Electrochemistry

Electrochemistry By Mac McGuire

What is Electrochemistry? • The dictionary.com definition is “the branch of chemistry that deals with the chemical changes produced by electricity and the production of electricity bychemical changes.” • In other words, it is the study of how electricity is generated and what that electricity does

Why is Electrochemistry important? • Electrochemistry is important because it is essential to batteries, which power many useful machines such as flashlights or cars • Other technologies that use electrochemistry are dry cells, lead storage batteries and fuel cells

Electrochemical Processes • An electrochemical process is any conversion between chemical energy and electrical energy • Any electrochemical process must involve something called a redox reactions • Redox reactions occur in electrochemical cells, which are any device that convert electrical energy into chemical energy or vice versa

Voltaic Cells • Voltaic Cells were invented by the Italian physicist Alessandro Volta in the year 1800 • They are a specific type of electrochemical cell that turn chemical energy into electrical energy • They produce electrical energy by spontaneous redox reactions inside the cell • They power flashlights and calculators

Parts of the Voltaic Cell • Voltaic cells contain the following: • Half Cells • Salt Bridges • Electrodes • Anodes (part of electrode) • Cathodes (part of electrode)

Half Cells • Half-Cells are the part of the Voltaic cell that handle either the oxidation or reduction process • Typically, the half cell is made up of a piece of metal immersed in a solution of its ions

Salt Bridges • Half cells are connected by Salt Bridges • Salt Bridges are tubes containing a strong electrolyte, usually Potassium Sulfate • They are often made up of agar, which is a gelatinous substance • A porous plate, which allows ions to pass from one cell to another but prevents the solutions from mixing completely, may also be used

The Electrodes • Electrodes are a conductor in a circuit that transports electrons either to or from a non-metallic substance • The electrode that handles the oxidation process is known as the anode • The anode produces electrons and is hence labeled the negative electrode • The electrode that handles the reduction process is referred to as the cathode • The cathode consumes electrons and is hence labeled the positive electrode • However, neither electrode, despite their name, are truly charged. In fact, the entire Voltaic cell remains balanced in charge at all times • This is because the constantly moving electrons serve to balance any charge that could accumulate through the oxidation-reduction process

How A Voltaic Cell Works • To best understand how a Voltaic cell works one could use the example of a zinc-copper Voltaic cell • http://www.glencove.k12.ny.us/highschool/ChemRev/voltaic.jpg • The reaction itself works in a multitude of simultaneous steps: • 1. Electrons are produced at the zinc rod as part of the oxidation half-reaction. The oxidation makes the zinc the anode • 2. The previously made electrons leave the zinc rod and pass through the external circuit (for example: a lightbulb) to the copper rod • 3. Electrons in the copper rod interact with the copper ions in solution and are reduced, making the copper rod the cathode • 4. The circuit is completed when both the positive and negative ions move through the solution via the salt bridge

Modern Uses of Voltaic Cells • Although the Zinc-Copper model is good for instructional purposes, it is no longer truly commercially relevant • However, some modern technologies still implement the electrochemical processes that produce electrical energy that are created by a voltaic cell • These technologies include dry cells, lead storage batteries, and fuel cells

Dry Cells • Dry cells are useful for situations demanding a compact and portable energy source • The dry cell is a variation on the copper-zinc model of the voltaic cell that uses a paste for electrodes • A common dry cell is a flashlight battery • http://media-2.web.britannica.com/eb-media/89/24089-004-FAE11DAE.gif

Lead Storage Batteries • A battery is a group of cells connected together • For example, a 12 volt car battery consists of 6 voltaic cells that are connected together; with each cell producing 2 volts • Lead storage batteries use the same electrolyte for both the half cells and thus don’t require a salt bridge or another variant of porous separator • http://www.rfcafe.com/references/electrical/Electricity%20-%20Basic%20Navy%20Training%20Courses/images/Figure%2029.jpg

Pros and Cons of Lead Storage Batteries • In theory, because of its nature, a lead storage battery can be discharged and then recharged limitlessly, ensuring it’s infinite usefulness • However, this is not the case because minuscule amounts of lead sulfate fall from the electrodes and the cells eventually lose so much of the precious lead sulfate that they are unable to recharge and hence lose all effetcivness

Fuel Cells • Fuel cells feature renewable electrodes, which aid them to overcome many of the disadvantages associated with the use of a lead storage battery • They are voltaic cells that bring about a reaction in which any type of fuel substance • Is oxidized and the electrical energy is continuously produced

Hydrogen-Oxygen Fuel Cell • A hydrogen-oxygen fuel cell is a simple example of a fuel cell • Open book to page 670 • The oxygen (which is, of course, the oxidizer) is fed into the cathode compartment while the hydrogen (the fuel) goes into the anode compartment • The two gases then diffuse through the electrode and the ions from the half reactions mix • These fuel cells were used on the Apollo Space Mission, each one weighed 100 kg!

Electric Potential • All voltaic cells have what is called electric potential • Electric potential is the measure of the cells ability to produce an electric current • The standard unit for the measurement is in volts • The electric potential of a single half cell cannot be measured, so, in the case of the zinc-copper reaction, both the zinc half cell and the copper half cell would have to connect in the voltaic cell in order to be measured • Electric potential results from the competition of electros between the two half cells, with the one possessing the superior tendency to acquire electrons being the cell to house the reduction process

Reduction Potential • Reduction potential is the tendency of a half cell to house the reduction process • Thus, the half cell with more reduction potential will have the reduction occur and one with less will have the oxidation process occur

Cell Potential • The cell potential is the difference between the reduction process of the two half cells • Turn to Page 671 for helpful diagram • If the cell potential for a give reaction is positive then it is considered spontaneous but if it is negative it will be labeled as nonspontaneous

Standard Cell Potential • The standard cell potential is the measured cell potential when the ion concentrations are specifically 1M and the conditions are STP • Because one cannot measure the potential of a individual half cell, scientists have chosen the standard hydrogen electrode to serve as a reference so one could in fact measure a single half cell • The standard hydrogen electrode has a set reduction potential of 0 V

Standard Reduction Potential • In order to figure out the standard reduction potential of a given reaction one must identify the half cell in which the reduction process takes place • One can determine the standard reduction potential of a particular half cell by using the equation for standard cell potential from page 671 and then the standard hydrogen electrode • Page 674

Electrolytic Cells • Electrolysis is the process in which electrical energy is used to make a non spontaneous redox reaction move forward • Electrolysis is essential in the making of silver utensils, chrome plated cars and gold plated jewelry as well as many others examples • Electrolytic cells are cells that cause a chemical change through the application of electric energy • They use this energy to force a non spontaneous reaction to finish its reaction

Electrolytic Cells vs. Voltaic Cells • Both types of cells feature electrons flowing from an anode to the cathode in a external circuit and in both types the reduction occurs at the cathode and the oxidation occurs at the anode • The main difference is that the flow of electrons in a voltaic cell results because of a spontaneous reaction while electrolytic cells push the electrons forward through use of an external power source and is not spontaneous

Examples of Electrolysis • Electrolysis of water produces hydrogen gas and oxygen gas • The electrolysis of brine produces chlorine gas

Electrochemistry