Faraday's laws of electrolysis

The after its discoverer Michael Faraday (1791-1867) named Faraday's laws describe the relationship between electric charge and mass conversion in electrochemical reactions, eg in electrolysis. They are therefore fundamental laws of electrochemistry and electrolysis. Traditionally called the laws " Faraday's laws of electrolysis ," but they are also for the conversion processes in galvanic cells, ie batteries, accumulators and fuel cells, valid.

Principles of electrolysis

The electrolysis, a method of electrochemistry, allowed in the 19th century for the first time to present some newly discovered metals in elemental form. This causes an electrical current is passed through a melt of compounds of these metals. This method called Faraday electrolysis ( after a Greek term for " free means of electricity ").

Faraday called a liquid or a solution having electric conductivity as an electrolyte. The metal rods that were immersed in the liquid or solution, he called electrodes ( after the Greek word for " the road of electricity" ). He called the positively charged anode electrode ( ἄνοδος "rise" ), the negatively charged cathode electrode ( κάθοδος " Hinabweg ").

Faraday compared the flow of electricity with water flowing from the top ( ie electricity at the anode ) to bottom ( the cathode ). He followed in the footsteps of Franklin, who defines a flow of electricity from positive to negative. This is also the direction of transport of positively charged carriers, the definition of the direction of flow gives no indication of the sign of the charge of the charge carriers. In electrolytes, there are positive and negative charge carriers, called ions, while in metallic conductors, the charge carriers are negatively charged electrons. Positively charged ions are called cations and negatively charged anions. The name is derived from the name of the electrode of opposite sign, to which these ions are deposited.

Faraday's laws

In 1834 Faraday published the basic laws of electrolysis, now known as Faraday's law:

Today's formulation

In order to deposit one mole of a monovalent ion electrolytically, it needs per mole of the amount of charge or charge Q1:

Where e is the elementary charge, and NA is Avogadro's number, which indicates how many of the particles contains one mole. F is the Faraday constant of 96485 C / mol, and it is equal to the load required for the separation of one mole of a monohydric substance. It is also equal to the magnitude of the charge of one mole of electrons required for the deposition or is discharged.

In order to deposit any amount of substance of a z- valent ion electrolytically it takes the charge:

With the atomic number Z of the ion used, the amount of substance n and the Faraday constant F

Because of the definition of the molar mass M can be written for the mass m of a substance:

With mass m of the substance, the molar mass M and the amount n of the substance. Now, the second equation is changed in accordance with n and used in the equation of the mass m, as follows:

This here is the mass m is the mass of deposited by electrolysis substance.

This equation combines the two Faradaygesetze together in a relationship. Therefore, such equations can also use the singular term " the Faradaygesetz " are called ..

If we define the electrochemical equivalent AEE

We obtain the equation

This equation follows from the first Faraday's law, however, the proportionality of the charge to the mass of the substance is in her words.

Rearranging the above equation yields the charge Q, which is necessary to deposit a given mass m of the substance by electrolysis:

Constant-current I, the charge Q is proportional to the electrolysis time t:

This is used in the equation of the mass m of the electrodeposited material as follows:

This equation tells us how big the deposited mass m of the substance as a function of the (constant) current and electrolysis time. Here, M and F constants. By rearranging this equation is obtained for the electrolysis time t:

This equation tells us how long the electrolysis time must be to deposit a certain mass of a substance deposited electrolytically at a given constant current.

Applications

The Faraday's laws serve as a pillar of the atomic theory, ie as a strong indication that there are atoms and ions: As is well known because of the Millikan experiment, the electric charge is quantized, that is, there is a smallest electric charge, the elementary charge. Since according to the Faraday's law the amount of material is proportional to the charge, it immediately follows that in the electrolysis, the materials are reacted in small portions, just the atoms or ions, which carry a charge, which corresponds to either of the unit load or a multiple thereof.

Other historically important applications are determination of the relative molar masses and charge numbers for this purpose are, for example, two series-connected electrolytic cells used for example in the dive being a two silver electrodes in a silver salt solution. Since the cells are connected in series, flows through both cells have the same charge, and when in an one mol of silver is reacted moles / z is converted into the other one.

Of course, the Faraday's laws are applied in the electroplating, where they permit the assessment of the layer thickness d, for example with a known geometric surface area A of a workpiece. Applies According to the definition of the density ()

Thus one obtains

Historical

1833 reported Michael Faraday, that the converted amount of substance not of amperage, but the charge was proportional ( "When electro - chemical decomposition takes place, there is great reason to believe did the quantity of matter decomposed is not proportionate to the intensity, but to the quantity of electricity passed " ). In his summary work of 1834 he set out clearly the laws. Although some scientists the importance and accuracy of Faradaygesetze soon realized they were largely disregarded 1834-1880, mainly because of the recognized chemist Jöns Jakob Berzelius thought she was wrong because he had current and charge not distinguish properly.

The Faradaygesetze may have been independently discovered by Carlo Matteucci. Matteucci himself wrote in 1839 that he had discovered independently. Because his works were, however, published in October 1834 and 1835, so that can not be ruled out that he knew before Faraday published results, Faraday is regarded as explorers, so that the laws only bear his name.

As of 1881, Faraday's laws in science and technology have been widely used, particularly the electrolysis was also used for the determination of charges and currents in the DC circuit. The devices used for this purpose were called in the 19th century voltameter, later coulometer. As of 1938, the charge measurement for the quantitative analysis was used conversely, the method is called coulometry.