S17E1 - Redox Reaction Examples and the Galvanic Cell

Electrochemistry and the Galvanic Cell

Electrochemistry  =  the study of the interchange of chemical and electrical energy.

Electrochemistry involves oxidation-reduction (redox) reactions, where two things can happen:

1. Generation of electricity from a spontaneous chemical reaction,  or...

2. Use of electricity to produce a non-spontaneous chemical reaction.

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Redox Reaction Example

Here's a typical redox reaction example:

Oxidizing and Reducing Agents

➞  Oxidizing agent - gets reduced (MnO₄^-)

➞  Reducing agent - gets oxidized (Fe²⁺)

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Redox Reactions and Two Half-Reactions

Redox reactions can be broken down into two half-reactions:

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So...

How Does a Redox Reaction Generate Electricity?

In the previous example, if MnO₄^- and Fe²⁺ are present together in the same solution, the 5e^- are transferred directly upon collision.

Direct collisions are not productive because the energy generated is lost as heat.

So instead, we must separate the oxidizing agent (MnO₄^-) and the reducing agent (Fe²⁺).

This separation forces the electron-transfer from reducing agent (Fe²⁺) to oxidizing agent (MnO₄^-) to occur through a wire.

The wire can then be directed through a motor, to provide useful work.

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Separating the Oxidizing Agent and Reducing Agent

How do we separate the oxidizing and reducing agents?

How to we keep the oxidation process and reduction process separate from each other?

This is done by using a Galvanic cell.

Galvanic cells are often referred to as "Electrochemical cells."

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The Galvanic Cell Set-Up

Galvanic Cell  =  a device in which chemical energy from a spontaneous redox reaction is changed to electrical energy that can be used to do work.

Here's a sketch of a typical Galvanic cell setup...

The two "beaker-like" compartments are separated by a salt bridge.

Salt Bridge - a porous disk (semi-permeable membrane) that permits ions to flow back and forth, without allowing extensive mixing of the two solutions (i.e. the two compartments).

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Notes About the Galvanic Cell (Electrochemical Cell).

1. Salt bridge - completes the electrical circuit by balancing the charges in each compartment of the galvanic cell

Without it, electricity would not flow (e^- 's) because there'd be no circuit, and a build-up of negative charge at the cathode.

2. Galvanic cell - device in which chemical energy is converted to electrical energy.

This electrical energy is measured with a voltmeter.

3. Other ions (see overall balanced redox reaction) besides the oxidizing agent and reducing agent are also present in the two compartments.

4. The oxidizing agent (MnO₄^-) gets reduced by "pulling" electrons towards it from the reducing agent (Fe²⁺) via the wire.

➞ the "pull" or driving force of the electrons is called the cell potential (E_cell).

➞ cell potential (E_cell) is also called the electromotive force (emf).

Because E_cell represents the amount of electrical energy running through the wire, it's reported in units of volts (V).

Also, 1 volt is the same as 1 Joule per coulomb.

Or, 1 V = 1 J/C

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In my next blog post on the subject of SECTION 17 - Electrochemistry,

We'll discuss HOW to calculate cell potentials (E_cell) ...

See you there!