Unit 7: Redox

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Unit 7 Redox Electrochemistry
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Whats the point ?
REDOX reactions are important in

• Purifying metals (e.g. Al, Na, Li)

• Producing gases (e.g. Cl2, O2, H2)

• Electroplating metals

• Electrical production (batteries, fuel cells)

• Protecting metals from corrosion

• Balancing complex chemical equations

• Sensors and machines (e.g. pH meter)

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What is Redox?

• REDOX stands for REDuction/OXidation
• Oxidation is often thought of as a combination of
  a substance with oxygen (rusting, burning)
• Just like with acid/base definitions the
  definition of oxidation is expanded
• Oxidation refers to a loss of electrons
• Reduction refers to a gain of electrons
• As a mnemonic remember LEO says GER

• Loss Electrons Oxidation
• Gain Electrons Reduction
• Read 12.1 (pg. 443 - 445)

4
Testing concepts

• Q- What is oxidation? What is reduction?
  Represent each as a chemical equation.
• A- oxidation loss of e X? X e
• reduction gain of e X e? X
• Q- Why are 2Na Cl2 ? 2NaCl 2H2 O2 ? H2O
  considered redox reactions?
• A- Both involve the transfer of electron density
  (Na has no charge, the atoms in diatomic
  molecules have no partial charge. After reaction
  the atoms have different shares of the electrons
  because of different EN values)
• Q- Is it possible to oxidize a material without
  reducing something else?
• A- No. A lost e is taken up by something else.

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Testing concepts

• Q- Define oxidizing agent, reducing agent.
• A- An oxidizing agent causes oxidation by being
  reduced itself
• A reducing agent causes reduction by being
  oxidized itself
• Q- PE 1
• A- CaCl2 is an ionic compound with a positive
  calcium ion and negative chlorine ions
• Ca Cl2 ? CaCl2
• Ca ? Ca2 2e, Cl2 2e? 2Cl.
• Thus Ca is losing electrons (oxidation) and Cl
  is gaining electrons (reduction).
• These are called half reactions

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Oxidation numbers

• We will see that there is a simple way to keep
  track of oxidation and reduction
• This is done via oxidation numbers
• An oxidation number is the charge an atom would
  have if electrons in its bonds belonged
  completely to the more electronegative atom
• E.g. in HCl, Cl has a higher EN (pg. 255). Thus,
  oxidation numbers are Cl -1, H 1
• Notice that oxidation numbers are written as 1
  vs. 1 to distinguish them from charges.
• Instead of referring to EN chart, a few rules are
  followed to assign oxidation numbers
• Refer also to study note

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Rules (and rationale - 12.2)

• 1. Any element, when not combined with atoms of a
  different element, has an oxidation of zero. (O
  in O2 is zero)
• 2. Any simple monatomic ion (one-atom ion) has an
  oxidation number equal to its charge (Na is 1,
  O2 is 2)
• 3. The sum of the oxidation numbers of all of the
  atoms in a formula must equal the charge written
  for the formula. (if the oxidation number of O is
  2, then in CO32 the oxidation number of C is 4)

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Rules (and rationale - 12.2)

• 4. In compounds, the oxidation of IA metals is
  1, IIA is 2, and aluminum (in IIIA) is 3
• 5. In ionic compounds, the oxidation of a
  nonmetal or polyatomic ion is equal to the charge
  of its associated ion. (CuCl2, Cl is 1)
• 6. F is always 1, O is always 2 (unless
  combined with F), H is usually 1
• rule
• total
• Ox.

or rule 5
6
6
3
4
6
3
6
6
3
5
3
6
6
3
1
-6
5
2
-14
12
6
-2
-4
-1
1
2
-8
5
1
-2
5
1
-2
6
1
-2
-2
-1
1
1
-2
5
H2PO4
H N O 3
K2Cr2O7
C2H6O
AgI
PE 2 (450), 12.9, 12.12 (484), 12.10, 12.11,
12.13 (484)
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More practice

• PE 2 pg. 450 answers in back of book
• 12.9, 12.12 (pg. 484) answers in back of book
• 12.10 a) Cl7, b) Cr3, c) Sn4, d) Au3
• 12.11 a) Na 1, H 1, P 5, O -2
• b) Ba 2, Mn 6, O -2
• c) Na 1, S 2.5, O -2
• d) Cl 3, F -1
• 12.13 a) 2, b) 4, c) 3, d) 5, e) -2, f) 0,
• g) 1, h) -3 I) 1/3

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