Electrochemical potential equation and its meaning. Galvani potential. electrochemical potential. Transport lipoproteins in the blood: features of the structure, composition and functions of different lipoproteins. Role in the metabolism of fats and cholesterol. Limits of changes concentration

General provisions of electrochemistry.

Electrolyte solutions.

Electrolytic dissociation of water.

Galvanic cell. Daniel–Jacobi element.

The emergence of electrode potentials.

Hydrogen electrode. Measurement of electrode potentials. Standard electrode potentials.

Nernst equation.

Electrode potential of a hydrogen electrode.

Classification of electrochemical systems.

Chemical chains.

Redox reactions.

Electrolysis.

GENERAL PROVISIONS OF ELECTROCHEMISTRY.

Electrochemistry- is a science that studies processes that occur due to electricity, or in the process of which electricity is formed, i.e. mutual transitions of chemical and electrical energy.

The reaction below can be carried out chemically or electrochemically.

Fe 3+ +Cu + = Fe 2+ +Cu 2+

Chemical reaction.

1. The direct collision of the participants leads to the fact that the path of the electron is very short.

2. The collision occurs in any part of the reaction system. This implies non-directionality of interaction.

3. Energy effects are expressed as heat.

Electrochemical reaction.

1. The path of the electron must be large compared to the size of the electron. It follows that the participants in the reaction must be spatially separated.

2. Direct contact between the reaction participants is replaced by contact with metal electrodes.

3. Reaction space required.

4. The electrode is a catalyst - this leads to a decrease in the activation energy of the process.

Components of an electrochemical system:

1. electrolytes- substances whose melts or solutions conduct electric current,

2. electrodes- electronic conductors of electric current,

3. external circuit- metal conductors connecting the electrodes.

Electrolyte solutions.

Theory of electrolytic dissociation.

The theory was proposed by Arrhenius in 1883-1887 and was later developed in the works of Mendeleev and Kablukov.

According to this theory, when an electrolyte is dissolved in water, it disintegrates into oppositely charged ions. Positively charged ions are called cations; these include, for example, metal and hydrogen ions (H+). Negatively charged ions are called anions, these include ions of acidic residues and hydroxide ions.

The process of decomposition of substance molecules into ions is called under the influence of polar solvent molecules, and also when they melt is called electrolytic dissociation. Only those molecules whose chemical bond has a sufficiently high degree of ionicity can disintegrate into ions.

The fraction of molecules that break up into ions is called degree of dissociation and is usually denoted .

Where n- number of decayed molecules, N- total number of molecules

 – quantitative characteristics of electrolytes:

 >30% - strong electrolytes;

3%<  >30% - medium strength electrolytes:

 <3% - слабые электролиты.

Strong electrolytes include acids: HClO 4, HCl, HNO 3, HBr, H 2 SO 4; hydroxides of alkali and alkaline earth metals, many salts with an ionic crystal lattice, highly soluble in water.

For electrolytes of medium strength - acids: HF, H 3 PO 4, H 2 SO 3; poorly soluble metal hydroxides, various salts.

Weak electrolytes are acids: H 2 CO 3, H 2 S, HCN; most organic acids, hydroxides of d-elements, salts slightly soluble in water.

The degrees of dissociation of various electrolytes are given in the corresponding tables.

The second quantitative characteristic of electrolytes is dissociation constant. Connection TO d And  can be considered using the example of a binary electrolyte. Binary electrolyte is an electrolyte consisting of a singly charged anion and a cation.

WITH- total concentration of the solution;

WITH A- concentration of undissociated molecules;

WITH+ - concentration of cations;

WITH– - concentration of anions.

, Where WITH + = C- = WITH ; C A = (1- )WITH

- Ostwald's law of dilution.

Degree of dissociation  depends on electrolyte concentration, temperature and pressure.

Dissociation constant K d depends on temperature and pressure.

Based on their ability to dissociate, there are 4 classes of electrolytes:

1. Grounds dissociate to form OH – and the main residue. Dissociation occurs stepwise and reversibly.

Dissociation at the first stage.

By the second stage

2. Acids dissociate stepwise and reversibly to form H + and an acid residue.

1st stage;

- 2nd stage.

3. Dissociation ampholytes occurs via a basic or acidic mechanism depending on conditions.

4. Salts are strong electrolytes. Their dissociation is irreversible; type of connection - ionic.

The ability for electrolytic dissociation depends not only on the electrolyte itself, but also on the solvent.

According to Coulomb's law, electrostatic attraction ( F) two opposite charges (q 1 and q 2), the distance between which is r:

 is the dielectric constant of the medium, that is, the greater the dielectric constant of the medium, the weaker the particles interact with each other, the more likely ionization of molecules will occur. Therefore, solvents with high dielectric constants have strong ionizing properties. The high dielectric constant of water is not the only reason for its high ionizing effect. The dipole nature of water molecules, which have lone electron pairs, determines its significant ability to form hydrated ions due to donor-acceptor interaction, and the ion hydration energy released in this case compensates for the energy necessary to overcome the forces of electrostatic attraction of ions in the crystal lattice of the substance. In solvents with low dielectric constant, ions continue to be quite strongly attracted to each other, remaining in the form of ion pairs.

Lecture 15

1. Concept of electrochemistry. Atoms consist of charged particles - nuclei (+) and electrons (-), but in general they are electrically neutral. The presence of electrical charges may not be noticeable. But sometimes we encounter electrification. We comb our hair, but the hair on our head is flying away. Clothes stick to the body, and you can even hear the crackling sound of electrical discharges. This reveals one universal phenomenon - the emergence of electric charges at phase boundaries. Contacting surfaces sometimes spontaneously, sometimes with the expenditure of work (the case of electrification by friction) acquire opposite electrical charges. Besides the obvious examples, surface charges are the cause of electric current in batteries; operation of thermoelements; charges on the membranes of nerve cells ensure the conduction of nerve impulses; charges on nanoparticles stabilize dispersed systems, etc. The name electricity itself arose from the ability of amber to electrify (in Greek hlektro - amber.)

The branch of physical chemistry that studies the relationship between chemical and electrical phenomena is called electrochemistry. The main problems of electrochemistry are the occurrence of electrical phenomena during chemical reactions and the occurrence of chemical reactions when exposed to electricity.

The founders of electrochemistry are considered to be two Italian doctors, Luigi Galvani (1737–1798, Bologna) and Alessandro Volta (1745–1827). To the root galvano there are 15 articles in the BME.

Galvanocaustics

Galvanization

Galvanotropism, etc.

The name galvanic cell comes from the surname Galvani.

An electrochemical system is a heterogeneous system in which an electric current arises due to a spontaneous reaction (galvanic cell) or a non-spontaneous reaction occurs due to the expenditure of electrical work (electrolyzer). A dual action of the system is possible: in the charged state it acts as a current source, and during the charging process as an electrolyzer. This device is called a battery. All curious people know this.

An electrochemical reaction is a reaction accompanied by the transfer of charges across the phase boundary.

2. Varieties of surface potentials. Depending on the nature of the contacting phases, several types of surface potentials are distinguished.

– Contact potential occurs at the interface between two metals. In the case of contact between zinc and copper, zinc, which more easily gives up electrons, is charged positively, and copper negatively. Excess charges are concentrated along the metal interface, forming an electrical double layer.

If such a bimetal is immersed in acid, then electrons that reduce H + ions leave the copper surface, and at the same time zinc ions from the metal surface go into solution:

– Diffusion potential occurs at the interface between two liquid electrolytes. These can be solutions of the same substance with different concentrations, or solutions of different substances, or a solution and a solvent. Obviously, such a boundary is unstable. Ion diffusion occurs, which leads to the emergence of a potential difference. Let us assume that the system consists of solutions of potassium chloride and hydrogen chloride of the same concentration of 1 mol/l. There is diffusion of K + ions into the HCl solution and counter diffusion of H + ions into the KCl solution. The diffusion of hydrogen ions occurs at a higher rate (the direction is shown by a longer arrow), as a result of which there is an excess of positive charge on the side of the KCl solution, and a negative charge on the side of the acid solution. A potential jump φ diff occurs.

– Membrane potential occurs on a membrane characterized by selective permeability towards ions of different nature. Let us imagine solutions of chloride of different concentrations, separated by a membrane that allows chlorine ions to pass through, but sodium ions not to pass through. Then a certain amount of Cl – ions will move from a solution with a higher concentration to a solution with a lower concentration. The remaining excess Na + ions attracts Cl – ions and stops transport across the membrane. A certain potential jump is established, corresponding to the equilibrium state.

– Electrode potential arises at the interface between metal (conductor of the 1st kind) and electrolyte (conductor of the 2nd kind). Electrode potential is of greatest importance in electrochemistry, since the operation of chemical current sources is based on this phenomenon. A system consisting of metal and electrolyte is called an electrode. Next we will talk about a number of types of electrodes. Now, as an example, consider a metal ion electrode (electrode of the 1st kind) Cu / Cu 2+. A copper metal plate is immersed in a copper salt solution, for example CuSO 4 . The electrode is conventionally written with the symbol Cu | Cu 2+, where the vertical line means the interface between the metal and the electrolyte.

The concentration of copper ions in the metal and, accordingly, their chemical potential is higher than in solution. Therefore, a certain number of Cu 2+ ions pass from the metal surface to the electrolyte. An excess of electrons remains on the metal. From the electrolyte side, ions with positive charges are attracted to the metal surface. An electric double layer (EDL) appears. As a result of the movement of ions in the solution, a certain number of ions are removed from the surface, being in the diffusion layer. The equilibrium value of the potential jump in the electrical double layer is established. This potential jump j is called the electrode potential.

Let's consider what the value of the electrode potential depends on. The separation of charges in a DEL means the expenditure of electrical work, and the transfer of substance particles in the form of ions from the metal into a solution is a spontaneous chemical process that overcomes electrical resistance. In a state of balance

W el = –W chemical

Let us transform this equation for one mole of metal ions Me z+ (in our example this is Cu 2+):

Where F– Faraday constant 96485.3383 C mol –1 (according to the latest data). In physical terms, this is a charge of 1 mole of elementary charges. Metal ion activity a(Me z+) in the case of sufficiently dilute solutions can be replaced by concentration With(Me z+). By dividing the written expression by zF we obtain the equation for calculating the electrode potential:

At A(Me z +) = 1; j = j o = DG°/zF. We make the substitution:

This equation is called the Nernst equation. According to this equation, the electrode potential depends on the activity (concentration) of electrolyte ions a(Me z+), temperature E and the nature of the Me/Me system z+ , which is implied in the value of the standard electrode potential j o.

Let us take for comparison another electrode obtained by immersing a zinc plate in a solution of zinc sulfate, denoted by the symbol Zn|Zn 2+:

Zinc is a more active metal than copper. A larger number of Zn 2+ ions pass from the metal surface to the electrolyte, and a greater excess of electrons remains on the metal (all other things being equal). As a result, it turns out that

j o (Zn 2+)< j о (Cu 2+)

In the activity series you know, metals are arranged in order of increasing standard electrode potentials.

3. Galvanic cell

Let's consider a system composed of two electrodes - copper and zinc. The electrolytes are connected by a curved tube filled with a potassium chloride solution. Ions can move through such a bridge. The mobilities of K + and Cl – ions are almost the same, and thus the diffusion potential is minimized. Metals are connected with copper wire. The contact between metals can be opened if necessary. A voltmeter can also be placed in the circuit. This system is an example of a galvanic cell, or chemical current source. The electrodes in a galvanic cell are called half-elements.

When the contact between the metals is open, equilibrium values ​​of the electrode potentials are established at the metal-electrolyte interfaces. There are no chemical processes in the system, but there is a potential difference between the electrodes

Δφ = jо (Cu 2+) – jо (Zn 2+)

When the contact is closed, electrons begin to move from the zinc plate, where their surface concentration is higher and the potential is lower, to the copper plate. The potential on copper decreases, and on zinc it increases. The balance has been disrupted. On the copper surface, electrons react with ions in the electrical double layer to form atoms:

Cu 2+ + 2e – = Cu

The copper potential is again approaching equilibrium. On the surface of zinc, the lack of electrons is compensated by the transition of ions into the electrical double layer, and from it into the electrolyte:

Zn = Zn 2+ + 2e –

The potential on zinc is again approaching equilibrium. Processes on the electrodes maintain a potential difference between them, and the flow of electrons does not stop. Electric current flows in the circuit. In a copper half-cell, copper is deposited on the metal surface and the concentration of Cu 2+ ions in the solution decreases. In the zinc half-cell, the mass of the metal decreases, and at the same time the concentration of Zn 2+ ions in the solution increases. The galvanic cell operates as long as the conductor is closed, and until the initial components - zinc metal and copper salt - are used up. Adding up the reactions occurring at the electrodes, we obtain the overall reaction equation in the galvanic cell:

Zn + Cu 2+ = Zn 2+ + Cu, Δ r H= -218.7 kJ; Δr G= -212.6 kJ

If the same reaction is carried out under normal conditions between zinc and copper sulfate, then all the energy is released in the form of heat equal to 218.7 kJ. The reaction in the galvanic cell produces electrical work of 212.6 kJ, leaving only 6.1 kJ as heat.

The potential difference between the electrodes in a galvanic cell is a measurable quantity called electromotive force, EMF. This is a positive value:

The potentials of the electrodes and the EMF of the element do not depend on the dimensions of the system, but only on the materials and conditions. Therefore, current sources have different sizes depending on their purpose, as we see in commercially available batteries. Electrodes for practical and scientific measurements can be microsized, allowing them to be inserted into a cell to measure membrane potentials.

The considered galvanic cell in the standard state has EMF = 1.1 V.

EMF = |j o (Cu 2+ /Cu) - j o (Zn 2+ /Zn)| = 1.1 V.

The following conventional notation for the galvanic circuit is used:

cathode

anode

-Zn| Zn 2+ || Cu 2+ | Cu+

Anode is an electrode at which oxidation occurs.

Cathode is the electrode at which reduction occurs.

The potential difference between the electrodes is measured with a voltmeter, but the electrode potential of an individual electrode cannot be determined experimentally. Therefore, the potential of a conventionally selected electrode is taken as zero, and the potentials of all other electrodes are expressed relative to it. A standard hydrogen electrode was taken as the zero electrode. It consists of a platinum plate coated with platinum black and dipped into an acid solution, into which hydrogen is passed under a pressure of 101.3 kPa. The electrode is written as follows:

By convention, jº(Pt, H 2 | H+) = 0 V.

If the hydrogen electrode in the galvanic cell under study turns out to be the cathode, then the second electrode in this element is the anode, and its potential is negative. In the opposite case, when the hydrogen electrode turns out to be the anode, the second electrode has a positive potential (cathode). In the series of metal activities, hydrogen is located between metals with negative and positive standard potentials. Standard electrode potentials, expressed relative to the hydrogen electrode, are given in the tables. We can use the table to find the potentials and calculate the emf of a copper-zinc galvanic cell:

j o (Cu 2+ /Cu) = +0.34 V; j o (Zn 2+ /Zn) = –0.76 V; EMF = 0.34 V – (–0.76 V) = 1.1 V.

Let us consider in more detail the mechanism of occurrence of galvanic potentials using the example of a hydrogen electrode. The hydrogen electrode belongs to the electrodes of the first kind. Hydrogen electrode is platinized platinum immersed in an acid solution, such as HC1, and blown with a stream of hydrogen gas. A reaction occurs at the electrode

where H+ q denotes the solvated proton in aqueous solution (i.e. the hydronium ion H e O +), and e(Pt) is the electron remaining in the platinum. At such an electrode, a hydrogen molecule dissociates to form a hydronium ion in solution and a conduction electron in platinum. In this case, the metal platinum is charged negatively, and the solution - positively. As a consequence, an electrical potential difference arises between the platinum and the solution. A double layer appears, consisting of negative and positive charges, reminiscent of a flat electric capacitor. The hydrogen electrode is reversible with respect to the cation.

When considering the equilibrium for the given dissociation reaction, it is necessary to take into account that the resulting H + cation, leaving the platinum, does work against electrical forces. This work is done due to the thermal energy of the solution. It is equal to the stored electrical energy. Therefore, the chemical potential of aquated protons, p(H ^ q), will not be equal to the simple sum p°(Hgq) + R71ntf(Hg q), since the solution has an electrical potential different from platinum. Taking into account the work against the electric field forces in the process of proton transfer, for p(H* a) we obtain

where cp(Pt) is the electrical potential of the platinum electrode; (p) is the electric potential of the solution; d(HM - activity of hydrogen cations in solution; F- Faraday number (F= 96485 C/mol); value sr (R)- f(P0 is the galvanic potential at the platinum-solution boundary D^f. The Faraday number arose because chemical potentials are usually calculated per mole, and not per electron. Work against electric field forces P[f(/>) - - f(P0] is accomplished due to the thermal energy of the solution. It is this work that ensures the charging of the electrodes, the discharge of which, when the external circuit is closed, is accompanied by the production of electrical energy.

A quantity of type p(H + q) is called electrochemical potential. Equating the chemical potentials for substances in the left and right sides of reaction (16.1) in equilibrium, we obtain

where p)