chemistry lesson

Factors affecting the speed of chemical reactions - CHEMICAL REACTIONS - LESSON PLANS for CHEMISTRY class 11 - lesson plans - lesson plans-author's lessons-lesson plan-lesson summary - chemistry

The purpose of the lesson: to teach you to explain the influence of various factors on the speed of chemical reactions based on the concept of activation energy; to solve computational problems of this topic.

Basic concepts: velocity constant, acting mass law, temperature coefficient, catalyst, and catalysis: homogeneous and heterogeneous.

Equipment: reagents for demonstration experiments: a) Na₂SO₄and ВаСl₂; b) Mg and HCl; Zn and Hcl; C) Na and N₂O; Na and alcohol; d) Mg and Hcl d) h and CH₃COOH; e) HCl, Zn (pellets.), Zn (powder); W) H₂SO₄(p), sodium thiosulfate; S and O₂; CACO₃(chalk), CACO₃(chopped.); h) raw potatoes, boiled (or meat), alcohol, sugars, ash.

Lesson progress

I. Front-end survey

1. To define the speed of homogeneous reaction. Formula for the speed of a homogeneous reaction.

2. To define the speed of heterogeneous reactions. Formula for the rate of heterogeneous reaction.

3. What is the activation energy (E_a)? Why is it important to know the_аamount of substances?

4. Calculation task:

Answer: C_{A kom}= 2.88 mol/l

II. Learning new material

Plan of presentation

1. Factors affecting the rate of chemical reaction.

a) the nature of the reactants,

b) the concentration of the reactants: pressure (for gases)

C) the contact surface of reactants.

d) temperature.

e) catalyst, inhibitors.

2. Solving calculation problems.

The speed of the chemical reaction depends on many factors. The main ones are the nature and concentration of reactants, pressure — if the reaction involves gaseous substances, the contact surface of reactants, temperature, and the action of the catalyst.

We need to consider the influence of each of these factors on the rate of chemical reaction, confirming experimentally.

The first factor — the nature of reacting substances and activation energy are interrelated. If the_а ion Density is 40 kJ/mol, this means that most particle collisions result in their rapid interaction. Ion exchange reactions occur instantly, the rate of such reactions is very high, because the reactions involve anions and cations, which_аhave a very small E and A.

If E_ais very large, > 120 kJ/mol, this means that only a small number of collisions of interacting particles leads to the formation of new substances. The rate of such a reaction is very small. For example, the interaction of nitrogen with hydrogen to form ammonia is not possible under normal conditions. If E_ahas an intermediate value of 40 < E_aExamples of such reactions are the interaction of metals with acids, the hydrolysis of sucrose, the interaction of sodium with water, alcohol, and so on.

Thus, the nature of the reacting substances should be understood as:

a) Features of the structure of atoms of elements of metals and non-metals.

Example. K is more active than Na; Mg is more active than Zn, since the radii of the atoms are different, the reducing capacity is different, and the E_{and a differ}.

Comparative experiment of K and Na interaction with water:

Interaction of Mg and Zn with hydrochloric acid:

b) Features of chemical bond: types of bond; σ-or π-bond; multiplicity of bond, bond length.

Example. In the nitrogen molecule, N₂is a covalent nonpolar bond, the multiplicity of the bond is three; one σ-bond and 2π-bond, the bond is short; E_andN₂is very high.

In compounds with an ionic bond, e_ais very small for ions. Reactions are instantaneous.

Experience: 

 — precipitates white instantly.

C) the mutual influence of atoms and groups of atoms is very important in compounds. In the ethanol molecule, the mobility of hydrogen in the group —IT is influenced by a hydrocarbon radical, which slows the reaction rate, if you compare the interaction of sodium in water and sodium with alcohol.

d) If electrons are involved, then the strength of the electrolyte affects the reaction rate. Compare the reaction rate of magnesium with hydrochloric acid-strong acid and magnesium with acetic acid-weak acid.

The second factor is the concentration of reactants. In order for the chemical interaction between some particles to take place, they must collide effectively. The greater the concentration of reactants, the greater the number of collisions and, consequently, the higher the reaction rate. On the basis of experiments conducted in 1867 by the Norwegians K. Guldberg and P. In 1865, the basic law of chemical kinetics was formed by the Russian scientist N. I. Beketov and independently of them, and the dependence of the reaction rate on the concentration of reacting substances was established.

The rate of chemical reaction is proportional to the product of the concentrations of reacting substances taken in degrees equal to their coefficients in the reaction equation.

This law is also called the law of active masses (at the end of the XIX century, the term "concentration" was not yet introduced, and chemists used the term "active masses" instead).

For the reaction A + B = D, it is calculated

and the reaction rate A + 2B = D is defined as

where C_A, C_B— concentrations of reactants, mol/l, k₁, k₂-reaction rate constants.

The velocity constant depends only on temperature and does not depend on the concentration of substances. The velocity constant at constant temperature is numerically equal to the reaction rate, in which the concentration of reactants is equal to 1 mol/L. The law of active masses does not take into account the concentration of reacting substances in the solid state, because they react on surfaces and their concentrations are constant.

Example: 

V = k · With_O2, the reaction rate is proportional only to the oxygen concentration. Thus, the law of active masses takes into account only the concentrations of gaseous or dissolved substances.

Example:

A change in pressure due to the participation of gaseous substances in the reaction also leads to a change in the concentration of these substances. Thus, when the pressure increases by 2, 3 or more times, the volume decreases, and the concentration of substances per unit volume increases by the same number of times, and Vice versa, when the pressure decreases, the volume increases, and the concentration per unit volume will also decrease.

Example:

a —- without changing the pressure

b) the pressure is increased 3 times,

In General, the reaction rate is increased:

 - in 27 times;

C) the pressure was reduced by 2 times.

 - the reaction speed decreases by 8 times.

The third factor is the contact surface of the reactants. Reactions take place on the surface — these are heterogeneous reactions.

Speed formulathe smaller the unit of area S, the greater the reaction speed.

Solid reactants should be crushed, ground, i.e. break the structure of the crystal lattice, because particles on a microcrystal are more reactive than the same particles on a "smooth surface".

Experience. To compare the rate of reaction (via overhead projector)

In industry, a "fluidized bed" is used to conduct heterogeneous reactions, increasing the contact surface of reacting substances and removing the reaction products. In the production of sulfuric acid, pyrrolidane is fired in the "fluidized bed"; in organic chemistry, catalytic cracking of petroleum products and catalyst regeneration are carried out in the" fluidized bed".

The fourth factor is temperature.

The higher the temperature, the more active particles, as E increases,_butincreases their speed, which leads to a large number of collisions at the interaction with each other. There is an increase in the reaction speed. Reaction between oxygen and hydrogen at t° = 20 °C for 54 billion cubic meters. years pass by 15%, at 500 °C - in 50 minutes, at 700 °C-instantly.

In 1844, J. Vanth-Hoff found that when the temperature rises for every 10°, the rate of chemical reaction increases by 2-4 times. This value is called the temperature coefficient of the reaction, denoted y-gamma. For each reaction with a number from 2 to 4. when the temperature rises from t°₁to t°₂, the reaction acceleratesseveral times, i.e. the reaction rate at 

Experience. Interaction of Hcl with zinc under: a) normal conditions (V₁); b) when heated (V₂). V₂ > V₁.

For more precise calculations use the Arrhenius equation:

where K — the rate constant; Z — the number of collisions of molecules, particles per second per unit volume; R — stereochemistry of the multiplier; e — base of natural logarithm (e = 3,718); R— universal gas constant (R = 8,314 j/Molk); T — temperature in Kelvin (273° + t°C); E_ais the activation energy.

However, increasing the temperature is not always applicable, because the raw materials can start to decompose, evaporate, solvents, or the substances themselves evaporate.

Experience 1. Interaction of sodium thiosulfate Na₂S₂O₃with sulfuric acid:

a) at room temperature;

b) when the temperature rises by 10° above room temperature;

C) when the temperature rises by 20° above room temperature.

For each stage, determine the time of turbidity, precipitation of sulfur, we note t_a> t>_b> t>_вC .

Experience 2. It is possible to show the burning of sulphur in air and in pure oxygen. Pre-collect the oxygen in the flask (either by decomposing Cmpo₄, or by decomposing H₂O₂on the catalyst).

The bright glow — blue flame when burning sulfur in pure oxygen is due to the fact that in this case the reaction rate will be higher, because the concentration of oxygen is high, and in the air it is about 20 %.

The fifth factor is a catalyst, an inhibitor. Catalysts are substances that reduce the reaction mechanism and direct it along the energy path in a more profitable way, with a lower activation energy.

Catalysts are substances that participate in a chemical reaction and change its speed or direction, while remaining unchanged qualitatively and quantitatively at the end of the reaction. If the catalyst and reagents are in the same aggregate state — it is a homogeneous catalysis.

Example.

If the catalyst and reagents are in different aggregate States — this is heterogeneous catalysis.

The mechanism of action of the catalyst is explained by the formation of intermediate compounds, which then easily enter into a chemical reaction.

 they flow at a high speed

AK-intermediate connection.

In the presence of a catalyst, the path along which the total reaction takes place changes, so its speed changes. Positive catalysts increase the reaction rate.

Negative catalysts — inhibitors-slow down the speed of the chemical reaction. They react at a high rate with active particles and form low-activity compounds. The rate of reaction slows down, and the chemical reaction stops. Inhibitors stabilize acids when they are transported in steel containers, suppress the action of hydrogen peroxide, and monomers prevent premature polymerization.

Catalysts and inhibitors work very effectively in a living organism. Catalysts in a living organism are called enzymes. Thanks to them, many complex reactions take place in a living organism at a low temperature with high speed. Enzymes differ in their specificity, each of them accelerates only one chemical reaction with a yield of 100 %. Inhibitors in a living organism suppress various harmful oxidation reactions in tissue cells that can be initiated by radioactive radiation.

Experience.

1) a Piece of sugar quickly ignites if ash is applied to it (in ash, Li⁺is the catalyst).

2) the Enzyme catalase. the effect of catalase on hydrogen peroxide.

In test tubes put a piece of raw potatoes and boiled, add hydrogen peroxide. The rate of elimination reaction of O₂, where raw potatoes, is greater than with boiled potatoes. Raw potato-catalase-enzyme is not destroyed.

You can use a piece of meat-cooked and raw (for comparison).

Thus, to increase the rate of chemical reaction, factors must be taken into account: the nature of the reacting substances, the contact surface, concentration, temperature, and catalysts.

III. Solving calculation problems

Task 1. Write mathematical expressions for the rates of reactions occurring according to the equations:

Task 2. How will the reaction rate changeif the concentration of hydrogen is increased by 2 times?

 — before increasing the concentration of hydrogen.

 — with an increase in the concentration of hydrogen.

The reaction rate (2) increased by 4 times.

Task 3. How will the reaction rate changeif the pressure is reduced by 100 times?

 When the pressure is reduced by 100 times the concentrations of H₂and CL₂ decrease by 100 times⇒

⇒ the reaction speed will decrease by 10,000 times.

Task 4. How many times will the reaction rate increase when the temperature rises from 50° to 100°C, if the temperature coefficient of the speed is equal to 2?

The reaction speed will increase by a factor of 32.

Task 5. Determine the temperature coefficient of the reaction rate, if when the temperature rises from 10° to 50°, the reaction rate increased by 16 times.

Answer: γ = 2.

III. Homework assignment

§ 13. Tasks # 1-5, verbally 6-10.

Be prepared to work independently on all assignments.