the wavelengths of radiation
reaching our eyes and the sensitivity of the receptors in our eyes.

Retinal
Chromophores convert light into electrical impulse.
https://www.aao.org/eye-health/anatomy/cones15Rods work under low intensity conditions →black, grey, and white.

Human eye is the mostsensitive to green color

Cones work under high intensity conditions →colors.
https://www.unm.edu/~toolson/human_cone_response.htm
https://commons.wikimedia.org/wiki/File:Eyesensitivity.png
https://www.flickr.com/photos/veisto/8389740249

16

Complementary Colors
If wavelengths of light from acertain region of the spectrum
are absorbed by a material,
then the material will appear tobe the complementary color.

Theopold, P. F., Klaus, & Richard Langley et al. (2021, May 21).
Color and the Colors of Complexes. OpenStax CNX.
https://chem.libretexts.org/@go/page/24356[Cu(NH3)4]2+17

AbsorbanceWavelength (nm)UV-Vis spectrum of
ꞵ-Carotene in methanol

Why do atoms produce
line spectra, whilemolecules
produceband spectra?

http://lipidbank.jp/cgi-bin/detail.cgi?id=VCA0001
https://en.wikipedia.org/wiki/%CE%92-Carotene
18Each electronic energy level has
many sublevels associated withvibration and rotation of themolecule19

UV-Vis spectrophotometer

Intensity I0
Wavelength 𝜆Transmitted lightintensity IAbsorbance

Detector

Wavelength (nm)https://en.wikibooks.org/wiki/Methods_and_Concepts_in_the_Life_Sciences/Spectroscopy

20Lambert-Beer law
Absorbancesample lengthcm
molar absorption (extinction)coeﬃcient
dm3 mol−1 cm−1

molar concentration
mol dm−3Transmittance

𝜀 depends on wavelength of the light 𝜆, solvent, and temperature
𝜀 can be evaluated by plotting A with [J] (linear)21

Absorption spectra of conjugated systems

𝜋-𝜋*transition

1 kcal/mol =4.184 kJ/mol

22

23

The longer the conjugated system, the smaller the gap between thehighest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO).
Short conjugated systems exhibit absorption in UV.
24beta carotene

In molecules with extended pi systems, the HOMO-LUMO
energy gap becomes so small that absorption occurs in thevisible rather then the UV region of the electromagneticspectrum.
25

Anthocyanin

beta carotene

Lycopene

Chlorophyll

Indigo dye26

Experiment 0: UV-Vis spectrum of Phenolphthalein (skip)+ H2O

Colorless
pH = 0−8.3+ H3O+Pink
pH = 8.3−10.0Can you see the diﬀerence in the length of the conjugated pi-electron system?
Where are the two form absorption peaks located in the electromagnetic spectrum?
Moore, Justin Shorb, Xavier Prat-Resina, Tim Wendorff, E. V., John W., & Hahn, A.
(2020, November 6). Indicators. Chemical Education Digital Library (ChemEd DL).
https://chem.libretexts.org/@go/page/49692

27

Experiment 1: UV-Vis spectra of Cu2+ and [Cu(NH3)4]2+
𝜆max = 615 nm

0.6

𝜀 of [Cu(NH3)4]2+ is severaltimes higherAbsorbance

0.5
0.4
0.30.2𝜆max = 810 nm

0.1

500

600

700

800900

Wavelength (nm)28

Experiment 1: Determine the concentration of Cu2+

AbsorbanceHighconcentrations,strongerintermolecularinteraction

Unknown
sample362+

912[Cu ] (×10−3 M)

Calibration curve for Cu2+
A = k[Cu2+] + b

High sensitivity (can be upto 10−6 M)
29

Technical Details
Results are sensitive with respect to impurities:● Highly puriﬁed solvents and chemicals are necessary.
● Clean glassware.
➢➢➢➢

Avoid contact with clear sides of cuvettes
with any hard surface.
Avoid spillage of the solution on the outerside of the cuvettes.
Hold the cuvettes such that transparent
sides are not being touched.
Ensure that transparent sides of thecuvettes are in the optical path.
30

Technical Details
The absorption of solvent must be taken into account by measuring theabsorption spectrum of a reference solution (containing the same volumeof solvent—2.4 M aqueous NH3).
Absorbance of the reference solution is set to 0
Absorbance of the sample solution is 0.3–0.6 (optimal range)

31

AbsorbanceExperiment 2: Absorption spectrum of bromothymol blue(BTB) at diﬀerent pH

Can you see the diﬀerence in the length ofthe conjugated pi-electron system?

Shimada, T., Hasegawa, T. (2017). Spectrochimica Acta Part A
Molecular and Biomolecular Spectroscopy, 185, 104-110.
https://en.wikipedia.org/wiki/Bromothymol_blue

32

Totalconcentration of
BTB (HL and L−)

cidi ionc
A lutso(Ignore sample length)

Baso siclution

33

Intercept34

Chapter 6: Rate of Chemical Reactions andEnergy

35

Main theme: In this experiment, you will learn
The Arrhenius equation (the relationship between temperature and thereaction rate) and the Belousov-Zhabotinsky reaction (chemical oscillationreactions).

36

Rate of reaction
The rate of a chemical reaction or reaction rate
● speed at which a chemical reaction takes place● how quickly amounts (moles) of reactants are converted intoamounts (moles) of products

Rusting - slow
https://en.wikipedia.org/wiki/Iron_oxide

Sodium in water - fast
https://www.youtube.com/watch?v=dmcfsEEogxs

37

A + 2B→Rate of consumptionof reactants

Rate of the reaction

3C + D

Rate of formationof products

http://staff.um.edu.mt/jgri1/teaching/che2372/notes/06/01/intro_kinetics.html

38

aA + bB

Unit:→cC + dD

mol dm−3 s−1 (liquid reactions)molecule cm−3 s−1 (gas phase reaction)

39

Rate lawaA + bB + ... → productorder of the reaction withrespect to A
order of the reaction
with respect to B

Rate constantGenerally, there is no connection between the order of the reaction
and the stoichiometry coeﬃcients!
The overall order = m + n + …
40

Order

Unit of kr

0mol·L−1·s−11s−12

L·mol−1·s−1

3L2·mol−2·s−1

41Arrhenius Equation
Most reaction rates increase with T: in solution, by about afactor of 2 to 4 with an increase of 10 ºC.
Low THigh T

Milk turns sour much more rapidly if
stored at room temperature.

Less collisions

More collisions

https://en.wikipedia.org/wiki/Soured_milk

https://igcseandialchemistry.com/factorsaffecting-the-rate-of-reaction/42

frequency factor or the
pre-exponential factor
Unit: same of kr
A>0Activation energy
Unit: kJ mol−143

the activated complex
or the transition stateThe activation energy is theminimum energy reactants
must have in order to form
products.

Potential Energy Diagrams. (2021, July 10).
https://chem.libretexts.org/@go/page/53890

44

Oscillating reactions[A]

http://staff.um.edu.mt/jgri1/teaching/che2372/notes/09/04/catalysis.html

[C]

[B]

Normal reactions:the concentration of species displays
a single maximum during the courseof the reaction.

Oscillating reactions:the concentration of species
oscillates with time.
Is it possible?
45

Lotka-Volterra Model
step 1:step 2:step 3:

A + X → 2X
X + Y → 2Y
Y→BA→B

autocatalytic reaction[X] and [Y] accelerates
its own production

Assumption:[A] = constant, A is continuously replaced from an external source
as it is consumed in the reaction
B plays no part in the reaction
X: activation species
Y: inhibitor species
46

Concentration[Y][X]Time47

Concentration

Prey-Predator ModelTime

48

Experiment 3: Belousov-Zhabotinsky (BZ) reaction (online)
Oxidation of malonic acid (CH2(COOH)2) with bromate ion (BrO3–) in an acidicsolution with the presence of cerium ions (catalyst)
3 BrO3– + 5 CH2(COOH)2 + 3 H+ → 3 BrCH(COOH)2 + 2 HCOOH + 4 CO2 + 5 H2O

Ce4+
https://en.wikipedia.org/wiki/Ammonium_cerium(IV)_sulfate

49

FNK (Field, Körös and Noyes) mechanism

Miyazaki, J. (2013). Belousov–Zhabotinsky Reaction. In PatternFormations and Oscillatory Phenomena (pp. 61-83). Elsevier.

50

Process 1: dominant at high [Br−]

Process 2: dominant at low [Br−]Bromous acid51

Br− is not presented at the beginning of the reaction → induction periodInduction period (s)Pind (T)Period of oscillation (s)Posc (T)
Yellow
Colorless

0Time52

Task: measure Posc and Pind at diﬀerent temperatures → Activation energy
Induction period (s)Pind (T)Period of oscillation (s)Posc (T)
Yellow
Colorless

0Time53https://www.youtube.com/watch?v=LL3kVtc-4vY54