Electrochemistry

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Electrochemistry

• Electrons in Chemical Reactions

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REDOX REACTIONS

• Examples
• Car Battery
• Electrolysis of water
• Rusting of an old Chevette
• Cellular respiration
• What do all these things have in common?
• Electrochemistry is the study of how electrons
  are transferred in chemical reactions
• The chemical changes that occur when electrons
  are transferred between reactants are known as
  reduction-oxidation (REDOX) reactions
• The of e- gained by one entity the of e-
  lost by another
• Example 2Na (s) Cl2(g) 2NaCl (s)

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• Writing Half- Reactions
• Redox reactions can be separated into an
  oxidation reaction and a reduction reaction which
  are called half-reactions because they involve
  only one-half of the redox reaction taking place
• The half-reaction must be balanced by adding
  electrons to the side with the more positive
  charge
• Elements have a charge of zero
• The charge of an ion is written as part of the
  symbol ! eg. charge on N3- is -3

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Reduction Half-Reactions

• Electrons are GAINED in a reduction reaction and
  are written on the REACTANT side of the equation
• Reduction
• Fe 3 3 e- Fe (s)
• Cl2 2 e- 2Cl-
• REDUCTION is a GAIN in electrons(RIG)

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Oxidation Half-Reactions

• Electrons LOST in oxidation are placed on the
  PRODUCT side of the equation
• Oxidation
• Mg Mg 2 2e-
• 2F 1- F2 (g) 2e-
• OXIDATION is a LOSS in electrons(OIL)

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Remembering Half-Reactions

• Oxidation OIL
• Reduction RIG
• So ... OIL RIG

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• Now try these
• Sulfur to sulphide ions
• Sodium ion to sodium metal
• Nickel to nickel ions (2 answers)

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Reactivity of Metals and Their Ions
The more stable a metal atom is, the more
reactive it is as an ion. The more stable a
metal ion is, the more reactive it is as a
metal. If you create a list that organizes metal
ions from most reactive to least reactive, you
end up creating a list that organizes metal atoms
from least reactive to most reactive as shown in
the table below. The resulting table is a table
of reduction half-reactions based on the
reactivity of atoms or ions, or an activity
series.
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Activity Series for Metals and Metal Ions(page
80 in text, page 5 in data booklet)
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Using the Activity Series

• The ranking of the reactivity of metal ions is
  shown on the left-hand side of the table, with
  the most reactive metal ions placed higher than
  less reactive metal ions.
• Which is more reactive tin ions or iron ions?
• The right side of the table lists metals in order
  of reactivity. Metals lower in the series are
  more reactive than the metals higher in the
  series.
• Which is more reactive copper or zinc?

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Using the Activity Series

• The table is most useful for determining if a
  redox reaction is
• Spontaneous a chemical reaction that occurs
  without the addition of external energy
• or
• Nonspontaneous a chemical reaction that does
  not occur without the addition of external energy
• How do we tell if a reaction is spontaneous or
  nonspontaneous

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Using the Activity Series
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Spontaneous Redox Reaction Example
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Zinc copper reaction in real life !
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Potato Clock
Two potatoes are pierced by zinc and copper
electrodes. A small clock runs on the voltage
produced.
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Voltaic Cells

• Applying our understanding of Redox Reactions

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Voltaic Cells

• also called GALVANIC cells.
• electrochemical cells that are used to convert
• chemical energy into electrical energy
• energy produced by SPONTANEOUS redox reactions
• In a spontaneous redox reaction, electrons are
  transferred from the substance oxidized to the
  substance reduced
• If reactants are arranged in a certain way, these
  electrons can be made to move through a wire
• The reactants must be separated and yet in
  contact with each other so that the reaction will
  occur s l o w l y
• They can be separated by a salt bridge or a
  porous cup

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Salt Bridge containing unreactive sodium sulfate
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Porous Cup Unglazed porcelain cup
Allows for ion transfer just like a salt bridge
does!
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Parts of a Voltaic Cell

• Half-cell - one part of a voltaic cell in which
  either oxidation or reduction occurs contains a
  solid (electrode) and a solution (electrolyte)
• Electrodes - solid conductors that connect the
  electrolyte solution to the external circuit
• Anode oxidation occurs here, electrons are
  produced
• negative (-) terminal
• Cathode reduction occurs here, electrons are
  used
• positive () terminal
• Electrolytes solutions that conduct electricity
  must contain ions!
• Salt bridge or porous cup - allows the passage of
  electrons without contamination of the two
  half-cells

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How does the electricity flow?How does a voltaic
cell work?

• The electrons move from the anode to the cathode
  through a wire. This is the external circuit.
• Another circuit keeps the charges moving the
  internal circuit is the flow of ions in the
  solutions through the salt bridge or porous cup.
• Cations (positively-charged ions) move to the
  cathode
• Anions (negatively-charged ions) move to the
  anode

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• Voltaic cells consist of
• Anode Zn(s) ? Zn2(aq) 2e (loses
  mass)
• Cathode Cu2(aq) 2e- ? Cu(s) (gains
  mass)
• As oxidation occurs, Zn is converted to Zn2 and
  2e-. The electrons flow towards the cathode
  where they are used in the reduction reaction.

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Cell Notation
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Identify the anode and cathode!
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Batteries

• A BATTERY is a self-contained VOLTAIC CELL.
• 1. Primary Batteries
• converts stored chemical potential energy into
  electrical energy when the two half cells within
  the battery are connected by an external circuit.
• usually the container is the anode and the
  graphite center is the cathode.
• the two are separated by a thick moist paste,
  which is the electrolyte.
• are not rechargeable

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2. Secondary Batteries

• are rechargeable by passing a direct current
  through it.
• Hg and Pb are often used in these batteries, are
  an environmental concern.
• cathode grill is filled with lead (IV) oxide, the
  anode grill is filled with spongy lead.

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Electrolytic Cells
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The Electrolytic Plant, which is the size of four
football fields, consumes the same amount of
power as a city of 250,000 people.
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Copper is electroplated onto a strip of silver.
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Note the differences.