Factors that affect the rate of a reaction. The rate of chemical reactions
Name at least 5 factors that affect the rate of a chemical reaction.
- Temperature, pressure, fineness (contact area), presence of catalysts, concentration of reactants
- nightmares)))))))))))) 0
until you read a hundred times pissed off - tin
- 1 catalyst
2 valency
3 chem. activity to other elements
4 impurities
5 temperature
6 external influence - Factors affecting the rate of chemical reactions.
1. The nature of the reactants. An important role is played by the nature of chemical bonds and the structure of the molecules of the reagents. Reactions proceed in the direction of the destruction of less strong bonds and the formation of substances with stronger bonds. For example, high energies are required to break bonds in H2 and N2 molecules; such molecules are not very reactive. To break bonds in highly polar molecules (HCl, H2O), less energy is required, and the reaction rate is much higher. Reactions between ions in electrolyte solutions proceed almost instantaneously.
Fluorine reacts explosively with hydrogen at room temperature; bromine reacts with hydrogen slowly even when heated.
Calcium oxide reacts vigorously with water, releasing heat; copper oxide - does not react.
2. Concentration. With an increase in concentration (the number of particles per unit volume), collisions of molecules of reacting substances occur more often - the reaction rate increases.
The law of 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.
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 reaction rate expression.
3. Temperature. With an increase in temperature for every 10C, the reaction rate increases by 2-4 times (Van't Hoff's Rule). As the temperature increases from t1 to t2, the change in the reaction rate can be calculated using the formula:
(t2 - t1) / 10
(where Vt2 and Vt1 are the reaction rates at temperatures t2 and t1, respectively; g is the temperature coefficient of this reaction).
Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation:
A is a constant depending on the nature of the reactants;
R is the universal gas constant 8.314 J / (mol K) \u003d 0.082 l atm / (mol K);
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.
exothermic reaction
Endothermic 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. The contact surface of the reactants. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction proceeds. The surface 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 rate, remaining unchanged by 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 make up one phase (they are in the same state of aggregation), while in heterogeneous catalysis they are different phases (they are in different states of aggregation). In some cases, the course of undesirable chemical processes can be drastically slowed down by adding inhibitors to the reaction medium (the phenomenon of "negative catalysis").
Question number 3
What factors affect the rate constant of a chemical reaction?
Reaction rate constant (specific reaction rate) is the coefficient of proportionality in the kinetic equation.
The physical meaning of the reaction rate constant k follows from the equation of the law of mass action: k numerically equal to the reaction rate at a concentration of each of the reactants equal to 1 mol / l.
The reaction rate constant depends on the temperature, on the nature of the reactants, on the presence of a catalyst in the system, but does not depend on their concentration.
1. Temperature. With an increase in temperature for every 10 ° C, the reaction rate increases by 2-4 times (Van't Hoff's Rule). With an increase in temperature from t1 to t2, the change in the reaction rate can be calculated by the formula: (t2 - t1) / 10 Vt2 / Vt1 = g (where Vt2 and Vt1 are the reaction rates at temperatures t2 and t1, respectively; g is the 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 reactants; 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. Exothermic reaction Endothermic 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. 2. The contact surface of the reactants. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction proceeds. The surface of solids can be increased by grinding them, and for soluble substances by dissolving them. 3. Catalysis. Substances that participate in reactions and increase its rate, remaining unchanged by 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 make up one phase (they are in the same state of aggregation), while in heterogeneous catalysis they are different phases (they are in different states of aggregation). In some cases, the course of undesirable chemical processes can be drastically slowed down by adding inhibitors to the reaction medium (the phenomenon of "negative catalysis").
Question number 4
Formulate and write down the law of mass action for the reaction:
2 NO+O2=2NO2
LAW OF MASS ACTION: The rate of a chemical reaction is proportional to the product of the concentrations of the reactants. for the reaction 2NO + O2 2NO2, the law of mass action will be written as follows: v=kС2(NO)·С(O2), where k is the rate constant, depending on the nature of the reactants and temperature. The rate in reactions involving solids is determined only by the concentration of gases or dissolved substances: C + O2 \u003d CO2, v \u003d kCO2
The rate of a chemical reaction depends on the nature of the reactants and the reaction conditions: concentration c, temperature /> t />, /> the presence of catalysts, as well as on some other factors (for example, pressure for gas reactions, grinding for solids , from radioactive exposure)./>
Influence of concentrations of reactants./> In order for substances A and B to interact chemically, their molecules (particles) must collide. The more collisions, the faster the reaction proceeds. The number of collisions is the greater, the higher the concentration of reactants. Hence, on the basis of extensive experimental material, the basic law of chemical kinetics is formulated, which establishes the dependence of the reaction rate on the concentration of reactants:
The rate of a chemical reaction is proportional to the product of the concentrations of the reactants.
For the reaction (/> I/>) this law is expressed by the equation />
v/> = />kc A /> />c B /> , /> (1)/>
where c A and c B are the concentrations of substances A and B, mol/l; />k/> - />coefficient/> proportionality, called the rate constant of the reaction. The basic law of chemical kinetics is often referred to as law of acting masses. />
From equation (1) it is easy to establish physical meaning rate constant />k/> : it is numerically equal to the reaction rate when the concentrations of each of the reactants are 1 mol/l or when their product is equal to unity./>
The reaction rate constant />k/> />depends on the nature of the reactants and on the temperature, but does not depend on their concentrations./>
Equation (1), which relates the reaction rate to the concentration of reactants, is called reaction kinetic equation. If the kinetic equation of the reaction is experimentally determined, then with its help it is possible to calculate the rates at other concentrations of the same reactants.
Temperature influence/> ./>
The dependence of the reaction rate on temperature is determined van't Hoff's rule:/>
For every 10° increase in temperature, the rate of most reactions increases by a factor of 2-4.
Mathematically, this dependence is expressed by the relation />
v t />/> 2/> = />v t /> 1/> γ/> , />
where />v t /> 1/> />, />v t /> 2/> - /> the reaction rate, respectively, at the initial (/> t/> 1/>) and final (/> t/> 2/>) temperatures, and />γ/> - /> the temperature coefficient of the reaction rate, which shows how many times the reaction rate increases with an increase in the temperature of the reacting substances by 10 °./>
The van't Hoff rule is approximate and is applicable only for an approximate assessment of the effect of temperature on the reaction rate. Temperature affects the rate of a chemical reaction by increasing the rate constant.
In life, we are faced with different chemical reactions. Some of them, like the rusting of iron, can go on for several years. Others, such as the fermentation of sugar into alcohol, take several weeks. Firewood in the stove burns out in a couple of hours, and gasoline in the engine burns out in a split second.
To reduce equipment costs, chemical plants increase the rate of reactions. And some processes, such as food spoilage, metal corrosion, need to be slowed down.
The rate of a chemical reaction can be expressed as change in the amount of matter (n, modulo) per unit time (t) - compare the speed of a moving body in physics as a change in coordinates per unit time: υ = Δx/Δt . So that the rate does not depend on the volume of the vessel in which the reaction takes place, we divide the expression by the volume of reacting substances (v), i.e., we obtain 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
υ = –––––––––– = –––––––– = Δс/Δt (1)
(t 2 − t 1) v Δt vwhere c = n / v is the concentration of the substance,
Δ (pronounced "delta") is the generally accepted designation for a change in magnitude.
If substances have different coefficients in the equation, the reaction rate for each of them, calculated by 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:
2SO 2 + O 2 \u003d 2SO 3
The oxygen velocity will be: υ \u003d 1: (10 1) \u003d 0.1 mol / l s
Sour gas speed: υ \u003d 2: (10 1) \u003d 0.2 mol / l s- this does not need to be memorized and spoken in the exam, an example is given in order 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)
ΔtSReactions are called heterogeneous when the reactants are in different phases:
- a solid with another solid, liquid or gas,
- two immiscible liquids
- gas liquid.
Homogeneous reactions occur between substances in the same phase:
- between well-miscible liquids,
- gases,
- substances in solutions.
Conditions affecting the rate of chemical reactions
1) The reaction rate depends on the nature of the reactants. Simply put, different substances react at different rates. For example, zinc reacts violently with hydrochloric acid, while iron reacts rather slowly.
2) The reaction rate is greater, the higher concentration substances. With a highly dilute acid, the zinc will take significantly longer to react.
3) The reaction rate increases significantly with increasing temperature. For example, in order to burn fuel, it is necessary to set it on fire, that is, to increase the temperature. For many reactions, an increase in temperature by 10°C is accompanied by an increase in the rate by a factor of 2–4.
4) Speed heterogeneous reactions increases with increasing surfaces of reactants. Solids for this are usually crushed. For example, in order for iron and sulfur powders to react when heated, iron must be in the form of small sawdust.
Note that formula (1) is implied in this case! Formula (2) expresses the speed per unit area, therefore it cannot depend on the area.
5) The reaction rate depends on the presence of catalysts or inhibitors.
Catalysts Substances that speed up chemical reactions but are not themselves consumed. An example is the rapid decomposition of hydrogen peroxide with the addition of a catalyst - manganese (IV) oxide:
2H 2 O 2 \u003d 2H 2 O + O 2
Manganese (IV) oxide remains on the bottom and can be reused.
Inhibitors- substances that slow down the reaction. For example, to extend the life of pipes and batteries, corrosion inhibitors are added to the water heating system. In automobiles, corrosion inhibitors are added to the brake fluid.
A few more examples.
Chemical Methods
Physical Methods
Methods for measuring the reaction rate
In the example above, the reaction rate between calcium carbonate and acid was measured by studying the volume of gas evolved as a function of time. Experimental data on reaction rates can be obtained by measuring other quantities.
If during the course of the reaction the total amount of gaseous substances changes, then its course can be observed by measuring the pressure of the gas at a constant volume. In cases where one of the starting materials or one of the reaction products is colored, the progress of the reaction can be monitored by observing the change in color of the solution. Another optical method is to measure the rotation of the plane of polarization of light (if the initial substances and reaction products have different rotational abilities).
Some reactions are accompanied by a change in the number of ions in the solution. In such cases, the reaction rate can be studied by measuring the electrical conductivity of the solution. In the next chapter some other electrochemical methods that can be used to measure reaction rates will be discussed.
The progress of the reaction can be monitored by measuring the concentration of one of the participants in the reaction over time using a variety of chemical analysis methods. The reaction is carried out in a thermostated vessel. At certain intervals, a sample of the solution (or gas) is taken from the vessel and the concentration of one of the components is determined. To obtain reliable results, it is important that no reaction occurs in the sample taken for analysis. This is achieved by chemical binding of one of the reagents, rapid cooling or dilution of the solution.
Experimental studies show that the reaction rate depends on several factors. Let us first consider the influence of these factors at a qualitative level.
1.The nature of the reactants. From laboratory practice, we know that the neutralization of an acid by a base
H + + OH - ® H 2 O
interaction of salts with the formation of a sparingly soluble compound
Ag + + Cl – ® AgCl
and other reactions in electrolyte solutions occur very quickly. The time required for such reactions to complete is measured in milliseconds and even microseconds. This is quite understandable, because the essence of such reactions is the approach and combination of charged particles with charges of the opposite sign.
In contrast to ionic reactions, the interaction between covalently bonded molecules usually proceeds much more slowly. Indeed, in the course of the reaction between such particles, the bonds in the molecules of the starting substances must break. To do this, colliding molecules must have a certain amount of energy. In addition, if the molecules are complex enough, in order for a reaction to occur between them, they must be oriented in space in a certain way.
2. Reactant concentration. The rate of a chemical reaction, ceteris paribus, depends on the number of collisions of the reacting particles per unit time. The probability of collisions depends on the number of particles per unit volume, i.e. from concentration. Therefore, the reaction rate increases with increasing concentration.
3. The physical state of substances. In homogeneous systems, the reaction rate depends on the number of particle collisions in solution volume(or gas). In heterogeneous systems, chemical interaction occurs at the interface. An increase in the surface area of a solid during its grinding facilitates the access of the reacting particles to the particles of the solid, which leads to a significant acceleration of the reaction.
4. Temperature has a significant impact on the rate of various chemical and biological processes. With an increase in temperature, the kinetic energy of the particles increases, and, consequently, the fraction of particles whose energy is sufficient for chemical interaction increases.
5. Steric factor characterizes the need for mutual orientation of the reacting particles. The more complex the molecules, the lower the probability of their proper orientation, the lower the efficiency of collisions.
6. Availability of catalysts.Catalysts are substances that change the rate of a chemical reaction. Introduced into the reaction system in small amounts and remaining unchanged after the reaction, they are capable of extremely changing the rate of the process.
The main factors on which the reaction rate depends will be discussed in more detail below.
