What is called the rate of a chemical reaction in what. What is the rate constant of a chemical reaction? Factors affecting the reaction rate
Movements are all an unbroken chain
form And arise one of
another in a known order.
Lucretius
What is a mechanism chemical reaction? What is the kinetic equation of the reaction and what is its meaning? What is the mechanism of action of a catalyst? What are inhibitors?
Lesson-lecture
CHEMICAL REACTION AS AN EXAMPLE OF MOVEMENT. Remember what the rate of a chemical reaction is and what factors it depends on.
Chemical reactions proceed at different rates. The range of their rates is extremely wide - from almost instantaneous reactions (an explosion, many reactions in solutions) to extremely slow ones that take centuries (for example, the oxidation of bronze in air).
Engraving. Alchemists
In the 19th century it was found that the vast majority of chemical reactions are multi-stage processes, i.e., they are not carried out by direct simultaneous collision of reactant particles with the formation of products, but through a series of simple (elementary) processes. Indeed, if, for example, the ammonia oxidation reaction proceeded in one stage, then this would require huge energy expenditures for the simultaneous breaking of bonds in ammonia and oxygen molecules. In addition, the probability of a collision of three particles is very small, four - almost equal to zero. The simultaneous collision of seven particles (four ammonia molecules and three oxygen molecules) is simply impossible.
Each elementary stage of a chemical reaction is either a chemical process (say, the decay of one molecule or a collision of two particles), or the transition of a particle to an excited state (or, conversely, its transition from an excited to a ground or low-excited state).
Even a seemingly simple reaction
progresses in stages, each stage proceeding at its own pace.
1st stage (fast):
2nd stage (relatively slow):
Remember what particles are called radicals. What reactions are called chain reactions and what is the activation energy?
The set of elementary stages of a chemical reaction, following one after another (i.e., sequentially) or occurring in parallel, is called mechanism chemical reaction. The reaction mechanisms are different.
It is very important for a chemist to know what factors determine the rate of a chemical reaction. The dependence of the reaction rate (or its stages) on the concentrations of reactants is especially important. This dependency is called kinetic equation. For a hypothetical reaction aA + bB = dD + eE, the mathematical expression (kinetic equation) has the form
where V is the rate of a chemical reaction; c is the concentration of the substance, mol/l; a, b - exponents (these values are determined experimentally). The proportionality coefficient k in the kinetic equation is called rate constant chemical reaction. It is numerically equal to the rate of a chemical reaction at concentrations of reactants equal to 1 mol/l.
The rate of the elementary stages of the reaction is proportional to the product of the concentrations of reactant particles, for example:
The rate of the overall reaction can vary, sometimes in a very complex way, depending on the concentration of the reagents.
Thus, the transformation of some substances into others is not a one-time event, but a process that unfolds in time, that is, it has its own temporal structure, which is expressed by the reaction mechanism. At the same time, the reaction mechanism takes into account not only changes in the composition of substances participating in the reaction, but also changes in the positions of atoms in space as the reaction proceeds. Therefore, we can talk about the spatio-temporal structure of the reaction.
The development of chemical kinetics, the field of chemistry that studies the rates and mechanisms of chemical reactions, began in the second half of the 19th century. The foundation of this discipline was laid in the 1880s. Dutch physical chemist Jacob van't Hoff and Swedish scientist Svante Arrhenius.
CATALYSIS. It has long been noted that some substances are capable of significantly increasing the rate of a chemical reaction, although they themselves do not change their chemical composition. Such substances are called catalysts. For example, hydrogen peroxide decomposes slowly at room temperature: 2H 2 0 2 \u003d 2H 2 0 + 0 2. In the presence of platinum, the rate of its decomposition increases by more than 2000 times, and the catalase enzyme (found in the blood) increases the reaction rate by 90 billion times!
The catalyst is not consumed in the chemical process. It is included in the intermediate stages of the process and regenerates at the very end. Therefore, the reaction equation itself does not include it.
The world of catalysts is wide and varied, as are their modes of action. But in general, it can be said that the catalyst, being included in the reaction mechanism, changes it and directs the process along an energetically more favorable path. At the same time, which is especially important, catalysts can cause such processes to occur at a noticeable rate, which practically do not occur without them.
Each catalyst can accelerate only certain types of reactions, and in some cases only certain reactions. This feature of catalysts is called selectivity (selectivity). The selectivity of the action of catalysts makes it possible to obtain only a certain desired product in a certain way: to “direct” the action of the drug, etc. Biological catalysts are distinguished by the highest selectivity and efficiency - enzymes that catalyze biochemical reactions in living organisms.
There are substances that slow down or even stop chemical processes. They are called inhibitors. However, unlike catalysts, inhibitors are consumed during the course of the reaction.
- On what factors do the rates of chemical reactions depend?
- Can the rate of any reaction be proportional to the square of the concentration of any substance? If yes, please provide examples.
- Suggest a hypothesis explaining why, unlike catalysts, inhibitors are consumed during the reaction.
1. What is called the rate of a chemical reaction? In what units is it measured? What factors does it depend on?
2. Compare the concepts of "speed of movement" and "speed of a chemical reaction." What do they have in common? 
3. What two classifications of reactions according to the state of aggregation of the reactants and according to the participation of the catalyst in them can you suggest? Give examples of such reactions, write down their equations. 
4. Formulate the law of mass action. What kind of reactions does it apply to? 
5. Formulate Van't Hoff's law.
6. What are catalysts? What groups can they be divided into? Where can inhibitors be used most effectively? 
7. What are enzymes? Compare them with inorganic catalysts. List the uses of enzymes. 
8. Why is there a violent “boiling” of hydrogen peroxide when treating cuts and other wounds?
9. Dry chlorine is stored in iron cylinders. Wet chlorine destroys iron. What role does water play in this process?
10. Substances were taken for the reaction at a temperature of 40°C. Then they were heated to a temperature of 70°C. How will the rate of a chemical reaction change if its temperature coefficient is 2? 
11. Write down an equation that reflects the law of mass action for reactions whose equations are:
a) 2NO+O₂↔2NO₂;
b) I₂+H₂↔2HI 
12. Why is food stored in refrigerators? 
What is viscosity and what factors does it depend on? In what units is viscosity measured?
viscosity- one of the phenomena of transfer, the property of fluid bodies (liquids and gases) to resist the movement of one of their parts relative to another. As a result, the work expended on this movement is dissipated in the form of heat.
The mechanism of internal friction in liquids and gases lies in the fact that randomly moving molecules transfer momentum from one layer to another, which leads to the alignment of velocities - this is described by the introduction of a friction force.
Viscosity depends on the composition and structure of the liquid, as well as temperature and pressure. In order to take into account the influence of the composition, it is necessary to choose a common temperature for comparing liquids. Due to the diverse temperature range of their existence and the different dependence of the viscosity of liquids on temperature, it is impossible and difficult to find such a temperature for all liquids even for liquids with similar composition.
Distinguish dynamic viscosity (unit in international system units (SI) - Pa s, in the CGS system - poise; 1 Pa s = 10 poise) and kinematic viscosity (SI unit - m/s, CGS - stokes, off-system unit - degree Engler). Kinematic viscosity can be obtained as the ratio of dynamic viscosity to the density of a substance and owes its origin to the classical methods of measuring viscosity, such as measuring the time it takes a given volume to flow through a calibrated orifice under the influence of gravity. A device for measuring viscosity is called a viscometer.
