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His accidental observation of the twitching of frog legs when they were in contact with an iron scalpel while the legs hung on copper hooks led to studies on electrical conductivity in muscles and nerves.

A voltaic cell is an electrochemical cell that uses a spontaneous redox reaction to produce electrical energy. The voltaic cell see Figure above consists of two separate compartments. A half-cell is one part of a voltaic cell in which either the oxidation or reduction half-reaction takes place.

The left half-cell is a strip of zinc metal in a solution of zinc sulfate. The right half-cell is a strip of copper metal in a solution of copper II sulfate.

The strips of metal are called electrodes. An electrode is a conductor in a circuit that is used to carry electrons to a nonmetallic part of the circuit. The nonmetallic part of the circuit is the electrolyte solutions in which the electrodes are placed.

A metal wire connects the two electrodes. A switch opens or closes the circuit. A porous membrane is placed between the two half-cells to complete the circuit. The various electrochemical processes that occur in a voltaic cell occur simultaneously. It is easiest to describe them in the following steps, using the above zinc-copper cell as an example.

Zinc atoms from the zinc electrode are oxidized to zinc ions. This happens because zinc is higher than copper on the activity series and so is more easily oxidized. The electrode at which oxidation occurs is called the anode. The zinc anode gradually diminishes as the cell operates due to the loss of zinc metal.

The zinc ion concentration in the half-cell increases. Because of the production of electrons at the anode, it is labeled as the negative electrode. The electrons that are generated at the zinc anode travel through the external wire and register a reading on the voltmeter.

They continue to the copper electrode. Electrons enter the copper electrode where they combine with the copper II ions in the solution, reducing them to copper metal. The electrode at which reduction occurs is called the cathode.

The cathode gradually increases in mass because of the production of copper metal. The concentration of copper II ions in the half-cell solution decreases. The cathode is the positive electrode. Ions move through the membrane to maintain electrical neutrality in the cell. The two half-reactions can again be summed to provide the overall redox reaction occurring in the voltaic cell. How many volts is that? The first meters were called galvanometers and they used basic laws of electricity to determine voltage.

They were heavy and hard to work with, but got the job done. The first multimeters were developed in the s, but true portability had to wait until printed circuits and transistors replaced the cumbersome wires and vacuum tubes. Electrical potential is a measurement of the ability of a voltaic cell to produce an electric current.

Electrical potential is typically measured in volts V. The voltage that is produced by a given voltaic cell is the electrical potential difference between the two half-cells.

It is not possible to measure the electrical potential of an isolated half-cell. For example, if only a zinc half-cell were constructed, no complete redox reaction can occur and so no electrical potential can be measured. It is only when another half-cell is combined with the zinc half-cell that an electrical potential difference, or voltage, can be measured. The electrical potential of a cell results from a competition for electrons.

In a zinc-copper voltaic cell, it is the copper II ions that will be reduced to copper metal. Instead, the zinc metal is oxidized. The reduction potential is a measure of the tendency of a given half-reaction to occur as a reduction in an electrochemical cell. In a given voltaic cell, the half-cell that has the greater reduction potential is the one in which reduction will occur. In the half-cell with the lower reduction potential, oxidation will occur. The cell potential E cell is the difference in reduction potential between the two half-cells in an electrochemical cell.

What is a standard? We all compare ourselves to someone. Can I run faster than you? Am I taller than my dad? When we use a standard for our comparisons, everybody can tell how one thing compares to another. One meter is the same distance everywhere in the world, so a meter track in one country is exactly the same distance as a meter track in another country.

We now have a universal basis for comparison. The activity series allows us to predict the relative reactivities of different materials when used in oxidation-reduction processes. We also know we can create electric current by a combination of chemical processes. But how do we predict the expected amount of current that will flow through the system? We measure this flow as voltage an electromotive force or potential difference. In order to do this, we need some way of comparing the extent of electron flow in the various chemical systems.

The best way to do this is to have a baseline that we use — a standard that everything can be measured against. For determination of half-reaction current flows and voltages, we use the standard hydrogen electrode. The Figure below illustrates this electrode. A platinum wire conducts the electricity through the circuit. The wire is immersed in a 1. The half-reaction at this electrode is.

Under these conditions, the potential for the hydrogen reduction is defined as exactly zero. We call this , the standard reduction potential. We can then use this system to measure the potentials of other electrodes in the half-cell.

A metal and one of its salts sulfate is often used is in the second half-cell. We will use zinc as our example see Figure below. The standard hydrogen half-cell paired with a zinc half-cell. As we observe the reaction, we notice that the mass of solid zinc decreases during the course of the reaction.

This suggests that the reaction occurring in that half-cell is. So, we have the following process occurring in the cell:. We define the standard emf electromotive force of the cell as:. We can do the same determination with a copper cell Figure below. The standard hydrogen half-cell paired with a copper half-cell. As we run the reaction, we see that the mass of the copper increases, so we write the half-reaction:.

This makes the copper electrode the cathode. We now have the two half-reactions:. Now we want to build a system in which both zinc and copper are involved.

We know from the activity series that zinc will be oxidized and cooper reduced, so we can use the values at hand:. Keeping rust away. When exposed to moisture, steel will begin to rust fairly quickly.

This creates a significant problem for items like nails that are exposed to the atmosphere. The nails can be protected by coated them with zinc metal, making a galvanized nail. The zinc is more likely to oxidize than the iron in the steel, so it prevents rust from developing on the nail. In order to function, any electrochemical cell must consist of two half-cells.

The Table below can be used to determine the reactions that will occur and the standard cell potential for any combination of two half-cells without actually constructing the cell. The half-cell with the higher reduction potential according to the table will undergo reduction within the cell. The half-cell with the lower reduction potential will undergo oxidation within the cell.

If those specifications are followed, the overall cell potential will be a positive value. The cell potential must be positive in order for redox reaction of the cell to be spontaneous. If a negative cell potential were to be calculated, that reaction would be spontaneous in the reverse direction. Write the balanced equation for the overall cell reaction that occurs. Identify the anode and the cathode. Step 1: List the known values and plan the problem.

The silver half-cell will undergo reduction because its standard reduction potential is higher. The tin half-cell will undergo oxidation. The overall cell potential can be calculated by using the equation.

Before adding the two reactions together, the number of electrons lost in the oxidation must equal the number of electrons gained in the reduction. The silver half-cell reaction must be multiplied by two.

After doing that and adding to the tin half-cell reaction, the overall equation is obtained. Step 3: Think about your result. The standard cell potential is positive, so the reaction is spontaneous as written.

Tin is oxidized at the anode, while silver ion is reduced at the cathode. Note that the voltage for the silver ion reduction is not doubled even though the reduction half-reaction had to be doubled to balance the overall redox equation. A substance which is capable of being reduced very easily is a strong oxidizing agent. Conversely, a substance which is capable of being oxidized very easily is a strong reducing agent.

According to the standard cell potential table, fluorine F 2 is the strongest oxidizing agent. It will oxidize any substance below on the table. For example, fluorine will oxidize gold metal according to the following reaction.

Lithium metal Li is the strongest reducing agent. It is capable of reducing any substance above on the table. For example, lithium will reduce water according to this reaction.

Using the Table above will allow you to predict whether reactions will occur or not. For example, nickel metal is capable of reducing copper II ions, but is not capable of reducing zinc ions. Read the material at the link below and answer the questions at the end:. This battery consisted of alternating disks of zinc and silver with pieces of cardboard soaked in brine separating the disks.

He found that the intensity of the shock increased with the number of metal plates in the system. Devices with twenty plates produced a shock that was quite painful. Two variations on the basic voltaic cell are the dry cell and the lead storage battery. Many common batteries, such as those used in a flashlight or remote control, are voltaic dry cells. These batteries are called dry cells because the electrolyte is a paste.

They are relatively inexpensive, but do not last a long time and are not rechargeable. In the zinc-carbon dry cell, the anode is a zinc container, while the cathode is a carbon rod through the center of the cell. The half-reactions for this dry cell are:.

Anode oxidation :. Cathode reduction :. The paste prevents the contents of the dry cell from freely mixing, so a salt bridge is not needed. The carbon rod is a conductor only and does not undergo reduction. The voltage produced by a fresh dry cell is 1. An alkaline battery is a variation on the zinc-carbon dry cell.

The alkaline battery has no carbon rod and uses a paste of zinc metal and potassium hydroxide instead of a solid metal anode. The cathode half-reaction is the same, but the anode half-reaction is different. Advantages of the alkaline battery are that it has a longer shelf life and the voltage does not decrease during use. A battery is a group of electrochemical cells combined together as a source of direct electric current at a constant voltage. Dry cells are not true batteries since they are only one cell.

The lead storage battery is commonly used as the power source in cars and other vehicles. It consists of six identical cells joined together, each of which has a lead anode and a cathode made of lead IV oxide PbO 2 packed on a metal plate.

A lead storage battery, such as those used in cars, consists of six identical electrochemical cells and is rechargeable. The cathode and anode are both immersed in an aqueous solution of sulfuric acid, which acts as the electrolyte. The cell reactions are:.

Each cell in a lead storage battery produces 2 V, so a total of 12 V is generated by the entire battery. This is used to start a car or power other electrical systems. Unlike a dry cell, the lead storage battery is rechargeable. Note that the forward redox reaction generates solid lead II sulfate which slowly builds up on the plates. Additionally, the concentration of sulfuric acid decreases. When the car is running normally, its generator recharges the battery by forcing the above reactions to run in the opposite, or nonspontaneous direction.

This reaction regenerates the lead, lead IV oxide, and sulfuric acid needed for the battery to function properly. Theoretically, a lead storage battery should last forever.

Do we have heat yet? The hypothesis was that the fusion would produce more energy than was required to cause the process to occur. Their process involved the electrolysis of heavy water water molecules containing some deuterium instead of normal hydrogen on a palladium electrode. The experiments could not be reproduced and their scientific reputations were pretty well shot. However, in more recent years, both industry and government researchers are taking another look at this process.

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