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LECTURE 19

Page history last edited by Wilfred Husunukpe 10 years, 3 months ago

Dr. Cooper's comments: Where are the figures?

 

Wilfred Husunukpe                                                         EQUILIBRIUM ELECTROCHEMISTRY

 

 

   

 

 

This is one of the most practical applications of chemical thermodynamics An electrical cell consists of two electrodes which may or may not be solid, in contact with an electrolyte. An electrolyte is a material that allows the transport of charged species on an ionic conductor. This may be a liquid, a solution or a solid(lattice) An electrode and its electrolyte comprise an electrode compartment. Different electrode configurations have been classified.

 

 

 

 

 

1. The most commonly known configuration consists of a metal that participates in the electrochemical reaction

 

 

    M(s) |M+(aq) - metal/ metal ion type

 

 

 

 

 

2. An inert metal which is only present as a aource or sink of electrons. It takes no other part in the reaction other than acting as a catalyst.

 

 

    eg. Pt(s) |X2(g) |X+(aq) gas electrode

 

 

 

 

 

3. Metal /insoluble salt M(s) |MX(s) |X- (aq)

 

 

 

 

 

4. Redox reaction

 

 

   Pt(s) | M+(aq) |M2+(aq)

 

 

 

 

 

 The chemical reaction typically involved in this type of process is reduction oxidation reaction (redox reaction). A redox reaction involves the transfer of electrons. An      oxidizing agent is the electron acceptor and a reducing agent is the electron donor. Redox equations are expressed in terms of two half reactions, one oxidation and one reduction equation.

 

 

 

 

 

O I L R I G

 

 

 

 

 

Oxidation Is Loss, Reduction Is Gain

 

 

 

  

 

 

 

 

 

Figure 7.11

 

 

 

 

 

Galvanic cell- Electricity produced as a result of a spontaneous reaction resulting in a negative Gibbs free energy.

 

 

 

 

 

Electrolytic cell- these reactions in these cells are non-spontaneous and are driven by an external source of current. These reactions occur when work is done on the system when electrons are forced through the circuit. A potential difference is a measure of how much work is extracted out of a set of electrode compartments.

 

 

 

 

 

TYPES OF CELLS

 

 

 

 

 

1. The simplest cell has a single electrolyte common to both electrodes.

 

 

 

 

 

2. In some cells it is necessary to immerse electrodes in different electrolytes as in the Daniell cell comprised of Cu2+/Cu and Zn2+/Zn at the two electrodes. The is a porous barrier between the two electrolytes to allow charges to be carried across the barrier.

 

 

 

 

 

Fig 7.12

 

 

 

 

 

 

 

 

Zn(s) -> Zn2+(aq) + 2e-s anode

 

 

 

 

 

Cu2+(s) + 2e-s -> Cu(s) cathode

 

 

 

 

 

There is a buildup of Cu(s) as a result of the reaction at the cathode. In a cell with 2 different electrolyte solutions as in the Daniell cell, there is an additional source of potential difference across the interface of the 2 electrolytes. This potential is referred to as the liquid junction potential. It reduces the amount of work that can be obtained from the system. A way to reduce the this potential is by using a salt bridge to join the electrolyte compartments thus keeping the charges valanced.

 

 

 

 

 

Fig. 7.13

 

 

 

 

NOTATION Phase boundaries are represented by vertical bars | eg. CuSO4(aq) | Cu(s) A liquid junction is represented by three vertical dots. A double vertical line indicates an interface that in which it is assumed the junction potential has been eliminated (||). eg. Zn(s) | ZnSO4(aq) || CuSO4(aq) | Cu(s) The convention is to write the anode half cell first before the cathode half cell

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