Unit: Electrochemistry

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Unit Electrochemistry

• Half Reactions and Redox Equations

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Remember OIL RIG
Half-Reactions

• Oxidation
• Is
• Loss of electrons
• Reduction
• Is
• Gain of electrons

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Half-Reaction Example
2HCl Mg ? MgCl2 H2
1. Label all oxidation numbers 2. Determine what
is reduced and oxidized reduction gain of
electrons (more negative ) oxidation loss of
electrons (more positive )
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Half-Reaction Definitions
2HCl Mg ? MgCl2 H2
Reducing agent gives electrons and is
OXIDIZED Oxidizing agent removes electrons and
is REDUCED
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Example
2HCl Mg ? MgCl2 H2
The oxidation number of hydrogen becomes more
negative, so hydrogen is reduced. Therefore,
hydrogen is the oxidizing agent (it oxides
another element). The oxidation number of
magnesium becomes more positive, so magnesium is
oxidized. Therefore, magnesium is the reducing
agent (it reduces another element).
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Balancing Half Reactions in Acidic Solutions

• 1. Label all oxidation numbers.
• 2. Determine which element is reduced and which
  is oxidized.
• 3. Write the two half reactions (follow
  solubility rules!) and do the following to each
• Balance element in question using coefficients
• Balance oxygen by adding H2O
• Balance hydrogen by adding H
• leave a blank line here!
• Balance charge by adding e-
• Multiply reactions so that number of e- are equal
  in both

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H2S HNO3 ? H2SO4 NO2 H2O
Balancing Redox Reactions in Acidic Solutions
Example 1
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Balancing Redox Reactions in Acidic Solutions

• 1. Label all oxidation numbers.
• 2. Determine which element is reduced and which
  is oxidized.
• 3. Write the two half reactions (follow
  solubility rules!) and do the following to each
• Balance element in question using coefficients
• Balance oxygen by adding H2O
• Balance hydrogen by adding H
• leave a blank line here!
• Balance charge by adding e-
• Multiply reactions so that number of e- are equal
  in both
• 4. Add reactions together, recombine ions, cancel
  things on both sides, and make the equation look
  like the original

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H2S HNO3 ? H2SO4 NO2 H2O
Balancing Redox Reactions in Acidic Solutions
Example 1
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Cr2O7-2 (aq) Cl- (aq) ? Cr3 (aq) Cl2 (g)
Balancing Redox Reactions in Acidic Solutions
Example 2
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H2S HNO3 ? H2SO4 NO2 H2O

• 1. Assign oxidation numbers.
• 2. Write the half-reactions
• - If a compound contains an element that changes
  oxidation , write that compound in ionic form
  (follow solubility rules)
• 3. Balance
• - Balance element in question using coefficients
• - Balance oxygen by adding H2O
• - Balance hydrogen by adding H ions (in basic
  solutions add OH-)
• We can add water or H because we are assuming
  we are doing this in an acidic, aqueous solution
  where these species are readily available

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H2S HNO3 ? H2SO4 NO2 H2O

• 4. Add electrons to balance the charge
• 5. Repeat process with the other half-reaction
• 6. Multiply half reactions so that the number of
  electrons lost equals the number gained
• 7. Add the half reactions
• 8. Re-combine the ions to form the compounds
  found in the original formula.

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Balancing Redox Equations Practice 1

• Balance the redox equation below
• NaBr(aq) Cl2(g)? NaCl(aq) Br2(l)

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Balancing Redox Equations Practice 2

• Balance the redox equation below
• Fe2O3(s) CO(g) ? Fe(s) CO2(g)

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Balancing Redox Equations Practice 3

• Balance the redox equation below
• I2(s) OCl-(aq) ? IO3- (aq) Cl-(aq)

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Balancing Redox Equations Practice 4

• Balance the redox equation below
• HCOOH(aq) MnO4-(aq) ? CO2 (g) Mn2(aq)
• Note HCOOH is a weak acid.

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Balancing Redox Reactions in Basic Solutions

• 1. Label all oxidation numbers.
• 2. Determine which element is reduced and which
  is oxidized.
• 3. Write the two half reactions (follow
  solubility rules!) and do the following to each
• Balance element in question using coefficients
• Balance oxygen by adding H2O
• Balance hydrogen by adding H
• Add OH- to neutralize H
• Balance charge by adding e-
• Multiply reactions so that number of e- are equal
  in both
• 4. Add reactions together, recombine ions, cancel
  things on both sides, and make the equation look
  like the original

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Cr3 (aq) O2 (g) ? Cr2O7-2 (aq) H2O2 (l)
Balancing Redox Reactions in Basic Solutions
Example 1