Energy Quantization

1
Energy Quantization

• Finish last bits of Redox reactions. Be able to
  break a redox reaction into an oxidation half
  reaction and a reduction half reaction (p 172),
  be able to balance redox reactions in acidic
  solution (learn The Half Reaction Method for
  Balancing Equations for Oxidation-Reduction
  Reactions Occuring in Acidic Solution on p 172),
  be able to balance redox reaction in basic
  solutions (learn The Half Reaction Method for
  Balancing Equations for Oxidation-Reduction
  Reactions Occuring in Acidic Solution on p 177)
• Energy Quantization (ch 7.1-7.3)
• Focus on wave nature of light (ch 7.1) learn
  figure 7.1, be able to calculate wavelength of
  light from frequency and vice versa (?? c, p
  291, sample exercise 7.1), study the
  electromagnetic spectrum and its regions (Fig
  7.2).
• Focus on particle nature of light (ch 7.3) be
  able to calculate the energy of a single photon
  (Ephoton h ? hc/?, p 294, sample exercise
  7.3) energy of a mole of photons (lab next week)
• QUIZ next Monday will cover all Ch 4 problems
  EXCEPT FOR PROBLEMS 77 and 79 of chapter 4, which
  are no longer assigned.

2
Half Reactions

• Break the overall redox reaction into two half
  reactions, an oxidation half reaction and a
  reduction half reaction. How?
• Assign oxidation numbers, figure out which two
  elements are changing oxidation numbers and then
  write separate reactions for each of the two
  atoms, including the electrons explicitly as
  reactants (in reduction reactions) or products
  (in oxidation reactions)
• EG/ 2 Na Cl2 - 2 NaCl
• Na oxidation state 0 Na oxidation state 1
• Cl oxidation state 0 Cl oxidation state -1
• Na is being oxidized is being reduced.
• Oxidation half reaction Na - Na e-
• reduction half reaction Cl2 2 e- - 2 Cl-
• We are learning to break redox equations into
  half reactions for two reasons firstly, because
  it is a handy thing to do when we want to balance
  redox reactions. But secondly, we are doing it
  because it is the basis of our study later of
  things like batteries and electrochemical cells
  and even biological energy transduction.

3
Balancing Redox Equations

• For reactions in solution, it matters whether the
  reaction is occuring in acidic or basic solution,
  and so we need to know that.
• Follow method on p 172 for reactions occuring in
  acidic solution While explaining the steps, will
  work through example on p 173-174
• MnO4- (aq) Fe2 (aq) - Fe3 (aq) Mn2 (aq)
• Ox. states Mn, 7 Fe, 2 Fe, 3 Mn, 2
• 1) write separate equations for the oxidation and
  reduction half reactions
• Reduction reaction MnO4- (aq) 5 e- - Mn2
  (aq)
• Oxidation reaction Fe2 (aq) - Fe3 (aq) e-

4

• 2) For each half reaction
• Balance all of the elements except hydrogen and
  oxygen
• DONE!
• Balance oxygen using H2O
• Reduction reaction MnO4- (aq) 5 e- - Mn2
  (aq) 4 H2O
• Oxidation reaction Fe2 (aq) - Fe3 (aq) e-
• Balance hydrogen using H
• Reduction reaction 8 H MnO4- (aq) 5 e- -
  Mn2 (aq) 4 H2O
• Oxidation reaction Fe2 (aq) - Fe3 (aq) e-
• Balance charge using electrons
• 3) If necessary, multiply one or both of the
  half reactions by an integer to equalize the
  number of electrons transferred in the two half
  reactions
• Reduction reaction 8 H MnO4- (aq) 5 e- -
  Mn2 (aq) 4 H2O
• Oxidation reaction 5Fe2 (aq) - 5Fe3 (aq)
  5e-
• 4) Add the half reactions and cancel identical
  species
• 5) Check that the elements and charges are
  balanced.

5
Balancing in Basic Solution

• Similar set of instructions in box on p 177.

6
CHANGE OF SUBJECT

• Have been so far learning the language and basic
  skills of chemistry, a very broad sweeping survey
  of the subject of general chemistry.
• Now we will start to look at chemistry at a less
  basic, more sophisticated level and will continue
  to do this for the rest of chem 115-116.
• First big topic is Structure and Bonding (the
  material in ch 7-9)(we will study ch 5 and 6
  later on in the course)

7
What is Structure and Bonding?

• Chemistry has two big branches one is
  quantitative (physical and analytical chemistry)
  and the other is descriptive (organic, inorganic
  and biochemistry)
• For the rest of chem 115, we will be looking
  primarily at the descriptive branch, and
  chemistry 116 will be looking at the other
  branch.
• Structure and Bonding is the study of the
  relationship between the electronic structure and
  chemical (bonding) properties of the elements.
  
• We start with electronic structure.

8
What is Electronic Structure?

• There are lots of different kinds of atoms (100
  or so kinds) and they all have slightly different
  chemical behavior that is, react differently
  with other atoms. Why?
• Even though each atom behaves slightly
  differently from each other atom, still you can
  divide the atoms into groups that share similar
  chemical behavior Why?
• Less than a hundred years ago, people realized
  that it is the ELECTRONS in the atoms that
  determine the chemical behavior of the elements.
  How? If electrons are all identical bits of
  matter, then how can they be different enough
  from each other to cause the different chemical
  behavior of the different elements?
• Answer to these questions electrons in atoms
  occupy energy states that confer upon them a
  physical shape that determines how they interact
  with the electrons on other atoms. Electrons in
  atoms act like waves that have a physical shape.
  The electronic structure of an atom is the way
  the electrons occupy the energy states and the
  physical shapes that result.
• Answer to first question different atoms act
  chemically different because they have different
  numbers of electrons and therefore different
  electronic shapes.
• Answer to second question atoms divide into
  groups of similar electronic structure and that's
  why they behave similarly chemically.
• To understand all this, we need to understand
  what are the energy states of the electrons in an
  atom and what are the physical shapes that go
  along with these energy states?

9
Energy States??? Whazzat?

• Earliest insight from Planck and Einstein and
  others is that ENERGY IS QUANTIZED means that
  energy comes in little packets.
• In order for an electron to change its energy
  state, it can only absorb energy packets.
  Therefore electron cannot have just any energy.
• Energy level diagram
• Now to understand the energy states of electron,
  need to understand a bit about light.
• Light have both wavelike and particle like
  behavior.

10
Light as a Wave

• A wave of what? Electromagnetism.
• Characteristics of waves Fig 7.1

11
Light Wave Calculations

• FUNDAMENTAL EQUATION FOR LIGHT
• ?? c
• SKILL be able to calculate wavelength of light
  (?) from frequency and vice versa (n)
• EG/ sample exercise 7.1 The brilliant red
  colors seen in fireworks are due to the emission
  of light with wavelengths around 650 nm. What is
  the frequency of this light? Ans. 4.61 x 1014
  Hz.

12
The Electromagnetic Spectrum

• Fig 7.2

13
LIGHT AS PHOTONS

• Quantum view of light light is energy packets
  called photons

14
Calculations with Photons

• Energy of a single photon
• Ephoton h ? hc/?
• h is Planck's constant, 6.62608 x 10-34 J s
• J is SI unit of energy. 1 J 1 kg m2/s2
• EG/ p 294, sample exercise 7.3. The blue light
  of fireworks has a wavelength of 450 nm. What is
  the energy of this blue light? (frequency is
  6.66 x 1014 s-1 and energy is 4.41 x 10-19 J)

15
energy of a mole of photons (lab next week)

• Energy of a mole of photons is the energy of a
  single photon times avogadro's number.