chemistry The by pengxiuhui

VIEWS: 6 PAGES: 17

									The nucleus is very small and very dense. It contains
protons, which have a mass of 1 amu and a charge of +1,
and neutrons, which also have a mass of 1 amu, but are
neutral. If a nucleus were the size of an average pea, it
would have a mass of 250 million tons. I

Electrons have almost no mass (0.00055 amu)and have a
charge of -1 yet they take up most of the space of an atom.
If the nucleus were the size of a marble (~1 cm) the
electrons would move around in space out to about 6 city
blocks from the nucleus (~1 km).i.e. atoms are mostly
empty space.
• The identity of the atom is determined by
  the # of protons in the nucleus.
• One can change one kind of atom into
  another by changing the number of
  protons in the nucleus These changes
  involve enormous amounts of energy.
  This is the source of nuclear energy and
  the atomic bomb.
• Most of these reactions result from nuclear
  decay and involve changes of only one or
  two protons in the nucleus.
 Can we turn copper into gold?

• On a Periodic Table the Atomic Number
  also tells us the number of protons in the
  nucleus.
• Copper has 29 protons in its nucleus.
• Gold has 79 protons in its nucleus
Clearly we can’t really turn copper into gold
• Alchemists didn’t really have a prayer to
  actually convert a “base metal” into gold
  either.
• Most chemical reactions occur from
  interactions of the electrons.
• A neutral atom has the same number of
  electrons as protons.
• However, an atom can gain and lose
  electrons fairly easily to form ions, which
  can have positive or negative charge.
• Electrons are also responsible for the
  forces (bonds) which hold atoms together
  in a molecule.
• The majority of chemical reactions can be
  classified as one of two kind of major
  reaction types.

• Acid/Base Reactions

• Oxidation/Reduction (Redox) Reactions.
              The Proton
• The hydrogen atom has a mass of about 1
  amu, as its nucleus contains only one
  proton and no neutrons. So the hydrogen
  atom without its electron has a charge of
  +1 and can be called a hydrogen ion, or
  more commonly a proton.
       Acid Base Reactions
• In an Acid Base reaction, the hydrogen ion
  is simply transferred from one molecule or
  ion to another molecule or ion.
• Any examples?
      This afternoon’s examples
•   AcOH + HCO3-  AcO- + H2CO3
•   H2CO3  CO2 + H2O
•   This reaction is reversible so it is also true that
•   CO2 + H2O H2CO3  H+ + HCO3-



• 2AcOH + CaCO3 Ca(AcO)2 + H2CO3
• H2CO3  CO2 + H2O
Oxidation-Reduction Reactions
• In a Redox reaction electrons are
  transferred from one atom, ion or molecule
  to another atom, ion or molecule.

• This is the reaction we will use to turn the
  copper penny into “silver”, or more
  accurately coat the penny with zinc.
       How does this happen?
• Zn(s) + 4-OH  ZnO22- + 2 H2O + 2e-
• 2H2O + 2e-  H2(g) + 2OH-
• Zn(s) + 2OH-  H2(g) + ZnO22-

•   2H2O + ZnO2- Zn2+ + 4OH-
•   Zn2+ + 2e-  Zn(s)
•   2OH- + H2  2H2O + 2e-
•   2OH- + H2 + Zn2+  Zn(s) + 2H2O
        Silver Penny to Gold
• This process is neither acid/base nor
  redox.
• In this reaction we form an alloy, i.e. a
  metal mixture. When we heat the zinc
  coated penny, the zinc atoms mix with the
  copper atoms to form brass, which has a
  gold-like color.
• We can use colorless nail polish to prevent
  tarnishing. Real gold does not tarnish.
 What do we mean by tarnish?
• When a metal tarnishes, it is getting
  oxidized by O2 to a metal oxide.
• e.g. 4Ag + O2  2 Ag2O
• 2Zn + O2  2ZnO
2Cu + O2  2CuO
Most metals, like these oxidize easily and
  form a smooth layer of oxides on the
  surface of the metal that actually protects
  it from further oxidation.
  Iron doesn’t tarnish, it rusts and
             corrodes
4 Fe + 3O2  2 Fe2O3 (rust)

The difference here is that rust does not
 form a smooth surface on Iron, it flakes off,
 exposes more metal surface to oxidation.
 We call this corrosion.
  How is this a redox reaction?
4 Fe + 3O2  2 Fe2O3 (rust)

• 4 Fe  4Fe3+ + 12e- iron gets oxidized
• 3O2 + 12e-  6”O2-” oxygen gets reduced
• 4Fe3+ + 6O2-  2 Fe2O3
 How does acid rain affect this?
• Iron needs water to get oxidized by
  air(O2). The water allows ions to move
  around, which is necessary for iron to
  corrode.

• Iron will not rust in anaerobic water.
• Iron will not rust in anhydrous air.

• Salt or acid, increase the rate of the redox
  process.
         Creating Acid Rain
• Real acid rain is caused by the reaction of
  water with Nitrogen and sulfur oxides
  formed from car exhaust and industrial
  waste.
• e.g. SO3 + H2O  H2SO4(sulfuric acid!)
            Making acid rain
• We will make “fake” acid rain, by first
  generating CO2, then bubbling it into
  water.
• AcOH + HCO3-  AcO- + H2CO3
• H2CO3  CO2 + H2O
• This reaction is reversible so it is also true
  that
• CO2 + H2O H2CO3  H+ + HCO3-

								
To top