Standard electrode potential (data page)

The electrochemical series is a collection of redox pairs according to their standard electrode potential ( redox potential under standard conditions ). Especially in metals it is also called redox.

From the electrochemical series, the redox behavior of a substance can be derived.

Each redox reaction can thus be described by two pairs and predict from the electrochemical series, the direction of reactions.

• 2.1 mnemonic for the voltage range of selected items

Interpretation and Meaning

Metals

In metals, the metal itself and its associated ion forms a redox couple. In the example

Cu is the reduced form ( "Red" ) and Cu2 , the oxidized form ( "ox "). The redox potential is a measure of the readiness of the ions to absorb the electrons. The ions of the noble metals accept electrons more readily than the ions of base metals, which is why the redox potential of Cu/Cu2 pair is clearly positive under standard conditions with 0.35 V than that of Zn/Zn2 pair with -0.76 V. and that in turn means that Zn among the noble metals and a stronger reducing agent, is therefore reduced its reactants and of oxidized and emits electrons.

( " Under standard conditions " means that the concentration - more precisely, activity - the ions 1 mol / l must be so that the redox potential takes the tabulated values ​​This restriction is necessary because it is equilibrium reactions After the Le Chatelier 's principle has one.. larger amount of metal ions, a greater willingness to be reduced to the metal, and therefore a higher redox potential. The Nernst equation describes this relationship mathematically. )

Redox potentials themselves are not measurable. Measurable contrast is the difference between two electrode potentials. An electrode under standard conditions, is simply realized by means of immersing a metal in a solution containing its ions in a concentration of 1 mol / l. Two such electrodes are electrically conductively connected (ion bridge ), a galvanic cell is formed, and a voltage can be measured between the metals. This voltage is equal to the difference of the standard electrode potentials associated with the redox couples in the electrode spaces and are tabulated in the electrochemical series. For example, the combination of the redox couples Cu/Cu2 and Zn/Zn2 creates a Daniell cell with the voltage 1.11 V.

Ion-/Gas-Elektroden ( normal hydrogen electrode)

Gaseous hydrogen and proton are also a redox couple:

Electrodes for redox couples with gaseous substances can be realized by an inert metal (Pt), immersed in a 1 mol / l solution of the ions (H ) and the associated gas (H2) at a pressure of 1 bar is lapped. In this particular case results in a normal hydrogen electrode. This electrode is easy to set up and provides a constant, reproducible potential. Since the redox couple H2 / H also the effect of acids describes ( it always immersed in the dissolution of metals in acids: eg Mg 2H → Mg2 H2), the standard potential of the standard hydrogen electrode, for practical reasons, it was defined as zero.

All other standard potentials are therefore the voltages is measured in a galvanic cell when the left normal hydrogen electrode and to the right, the electrode of the redox pair are joined together. (All expressed under standard conditions! )

Applications

The electrochemical series allows the calculation of voltages that can deliver a maximum of batteries and accumulators. Conversely, these are the voltages that must be applied at least for the driving of electrolysis or charging the batteries.

Furthermore, the calculation of reaction direction and strength are possible. Combining any two redox couples in a reaction solution, the reduction is for the pair with the higher redox potential run for the pair with the lower redox potential of the oxidation. If you dive, for example, a zinc plate in a CuSO4 solution, zinc is oxidized due to its lower redox potential ( -0.76 V) and goes as zinc ions in solution, whereas the same copper ions ( 0.35 V) are reduced and deposited as a coating on the copper - zinc sheet. ( This was fond of quoting example disregards the requirement for standard conditions. Way, is also a copper plate which is immersed in a ZnSO4 solution, over a little pull with zinc, because initially no zinc present and the Cu2 concentration are zero., The effect can be calculated using the Nernst equation, but is immeasurably small, so that the example has a certain justification. ) a measure of the strength of the reaction is the Gibbs free energy ( Gibbs free energy ) of the associated reaction after

Can be calculated. Is the number of exchanged electrons = 96 485 C mol-1 is the Faraday constant, and? E °, the difference of the standard potentials.

The reduced form of a redox couple with very negative standard potential represents a very strong reducing agent because it strives to electron donation (eg, sodium ). However, the oxidised form of a redox couple with very positive standard potential is a strong oxidizing agent (such as fluorine as the strongest oxidising agent known, i.e., the highest standard potential ) because it strives to electron acceptance. Upon contact of metals with different standard electrode potential may, therefore, to contact corrosion. The electrochemical series is thus a collection of oxidants on the oxidation strength and at the same time a reverse listing of reducing agents by reducing strength.

In addition, the electrochemical series contains a gradation of metals ( " very noble metal ", " noble metal ", " less noble metal ", " base metal ", "very base metal " ) to their quest to can be oxidized into acids. The standard potentials of the noble metals have a positive sign that the base on the other hand a negative. Common metals therefore dissolve in acids because acids contain H . ( The arguments, for example, Zn / Cu analogously apply. )

Noble metals dissolve, however, only in oxidizing acids.

Electrochemical Series

( Standard potentials at 25 ° C, 101.3 kPa, pH = 0; ion activity = 1)

Mnemonic for the voltage range of selected items

Emperor Napoleon like all jagged soldiers - Lead Cuban agents also have (K - Na - Mg - Al - Zn - Sn - Pb - H - Cu - Ag - Au) Alternative: Can coatis saturate all possible goats? - Lead Cuban agents have also