In life we encounter different chemical reactions. Some of them, like the rusting of iron, can last for several years. Others, such as fermenting sugar into alcohol, take several weeks. Firewood in a stove burns in a couple of hours, and gasoline in an engine burns in a split second.
To reduce equipment costs, chemical plants increase the speed of reactions. And some processes, for example, damage food products, corrosion of metals - needs to be slowed down.
Speed chemical reaction can be expressed as change in the amount of matter (n, modulo) per unit of time (t) - compare the speed of a moving body in physics as a change in coordinates per unit of time: υ = Δx/Δt. So that the speed does not depend on the volume of the vessel in which the reaction takes place, we divide the expression by the volume of the reacting substances (v), i.e. we get change in the amount of a substance per unit time per unit volume, or change in the concentration of one of the substances per unit time:
n 2 − n 1 Δn
υ = –––––––––– = –––––––– = Δс/Δt (1)
(t 2 − t 1) v Δt v
where c = n / v is the concentration of the substance,
Δ (read “delta”) is a generally accepted designation for a change in value.
If in the equation for substances different odds, the reaction rate for each of them, calculated using this formula, will be different. For example, 2 moles of sulfur dioxide reacted completely with 1 mole of oxygen in 10 seconds in 1 liter:
2SO2 + O2 = 2SO3
The oxygen rate will be: υ = 1: (10 1) = 0.1 mol/l s
Speed for sulfur dioxide: υ = 2: (10 1) = 0.2 mol/l s- this does not need to be memorized and said during the exam, the example is given so as not to get confused if this question arises.
The rate of heterogeneous reactions (involving solids) is often expressed per unit area of contacting surfaces:
Δn
υ = –––––– (2)
Δt S
Reactions are called heterogeneous when the reactants are in different phases:
- a solid with another solid, liquid or gas,
- two immiscible liquids
- liquid with gas.
Homogeneous reactions occur between substances in one phase:
- between well-mixed liquids,
- gases,
- substances in solutions.
Conditions affecting the rate of chemical reactions
1) The reaction speed depends on nature of reactants. Simply put, different substances react at different speeds. For example, zinc reacts violently with hydrochloric acid, while iron reacts rather slowly.
2) The higher the reaction speed, the faster concentration substances. Zinc will react much longer with a highly dilute acid.
3) The reaction speed increases significantly with increasing temperature. For example, for fuel to burn, it must be ignited, i.e., the temperature must be increased. For many reactions, a 10°C increase in temperature is accompanied by a 2- to 4-fold increase in rate.
4) Speed heterogeneous reactions increases with increasing surfaces of reacting substances. Solids are usually ground for this purpose. For example, in order for iron and sulfur powders to react when heated, the iron must be in the form of fine sawdust.
Please note that in in this case formula (1) is implied! Formula (2) expresses the speed per unit area, therefore it cannot depend on the area.
5) The rate of reaction depends on the presence of catalysts or inhibitors.
Catalysts- substances that accelerate chemical reactions, but are not consumed themselves. An example is the rapid decomposition of hydrogen peroxide with the addition of a catalyst - manganese (IV) oxide:
2H 2 O 2 = 2H 2 O + O 2
Manganese(IV) oxide remains at the bottom and can be reused.
Inhibitors- substances that slow down the reaction. For example, corrosion inhibitors are added to a water heating system to extend the life of pipes and batteries. In cars, corrosion inhibitors are added to brake and coolant fluid.
A few more examples.
Purpose of the work: study of the rate of a chemical reaction and its dependence on various factors: nature of reactants, concentration, temperature.
Chemical reactions occur at different rates. Speed of chemical reaction is called the change in the concentration of a reactant per unit time. It is equal to the number of interaction events per unit time per unit volume for a reaction occurring in a homogeneous system (for homogeneous reactions), or per unit interface surface for reactions occurring in a heterogeneous system (for heterogeneous reactions).
Average speed reactions v avg. in the time interval from t 1 to t 2 is determined by the relation:
Where C 1 And C 2– molar concentration of any reaction participant at time points t 1 And t 2 respectively.
The “–” sign before the fraction refers to concentration starting materials, Δ WITH < 0, знак “+” – к концентрации продуктов реакции, ΔWITH > 0.
The main factors influencing the rate of a chemical reaction: the nature of the reactants, their concentration, pressure (if gases are involved in the reaction), temperature, catalyst, interface area for heterogeneous reactions.
Most chemical reactions are complex processes occurring in several stages, i.e. consisting of several elementary processes. Elementary or simple reactions are reactions that occur in one step.
For elementary reactions, the dependence of the reaction rate on concentration is expressed by the law of mass action.
At constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, taken in powers equal to the stoichiometric coefficients.
For reaction in general view
a A + b B… → c C,
according to the law of mass action v expressed by the ratio
v = К∙с(А) а ∙ с(В) b,
Where c(A) And s(B)– molar concentrations of reactants A and B;
TO– rate constant of this reaction, equal to v, If c(A)a=1 and c(B)b=1, and depending on the nature of the reactants, temperature, catalyst, and interface area for heterogeneous reactions.
The expression of the reaction rate as a function of concentration is called the kinetic equation.
In the case of complex reactions, the law of mass action applies to each individual stage.
For heterogeneous reactions, the kinetic equation includes only the concentrations of gaseous and dissolved substances; yes, for burning coal
C (k) + O 2 (g) → CO 2 (g)
the velocity equation has the form
v = K∙s(O 2)
A few words about the molecularity and kinetic order of the reaction.
Concept "molecularity of reaction" apply only to simple reactions. The molecularity of a reaction characterizes the number of particles participating in an elementary interaction.
There are mono-, bi- and trimolecular reactions, in which one, two and three particles participate, respectively. The probability of three particles colliding simultaneously is small. The elementary process of interaction of more than three particles is unknown. Examples of elementary reactions:
N 2 O 5 → NO + NO + O 2 (monomolecular)
H 2 + I 2 → 2HI (bimolecular)
2NO + Cl 2 → 2NOCl (trimolecular)
The molecularity of simple reactions coincides with the general kinetic order of the reaction. The order of the reaction determines the nature of the dependence of rate on concentration.
The general (total) kinetic order of a reaction is the sum of the exponents at the concentrations of the reactants in the reaction rate equation, determined experimentally.
As temperature increases, the rate of most chemical reactions increases. The dependence of the reaction rate on temperature is approximately determined by the Van't Hoff rule.
For every 10-degree increase in temperature, the rate of most reactions increases by 2–4 times.
where and are the reaction rate, respectively, at temperatures t 2 And t 1 (t 2 >t 1);
γ is the temperature coefficient of the reaction rate, this is a number showing how many times the rate of a chemical reaction increases when the temperature increases by 10 0.
Using Van't Hoff's rule, it is only possible to approximately estimate the effect of temperature on the reaction rate. A more accurate description of the dependence of the temperature reaction rate is feasible within the framework of the Arrhenius activation theory.
One of the methods of accelerating a chemical reaction is catalysis, which is carried out using substances (catalysts).
Catalysts- these are substances that change the rate of a chemical reaction due to repeated participation in intermediate chemical interactions with reaction reagents, but after each cycle of intermediate interaction they restore their chemical composition.
The mechanism of action of the catalyst is reduced to a decrease in the activation energy of the reaction, i.e. reducing the difference between the average energy of active molecules (active complex) and the average energy of molecules of the starting substances. The rate of the chemical reaction increases.
The mechanisms of chemical transformations and their rates are studied by chemical kinetics. Chemical processes occur over time at different rates. Some happen quickly, almost instantly, while others take a very long time to occur.
Reaction speed- the rate at which reagents are consumed (their concentration decreases) or reaction products are formed per unit volume.
Factors that can influence the rate of a chemical reaction
The following factors can affect how quickly a chemical reaction occurs:
- concentration of substances;
- nature of reagents;
- temperature;
- presence of a catalyst;
- pressure (for reactions in a gas environment).
Thus, by changing certain conditions of a chemical process, you can influence how quickly the process will proceed.
In the process of chemical interaction, particles of reacting substances collide with each other. The number of such coincidences is proportional to the number of particles of substances in the volume of the reacting mixture, and therefore proportional to the molar concentrations of the reagents.
Law of mass action describes the dependence of the reaction rate on the molar concentrations of the substances that interact.
For an elementary reaction (A + B → ...) this law is expressed by the formula:
υ = k ∙С A ∙С B,
where k is the rate constant; C A and C B are the molar concentrations of reagents A and B.
If one of the reacting substances is in a solid state, then the interaction occurs at the interface; therefore, the concentration of the solid substance is not included in the equation of the kinetic law of mass action. To understand the physical meaning of the rate constant, it is necessary to take C, A and C B equal to 1. Then it becomes clear that the rate constant is equal to the reaction rate at reactant concentrations equal to unity.
Nature of the reagents
Since in the process of interaction they are destroyed chemical bonds reactants and new bonds of reaction products are formed, then the nature of the bonds involved in the reaction of the compounds and the structure of the molecules of the reacting substances will play a large role.
Surface area of contact of reagents
Such a characteristic as the surface area of contact of solid reagents affects the course of the reaction, sometimes quite significantly. Grinding a solid allows you to increase the surface area of contact of the reagents, and therefore speed up the process. The contact area of soluble substances is easily increased by dissolving the substance.
Reaction temperature
As the temperature increases, the energy of colliding particles will increase; it is obvious that with increasing temperature the chemical process will speed up. A clear example of how an increase in temperature affects the process of interaction of substances can be considered the data given in the table.
Table 1. Effect of temperature changes on the rate of water formation (O 2 +2H 2 →2H 2 O)
To quantitatively describe how temperature can affect the rate of interaction of substances, the Van't Hoff rule is used. Van't Hoff's rule is that when the temperature increases by 10 degrees, an acceleration occurs by 2-4 times.
The mathematical formula describing van't Hoff's rule is as follows:
Where γ is the temperature coefficient of the rate of the chemical reaction (γ = 2−4).
But the Arrhenius equation describes the temperature dependence of the rate constant much more accurately:
Where R is the universal gas constant, A is a multiplier determined by the type of reaction, E, A is the activation energy.
Activation energy is the energy that a molecule must acquire for a chemical transformation to occur. That is, it is a kind of energy barrier that molecules colliding in the reaction volume will need to overcome in order to redistribute bonds.
The activation energy does not depend on external factors, but depends on the nature of the substance. The activation energy value of up to 40 - 50 kJ/mol allows substances to react with each other quite actively. If the activation energy exceeds 120 kJ/mol, then the substances (at ordinary temperatures) will react very slowly. A change in temperature leads to a change in the number of active molecules, that is, molecules that have reached an energy greater than the activation energy, and therefore are capable of chemical transformations.
Catalyst action
A catalyst is a substance that can speed up a process, but is not part of its products. Catalysis (acceleration of a chemical transformation) is divided into homogeneous and heterogeneous. If the reactants and catalyst are in the same states of aggregation, then catalysis is called homogeneous, if different, then heterogeneous. The mechanisms of action of catalysts are varied and quite complex. In addition, it is worth noting that catalysts are characterized by selectivity of action. That is, the same catalyst, while accelerating one reaction, may not change the rate of another.
Pressure
If gaseous substances are involved in the transformation, then the rate of the process will be affected by changes in pressure in the system . This happens because that for gaseous reagents, a change in pressure leads to a change in concentration.
Experimental determination of the rate of a chemical reaction
The speed of a chemical transformation can be determined experimentally by obtaining data on how the concentration of substances entering the reaction or products changes per unit time. Methods for obtaining such data are divided into
- chemical,
- physico-chemical.
Chemical methods are quite simple, accessible and accurate. With their help, the speed is determined by directly measuring the concentration or amount of the substance of the reactants or products. In case of a slow reaction, samples are taken to monitor how the reagent is consumed. Then the content of the reagent in the sample is determined. By taking samples at regular intervals, it is possible to obtain data on changes in the amount of a substance during the interaction process. The most commonly used types of analysis are titrimetry and gravimetry.
If the reaction proceeds quickly, then it has to be stopped in order to take a sample. This can be done using cooling, abrupt removal of the catalyst, it is also possible to dilute or transfer one of the reagents to a non-reactive state.
Methods of physicochemical analysis in modern experimental kinetics are used more often than chemical ones. With their help, you can observe changes in the concentrations of substances in real time. In this case, there is no need to stop the reaction and take samples.
Physicochemical methods are based on measurement physical properties, depending on the quantitative content of a certain compound in the system and changing over time. For example, if gases are involved in a reaction, then pressure may be such a property. Electrical conductivity, refractive index, and absorption spectra of substances are also measured.
Like any processes, chemical reactions occur over time and are therefore characterized by one or another speed.
The branch of chemistry that studies the rate of chemical reactions and the mechanism of their occurrence, called chemical kinetics. Chemical kinetics operates with the concepts of “phase” and “system”. Phase – it is a part of a system separated from its other parts by an interface.
Systems can be homogeneous or heterogeneous. Homogeneous systems consist of single phase. For example, air or any mixture of gases, salt solution. Heterogeneous systems consist of two or more phases. For example, liquid water – ice – steam, salt solution + sediment.
Reactions occurring in a homogeneous system, are called homogeneous. For example, N 2 (g) + 3H 2 (g) = 2NH 3 (g). They flow throughout. Reactions occurring in a heterogeneous system, are called heterogeneous. For example, C (k) + O 2 (g) = CO 2 (g). They flow at the phase interface.
Chemical reaction rate determined the amount of substance that reacts or is formed during a reaction per unit time per unit volume(for homogeneous reaction) or per unit interface(for a heterogeneous system).
The reaction rate depends on the nature of the reactants, their concentration, temperature, and the presence of catalysts.
1. The nature of the reacting substances.
Reactions proceed in the direction of destruction of less strong bonds and the formation of substances with stronger bonds. Thus, breaking bonds in H 2 and N 2 molecules requires high energies; such molecules are slightly reactive. Breaking bonds in highly polar molecules (HCl, H 2 O) requires less energy, and the reaction rate is much higher. Reactions between ions in electrolyte solutions occur almost instantly.
2. Concentration.
As the concentration increases, collisions of molecules of reacting substances occur more often - the reaction rate increases.
The dependence of the rate of a chemical reaction on the concentration of reactants is expressed law of mass action (LMA): at constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances.
In general, for homogeneous reactions
nA (g) + mB (g) = pAB (g)
the reaction rate dependence is expressed by the equation:
where C A and C B are the concentrations of reactants, mol/l; k is the reaction rate constant. For a specific reaction 2NO (g) + O 2 (g) = 2NO 2 (g), the mathematical expression for the ZDM is:
υ = k∙∙
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 (when substances are in different states of aggregation), the reaction rate depends only on the concentration of gases or dissolved substances, and the concentration of the solid phase is not included in the mathematical expression of EDM:
nA (k) + mB (g) = pAB (g)
For example, the rate of combustion of carbon in oxygen is proportional only to the oxygen concentration:
C (k) + O 2 (g) = CO 2 (k)
3. Temperature.
As the temperature increases, the speed of movement of molecules increases, which in turn leads to an increase in the number of collisions between them. For a reaction to take place, the colliding molecules must have a certain excess energy. The excess energy that molecules must have before their collision can lead to the formation of a new substance, called activation energy. Activation energy ( E a) are expressed in kJ/mol. Its value depends on the nature of the reacting substances, i.e. Each reaction has its own activation energy. Molecules with activation energy, called active. Increasing the temperature increases the number of active molecules, and therefore increases the rate of the chemical reaction.
The dependence of the rate of a chemical reaction on temperature is expressed van't Hoff's rule: for every 10 °C increase in temperature, the reaction rate increases by 2-4 times.
where υ 2 and υ 1 are reaction rates at temperatures t 2 and t 1,
γ is the temperature coefficient of the reaction rate, showing how many times the reaction rate increases when the temperature increases by 10 0 C.
4. Contact surface of reacting substances.
For heterogeneous systems, than more surface contact, the faster the reaction occurs. The surface area of solids can be increased by grinding them, and for soluble substances by dissolving them.
5. Catalysts.
Substances that participate in reactions and increase its speed, remaining unchanged at the end of the reaction, are called catalysts. The change in reaction rate under the influence of catalysts is called catalysis. There are catalysis homogeneous And heterogeneous.
TO homogeneous These include processes in which the catalyst is in the same state of aggregation as the reactants.
2SO 2 (g) + O 2 (g) 2SO 3 (g)
The action of a homogeneous catalyst is to form more or less strong intermediate active compounds, from which it is then completely regenerated.
TO heterogeneous Catalysis refers to processes in which the catalyst and reactants are in different states of aggregation, and the reaction occurs on the surface of the catalyst.
N 2(g) + 3H 2(g) 2NH 3(g)
The mechanism of action of heterogeneous catalysts is more complex than homogeneous ones. A significant role in these processes is played by the phenomena of absorption of gaseous and liquid substances on the surface of a solid substance - the phenomenon of adsorption. As a result of adsorption, the concentration of reacting substances increases, their chemical activity increases, which leads to an increase in the reaction rate.
Physical chemistry: lecture notes Berezovchuk A V
2. Factors affecting the rate of a chemical reaction
For homogeneous, heterogeneous reactions:
1) concentration of reacting substances;
2) temperature;
3) catalyst;
4) inhibitor.
Only for heterogeneous:
1) the rate of supply of reacting substances to the phase interface;
2) surface area.
The main factor is the nature of the reactants - the nature of the bonds between atoms in the molecules of the reactants.
NO 2 – nitrogen oxide (IV) – fox tail, CO – carbon monoxide, carbon monoxide.
If they are oxidized with oxygen, then in the first case the reaction will occur instantly, as soon as you open the cap of the vessel, in the second case the reaction is extended over time.
The concentration of reactants will be discussed below.
Blue opalescence indicates the moment of sulfur precipitation; the higher the concentration, the higher the speed.
Rice. 10
The higher the concentration of Na 2 S 2 O 3, the less time the reaction takes. The graph (Fig. 10) shows a directly proportional relationship. The quantitative dependence of the reaction rate on the concentration of the reacting substances is expressed by the LMA (law of mass action), which states: the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances.
So, basic law of kinetics is an experimentally established law: the rate of a reaction is proportional to the concentration of the reactants, example: (i.e. for a reaction)
For this reaction H 2 + J 2 = 2HJ – the rate can be expressed in terms of a change in the concentration of any of the substances. If the reaction proceeds from left to right, then the concentration of H 2 and J 2 will decrease, and the concentration of HJ will increase as the reaction progresses. For the instantaneous reaction rate, we can write the expression:
square brackets indicate concentration.
Physical meaning k– molecules are in continuous motion, collide, fly apart, and hit the walls of the vessel. In order for the chemical reaction to form HJ to occur, the H2 and J2 molecules must collide. The number of such collisions will be greater, the more molecules of H 2 and J 2 are contained in the volume, i.e., the greater the values [H 2 ] and . But the molecules move at different speeds, and the total kinetic energy of the two colliding molecules will be different. If the fastest molecules H 2 and J 2 collide, their energy can be so high that the molecules break into atoms of iodine and hydrogen, which fly apart and then interact with other molecules H 2 + J 2 ? 2H+2J, then H + J 2 ? HJ + J. If the energy of the colliding molecules is less, but high enough to weaken the H – H and J – J bonds, the formation reaction of hydrogen iodide will occur:
For most colliding molecules, the energy is less than that required to weaken the bonds in H 2 and J 2. Such molecules will “quietly” collide and also “quietly” disperse, remaining what they were, H 2 and J 2. Thus, not all, but only part of the collisions lead to a chemical reaction. The proportionality coefficient (k) shows the number of effective collisions leading to a collision reaction at concentrations [H 2 ] = 1 mol. Magnitude k–const speed. How can speed be constant? Yes, uniform speed rectilinear movement called a constant vector quantity, equal to the ratio movement of a body over any period of time to the value of this interval. But molecules move chaotically, then how can the speed be const? But constant speed can only be done at a constant temperature. With increasing temperature, the proportion of fast molecules whose collisions lead to a reaction increases, i.e., the rate constant increases. But the increase in the rate constant is not unlimited. At a certain temperature, the energy of the molecules will become so great that almost all collisions of the reactants will be effective. When two fast molecules collide, a reverse reaction will occur.
There will come a moment when the rates of formation of 2HJ from H 2 and J 2 and decomposition will be equal, but this is already chemical equilibrium. The dependence of the reaction rate on the concentration of the reactants can be traced using the traditional reaction of interaction of a solution of sodium thiosulfate with a solution of sulfuric acid.
Na 2 S 2 O 3 + H 2 SO 4 = Na 2 SO 4 + H 2 S 2 O 3, (1)
H 2 S 2 O 3 = S? + H 2 O + SO 2?. (2)
Reaction (1) occurs almost instantly. The rate of reaction (2) depends at a constant temperature on the concentration of the reactant H 2 S 2 O 3. This is exactly the reaction we observed - in this case, the speed is measured by the time from the beginning of the solutions to merge until the appearance of opalescence. In the article L. M. Kuznetsova The reaction of sodium thiosulfate with hydrochloric acid is described. She writes that when solutions are drained, opalescence (turbidity) occurs. But this statement by L.M. Kuznetsova is erroneous since opalescence and turbidity are two different things. Opalescence (from opal and Latin escentia– suffix meaning weak effect) – scattering of light by turbid media due to their optical inhomogeneity. Light scattering– deviation of light rays propagating in a medium in all directions from the original direction. Colloidal particles are capable of scattering light (Tyndall-Faraday effect) - this explains opalescence, a slight turbidity of the colloidal solution. When carrying out this experiment, it is necessary to take into account the blue opalescence, and then the coagulation of the colloidal suspension of sulfur. The same density of the suspension is noted by the visible disappearance of any pattern (for example, a grid on the bottom of a cup) observed from above through the layer of solution. Time is counted using a stopwatch from the moment of draining.
Solutions of Na 2 S 2 O 3 x 5H 2 O and H 2 SO 4.
The first is prepared by dissolving 7.5 g of salt in 100 ml of H 2 O, which corresponds to a 0.3 M concentration. To prepare a solution of H 2 SO 4 of the same concentration, you need to measure 1.8 ml of H 2 SO 4 (k), ? = = 1.84 g/cm 3 and dissolve it in 120 ml of H 2 O. Pour the prepared Na 2 S 2 O 3 solution into three glasses: 60 ml in the first, 30 ml in the second, 10 ml in the third. Add 30 ml of distilled H 2 O to the second glass, and 50 ml to the third glass. Thus, in all three glasses there will be 60 ml of liquid, but in the first the salt concentration is conditionally = 1, in the second – ½, and in the third – 1/6. After the solutions have been prepared, pour 60 ml of H 2 SO 4 solution into the first glass with a salt solution and turn on the stopwatch, etc. Considering that the reaction rate drops with dilution of the Na 2 S 2 O 3 solution, it can be determined as a quantity inversely proportional to time v = 1/? and construct a graph, plotting the concentration on the abscissa axis, and the reaction rate on the ordinate axis. The conclusion from this is that the reaction rate depends on the concentration of substances. The data obtained are listed in Table 3. This experiment can be performed using burettes, but this requires a lot of practice from the performer, because the graph may be incorrect.
Table 3
Speed and reaction time
The Guldberg-Waage law is confirmed - professor of chemistry Gulderg and young scientist Waage).
Let's consider the next factor - temperature.
As temperature increases, the rate of most chemical reactions increases. This dependence is described by Van't Hoff's rule: “For every 10 °C increase in temperature, the rate of chemical reactions increases by 2 to 4 times.”
Where ? – temperature coefficient showing how many times the reaction rate increases when the temperature increases by 10 °C;
v 1 – reaction rate at temperature t 1 ;
v 2 – reaction rate at temperature t2.
For example, a reaction at 50 °C takes two minutes, how long will it take for the process to complete at 70 °C if the temperature coefficient ? = 2?
t 1 = 120 s = 2 min; t 1 = 50 °C; t 2 = 70 °C.
Even a slight increase in temperature causes sharp increase reaction rates of active collisions of molecules. According to activation theory, only those molecules whose energy is greater than the average energy of molecules by a certain amount participate in the process. This excess energy is activation energy. Its physical meaning is the energy that is necessary for the active collision of molecules (orbital rearrangement). The number of active particles, and therefore the reaction rate, increases with temperature according to an exponential law, according to the Arrhenius equation, which reflects the dependence of the rate constant on temperature
Where A - Arrhenius proportionality coefficient;
k– Boltzmann's constant;
E A – activation energy;
R – gas constant;
T- temperature.
A catalyst is a substance that accelerates the rate of a reaction without being consumed.
Catalysis– the phenomenon of changing the reaction rate in the presence of a catalyst. There are homogeneous and heterogeneous catalysis. Homogeneous– if the reagents and the catalyst are in the same state of aggregation. Heterogeneous– if the reagents and catalyst are in different states of aggregation. About catalysis, see separately (further).
Inhibitor– a substance that slows down the rate of reaction.
The next factor is surface area. The larger the surface area of the reactant, the more speed. Let's look at an example of the effect of the degree of dispersion on the reaction rate.
CaCO 3 – marble. We will lower the tile marble into hydrochloric acid HCl, wait five minutes, it will dissolve completely.
Powdered marble - we will do the same procedure with it, it will dissolve in thirty seconds.
The equation for both processes is the same.
CaCO 3 (solid) + HCl (g) = CaCl 2 (solid) + H 2 O (liquid) + CO 2 (g) ?.
So, when adding powdered marble, the time is less than when adding slab marble, for the same mass.
With an increase in the interface surface, the rate of heterogeneous reactions increases.
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