Electrochemical activity series of metals (voltage range, range of standard electrode potentials) - sequence in which metals are arranged in order of increasing their standard electrochemical potentials φ 0, corresponding to the half-reaction of reduction of the metal cation Me n+: Me n+ + nē → Me

A number of voltages characterize the comparative activity of metals in redox reactions in aqueous solutions.

Story

The sequence of arrangement of metals in the order of changes in their chemical activity was already known in general terms to alchemists. The processes of mutual displacement of metals from solutions and their surface deposition (for example, the displacement of silver and copper from solutions of their salts by iron) were considered as a manifestation of the transmutation of elements.

Later alchemists came close to understanding the chemical side of the mutual precipitation of metals from their solutions. Thus, Angelus Sala in his work “Anatomia Vitrioli” (1613) came to the conclusion that the products of chemical reactions consist of the same “components” that were contained in the original substances. Subsequently, Robert Boyle proposed a hypothesis about the reasons why one metal displaces another from solution based on corpuscular concepts.

In the era of the emergence of classical chemistry, the ability of elements to displace each other from compounds became an important aspect of understanding reactivity. J. Berzelius, based on the electrochemical theory of affinity, built a classification of elements, dividing them into “metalloids” (the term “non-metals” is now used) and “metals” and placing hydrogen between them.

The sequence of metals according to their ability to displace each other, long known to chemists, was especially thoroughly and comprehensively studied and supplemented by N. N. Beketov in the 1860s and subsequent years. Already in 1859, he made a report in Paris on the topic “Investigation of the phenomena of the displacement of some elements by others.” In this work, Beketov included a number of generalizations about the relationship between the mutual displacement of elements and their atomic weight, connecting these processes with “ the original chemical properties of elements - what is called chemical affinity". Beketov's discovery of the displacement of metals from solutions of their salts by hydrogen under pressure and the study of the reducing activity of aluminum, magnesium and zinc at high temperatures (metallothermy) allowed him to put forward a hypothesis about the connection between the ability of some elements to displace others from compounds with their density: lighter simple substances are able to displace more heavy (therefore this series is often also called Beketov's displacement series, or just Beketov series).

Without denying Beketov’s significant merits in the development of modern ideas about the activity series of metals, the idea of ​​him as the only creator of this series, existing in domestic popular and educational literature, should be considered erroneous. Numerous experimental data obtained at the end of the 19th century refuted Beketov’s hypothesis. Thus, William Odling described many cases of “reversal of activity.” For example, copper displaces tin from a concentrated acidified solution of SnCl 2 and lead from an acidic solution of PbCl 2 ; it is also capable of dissolving in concentrated hydrochloric acid with the release of hydrogen. Copper, tin and lead are in the series to the right of cadmium, but can displace it from a boiling, slightly acidified solution of CdCl 2.

The rapid development of theoretical and experimental physical chemistry pointed to another reason for the differences in the chemical activity of metals. With the development of modern concepts of electrochemistry (mainly in the works of Walter Nernst), it became clear that this sequence corresponds to the “series of voltages” - the arrangement of metals according to the value of standard electrode potentials. Thus, instead of a qualitative characteristic - the “propensity” of a metal and its ion to certain reactions - Nerst introduced an exact quantitative value characterizing the ability of each metal to go into solution in the form of ions, as well as to be reduced from ions to the metal on the electrode, and the corresponding series got the name range of standard electrode potentials.

Theoretical foundations

The values ​​of electrochemical potentials are a function of many variables and therefore exhibit a complex dependence on the position of metals in the periodic table. Thus, the oxidation potential of cations increases with an increase in the atomization energy of the metal, with an increase in the total ionization potential of its atoms, and with a decrease in the hydration energy of its cations.

In the most general form, it is clear that metals located at the beginning of periods are characterized by low values ​​of electrochemical potentials and occupy places on the left side of the voltage series. In this case, the alternation of alkali and alkaline earth metals reflects the phenomenon of diagonal similarity. Metals located closer to the middle of the periods are characterized by large potential values ​​and occupy places in the right half of the row. A consistent increase in the electrochemical potential (from −3.395 V for the Eu 2+ /Eu [ ] to +1.691 V for the Au + /Au pair) reflects a decrease in the reducing activity of metals (the ability to donate electrons) and an increase in the oxidizing ability of their cations (the ability to gain electrons). Thus, the strongest reducing agent is metallic europium, and the strongest oxidizing agent is the gold cations Au+.

Hydrogen is traditionally included in the voltage series, since practical measurement of electrochemical potentials of metals is made using a standard hydrogen electrode.

Practical use of a range of voltages

A number of voltages are used in practice for comparative [relative] assessment of the chemical activity of metals in reactions with aqueous solutions of salts and acids and for assessment of cathodic and anodic processes during electrolysis:

• Metals to the left of hydrogen are stronger reducing agents than metals to the right: they displace the latter from salt solutions. For example, the interaction Zn + Cu 2+ → Zn 2+ + Cu is possible only in the forward direction.
• Metals in the row to the left of hydrogen displace hydrogen when interacting with aqueous solutions of non-oxidizing acids; the most active metals (up to and including aluminum) - and when interacting with water.
• Metals in the series to the right of hydrogen do not interact with aqueous solutions of non-oxidizing acids under normal conditions.
• During electrolysis, metals to the right of hydrogen are released at the cathode; the reduction of moderately active metals is accompanied by the release of hydrogen; The most active metals (up to aluminum) cannot be isolated from aqueous salt solutions under normal conditions.

Table of electrochemical potentials of metals

Metal	Cation	φ 0, V	Reactivity	Electrolysis (at the cathode):
	Li+	-3,0401	reacts with water	hydrogen is released
	Cs+	-3,026
	Rb+	-2,98
	K+	-2,931
	Fr+	-2,92
	Ra 2+	-2,912
	Ba 2+	-2,905
	Sr 2+	-2,899
	Ca2+	-2,868
	Eu 2+	-2,812
	Na+	-2,71
	Sm 2+	-2,68
	Md 2+	-2,40	reacts with aqueous solutions of acids
	La 3+	-2,379
	Y 3+	-2,372
	Mg 2+	-2,372
	Ce 3+	-2,336
	Pr 3+	-2,353
	Nd 3+	-2,323
	Er 3+	-2,331
	Ho 3+	-2,33
	Tm 3+	-2,319
	Sm 3+	-2,304
	PM 3+	-2,30
	Fm 2+	-2,30
	Dy 3+	-2,295
	Lu 3+	-2,28
	Tb 3+	-2,28
	Gd 3+	-2,279
	Es 2+	-2,23
	Ac 3+	-2,20
	Dy 2+	-2,2
	PM 2+	-2,2
	Cf 2+	-2,12
	Sc 3+	-2,077
	Am 3+	-2,048
	Cm 3+	-2,04
	Pu 3+	-2,031
	Er 2+	-2,0
	Pr 2+	-2,0
	Eu 3+	-1,991
	Lr 3+	-1,96
	Cf 3+	-1,94
	Es 3+	-1,91
	Th 4+	-1,899
	Fm 3+	-1,89
	Np 3+	-1,856
	Be 2+	-1,847
	U 3+	-1,798
	Al 3+	-1,700
	MD 3+	-1,65
	Ti 2+	-1,63		competing reactions: both the release of hydrogen and the release of pure metal
	Hf 4+	-1,55
	Zr 4+	-1,53
	Pa 3+	-1,34
	Ti 3+	-1,208
	Yb 3+	-1,205
	No 3+	-1,20
	Ti 4+	-1,19
	Mn 2+	-1,185
	V 2+	-1,175
	Nb 3+	-1,1
	Nb 5+	-0,96
	V 3+	-0,87
	Cr 2+	-0,852
	Zn 2+	-0,763
	Cr 3+	-0,74
	Ga 3+	-0,560

Metals that react easily are called active metals. These include alkali, alkaline earth metals and aluminum.

Position in the periodic table

The metallic properties of elements decrease from left to right in the periodic table. Therefore, elements of groups I and II are considered the most active.

Rice. 1. Active metals in the periodic table.

All metals are reducing agents and easily part with electrons at the outer energy level. Active metals have only one or two valence electrons. In this case, metallic properties increase from top to bottom with increasing number of energy levels, because The further an electron is from the nucleus of an atom, the easier it is for it to separate.

Alkali metals are considered the most active:

• lithium;
• sodium;
• potassium;
• rubidium;
• cesium;
• French

Alkaline earth metals include:

• beryllium;
• magnesium;
• calcium;
• strontium;
• barium;
• radium.

The degree of activity of a metal can be determined by the electrochemical series of metal voltages. The further to the left of hydrogen an element is located, the more active it is. Metals to the right of hydrogen are inactive and can only react with concentrated acids.

Rice. 2. Electrochemical series of voltages of metals.

The list of active metals in chemistry also includes aluminum, located in group III and to the left of hydrogen. However, aluminum is on the border of active and intermediately active metals and does not react with some substances under normal conditions.

Properties

Active metals are soft (can be cut with a knife), light, and have a low melting point.

The main chemical properties of metals are presented in the table.

Reaction	Equation	Exception
Alkali metals spontaneously ignite in air when interacting with oxygen	K + O 2 → KO 2	Lithium reacts with oxygen only at high temperatures
Alkaline earth metals and aluminum form oxide films in air and spontaneously ignite when heated	2Ca + O 2 → 2CaO
React with simple substances to form salts	Ca + Br 2 → CaBr 2; - 2Al + 3S → Al 2 S 3	Aluminum does not react with hydrogen
React violently with water, forming alkalis and hydrogen	- Ca + 2H 2 O → Ca(OH) 2 + H 2	The reaction with lithium is slow. Aluminum reacts with water only after removing the oxide film
React with acids to form salts	Ca + 2HCl → CaCl 2 + H 2; 2K + 2HMnO 4 → 2KMnO 4 + H 2
Interact with salt solutions, first reacting with water and then with salt	2Na + CuCl 2 + 2H 2 O: 2Na + 2H 2 O → 2NaOH + H 2; - 2NaOH + CuCl 2 → Cu(OH) 2 ↓ + 2NaCl

Active metals easily react, so in nature they are found only in mixtures - minerals, rocks.

Rice. 3. Minerals and pure metals.

What have we learned?

Active metals include elements of groups I and II - alkali and alkaline earth metals, as well as aluminum. Their activity is determined by the structure of the atom - a few electrons are easily separated from the external energy level. These are soft light metals that quickly react with simple and complex substances, forming oxides, hydroxides, and salts. Aluminum is closer to hydrogen and its reaction with substances requires additional conditions - high temperatures, destruction of the oxide film.

Test on the topic

Evaluation of the report

Average rating: 4.4. Total ratings received: 339.

Metal stress range- this is a series of metals arranged in increasing order of their standard electrode potential (). The position of a metal in the voltage series indicates its redox abilities in relation to other metals and their cations for reactions occurring in electrolyte solutions, i.e., in reactions with salts and bases. And also with non-metals, if these reactions occur in aqueous solutions; in particular, such processes include the processes of corrosion of metals ().

In the series of voltages:

1) The reducing ability of metals decreases.

2) Oxidizing capacity increases. As a consequence of this, metals that are in the voltage series before hydrogen displace it from solutions of acids (not oxidizing agents).

3) Metals standing to the left in the series (having a lower potential) displace metals standing to the right (having a higher potential) from solutions of their salts.

4) Metals in the voltage range up to Mg (having ) displace hydrogen from water.

Thus, the value of the electrode potential determines the redox abilities of metals in relation to each other and in relation to H and the cations containing it in electrolytes.

Measurement of electrode potentials. A range of standard electrode potentials, hydrogen electrode.

The absolute value of the electrode potential is almost impossible to measure. In this regard, the electrode potential is measured by measuring the EMF of a galvanic cell composed of the electrode under study and the electrode potential, of which the potential is known. The standard electrode potential is determined by the value of the emf of a galvanic cell composed of the electrode under study and a standard hydrogen electrode, the potential of which is conventionally assumed to be zero.

Standard hydrogen electrode– This is a system under normal conditions, consisting of a sponge plate into the pores of which hydrogen is pumped, placed in a one-molar solution of sulfuric acid H 2 SO 4 with C(H +) = 1 mol/kg

Standardizing the conditions and reproducing the potential of such an electrode is a difficult task, so this electrode is used for meteorological purposes. In laboratory practice, auxiliary electrodes are used to measure electrode potentials.

Example: calomel electrode - Hg,HgCl/Cl - ;

silver chlorine – Ag, AgCl/Cl - etc.

The potential of these electrodes is stably reproduced, that is, it retains its value during storage and operation.

Metals are always reducing agents in chemical reactions. The reduction activity of a metal reflects its position in the electrochemical voltage series.

Based on the series, the following conclusions can be drawn:

1. The further to the left a metal is in this row, the more powerful a reducing agent it is.

2. Each metal is capable of displacing from salts in solution those metals that are to the right

2Fe + 3CuSO 4 → 3Cu + Fe 2 (SO 4) 3

3. Metals located in the voltage series to the left of hydrogen are able to displace it from acids.

Zn + 2HCl → ZnCl 2 + H 2

4. Metals that are the strongest reducing agents (alkali and alkaline earth) in any aqueous solutions first react with water.

The reducing ability of a metal, determined by the electrochemical series, does not always correspond to its position in the periodic system, since the voltage series takes into account not only the radius of the atom, but also the energy of electron abstraction.

Aldehydes, their structure and properties. Preparation and use of formic and acetaldehydes.

Aldehydes are organic compounds whose molecules include a carbonyl group connected to hydrogen and a hydrocarbon radical.

Methanal (formaldehyde)

Physical properties

Methanal – gaseous substance, aqueous solution – formalin

Chemical properties

The reagent for aldehydes is Cu(OH) 2

Application

Methanal and ethanal are the most widely used. A large amount of methanal is used to produce phenol-formaldehyde resin, which is obtained by reacting methanal with phenol. This resin is necessary for the production of various plastics. Plastics made from phenol-formaldehyde resin in combination with various fillers are called phenolics. By dissolving phenol-formaldehyde resin in acetone or alcohol, various varnishes are obtained. When methanal reacts with urea CO(NH 2) 2, carbide resin is obtained, and aminoplasts are made from it. Microporous materials are made from these plastics for the needs of electrical engineering. Methanal is also used in the production of some medicinal substances and dyes. An aqueous solution containing 40% methanal in mass fractions is widely used. It's called formalin. Its use is based on its ability to fold proteins.

Receipt

Aldehydes are obtained by the oxidation of alkanes and alcohols. Ethanal is produced by hydration of ethine and oxidation of ethene.

Ticket No. 12

Higher oxides of chemical elements of the third period. Regularities in measuring their properties in connection with the position of chemical elements in the periodic table. Characteristic chemical properties of oxides: basic, amphoteric, acidic.

Oxides- these are complex substances consisting of two chemical elements, one of which is oxygen with an oxidation state of “-2”

Oxides of the third period include:
Na 2 O, MgO, Al 2 O 3, SiO 2, P 2 O 5, SO 3, Cl 2 O 7.

With an increase in the degree of oxidation of elements, the acidic properties of the oxides increase.

Na 2 O, MgO – basic oxides

Al 2 O 3 – amphoteric oxide

SiO 2 , P 2 O 5 , SO 3 , Cl 2 O 7 – acidic oxides.

Basic oxides react with acids to form salt and water.

MgO + 2CH 3 COOH → (CH 3 COO) 2 Hg + H 2 O

Oxides of alkali and alkaline earth metals react with water to form alkali.

Na 2 O + HOH → 2NaOH

Basic oxides react with acidic oxides to form a salt.
Na 2 O + SO 2 → Na 2 SO 3
Acidic oxides react with alkalis to form salt and water

2NaOH + SO 3 → Na 2 SO 4 + H 2 O

Reacts with water to form acid

SO 3 + H 2 O → H 2 SO 4

Amphoteric oxides react with acids and alkalis

Al 2 O 3 + 6HCl → 2AlCl 3 + 3H 2 O

With alkali

Al 2 O 3 + 2NaOH → 2NaAlO 2 + H 2 O

Fats, their properties and composition. Fats in nature, transformation of fats in the body. Products of technical processing of fats, the concept of synthetic detergents. Protecting nature from SMS pollution.

Fats are esters of glycerol and carboxylic acids.

General formula of fats:

Solid fats are formed predominantly by higher saturated carboxylic acids - stearic C 17 H 35 COOH, palmitic C 15 H 31 COOH and some others. Liquid fats are formed mainly by higher unsaturated carboxylic acids - oleic C17H33COOH, lenolic C17H31COOH

Fats, along with hydrocarbons and proteins, are part of the organisms of animals and plants. They are an important part of human and animal food. When fats are oxidized, energy is released in the body. When fats enter the digestive organs, under the influence of enzymes they are hydrolyzed into glycerol and corresponding acids.

The products of hydrolysis are absorbed by the intestinal villi, and then fat is synthesized, but already characteristic of the body. Fats are transported by the bloodstream to other organs and tissues of the body, where they accumulate or are again hydrolyzed and gradually oxidized to carbon monoxide (IV) and water.

Physical properties.

Animal fats are mostly solid substances, but there are also liquid ones (fish oil). Vegetable fats are most often liquid substances - oils; Solid vegetable fats – coconut oil – are also known.

Chemical properties.

Fats in animal organisms are hydrolyzed in the presence of enzymes. In addition to reactions with water, fats interact with alkalis.

Vegetable oils contain esters of unsaturated carboxylic acids and can be subjected to hydrogenation. They turn into ultimate connections
Example: Margarine is produced industrially from vegetable oil.

Application.
Fats are mainly used as a food product. Previously, fats were used to make soap
Synthetic detergents.

Synthetic detergents have a harmful effect on the environment because... they are stable and difficult to break.

If from the entire series of standard electrode potentials we select only those electrode processes that correspond to the general equation

then we get a series of metal stresses. In addition to metals, this series will always include hydrogen, which allows you to see which metals are capable of displacing hydrogen from aqueous solutions of acids.

Table 19. Series of metal stresses

A number of stresses for the most important metals are given in table. 19. The position of a particular metal in the stress series characterizes its ability to undergo redox interactions in aqueous solutions under standard conditions. Metal ions are oxidizing agents, and metals in the form of simple substances are reducing agents. Moreover, the further a metal is located in the voltage series, the stronger the oxidizing agent in an aqueous solution are its ions, and vice versa, the closer the metal is to the beginning of the series, the stronger the reducing properties of a simple substance - the metal.

Electrode process potential

in a neutral environment it is equal to B (see page 273). Active metals at the beginning of the series, having a potential significantly more negative than -0.41 V, displace hydrogen from water. Magnesium displaces hydrogen only from hot water. Metals located between magnesium and cadmium generally do not displace hydrogen from water. Oxide films are formed on the surface of these metals, which have a protective effect.

Metals located between magnesium and hydrogen displace hydrogen from acid solutions. At the same time, protective films are also formed on the surface of some metals, inhibiting the reaction. Thus, the oxide film on aluminum makes this metal stable not only in water, but also in solutions of certain acids. Lead does not dissolve in sulfuric acid at its concentration below, since the salt formed when lead reacts with sulfuric acid is insoluble and creates a protective film on the metal surface. The phenomenon of deep inhibition of metal oxidation, due to the presence of protective oxide or salt films on its surface, is called passivity, and the state of the metal in this case is called a passive state.

Metals are capable of displacing each other from salt solutions. The direction of the reaction is determined by their relative position in the series of stresses. When considering specific cases of such reactions, it should be remembered that active metals displace hydrogen not only from water, but also from any aqueous solution. Therefore, the mutual displacement of metals from solutions of their salts practically occurs only in the case of metals located in the series after magnesium.

Beketov was the first to study in detail the displacement of metals from their compounds by other metals. As a result of his work, he arranged metals according to their chemical activity into a displacement series, which is the prototype of a series of metal stresses.

The relative position of some metals in the stress series and in the periodic table at first glance does not correspond to each other. For example, according to the position in the periodic table, the chemical activity of potassium should be greater than sodium, and sodium - greater than lithium. In the series of voltages, lithium is the most active, and potassium occupies a middle position between lithium and sodium. Zinc and copper, according to their position in the periodic table, should have approximately equal chemical activity, but in the voltage series, zinc is located much earlier than copper. The reason for this kind of inconsistency is as follows.

When comparing metals occupying one or another position in the periodic table, the ionization energy of free atoms is taken as a measure of their chemical activity - reducing ability. Indeed, when moving, for example, from top to bottom along the main subgroup of group I of the periodic system, the ionization energy of atoms decreases, which is associated with an increase in their radii (i.e., with a greater distance of outer electrons from the nucleus) and with increasing screening of the positive charge of the nucleus by intermediate electronic layers (see § 31). Therefore, potassium atoms exhibit greater chemical activity - they have stronger reducing properties - than sodium atoms, and sodium atoms exhibit greater activity than lithium atoms.

When comparing metals in a series of voltages, the work of converting a metal in a solid state into hydrated ions in an aqueous solution is taken as a measure of chemical activity. This work can be represented as the sum of three terms: the atomization energy - the transformation of a metal crystal into isolated atoms, the ionization energy of free metal atoms and the hydration energy of the resulting ions. Atomization energy characterizes the strength of the crystal lattice of a given metal. The energy of ionization of atoms - the removal of valence electrons from them - is directly determined by the position of the metal in the periodic table. The energy released during hydration depends on the electronic structure of the ion, its charge and radius.

Lithium and potassium ions, having the same charge but different radii, will create unequal electric fields around themselves. The field generated near small lithium ions will be stronger than the field near large potassium ions. It is clear from this that lithium ions will hydrate with the release of more energy than potassium ions.

Thus, during the transformation under consideration, energy is expended on atomization and ionization and energy is released during hydration. The lower the total energy consumption, the easier the entire process will be and the closer to the beginning of the stress series the given metal will be located. But of the three terms of the general energy balance, only one - the ionization energy - is directly determined by the position of the metal in the periodic table. Consequently, there is no reason to expect that the relative position of certain metals in the stress series will always correspond to their position in the periodic table. Thus, for lithium, the total energy consumption turns out to be less than for potassium, according to which lithium comes before potassium in the voltage series.

For copper and zinc, the energy expenditure for the ionization of free atoms and the energy gain during ion hydration are close. But metallic copper forms a stronger crystal lattice than zinc, as can be seen from a comparison of the melting temperatures of these metals: zinc melts at , and copper only at . Therefore, the energy spent on the atomization of these metals is significantly different, as a result of which the total energy costs for the entire process in the case of copper are much greater than in the case of zinc, which explains the relative position of these metals in the stress series.

When passing from water to non-aqueous solvents, the relative positions of metals in the voltage series may change. The reason for this is that the solvation energy of different metal ions changes differently when moving from one solvent to another.

In particular, the copper ion is solvated quite vigorously in some organic solvents; This leads to the fact that in such solvents copper is located in the voltage series before hydrogen and displaces it from acid solutions.

Thus, in contrast to the periodic system of elements, a series of metal stresses is not a reflection of a general pattern, on the basis of which it is possible to give a comprehensive Characteristic of the chemical properties of metals. A series of voltages characterizes only the redox ability of the Electrochemical system “metal - metal ion” under strictly defined conditions: the values ​​​​given in it refer to an aqueous solution, temperature and unit concentration (activity) of metal ions.