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Energy Quantization

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Title: 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.
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