Speed chemical reactions.
The reaction rate is determined by a change in the molar concentration of one of the reactants:
V = ± ((C2 - C1) / (t2 - t1)) = ± (DC / Dt)
where C1 and C2 - molar concentrations substances at times t1 and t2, respectively (sign (+) - if the rate is determined by the reaction product, sign (-) - by the starting substance).
Reactions occur when molecules of reacting substances collide. Its speed is determined by the number of collisions and the likelihood that they will lead to transformation. The number of collisions is determined by the concentrations of the reacting substances, and the probability of a reaction is determined by the energy of the colliding molecules.
Factors influencing the rate of chemical reactions.
1. The nature of the reacting substances. Character plays a big role chemical bonds and the structure of reagent molecules. Reactions proceed in the direction of destruction of less strong bonds and the formation of substances with stronger bonds. Thus, to break bonds in H2 and N2 molecules, high energies; such molecules are slightly reactive. Breaking bonds in highly polar molecules (HCl, H2O) requires less energy and the reaction rate is much higher. Reactions between ions in electrolyte solutions occur almost instantly.
Fluorine reacts with hydrogen explosively at room temperature, bromine reacts with hydrogen slowly when heated.
Calcium oxide reacts with water vigorously, releasing heat; copper oxide - does not react.
2. Concentration. With increasing concentration (the number of particles per unit volume), collisions of molecules of reacting substances occur more often - the reaction rate increases.
Law active masses(K. Guldberg, P. Waage, 1867).
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.
aA + bB +… ®… .
V = k [A]a [B]b … .
The reaction rate constant k depends on the nature of the reactants, temperature and catalyst, but does not depend on the concentrations of the reactants.
Physical meaning The rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions the concentration of the solid phase is not included in the expression of the reaction rate.
3. Temperature. For every 10°C increase in temperature, the reaction rate increases by 2-4 times (van't Hoff's rule). As the temperature increases from t1 to t2, the change in reaction rate can be calculated using the formula:
(t2 - t1) / 10 Vt2 / Vt1 = g
(where Vt2 and Vt1 are reaction rates at temperatures t2 and t1, respectively; g- temperature coefficient this reaction).
Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation:
k = A e -Ea/RT
A is a constant depending on the nature of the reactants;
R is the universal gas constant;
Ea is the activation energy, i.e. energy that colliding molecules must have in order for the collision to result in chemical transformation.
Energy diagram of a chemical reaction.
A - reagents, B - activated complex ( transition state), C - products.
The higher the activation energy Ea, the more the reaction rate increases with increasing temperature.
4. Contact surface of reacting substances. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction occurs. Surface solids can be increased by grinding them, and for soluble substances - by dissolving them.
5. Catalysis. Substances that participate in reactions and increase its speed, remaining unchanged at the end of the reaction, are called catalysts. The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds. In homogeneous catalysis, the reactants and the catalyst form one phase (are in the same state of aggregation), with heterogeneous catalysis- different phases (are in different states of aggregation). Dramatically slow down the progression of unwanted chemical processes in some cases, inhibitors can be added to the reaction medium (the phenomenon of “negative catalysis”).
Chemical balance.
Reversible reactions- chemical reactions occurring simultaneously in two opposite directions.
Chemical equilibrium is a state of a system in which the rate of the forward reaction (V1) is equal to the rate of the reverse reaction (V2). In chemical equilibrium, the concentrations of substances remain unchanged. Chemical equilibrium is dynamic in nature: forward and reverse reactions do not stop at equilibrium.
State chemical equilibrium is quantitatively characterized by an equilibrium constant, which is the ratio of the constants of the forward (K1) and reverse (K2) reactions.
For the reaction mA + nB pC + dD the equilibrium constant is equal to
K = K1 / K2 = ([C]p [D]d) / ([A]m [B]n)
The equilibrium constant depends on the temperature and the nature of the reactants. The greater the equilibrium constant, the more balance shifted towards the formation of direct reaction products.
Ways to shift the balance.
Le Chatelier's principle. If a system in equilibrium is affected by external influence(concentration, temperature, pressure change), then it favors the occurrence of one of the two opposite reactions that weakens this effect
V1 A + B C V2
1. Pressure. An increase in pressure (for gases) shifts the equilibrium towards a reaction leading to a decrease in volume (i.e. the formation smaller number molecules).
V1 A + B C; an increase in P leads to V1 > V2 V2 2 1
2. An increase in temperature shifts the equilibrium position towards an endothermic reaction (i.e. towards a reaction that occurs with the absorption of heat)
V1 B + Q, then an increase in t°C leads to V2 > V1 A + B V2 V1 B - Q, then an increase in t°C leads to V1 > V2 A + B V2
3. Increased concentration starting materials and the removal of products from the reaction sphere shifts the equilibrium towards the direct reaction. Increasing the concentrations of the starting substances [A] or [B] or [A] and [B]: V1 > V2.
4. Catalysts do not affect the equilibrium position.
In chemical reactions, something similar to “ionization” occurs. For example, two substances and combine in the main substance; then, after thinking a little, we can call an atom ( - what we call an electron, and - what we call an ion). After such a replacement, as before, we can write the equilibrium equation
. (42.9)
This formula, of course, is inaccurate, because the “constant” depends on the volume to which it is allowed to combine, etc., but by turning to thermodynamic arguments, one can give meaning to the value in terms of an exponential factor, and then it turns out that it is closely related to the energy required for a reaction.
Let's try to understand this formula as a result of collisions, in approximately the same way as we comprehended the evaporation formula, counting the electrons escaping into space and those returning back per unit time. Suppose that in collisions and sometimes form a connection. And let us also assume that this is a complex molecule that participates in a general dance and is hit by other molecules, and from time to time it receives energy sufficient to explode and fall apart again and.
Note that in chemical reactions the situation is that if the approaching atoms have too little energy, then, although this energy is sufficient for the reaction, the fact of the collision of atoms does not necessarily mean the start of the reaction. Usually the collision is required to be more "hard", a "soft" collision between and may not be enough to start a reaction, even if the process releases enough energy for the reaction. Let's assume that common feature chemical reactions is a requirement according to which a simple collision is not enough to combine and form, but it is necessary that they collide with a certain amount of energy. This energy is called activation energy, that is, the energy needed to “activate” a reaction. Let be the excess energy that is necessary for collisions to cause a reaction. Then the speed with which and generate must contain the product of the number of atoms and , multiplied by the speed with which an individual atom hits a certain area of magnitude , and by the value (the probability that the atoms have sufficient energy):
. (42.10)
Now we need to find the speed of the reverse process. There is some possibility that they will break up again. To separate, they do not have enough energy to ensure their separate existence. But since it is not easy for molecules to connect, there must be some kind of barrier through which they must cross in order to fly apart. They must stock up not only with the energy necessary for their existence, but also take something in reserve. It turns out something like climbing a hill before descending into a valley; first you have to climb to a height, then go down, and only after that disperse (Fig. 42.1). Thus, the rate of transition in and is proportional to the product - the initial number of molecules per 
:
. (42.11)
The constant is the sum of the volume of atoms and the frequency of collisions; it can be obtained, as in the case of evaporation, by multiplying the area and thickness of the layer, but we will not do this now. What interests us now is the fact that when these speeds are equal, their ratio is equal to one. This suggests that, as before, , where contains cross sections, velocities and other factors that do not depend on the numbers .
Fig. 42.1. Energy ratio in a reaction.
Interestingly, the reaction rate still varies as 
, although this constant no longer has any relation to the one we encountered in the problem of concentrations; activation energy is very different from energy. Energy regulates the proportions and at which equilibrium is established, but if we want to know whether it quickly turns into , then this has nothing to do with equilibrium, and another energy appears, the activation energy, which, using the exponential, controls the rate of reaction.
Moreover, it is not a fundamental constant like . Suppose that the reaction occurs on the surface of the wall, or on some other surface, then they can spread over it in such a way that combining into will be an easier matter for them. In other words, you can dig a “tunnel” through a mountain or rip off the top of a mountain. Due to the conservation of energy, no matter which path we take, the result will be the same: from and we get , so the energy difference does not depend on the path along which there is a reaction, however, the activation energy is very dependent on this path. This is why the rates of chemical reactions are so sensitive to external conditions. You can change the reaction rate by changing the surface with which the reagents come into contact; you can make a “set of barrels” and use it to select any speeds if they depend on the properties of the surface. You can introduce a third object into the environment in which the reaction occurs; it can also greatly change the rate of a reaction, such substances with a slight change sometimes greatly influence the rate of a reaction; they are called catalysts. There may be practically no reaction at all, because it is too high for a given temperature, but if you add this special substance - a catalyst, then the reaction proceeds very quickly because it decreases. Therefore, the rate of the reverse reaction is proportional and drops out of the formula for equilibrium concentrations. The correctness of the equilibrium law (42.9), which we wrote in the first place, is absolutely guaranteed regardless of any possible reaction mechanism!
Kinematics of chemical reactions
The rate of chemical reactions.
The reaction rate is determined by a change in the molar concentration of one of the reactants:
V = ± ((C 2 - C 1 ) / (t 2 - t 1 )) = ± ( WITH / t)
where C 1 and C 2 — molar concentrations of substances at times t 1 and t 2 respectively (sign (+) - if the rate is determined by the reaction product, sign (-) - if the rate is determined by the starting substance).
Reactions occur when molecules of reacting substances collide. Its speed is determined by the number of collisions and the likelihood that they will lead to transformation. The number of collisions is determined by the concentrations of the reacting substances, and the probability of a reaction is determined by the energy of the colliding molecules.
Factors influencing the rate of chemical reactions.
1. The nature of the reacting substances. The nature of the chemical bonds and the structure of the reagent molecules play an important role. Reactions proceed in the direction of destruction of less strong bonds and the formation of substances with stronger bonds. Thus, to break bonds in molecules H 2 and N 2 high energies required; such molecules are slightly reactive. To break bonds in highly polar molecules (HCl, H 2 O) Less energy is required and the reaction rate is much faster. Reactions between ions in electrolyte solutions occur almost instantly.
Examples.
Fluorine reacts with hydrogen explosively at room temperature, bromine reacts with hydrogen slowly when heated.
Calcium oxide reacts with water vigorously, releasing heat; copper oxide - does not react.
2. Concentration. With increasing concentration (the number of particles per unit volume), collisions of molecules of reacting substances occur more often - the reaction rate increases.
Law of mass action (K. Guldberg, P. Waage, 1867).
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.
aA + bB + . . . . . .
V = k [A] a [B] b . . .
The reaction rate constant k depends on the nature of the reactants, temperature and catalyst, but does not depend on the concentrations of the reactants.
The physical meaning of the rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions, the concentration of the solid phase is not included in the expression of the reaction rate.
3. Temperature. For every 10°C increase in temperature, the reaction rate increases by 2-4 times (van't Hoff's rule). As the temperature increases from t 1 to t 2 the change in reaction rate can be calculated using the formula:
(where Vt 2 and Vt 1 — reaction rates at temperatures t 2 and t 1 respectively; - temperature coefficient of this reaction).
Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation:
k = A e -Ea/RT
Where
A is a constant depending on the nature of the reacting substances;
R is the universal gas constant;
Ea is the activation energy, i.e. the energy that colliding molecules must have in order for the collision to lead to a chemical transformation.
Energy diagram of a chemical reaction.
A - reagents, B - activated complex (transition state), C - products.
The higher the activation energy Ea, the more the reaction rate increases with increasing temperature.
4. Contact surface of reacting substances. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction occurs. The surface area of solids can be increased by grinding them, and for soluble substances by dissolving them.
5. Catalysis. Substances that participate in reactions and increase its speed, remaining unchanged at the end of the reaction, are called catalysts. The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds. In homogeneous catalysis, the reagents and the catalyst form one phase (are in the same state of aggregation); in heterogeneous catalysis, they are different phases (are in different states of aggregation). In some cases, the occurrence of undesirable chemical processes can be sharply slowed down by adding inhibitors to the reaction medium (the phenomenon of “negative catalysis”).
Chemical balance.
Reversible reactions are chemical reactions that occur simultaneously in two opposite directions.
Chemical equilibrium is a state of a system in which the rate of the forward reaction (V1) is equal to the rate of the reverse reaction (V 2 ). In chemical equilibrium, the concentrations of substances remain unchanged. Chemical equilibrium is dynamic in nature: forward and reverse reactions do not stop at equilibrium.
The state of chemical equilibrium is quantitatively characterized by an equilibrium constant, which is the ratio of straight line constants (K 1) and reverse (K 2) reactions.
For the reaction mA + nB pC + dD the equilibrium constant is equal to
K = K 1 /K 2 = ([C] p [D] d ) / ([A] m [B] n )
The equilibrium constant depends on the temperature and the nature of the reactants. The greater the equilibrium constant, the more the equilibrium is shifted towards the formation of direct reaction products.
Ways to shift the balance.
Le Chatelier's principle. If an external influence is applied to a system that is in equilibrium (concentration, temperature, pressure changes), then it favors the occurrence of whichever of the two opposite reactions weakens this influence
1. Pressure. Increasing pressure (for gases) shifts the equilibrium towards a reaction leading to a decrease in volume (i.e., the formation of fewer molecules).
2. An increase in temperature shifts the equilibrium position towards an endothermic reaction (i.e. towards a reaction that occurs with the absorption of heat)
3. An increase in the concentration of starting substances and the removal of products from the reaction sphere shifts the equilibrium towards a direct reaction. Increasing the concentrations of starting substances [A] or [B] or [A] and [B]: V 1 > V 2 .
4. Catalysts do not affect the equilibrium position.
