READ: Electrochemistry

Introduction

Previously, we have learned how atoms can gain or lose electrons forming charged ions. For instance, we specified that the oxidation state of an atom in the alkali family is +1. This occurs because these atoms always lose one electron to form an ion. In comparison, the charge of an ion in the halogen family is -1. Again, this is because a halogen atom tends to gain an electron to form its ions. In this chapter, we will re-examine the concept of charges in more detail.

Electrochemistry

Some types of chemical reactions are capable of producing an electrical current (or electricity). These are the type of reactions that occur inside batteries. When a reaction is arranged to produce an electric current as it runs, the arrangement is called an electrochemical cell or a Galvanic Cell.

If a strip of copper is placed in a solution of silver nitrate, the following reaction takes place:

2Ag++Cu→2Ag+Cu2+

In this reaction (illustrated below), copper atoms are donating electrons to silver ions so that the silver ions are reduced to silver atoms and the copper atoms are oxidized to copper(II) ions.

As the reaction occurs, an observer would see the solution slowly turn blue (Cu2+ions are blue in solution), and a mass of solid silver atoms would build up on the copper strip.

Although electrons are transferred (moved from the copper atoms to the silver ions) in this example, no electricity is being produced as the electrons are not flowing through a wire.

Electrochemical Cells

Because electrons are transferred, the reaction 2Ag++Cu→2Ag+Cu2+ is one that could be physically arranged to produce an external electric current. To do this, the two half-reactions must occur in separate compartments, and the separate compartments must remain in contact through an ionic solution and an external wire.

In the electrochemical cell illustrated above, the copper metal must be separated from the silver ions to avoid a direct reaction. Each electrode in its solution could be represented by the following half-reactions:

Cu→Cu2++2e−Ag→Ag++e−

On the copper (Cu) side, each atom loses two electrons. The electrons go through a wire to the other side where the silver ions (Ag+) gains the electrons to product solid silver (Ag). The net reaction for the entire cell is:

2Ag+(aq)+Cu(s)→Ag(aq)+Cu2+(aq)

The electrons that pass through the external circuit can do useful work, such as lighting lights, running cell phones, and so forth. If the light bulb is removed from the circuit with the electrochemical cell and replaced with a voltmeter (see illustration below), the voltmeter will measure the voltage (electrical potential energy per unit charge) of the combination of half-cells.

It may seem complicated to construct an electrochemical cell. Electrochemical cells, however, are actually easy to make and sometimes even occur accidentally. If you take two coins of different metal composition, one copper and one silver for example, and push them part way through the peel of a whole lemon (as illustrated below), upon connecting the two coins with a wire, a small electric current will flow.

Follow this link to see an electrochemical cell animation:

Summary

  • An endothermic reaction system absorbs heat from the surroundings. An exothermic reaction system releases heat to the surroundings.
  • An exothermic reaction is a chemical reaction in which less energy is needed to break bonds in the reactants than is released when new bonds form in the products.
  • During an exothermic reaction, energy is constantly given off, often in the form of heat.
  • All combustion reactions are exothermic reactions. During combustion, a substance burns as it combines with oxygen, releasing energy in the form of heat and light.
  • An endothermic reaction is a chemical reaction in which more energy is needed to break bonds in the reactants than is released when new bonds form in the products.
  • A constant input of energy, often in the form of heat, is needed to keep an endothermic reaction going.
  • One of the most important series of endothermic reactions is photosynthesis. The energy needed for photosynthesis comes from light.
  • “Some reactions produce a flow of electrons which can be harnessed in a wire to create an electrical current.”
CC-BY-NC http://www.ck12.org/user%3Adxrhac1vzxitzg9llnbhcnruzxjay2sxmi5vcmc./book/Chemistry/section/5.6/


After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete

Last modified: Thursday, 14 July 2016, 11:17 AM