Electrochemistry

1
Electrochemistry

• Day 1

2
Oxidation States

• indicates the number of electrons that it gains
  or loses when it forms a bond.
• 3 Important things to keep in mind with oxidation
  numbers
• 1. Free zero ex. Zn, H2, etc
• 2. Sum zero
• 3. Sum of polyatomic ion its charge

3
Oxidation States

• Most elements have different ox. s that can
  vary depending on the molecule they are part of.
  Here are some consistent ox. s
• Alkali metals (1A) 1
• Alkaline earths (2A) 2
• Group 3A 3
• Oxygen -2
• Halogens -1

4
Oxidation States

• Transition metals can have several oxidation
  states, which are shown by a Roman numeral in the
  compound name.
• Know these common polyatomic ions
• Hydroxide OH- Nitrate NO3-
• Perchlorate ClO4- Acetate C2H3O2-
• Carbonate CO3-2 Sulfate SO4-2
• Phosphate PO4-3

5
Oxidation-Reducation Reactions

• Electrons are exchanged by reactants and the
  oxidation states of some of the reactants are
  changed during the reaction.
• Fe 2HCl ? FeCl2 H2
• Fe 0 ? 2
• H 1 ? 0

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Oxidation and Reduction

• When an atom gains electrons, its ox.
  decreases, it is said to have been reduced
• When an atom loses electrons, its ox.
  increases, it is said to have been oxidized
• OIL
• RIG

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Oxidizing and Reducing Agents

• Oxidation and reduction go together. One cant
  happen without the other.
• The reactant that is oxidized is the reducing
  agent or reductant
• The reactant that is reduced is the oxidizing
  agent or oxidant

8
Half Reactions

• An equation can be written for oxidation and one
  for reduction.
• Fe 2HCl ? FeCl2 H2
• Oxidation Fe ? Fe2 2 e-
• Reduction 2 H 2 e- ? H2

9
Reduction Potentials

• Every half reaction has a potential, or voltage,
  associated with it. Look at chart of reduction
  potentials.
• The potentials are given as reduction half
  reactions, but you can reverse them, and change
  the sign to get the oxidation potential.
• ex. Zn2 2 e- ? Zn E -0.76 V
• Zn ? Zn2 2 e- E 0.76 V

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Reduction Potentials

• F2 (g) has a very large reduction potential,
  which means it is more likely to gain electrons
  and be reduced, making it a strong oxidizing
  agent.
• Li(s) has a very large oxidation potential, so it
  is likely to lose electrons and be oxidized, so
  it is a very strong reducing agent.

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Calculating the Potential of a Redox rxn

• You can calculate this if you know that values
  for the two half reactions.
• Two important things
• Add the potential for the oxidation half-reaction
  to the potential for the reduction half-reaction
• Never multiply the potential for a half-reaction
  by a coefficient.

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Example

• Zn 2 Ag ? Zn2 2 Ag
• The two half reactions are
• Zn ? Zn2 2 e- E 0.76 V
• Ag e- ? Ag E 0.80 V
• E Eoxidation Ereduction
• E 0.76 V 0.80 V 1.56 V
• (the coef. of 2 is ignored)