Chapter 20 electrochemistry
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Chapter 20 Electrochemistry. Oxidation-reduction reactions Oxidation numbers Oxidation of metals by acids and salts The activity series ALL these to be done in class. Oxidation-Reduction Reactions. In the reaction Zn( s ) + 2H + ( aq )  Zn 2+ ( aq ) + H 2 ( g ).

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Chapter 20 Electrochemistry

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Chapter 20 electrochemistry

Chapter 20Electrochemistry

Chapter 20


Chapter 20 electrochemistry

  • Oxidation-reduction reactions

  • Oxidation numbers

  • Oxidation of metals by acids and salts

  • The activity series

  • ALL these to be done in class

Chapter 20


Chapter 20 electrochemistry

Oxidation-Reduction Reactions

  • In the reaction

  • Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g).

  • Which element is oxidised and which one is reduced?

  • Oxidation – loss of e-

  • Reduction – gain of e-

Chapter 20


Chapter 20 electrochemistry

Balancing Oxidation-Reduction Reactions

  • Law of conservation of mass: the amount of each element present at the beginning of the reaction must be present at the end.

  • Conservation of charge: electrons are not lost in a chemical reaction.

Chapter 20


Chapter 20 electrochemistry

Half Reactions

Half-reactions are a convenient way of separating oxidation and reduction reactions.

Chapter 20


Chapter 20 electrochemistry

  • The half-reactions for

  • Sn2+(aq) + 2Fe3+(aq)  Sn4+(aq) + 2Fe3+(aq)

  • are………

Chapter 20


Chapter 20 electrochemistry

Balancing Equations by the Method of Half Reactions

The two incomplete half reactions are

MnO4-(aq)  Mn2+(aq)

C2O42-(aq)  2CO2(g)

Balance the overall reaction equation in an acidic solution

Chapter 20


Chapter 20 electrochemistry

  • Balancing Equations for Reactions Occurring in Basic Solution

  • We use OH- and H2O rather than H+ and H2O.

  • The same method as for acidic solution is used, but OH- is added to “neutralize” the H+ used.

Chapter 20


Chapter 20 electrochemistry

Voltaic Cells

  • If a strip of Zn is placed in a solution of CuSO4, Cu is deposited on the Zn and the Zn dissolves by forming Zn2+.

Chapter 20


Chapter 20 electrochemistry

  • Zn is spontaneously oxidized to Zn2+ by Cu2+.

  • The Cu2+ is spontaneously reduced to Cu0 by Zn.

Chapter 20


Chapter 20 electrochemistry

  • “Rules” of voltaic cells:

    • 1. At the anode electrons are products. (Oxidation)

    • 2. At the cathode electrons are reagents. (Reduction)

    • 3. Electrons can’t swim!

Chapter 20


Chapter 20 electrochemistry

  • Anions and cations move through a porous barrier or salt bridge.

  • Cations move into the cathodic compartment to neutralize the excess negatively charged ions

  • Anions move into the anodic compartment to neutralize the excess Zn2+ ions formed by oxidation

Chapter 20


Chapter 20 electrochemistry

A Molecular View of Electrode Processes

Chapter 20


Chapter 20 electrochemistry

Cell EMF

  • e- flow from anode to cathode because the cathode has a lower electrical potential energy than the anode.

  • 1 V is the potential difference required to impart 1 J of energy to a charge of one coulomb:

Chapter 20


Chapter 20 electrochemistry

  • 1 V is the potential difference required to impart 1 J of energy to a charge of one coulomb:

Chapter 20


Chapter 20 electrochemistry

  • Electromotive force (emf) is the force required to push electrons through the external circuit.

  • Cell potential: Ecell is the emf of a cell.

  • For 1M solutions at 25 C (standard conditions), the standard emf (standard cell potential) is called Ecell.

Chapter 20


Chapter 20 electrochemistry

  • Standard Reduction (Half-Cell) Potentials

  • Standard reduction potentials, Ered are measured relative to the standard hydrogen electrode (SHE).

Chapter 20


Chapter 20 electrochemistry

Chapter 20


Chapter 20 electrochemistry

  • For the SHE, we assign

  • 2H+(aq, 1M) + 2e- H2(g, 1 atm)

  • Ered = 0.

Chapter 20


Chapter 20 electrochemistry

  • For Zn:

  • Ecell = Ered(cathode) - Ered(anode)

  • 0.76 V = 0 V - Ered(anode).

  • Therefore, Ered(anode) = -0.76 V.

  • Standard reduction potentials must be written as reduction reactions:

  • Zn2+(aq) + 2e- Zn(s), Ered = -0.76 V.

Chapter 20


Chapter 20 electrochemistry

  • Changing the stoichiometric coefficient does not affect Ered.

  • Therefore,

  • 2Zn2+(aq) + 4e- 2Zn(s), Ered = -0.76 V.

Chapter 20


Chapter 20 electrochemistry

  • Reactions with Ered < 0 are spontaneous oxidations relative to the SHE.

  • The larger the difference between Ered values, the larger Ecell.

Chapter 20


Chapter 20 electrochemistry

  • Oxidizing and Reducing Agents

  • The more positive Ered the stronger the oxidizing agent on the left.

  • The more negative Ered the stronger the reducing agent on the right.

Chapter 20


Chapter 20 electrochemistry

Chapter 20


Chapter 20 electrochemistry

  • More generally, for any electrochemical process

Chapter 20


Chapter 20 electrochemistry

Example: For the following cell, what is the cell

reaction and Eocell?

Al(s)|Al3+(aq)||Fe2+(aq)|Fe(s)

Al3+(aq) + 3e-→ Al(s); EoAl = -1.66 V

Fe2+(aq) + 2e- → Fe(s); EoFe = -0.41 V

Chapter 20


Chapter 20 electrochemistry

Example: When an aqueous solution of CuSO4 is electrolysed, Cu metal is deposited:

Cu2+(aq) + 2e-→ Cu(s)

If a constant current was passed for 5.00 h and 404 mg of Cu metal was deposited, what was the current?

Ans: 6.81 x 10-2 A

Chapter 20


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