4.4-4.5: Chain Reactions & Photochemistry

Question 1

Free radicals (⦁)

Answer 1

atoms/molecules w/ unpaired e-

Question 2

are free radical highly reactive

Answer 2

YES- w/ short life

Question 3

how do free radicals gain stability?

Answer 3

Gains stability by pairing w/ e- on another atom (covalent bonding)

Question 4

Free radical formation reaction

Answer 4

(⦁) forms new bond at expense of already existing bond
- New radical attacks another molecule, forms new radical
- A⦁ + B-C –> A-B + C⦁

Question 5

Chain termination reaction

Answer 5

pair of radicals find each other = form stable molecule
- A⦁ + C⦁  A-C

Question 6

Catalyzed chain reaction

Answer 6

radical chain reaction circumvents kinetic barrier

Question 7

Initiation event

Answer 7

produces initial population of radicals
- Required to start radical reactions
- Somehow bond breaks –> fragments of unpaired e-

Question 8

Atmospheric chemistry is governed by ___ produced by ____

Answer 8

• free radicals
• solar radiation

Question 9

NOx is an exception to free radicals bc…

Answer 9

it’s still stable (even though has unpaired e-) bc… unpaired e- = in low-energy molecular orbit

Question 10

Spectrum

Answer 10

variation of light intensity w/ wavelength

Question 11

spectrum: visible light

Answer 11

wavelength = 400-750 nm (violet –> red)

Question 12

spectrum: ultraviolet light (UV)

Answer 12

wavelength = < 400 nm

Question 13

spectrum: infrared light (IR)

Answer 13

wavelength = > 750 nm

Question 14

Most of suns rays = w/in _______ light region

Answer 14

visible

Question 15

Solar Spectrum:

Answer 15

Records light arriving @ earth’s surface on clear day
- Sun emits more UV-rays (absorbed by stratospheric ozone layer)

Question 16

Photon

Answer 16

energy packets (how light rays are delivered)

Question 17

how light rays are delivered?

Answer 17

photons

Question 18

relationship b/w energy of photon and frequency of light

Answer 18

Energy of photon = proportional to frequency of light

Question 19

relationship b/w light and wavelength

Answer 19

inversely proportional

Question 20

E =

Answer 20

hv = (hc)/ƛ

Question 21

h =

Answer 21

Planck’s constant = 6.626 x 10-34 J*sec

Question 22

c =

Answer 22

speed of light = 2.998 x 108 m/s

Question 23

When 1 mol photons = absorbed: energy deposited in molecules (E) =

Answer 23

(119.627/ƛ)

Question 24

visible light/UV light absorption occurs when…

Answer 24

energy is sufficient to raise molecule (ground state) –> (excited state)

Question 25

molecules release their extra/excited energy by..

Answer 25

• re-emit photon
• collide w/ other molecule and transfer kinetic energy
• break bond releasing pair of e- fragments

Question 26

for a bond to break: the energy absorbed by photon ___ bond energy

Answer 26

energy (absorbed photon) ≥ bond energy

Question 27

Why are free radicals so reactive?

Answer 27

Bc of unpaired e-

Question 28

How do radical chain reactions terminate?

Answer 28

When a pair of radicals find each other = form stable molecule
- A⦁ + C⦁  A-C

Question 29

What are the requirements for a radical reaction to take place?

Answer 29

1. Present in low concentrations so must encounter another free radical
2. Initiation event- produces initial population of radicals
   - Somehow bond breaks –> fragments of unpaired e-

Question 30

What is the role of sunlight in atmospheric radical reactions?

Answer 30

Atmospheric chemistry = governed by free radicals produced by solar radiation
- (⦁) reacts w/ nitric oxide  NO2⦁ + sunlight = splits + O2 = ozone

Question 31

Where is the solar spectrum strongest?

Answer 31

Wavelength (ƛ) ≈ 483 nm

Question 32

In what region of the electromagnetic spectrum does the sun’s radiation occur?

Answer 32

Mostly visible light region (some/tails off at infrared and ultraviolet)

Question 33

How does sunlight react with molecules in the atmosphere?

Answer 33

Sunlight = delivered in photons
- Ultraviolet ƛ = splits O2 (O⦁ *2)
- O⦁ = energetically excited + O2 = bonds to form O3 (ozone)