Chapter 20 – Electrochemistry
Oxidation number – hypothetical charge assigned to an atom in a substance.
-charge of atom in molecule if all the bonding is considered ionic.
-oxidation numbers give us a way to account for electrons in a chemical reaction.
Determining Oxidation State (or Oxidation Number)
1. Elements in natural form – ox #= zero
2. Sum of ox numbers of a compound=zero
3. Polyatomic ions: sum of ox #=charge of ion
4. Monatomic ions: ox # = charge of ion
5. Oxygen in compound: ox #= -2
Peroxide exceptions  ox # is -1
6. Hydrogen in compound: ox#= +1
Hydride exceptions  ox # is -1
Oxidation – process of losing electrons
Reduction – process of gaining electrons
Oxidizing agent – substance that causes oxidation in another substance.
Reducing agent – substance that causes reduction in another substance.
**oxidizing agent gets reduced
**reducing agent gets oxidized
Balancing Redox Reactions
1. Divide equation into half-reactions.
2. Balance each ½ rxn
a) Balance atoms other than H and O
b) Balance oxygen atoms by adding H2O to appropriate side.
c) Balance hydrogen atoms by adding H+ to appropriate side.
d) Balance charge by adding e- to appropriate side.
3. Multiply each equation by integer so that
# e- lost = # e- gained
4. Add equation together, cancel spectator species.
a) If reaction occurs in basic solution, neutralize H+ by adding OH- to both sides and canceling extra
water molecules.
Electrochemical Cell – A system consisting of electrodes that dip into an electrolyte and ...
- a chemical reaction generates electric current.
- uses an electric current to drive a non-spontaneous reaction.
Voltaic Cell (galvanic) – An electrochemical cell in which a spontaneous reaction generates electric current.
Electrolytic Cell – An electrochemical cell in which an electric current drives an otherwise non-spontaneous reaction.
Anode – Electrode at which oxidation occurs.
Cathode – Electrode at which reduction occurs.
Voltaic cell notation is an abbreviated way of describing a voltaic cell.
1. All phase boundaries are indicated with a vertical line.
2. The salt bridge is indicated by a double vertical line.
3. The anode is written first (on the left), then the cathode on the right.
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Standard Cell Potentials and Standard Electrode Potentials
Cell Potential (
) – the potential difference between the electrodes of a voltaic cell.
– the strength of the reducing agent's e- pushing combined with the strength of the oxidizing agents e- pulling.
– Also called Electromotive Force (EMF)
1J
1V 
– Unit for cell potential is volt.
1C
Standard Cell Potential (
) – the cell potential (or EMF) of a voltaic cell operating under standard state conditions.
Standard Electrode Potential ( ) – the electrode potential when the solutes are at standard state conditions.
-standard electrode potentials are given in Table 19.1
**Calculation of cell potentials can be done using electrode potentials
Strength of Oxidizing and Reducing Agents
• The more (+) the reduction potential
– The stronger the reduction
– The better the oxidizing agent (F2, H2O2, and MnO4- are good oxidizing agents)
• The more (-) the reduction potential
– The more (+) the oxidation potential
– The stronger the oxidation
– The better the reducing agent (Li(s) and Mg(s) are good reducing agents)
Free Energy and Electrical Potential
Nernst Equation – Equation which relates cell potential (or EMF) with concentration of electrolyte
at 25 ºC