From qualitative considerations, it is clear that the rate of reactions should increase with increasing temperature, because at the same time, the energy of the colliding particles increases and the likelihood that a chemical transformation will occur during a collision increases. To quantitatively describe temperature effects in chemical kinetics, two main relationships are used - the Van't Hoff rule and the Arrhenius equation.
Van't Hoff's rule is that when heated by 10 o C, the rate of most chemical reactions increases by 2 to 4 times. Mathematically, this means that the reaction rate depends on temperature in a power-law manner:
, (4.1)
where is the temperature coefficient of speed ( = 24). Van't Hoff's rule is very rough and is applicable only in a very limited temperature range.
Much more accurate is Arrhenius equation, describing the temperature dependence of the rate constant:
, (4.2)
Where R- universal gas constant; A- pre-exponential factor, which does not depend on temperature, but is determined only by the type of reaction; E A - activation energy, which can be characterized as a certain threshold energy: roughly speaking, if the energy of colliding particles is less E A, then during a collision the reaction will not occur if the energy exceeds E A, the reaction will occur. The activation energy does not depend on temperature.
Graphically dependency k(T) looks in the following way:
At low temperatures Ah, chemical reactions hardly occur: k(T) 0. At very high temperatures, the rate constant tends to the limiting value: k(T)A. This corresponds to the fact that all molecules are chemically active and every collision results in a reaction.
The activation energy can be determined by measuring the rate constant at two temperatures. From equation (4.2) it follows:
. (4.3)
More accurately, the activation energy is determined from the values of the rate constant at several temperatures. To do this, the Arrhenius equation (4.2) is written in logarithmic form
and record experimental data in ln coordinates k - 1/T. The tangent of the angle of inclination of the resulting straight line is equal to - E A / R.
For some reactions, the pre-exponential factor depends weakly on temperature. In this case, the so-called experienced activation energy:
. (4.4)
If the pre-exponential factor is constant, then the experimental activation energy is equal to the Arrhenius activation energy: E op = E A.
Example 4-1. Using the Arrhenius equation, estimate at what temperatures and activation energies the Van't Hoff rule is valid.
Solution. Let's imagine Van't Hoff's rule (4.1) as a power-law dependence of the rate constant:
,
Where B- constant value. Let us compare this expression with the Arrhenius equation (4.2), taking the value ~ for the temperature coefficient of velocity e = 2.718:
.
Let's take the natural logarithm of both sides of this approximate equality:
.
Having differentiated the resulting relationship with respect to temperature, we find the desired connection between activation energy and temperature:
If the activation energy and temperature approximately satisfy this relationship, then the van’t Hoff rule can be used to assess the effect of temperature on the reaction rate.
Example 4-2. The first order reaction at a temperature of 70 o C is 40% complete in 60 minutes. At what temperature will the reaction be 80% complete in 120 minutes if the activation energy is 60 kJ/mol?
Solution. For a first-order reaction, the rate constant is expressed in terms of the degree of conversion as follows:
,
where a = x/a- degree of transformation. Let us write this equation at two temperatures taking into account the Arrhenius equation:
Where E A= 60 kJ/mol, T 1 = 343 K, t 1 = 60 min, a 1 = 0.4, t 2 = 120 min, a 2 = 0.8. Let's divide one equation by another and take the logarithm:
Substituting the above values into this expression, we find T 2 = 333 K = 60 o C.
Example 4-3. The rate of bacterial hydrolysis of fish muscles doubles when moving from a temperature of -1.1 o C to a temperature of +2.2 o C. Estimate the activation energy of this reaction.
Solution. An increase in the rate of hydrolysis by 2 times is due to an increase in the rate constant: k 2 = 2k 1 . The activation energy in relation to the rate constants at two temperatures can be determined from equation (4.3) with T 1 = t 1 + 273.15 = 272.05 K, T 2 = t 2 + 273.15 = 275.35 K:
130800 J/mol = 130.8 kJ/mol.
4-1. Using Van't Hoff's rule, calculate at what temperature the reaction will end in 15 minutes, if at 20 o C it takes 2 hours. Temperature coefficient speed is 3.(answer)
4-2. The half-life of the substance at 323 K is 100 minutes, and at 353 K it is 15 minutes. Determine the temperature coefficient of speed.(answer)
4-3. What should be the activation energy for the reaction rate to increase 3 times with an increase in temperature by 10 0 C a) at 300 K; b) at 1000 K? (answer)
4-4. The first order reaction has an activation energy of 25 kcal/mol and a pre-exponential factor of 5. 10 13 sec -1 . At what temperature will the half-life for this reaction be: a) 1 min; b) 30 days? (answer)
4-5. In which of the two cases does the reaction rate constant increase by larger number times: when heated from 0 o C to 10 o C or when heated from 10 o C to 20 o C? Justify your answer using the Arrhenius equation. (answer)
4-6. The activation energy of some reaction is 1.5 times greater than the activation energy of another reaction. When heated from T 1 to T 2 the rate constant of the second reaction increased by a once. How many times did the rate constant of the first reaction increase when heated from T 1 to T 2 ?(answer)
4-7. The rate constant of a complex reaction is expressed in terms of the rate constants of the elementary stages as follows:
Express the activation energy and pre-exponential factor of the complex reaction in terms of the corresponding quantities related to the elementary stages.(answer)
4-8. In an irreversible 1st order reaction in 20 minutes at 125 o C, the degree of conversion of the starting substance was 60%, and at 145 o C the same degree of conversion was achieved in 5.5 minutes. Find the rate constants and activation energy for this reaction.(answer)
4-9. The 1st order reaction at a temperature of 25 o C is completed by 30% in 30 minutes. At what temperature will the reaction be 60% complete in 40 minutes if the activation energy is 30 kJ/mol? (answer)
4-10. The 1st order reaction at a temperature of 25 o C is 70% complete in 15 minutes. At what temperature will the reaction be 50% complete in 15 minutes if the activation energy is 50 kJ/mol? (answer)
4-11. The first order reaction rate constant is 4.02. 10 -4 s -1 at 393 K and 1.98 . 10 -3 s -1 at 413 K. Calculate the pre-exponential factor for this reaction. (answer)
4-12. For the reaction H 2 + I 2 2HI, the rate constant at a temperature of 683 K is equal to 0.0659 l/(mol. min), and at a temperature of 716 K - 0.375 l/(mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 700 K.(answer)
4-13. For the reaction 2N 2 O 2N 2 + O 2 the rate constant at a temperature of 986 K is 6.72 l/(mol. min), and at a temperature of 1165 K - 977.0 l/(mol. min). Find the activation energy of this reaction and the rate constant at a temperature of 1053.0 K.(answer)
4-14. Trichloroacetate ion in ionizing solvents containing H + decomposes according to the equation
H + + CCl 3 COO - CO 2 + CHCl 3
The stage that determines the rate of the reaction is the monomolecular cleavage of the C-C bond in the trichloroacetate ion. The reaction proceeds in first order, and the rate constants have the following values: k= 3.11. 10 -4 s -1 at 90 o C, k= 7.62. 10 -5 s -1 at 80 o C. Calculate a) activation energy, b) rate constant at 60 o C. (answer)
4-15. For the reaction CH 3 COOC 2 H 5 + NaOH * CH 3 COONa + C 2 H 5 OH, the rate constant at a temperature of 282.6 K is equal to 2.307 l/(mol. min), and at a temperature of 318.1 K - 21.65 l /(mol min). Find the activation energy of this reaction and the rate constant at a temperature of 343 K.(answer)
4-16. For the reaction C 12 H 22 O 11 + H 2 O C 6 H 12 O 6 + C 6 H 12 O 6 the rate constant at a temperature of 298.2 K is equal to 0.765 l/(mol. min), and at a temperature of 328.2 K - 35.5 l/(mol min). Find the activation energy of this reaction and the rate constant at a temperature of 313.2 K.(answer)
4-17. The substance decomposes in two parallel paths with rate constants k 1 and k 2. What is the difference in activation energies of these two reactions if at 10 o C k 1 /k 2 = 10, and at 40 o C k 1 /k 2 = 0.1? (answer)
4-18. In two reactions of the same order, the difference in activation energies is E 2 - E 1 = 40 kJ/mol. At a temperature of 293 K the ratio of the rate constants is k 1 /k 2 = 2. At what temperature do the rate constants become equal? (answer)
4-19. The decomposition of acetone dicarboxylic acid in an aqueous solution is a first-order reaction. The rate constants of this reaction were measured at different temperatures:
Calculate the activation energy and pre-exponential factor. What is the half-life at 25 o C?
Van't Hoff's rule:
When the temperature increases by 10 degrees, the speed of a homogeneous chemical reaction increases by 2-4 times.
where V2 is the reaction rate at temperature T2, V1 is the reaction rate at temperature T1, is the temperature coefficient of the reaction (if it is equal to 2, for example, then the reaction rate will increase 2 times when the temperature increases by 10 degrees).
From the van't Hoff equation temperature coefficient calculated by the formula:
The theory of active collisions generalizes the laws dependence of the rate of chemical reaction on temperature:
1. Not all molecules can react, but only those in a special active state
2.Activation of a molecule occurs as a result of a biomolecular collision.
3. When particles with approximately the same amount of energy collide, its redistribution occurs, as a result of which the energy of one of the molecules reaches a value corresponding to the activation energy.
4. The influence of temperature on the reaction rate: a shift in the equilibrium between ordinary and active molecules towards an increase in the concentration of the former.
Energy profile of the reaction (dependence graph potential energy from the reaction coordinate)
Activation energy Ea- the minimum additional energy that must be imparted to a molecule above its average value in order for a chemical reaction to become possible. interaction.
Arrhenius equation sets the rate constant dependence chemical reaction k on temperature T.
Here A characterizes the frequency of collisions of reacting molecules, R is the universal gas constant.
7. Catalysis. Homogeneous and heterogeneous catalysis. Features of the catalytic activity of enzymes. Catalysis- a change in the rate of chemical reactions in the presence of substances that, after completion of the reaction, remain unchanged in form and quantity. An increase in the rate of reaction is called positive catalysis, decrease - negative catalysis (or inhibition). Catalysts name substances that cause positive catalysis; substances that slow down reactions - inhibitors. There are homogeneous and heterogeneous catalysis. Acceleration of the reaction of disproportionation of hydrogen peroxide in an aqueous solution in the presence of dichromate ions is an example of homogeneous catalysis (the catalyst forms one phase with the reaction mixture), and in the presence of manganese(IV) oxide is an example of heterogeneous catalysis (aqueous solution of hydrogen peroxide - liquid phase, manganese oxide -hard). Catalysts for biochemical reactions are protein in nature and are called enzymes. Enzymes differ from conventional catalysts in a number of features: 1) they have significantly higher catalytic efficiency; 2)high specificity, i.e. selectivity of action; 3) many enzymes exhibit catalytic activity towards only one substrate; 4) enzymes exhibit maximum efficiency only in mild conditions, characterized by a small range of temperatures and pH values. Enzyme activity = Reaction rate zero order. 8. Chemical equilibrium. Reversible and irreversible in the direction of the reaction. Chemical equilibrium: a dynamic state in which the rates of forward and reverse reactions are equal. Equilibrium constant: under constant external conditions in equilibrium, the ratio of the product of product concentrations to the product of reactant concentrations, taking into account stoichiometry, is a constant value that does not depend on chemical composition systems. Kc is related to the standard Gibbs E by the relation: Le Chatelier's principle: the impact of any factor (t, c, p) on an equilibrium system stimulates a shift in equilibrium in a direction that helps restore the original characteristics of the system. Thermodynamic equilibrium conditions: G 2 -G 1 =0S 2 -S 1 =0 Reversible direction: under given conditions, spontaneously flowing in both forward and reverse directions .Conditions for completion: - Sparely soluble precipitate - gas - slightly dissociating substance (water) - stable complex compound Irreversible district: under given conditions, it flows in one direction. Position chemical equilibrium depends on the following reaction parameters: temperature, pressure and concentration. The influence that these factors have on a chemical reaction is subject to a pattern that was expressed in general form in 1884 by the French scientist Le Chatelier. Modern formulation Le Chatelier's principle is:
9. The role of water and solutions in life. Thermodynamics of dissolution.Solution is a homogeneous system of variable composition of two or more substances that is in a state of equilibrium. Classification: 1) suspension(coarse dispersed system): suspensions (solid in liquid) and emulsions (liquid in liquid) 2) colloids, sols(finely dispersed systems). The value of solutions in life: many chemical processes occur only under the condition that the substances involved in them are in a dissolved state. The most important biological fluids (blood, lymph, urine, saliva, sweat) are solutions of salts, proteins, carbohydrates, lipids in water. The absorption of food is associated with the transition of nutrients into a dissolved state. Biochemical reactions in living organisms occur in solutions. Biofluids are involved in the transport of nutrients (fats, amino acids, oxygen), medicines to organs and tissues, as well as in the removal of metabolites from the body. In the liquid media of the body, a constant acidity, salt concentration and organic matter(concentration homeostasis). The most common solvent on our planet is water. Features of water: its heat capacity exceeds all substances; abnormal behavior during cooling - water becomes denser, begins to sink, then rises (all other substances sink when compacted); can sublimate (sublimation of water) - sublimation (under certain conditions, ice can turn into steam without first turning into liquid water, i.e. without melting); water dissolves all substances (the only question is how much?); high dielectric constant of water (a value indicating how many times the force of interaction between two charges in a given substance is less than in a vacuum); high critical temperature; water is an ampholyte (not an acid, not a base); participates in the creation of polymer structures of the body (protein, lipids...); the basis membrane transport. Thermodynamics of dissolution: according to the 2nd law of thermodynamics at p, T=const substances can spontaneously dissolve in any solvent if, as a result of this process, the Gibbs energy of the system decreases, i.e. . G=(H - T S)<0 . (H- enthalpy factor, T S-entropy factor of dissolution). When dissolving liquid and solid substances S>0. When dissolving gases in liquid S<0. The enthalpy change is the algebraic sum of the enthalpy change H cr as a result of destruction of the crystal lattice and changes in enthalpy H sol due to solvation by solvent particles H dist = H cr +H Sol . When gases dissolve, the enthalpy H cr =0, because there is no need to expend energy on destroying the crystal lattice. During dissolution, a change in both entropy and enthalpy can occur. 10 . Ideal solution- enthalpy of mixing is 0 (homogeneous mixtures of hydrocarbons; hypothetical solution, where all intermolecular interaction forces are equal.) Solubility constant or PR- this is the product of the concentrations of ions of a sparingly soluble electrolyte in a saturated solution at a given temperature - a constant value BaCO 3 = Ba + CO 3 , Ks=Conditions for dissolution and formation of precipitation Precipitation and dissolution are exchange reactions occurring in an electrolyte solution ---1) The electrolyte will precipitate if the product of the concentration of its ions in the solution is greater than the solubility constant c(Ba)*c(CO 3)>Kpr 2) Its precipitate will dissolve if all vice versa 11. Coligative properties of solutions. Colligative properties of solutions- these are those properties that, under given conditions, turn out to be equal and independent of chemical nature solute; properties of solutions, which depend only on the number of kinetic units and on their thermal movement. Raoult's law and its corollary- Vapor that is in equilibrium with a liquid is called saturated. The pressure of such vapor over a pure solvent (p0) is called pressure or elasticity saturated steam pure solvent. The vapor pressure of a solution containing a non-volatile solute is directly proportional to the mole fraction of solvent in the solution: p = p0 · χр-л, where p is the vapor pressure above the solution, PA; p0 is the vapor pressure above the pure solvent; χр-л is the mole fraction of the solvent. For electrolyte solutions, a slightly different form of the equation is used, which allows you to add an isotonic coefficient to it: Δp = i · p0 · χв -va, where Δp is the actual change in pressure compared to a pure solvent; χv-va is the mole fraction of the substance in the solution. From Raoult's law two arise consequences. According to one of them, the boiling point of the solution is higher than the boiling point of the solvent. This is due to the fact that the saturated vapor pressure of the solvent above the solution becomes equal to atmospheric pressure(liquid boiling condition) at more high temperature than in the case of a pure solvent. The increase in boiling point Tbp is proportional to the molality of the solution:. Boil=Ke cm where Ke is the ebullioscopic constant of the solvent, cm is the molal concentration. According to second consequence from Raoult's law, the freezing (crystallization) temperature of a solution is lower than the freezing (crystallization) temperature of a pure solvent. This is due to the lower vapor pressure of the solvent above the solution than above the solvent. The decrease in freezing temperature (crystallization) Tzam is proportional to the molality of the solution : Tzam= Kk cm where Kk is the cryoscopic constant of the solution Reducing the crystallization temperature of solutions. Condition crystallization is the equality of the saturated vapor pressure of the solvent above the solution to the vapor pressure above the solid solvent. Since the vapor pressure of the solvent above the solution is always lower than above the pure solvent, this equality will always be achieved at a temperature lower than the freezing point of the solvent. So, ocean water begins to freeze at a temperature of about minus 2 °C. The difference between the crystallization temperature of the solvent and the temperature at which crystallization of the solution begins is a decrease in the crystallization temperature. Increasing the boiling point of solutionsLiquid boils at the temperature at which the total pressure of the saturated vapor becomes equal to the external pressure. pressure saturated vapors above the solution at any temperature will be less than above the pure solvent, and equality with its external pressure will be achieved at a higher temperature. Thus, the boiling point of a solution of a nonvolatile substance T is always higher than the boiling point of a pure solvent at the same pressure T°. The increase in the boiling point of infinitely dilute solutions of nonvolatile substances does not depend on the nature of the solute and is directly proportional to the molal concentration of the solution. Spontaneous transition of the solvent through semipermeable membrane, separating a solution and a solvent or two solutions with different concentrations of solute is called by osmosis. Osmosis is caused by the diffusion of solvent molecules through a semi-permeable partition, which allows only solvent molecules to pass through. Solvent molecules diffuse from a solvent into a solution or from a less concentrated solution to a more concentrated one. Osmosis is characterized quantitatively osmotic pressure, equal strength, per unit surface area, and causing solvent molecules to penetrate through the semi-permeable partition. It is equal to the pressure of the solution column in the osmometer with height h. At equilibrium external pressure balances osmotic pressure. In this case, the rates of forward and reverse transitions of molecules through the semi-permeable partition become the same. Osmotic pressure increases with increasing solute concentration and temperature. Van't Hoff suggested that the equation of state can be applied to osmotic pressure ideal gas: pV = nRT or p = (n/V) RT whence p = with RT, where p is the osmotic pressure (kPa), c is the molar concentration of the solution. Osmotic pressure is directly proportional to the molar concentration of the solute and temperature. Osmosis plays very well important role in biological processes, ensuring the flow of water into cells and other structures. Solutions with the same osmotic pressure are called isotonic. If the osmotic pressure is higher than the intracellular pressure, then it is called hypertonic, if it is lower than the intracellular pressure, it is called hypotonic. The isotonic coefficient (also Van't Hoff factor; denoted i) is a dimensionless parameter that characterizes the behavior of a substance in solution. He is numerically equal to the ratio values of some colligative property of a solution of this substance and the values of the same colligative property of a nonelectrolyte of the same concentration with other system parameters unchanged. Izoosmia-relative constancy of osmotic pressure in liquid media and tissues of the body, due to the maintenance of this level concentrations of the substances they contain: electrolytes, proteins. This is one of the most important physiological constants of the body, provided by self-regulation mechanisms (Homeostasis). HEMOLYSIS- destruction of red blood cells, accompanied by the release of hemoglobin from them. Physical reasons includes the effects of high and low temperatures, ultrasound, and chemical ones - hemolytic poisons, certain medicines etc. Hemolysis can occur due to transfusion of incompatible blood or administration of hypotonic solutions. Plasmolysis-when cells are placed in a hypertonic solution, water leaves the cells to more concentrated solution and cell shrinkage is observed.
Elements of the theory of electrolyte solutions. Strong and weak electrolytes. Ionization constant of a weak electrolyte. Ostwald's law of breeding. Ionic strength solution. Activity and activity coefficient of ions. Electrolytes in the body, saliva as an electrolyte.
Electrolytes- these are substances with ionic or highly polar covalent bonds V aqueous solutions exposed electrolytic dissociation, resulting in the formation of cations and anions.
Strong electrolytes- substances that can dissociate completely. These include most salts, as well as some substances molecular structure(HCl).
Weak electrolytes dissociate to an insignificant extent, and their predominant form is molecular (H2S, organic acids).
The ability of a molecular electrolyte to dissociate is determined quantitatively degree of ionization( it depends on the electrolyte concentration ):
where Ntotal – total number molecules in solution; N ionization is the number of molecules that have broken up into ions.
Ionization constant:
Where [A],[B] are decayed ions
- a substance that has not broken down into ions.
Ostwald's dilution law:
K= α 2 c/1- α ,
Where α is the degree of ionization
C – molar concentration
Ionic strength of solution:
I=0.5∑с i z i 2 ,
Where c i is the molar concentration of the ion in solution, mol/l
z i is the charge of the ion.
Ion activity is its effective concentration.
Activity is related to molar concentration as follows:
where f – activity factor
Electrolytes in the body: Na and Cl participate in maintaining acid-base balance, osmotic balance in the body. Sa plays big role in the construction of bone tissue and teeth, in the regulation of blood acidity and its coagulation, in the excitability of muscle and nerve tissue. TO found primarily in body fluids and soft tissues, where it is a necessary element for maintaining osmotic pressure and regulating blood pH. Mg is a cofactor of many enzymatic reactions, is necessary at all stages of protein synthesis. In living organisms Fe is an important trace element that catalyzes oxygen exchange processes. Co is part of vitamin B 12, is involved in hematopoiesis, functions nervous system and liver, enzymatic reactions. Zn necessary for the metabolism of vitamin E, is involved in the synthesis of various anabolic hormones in the body, including insulin, testosterone and growth hormone. Mn affects growth, blood formation and function of the sex glands.
Saliva as an electrolyte is a complex biochemical environment. The number of H+ and OH ions determines the pH of saliva, which is normally 6.9. The value pH value varies depending on the nature of the pathological process in the oral cavity. So. in infectious diseases, the saliva reaction is acidic. Of the inorganic substances, saliva contains anions of chlorine, bromine, iodine, and fluorine. Phosphate and fluorine anions contribute to an increase in electrochemical potentials, chlorine anion - the transfer of ionic charges and is a depolarizer (a factor that accelerates anodic and cathodic processes). Microelements are determined in saliva: iron, copper, silver, manganese, aluminum, etc. - and macroelements: calcium, potassium, sodium, magnesium, phosphorus.
Factors influencing the reaction
In the human body, thousands of enzymatic reactions take place in a living cell. However, in a multi-stage chain of processes, the difference between the rates of individual reactions is quite large. Thus, the synthesis of protein molecules in a cell is preceded by at least two more stages: the synthesis of transfer RNA and the synthesis of ribosomes. But the time during which the concentration of t-RNA molecules doubles is 1.7 minutes, protein molecules - 17 minutes, and ribosomes - 170 minutes. The rate of the overall process of the slow (limiting) stage, in our example - the rate of ribosome synthesis. The presence of a limiting reaction ensures high reliability and the flexibility to control thousands of reactions occurring in the cell. It is enough to monitor and regulate only the slowest ones. This method of regulating the rate of multi-stage synthesis is called the minimum principle. It allows you to significantly simplify and make the auto-regulation system in the cage more reliable.
Classifications of reactions used in kinetics: reactions, homogeneous, heterogeneous and microheterogeneous; reactions are simple and complex (parallel, sequential, conjugate, chain). Molecularity of an elementary reaction act. Kinetic equations. Order of reaction. Half-life
Microheterogeneous reactions –
The molecularity of a reaction is determined by the number of molecules entering into chemical reaction in an elementary act of reaction. On this basis, reactions are divided into monomolecular, bimolecular and trimolecular.
Then reactions of type A -> B will be monomolecular, for example:
a) C 16 H 34 (t°C) -> C g H 18 + C 8 H 16 - hydrocarbon cracking reaction;
b) CaC0 3 (t°C) ->CaO + C0 2 - thermal decomposition calcium carbonate.
Reactions of type A + B -> C or 2A -> C - are bimolecular, for example:
a) C + 0 2 -> C0 2; b) 2H 2 0 2 -> 2H 2 0 + 0 2, etc.
Trimolecular reactions are described general equations type:
a) A + B + C D; b) 2A + B D; c) 3A D.
For example: a) 2H 2 + 0 2 2H 2 0; b) 2NO + H 2 N 2 0 + H 2 0.
The reaction rate, depending on molecularity, will be expressed by the equations: a) V = k CA - for monomolecular reaction; b) V = to C A C in or c) V = to C 2 A - for a bimolecular reaction; d) V = k C C in C e e) V = k C 2 A C in or f) V = k C 3 A - for a trimolecular reaction.
Molecularity is the number of molecules reacting in one elementary chemical act.
Often the molecularity of a reaction is difficult to establish, so more formal sign- order of chemical reaction.
Reaction order equal to the sum indicators of degrees of concentration in the equation expressing the dependence of the reaction rate on the concentration of the reactants (kinetic equation).
The order of the reaction most often does not coincide with molecularity due to the fact that the reaction mechanism, i.e., the “elementary act” of the reaction (see the definition of the sign of molecularity), is difficult to establish.
Let us consider a number of examples illustrating this position.
1. The rate of crystal dissolution is described by zero-order kinetics equations, despite the monomolecular nature of the reaction: AgCl (TB) ->Ag + + CI", V = k C(AgCl (TB p= k"C(AgCl (ra)) - p - density and is constant value, i.e. the rate of dissolution does not depend on the amount (concentration) of the solute.
2. The hydrolysis reaction of sucrose: CO + H 2 0 -> C 6 H 12 0 6 (glucose) + C 6 H 12 0 6 (fructose) is a bimolecular reaction, but its kinetics is described by the first-order kinetic equation: V = k*C cax, since under experimental conditions, including in the body, the concentration of water is a constant value C(H 2 0) - const.
3. 
The decomposition reaction of hydrogen peroxide, which occurs with the participation of catalysts, both inorganic ions Fe 3+, Cu 2+ metal platinum, and biological enzymes, for example catalase, has general form:
2H 2 0 2 -> 2H 2 0 + O i.e. it is bimolecular.
Dependence of reaction rate on concentration. Kinetic equations of first, second and zero order reactions. Experimental methods determining the rate and rate constant of reactions.
Dependence of reaction rate on temperature. Van't Hoff rule. Temperature coefficient of reaction rate and its features for biochemical processes.
γ-temperature coefficient of reaction rate.
Physical meaning The value of γ is that it shows how many times the reaction rate changes with a change in temperature for every 10 degrees.
15. The concept of the theory of active collisions. Energy profile of the reaction; activation energy; Arrhenius equation. The role of the steric factor. The concept of the theory of transition state.
The relationship between the rate constant, activation energy and temperature is described by the Arrhenius equation: k T = k 0 *Ae~ E / RT, where k t and k 0 are the rate constants at temperature T and T e is the base natural logarithm, A is the steric factor.
Steric factor A determines the probability of collision of two reacting particles in the active center of the molecule. This factor is particularly important for biochemical reactions with biopolymers. In acid-base reactions, the H + ion must react with the terminal carboxyl group - COO." However, not every collision of the H + ion with a protein molecule will lead to this reaction. Only those collisions that directly occur at some points of the macromolecules will be effective , called active centers.
From the Arrhenius equation it follows that the lower the activation energy E and the higher the temperature T of the process, the higher the rate constant.
Temperature and reaction rate
At a fixed temperature, a reaction is possible if the interacting molecules have a certain amount of energy. Arrhenius called this excess energy activation energy , and the molecules themselves activated.
According to Arrhenius rate constant k and activation energy Ea are related by a relationship called the Arrhenius equation:
Here A– pre-exponential factor, R– universal gas constant, T– absolute temperature.
Thus, at a constant temperature, the reaction rate determines Ea. The more Ea, those less number active molecules and the slower the reaction proceeds. When decreasing Ea speed increases, and when Ea= 0 the reaction occurs instantly.
Magnitude Ea characterizes the nature of the reacting substances and is determined experimentally from the dependence k = f(T). Having written equation (5.3) in logarithmic form and solving it for constants at two temperatures, we find Ea:
γ is the temperature coefficient of the rate of chemical reaction. Van't Hoff's rule has limited application, since the value of γ depends on temperature, and outside the region Ea= 50–100 kJ ∙ mol –1 this rule does not apply at all.
In Fig. 5.4 it is clear that the time spent on converting initial products into active state(A* – activated complex) the energy is then fully or partially released again during the transition to the final products. The energy difference between the initial and final products determines Δ H a reaction that does not depend on the activation energy.
Thus, on the way from the initial state to the final state, the system must overcome an energy barrier. Only active molecules that at the moment of collision have the necessary excess energy equal to Ea, can overcome this barrier and enter into a chemical interaction. With increasing temperature, the proportion of active molecules in the reaction medium increases.
Pre-exponential factorA characterizes the total number of collisions. For reactions with simple molecules A close to theoretical collision magnitude Z, i.e. A = Z, calculated from kinetic theory gases For complex molecules A ≠ Z, therefore it is necessary to introduce a steric factor P:
Here Z– number of all collisions, P– the proportion of collisions that are favorable in spatial terms (takes values from 0 to ), – the proportion of active, i.e., favorable in energetically collisions.
The dimension of the rate constant is obtained from the relation
Analyzing expression (5.3), we come to the conclusion that there are two fundamental possibilities for accelerating the reaction:
a) increase in temperature,
b) decrease in activation energy.
Problems and tests on the topic "Chemical kinetics. Temperature and reaction rate"
- The rate of a chemical reaction. Catalysts - Classification of chemical reactions and patterns of their occurrence, grades 8–9
Lessons: 5 Assignments: 8 Tests: 1
The rate of a chemical reaction depends on temperature, and as the temperature increases, the rate of reaction increases. The Dutch scientist Van't Hoff showed that with an increase in temperature by 10 degrees, the rate of most reactions increases by 2-4 times;
VT 2 =VT 1 *y (T2-T1)/10
Where VT 2 and VT 1 are the reaction rates at temperatures T 2 and T 1; y is the temperature coefficient of the reaction rate, which shows how many times the reaction rate increases when the temperature increases by 10K.
At a concentration of reactants of 1 mol/l, the reaction rate is numerically equal to the rate constant k. Then the equation shows that the rate constant depends on temperature in the same way as the rate of the process.
3. Write a version of the elimination reaction with the release of hydrogen halide.
C 2 H 5 Cl=C 2 H 4 +HCl
Ticket No. 4
1. What is “ atomic mass», « molecular mass", "mole of substance" and what is taken to be an atomic mass unit (amu)?
ATOMIC MASS - the mass of an atom in atomic units mass (a.m.u.). Per unit a. The e.m. is taken to be 1/12 of the mass of the carbon-12 isotope.
a.e.m. = 1/12 m 12 6 C = 1.66 * 10 -24
MOLECULAR MASS - molar mass of a compound divided by 1/12 molar mass carbon-12 atom.
MOL - an amount of substance containing the same number of particles or structural units(atoms, ions, molecules, radicals, electrons, equivalents, etc.), as in 12 a. e.m. carbon-12 isotope.
Formula for increasing reaction rate in the presence of a catalyst.
The value of Ea (activation energy) can be changed using catalysts. Substances that take part but are not consumed in the reaction process are called catalysts. This phenomenon itself is called catalysis. The increase in reaction rate in the presence of a catalyst is determined by the formula
Depending on whether the catalyst is in the same phase as the reactants or forms an independent phase, we speak of homogeneous or heterogeneous catalysis. The mechanism of catalytic action is not the same for them, however, in both cases the reaction is accelerated due to a decrease in Ea. There are a number of specific catalysts - inhibitors that reduce the reaction rate.
where are the parameters of the catalytic process, V, k, Ea are the parameters of the non-catalytic process.
Write the combustion reactions of carbon-containing inorganic substances in oxygen, indicating the oxidizing agent and reducing agent, as well as the oxidation state of carbon before and after the reaction.
C – reducing agent, oxidation process
O – oxidizing agent, reduction process
Ticket number 5
1. What is “electronegativity”, “valence”, “oxidation state” of an element and what are the basic rules for determining them?
OXIDATION DEGREE - the conditional charge of an atom of an element, obtained under the assumption that the compound consists of ions. It can be positive, negative, zero, fractional and is indicated by an Arabic numeral with a “+” or “-” sign in the form of the upper right index of the element symbol: C 1-, O 2-, H +, Mg 2+, N 3-, N 5+, Cr 6+.
To determine the oxidation state (s.o.) of an element in a compound (ion), use the following rules:
1 V simple substances(H2, S8, P4) p. O. equal to zero.
2 Constant s. O. have alkaline (E+) and alkaline earth (E2+) elements, as well as fluorine P-.
3 Hydrogen in most compounds has c. O. H+ (H2O, CH4, HC1), in hydrides - H- (-NaH, CaH2); With. O. oxygen, as a rule, is equal to -2 (O2-), in peroxides (-O-O-) - 1 (O-).
4 In binary compounds of non-metals, negative c. O. assigned to the element on the right).
5 Algebraic sum With. O. molecule is equal to zero, ion - its charge.
The ability of an atom to add or replace certain number other atoms are called VALENCE. The measure of valency is the number of hydrogen or oxygen atoms attached to an element, provided that hydrogen is monovalent and oxygen is divalent.