§3. Reaction equation and how to write it

Interaction hydrogen with oxygen, as Sir Henry Cavendish established, leads to the formation of water. Let's use this simple example to learn how to write equations of chemical reactions.
What comes from hydrogen and oxygen, we already know:

H 2 + O 2 → H 2 O

Now we take into account that the atoms of chemical elements in chemical reactions do not disappear and do not appear from nothing, do not turn into each other, but combine in new combinations to form new molecules. This means that in the equation of the chemical reaction of atoms of each type there must be the same number before reactions ( left from the equal sign) and after the end of the reaction ( on right from the equal sign), like this:

2H 2 + O 2 \u003d 2H 2 O

That's what it is reaction equation - conditional record of an ongoing chemical reaction using formulas of substances and coefficients.

This means that in the above reaction two moles hydrogen should react with by one mole oxygen, and the result will be two moles water.

Interaction hydrogen with oxygen- not a simple process at all. It leads to a change in the oxidation states of these elements. To select coefficients in such equations, one usually uses the method " electronic balance".

When water is formed from hydrogen and oxygen, this means that hydrogen changed its oxidation state from 0 before +I, a oxygen- from 0 before −II. At the same time, several (n) electrons:

Hydrogen donating electrons serves here reducing agent, and oxygen accepting electrons - oxidizing agent.

Oxidizing and reducing agents

Now let's see how the processes of giving and receiving electrons look like separately. Hydrogen, having met with the "robber" - oxygen, loses all its property - two electrons, and its oxidation state becomes equal to +I:

H 2 0 − 2 e− = 2Н + I

Happened oxidation half-reaction equation hydrogen.

And the bandit oxygen About 2, having taken the last electrons from the unfortunate hydrogen, is very pleased with his new oxidation state -II:

O 2 + 4 e− = 2O −II

This is reduction half-reaction equation oxygen.

It remains to add that both the "bandit" and his "victim" have lost their chemical identity and from simple substances - gases with diatomic molecules H 2 and About 2 turned into components of a new chemical substance - water H 2 O.

Further, we will argue as follows: how many electrons the reductant gave to the oxidizing bandit, that is how much he received. The number of electrons donated by the reducing agent must be equal to the number of electrons accepted by the oxidizing agent..

So you need equalize the number of electrons in the first and second half-reactions. In chemistry, the following conditional form of writing the equations of half-reactions is accepted:

	2 H 2 0 − 2 e− = 2Н + I
	1 O 2 0 + 4 e− = 2O −II

Here, the numbers 2 and 1 to the left of the curly bracket are factors that will help ensure that the number of given and received electrons is equal. We take into account that in the equations of half-reactions 2 electrons are given away, and 4 are accepted. To equalize the number of received and given electrons, the least common multiple and additional factors are found. In our case, the least common multiple is 4. Additional factors will be 2 for hydrogen (4: 2 = 2), and for oxygen - 1 (4: 4 = 1)
The resulting multipliers will serve as the coefficients of the future reaction equation:

2H 2 0 + O 2 0 \u003d 2H 2 + I O -II

Hydrogen oxidized not only when meeting oxygen. Approximately the same effect on hydrogen and fluorine F2, halogen and the famous "robber", and seemingly harmless nitrogen N 2:

H 2 0 + F 2 0 = 2H + I F −I

3H 2 0 + N 2 0 \u003d 2N -III H 3 + I

This results in hydrogen fluoride HF or ammonia NH3.

In both compounds, the oxidation state hydrogen becomes equal +I, because he gets partners in the molecule "greedy" for someone else's electronic good, with high electronegativity - fluorine F and nitrogen N. At nitrogen the value of electronegativity is considered equal to three conventional units, and y fluorine in general, the highest electronegativity among all chemical elements is four units. So it's no wonder they leave the poor hydrogen atom without any electronic environment.

But hydrogen maybe restore- accept electrons. This happens if alkali metals or calcium, in which the electronegativity is less than that of hydrogen, participate in the reaction with it.

Water (hydrogen oxide) is a binary inorganic compound with the chemical formula H 2 O. The water molecule consists of two hydrogen atoms and one oxygen, which are interconnected by a covalent bond.

Hydrogen peroxide.

Physical and chemical properties

The physical and chemical properties of water are determined by the chemical, electronic and spatial structure of H 2 O molecules.

The H and O atoms in the H 2 0 molecule are in their stable oxidation states, respectively +1 and -2; therefore, water does not exhibit pronounced oxidizing or reducing properties. Please note: in metal hydrides, hydrogen is in the -1 oxidation state.

The H 2 O molecule has an angular structure. H-O bonds are very polar. There is an excess negative charge on the O atom, and excess positive charges on the H atoms. In general, the H 2 O molecule is polar, i.e. dipole. This explains the fact that water is a good solvent for ionic and polar substances.

The presence of excess charges on H and O atoms, as well as unshared electron pairs at O ​​atoms, causes the formation of hydrogen bonds between water molecules, as a result of which they are combined into associates. The existence of these associates explains the anomalously high values ​​of mp. etc. kip. water.

Along with the formation of hydrogen bonds, the result of the mutual influence of H 2 O molecules on each other is their self-ionization:
in one molecule, a heterolytic break of the polar O-H bond occurs, and the released proton joins the oxygen atom of another molecule. The resulting hydroxonium ion H 3 O + is essentially a hydrated hydrogen ion H + H 2 O, therefore, the water self-ionization equation is simplified as follows:

H 2 O ↔ H + + OH -

The dissociation constant of water is extremely small:

This indicates that water very slightly dissociates into ions, and therefore the concentration of undissociated H 2 O molecules is almost constant:

In pure water, [H + ] = [OH - ] = 10 -7 mol / l. This means that water is a very weak amphoteric electrolyte that exhibits neither acidic nor basic properties to a noticeable degree.
However, water has a strong ionizing effect on the electrolytes dissolved in it. Under the action of water dipoles, polar covalent bonds in the molecules of solutes turn into ionic ones, the ions are hydrated, the bonds between them are weakened, resulting in electrolytic dissociation. For example:
HCl + H 2 O - H 3 O + + Cl -

(strong electrolyte)

(or excluding hydration: HCl → H + + Cl -)

CH 3 COOH + H 2 O ↔ CH 3 COO - + H + (weak electrolyte)

(or CH 3 COOH ↔ CH 3 COO - + H +)

According to the Bronsted-Lowry theory of acids and bases, in these processes, water exhibits the properties of a base (proton acceptor). According to the same theory, water acts as an acid (proton donor) in reactions, for example, with ammonia and amines:

NH 3 + H 2 O ↔ NH 4 + + OH -

CH 3 NH 2 + H 2 O ↔ CH 3 NH 3 + + OH -

Redox reactions involving water

I. Reactions in which water plays the role of an oxidizing agent

These reactions are possible only with strong reducing agents, which are able to reduce the hydrogen ions that are part of the water molecules to free hydrogen.

1) Interaction with metals

a) Under normal conditions, H 2 O interacts only with alkali. and alkali-earth. metals:

2Na + 2H + 2 O \u003d 2NaOH + H 0 2

Ca + 2H + 2 O \u003d Ca (OH) 2 + H 0 2

b) At high temperatures, H 2 O also reacts with some other metals, for example:

Mg + 2H + 2 O \u003d Mg (OH) 2 + H 0 2

3Fe + 4H + 2 O \u003d Fe 2 O 4 + 4H 0 2

c) Al and Zn displace H 2 from water in the presence of alkalis:

2Al + 6H + 2 O + 2NaOH \u003d 2Na + 3H 0 2

2) Interaction with non-metals having low EO (reactions occur under harsh conditions)

C + H + 2 O \u003d CO + H 0 2 (“water gas”)

2P + 6H + 2 O \u003d 2HPO 3 + 5H 0 2

In the presence of alkalis, silicon displaces hydrogen from water:

Si + H + 2 O + 2NaOH \u003d Na 2 SiO 3 + 2H 0 2

3) Interaction with metal hydrides

NaH + H + 2 O \u003d NaOH + H 0 2

CaH 2 + 2H + 2 O \u003d Ca (OH) 2 + 2H 0 2

4) Interaction with carbon monoxide and methane

CO + H + 2 O \u003d CO 2 + H 0 2

2CH 4 + O 2 + 2H + 2 O \u003d 2CO 2 + 6H 0 2

Reactions are used in industry to produce hydrogen.

II. Reactions in which water acts as a reducing agent

These reactions are possible only with very strong oxidizing agents that are capable of oxidizing oxygen CO CO -2, which is part of water, to free oxygen O 2 or to peroxide anions 2-. In an exceptional case (in reaction with F 2), oxygen is formed with c o. +2.

1) Interaction with fluorine

2F 2 + 2H 2 O -2 \u003d O 0 2 + 4HF

2F 2 + H 2 O -2 \u003d O +2 F 2 + 2HF

2) Interaction with atomic oxygen

H 2 O -2 + O \u003d H 2 O - 2

3) Interaction with chlorine

At high T, a reversible reaction occurs

2Cl 2 + 2H 2 O -2 \u003d O 0 2 + 4HCl

III. Reactions of intramolecular oxidation - reduction of water.

Under the action of an electric current or high temperature, water can be decomposed into hydrogen and oxygen:

2H + 2 O -2 \u003d 2H 0 2 + O 0 2

Thermal decomposition is a reversible process; the degree of thermal decomposition of water is low.

Hydration reactions

I. Hydration of ions. The ions formed during the dissociation of electrolytes in aqueous solutions attach a certain number of water molecules and exist in the form of hydrated ions. Some ions form such strong bonds with water molecules that their hydrates can exist not only in solution, but also in the solid state. This explains the formation of crystalline hydrates such as CuSO4 5H 2 O, FeSO 4 7H 2 O, etc., as well as aqua complexes: CI 3 , Br 4 , etc.

II. Hydration of oxides

III. Hydration of organic compounds containing multiple bonds

Hydrolysis reactions

I. Hydrolysis of salts

Reversible hydrolysis:

a) according to the salt cation

Fe 3+ + H 2 O \u003d FeOH 2+ + H +; (acidic environment. pH

b) by salt anion

CO 3 2- + H 2 O \u003d HCO 3 - + OH -; (alkaline environment. pH > 7)

c) by the cation and by the anion of the salt

NH 4 + + CH 3 COO - + H 2 O \u003d NH 4 OH + CH 3 COOH (environment close to neutral)

Irreversible hydrolysis:

Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 ↓ + 3H 2 S

II. Hydrolysis of metal carbides

Al 4 C 3 + 12H 2 O \u003d 4Al (OH) 3 ↓ + 3CH 4 netane

CaC 2 + 2H 2 O \u003d Ca (OH) 2 + C 2 H 2 acetylene

III. Hydrolysis of silicides, nitrides, phosphides

Mg 2 Si + 4H 2 O \u003d 2Mg (OH) 2 ↓ + SiH 4 silane

Ca 3 N 2 + 6H 2 O \u003d ZCa (OH) 2 + 2NH 3 ammonia

Cu 3 P 2 + 6H 2 O \u003d ZCu (OH) 2 + 2PH 3 phosphine

IV. Hydrolysis of halogens

Cl 2 + H 2 O \u003d HCl + HClO

Br 2 + H 2 O \u003d HBr + HBrO

V. Hydrolysis of organic compounds

Classes of organic substances	Hydrolysis products (organic)
Halogenalkanes (alkyl halides)
Aryl halides
Dihaloalkanes	Aldehydes or ketones
Metal alcoholates
Carboxylic acid halides	carboxylic acids
Anhydrides of carboxylic acids	carboxylic acids
Esters of carboxylic acids	Carboxylic acids and alcohols
	Glycerin and higher carboxylic acids
Di- and polysaccharides	Monosaccharides
Peptides and proteins	α-Amino acids
Nucleic acids

2. Write down the kinetic equation for the reaction: 2H2 + O2 = 2H2O. 3. How many times will the reaction rate increase if the temperature coefficient is 3 and the temperature is increased by 30 degrees? 4. When the temperature rises by 40 degrees, the reaction rate increases by 16 times. Determine the temperature coefficient.

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Reactions

"Speed ​​of reaction" - Factors affecting the speed. What did we study? Influence of the concentration of reactants (for homogeneous systems) 3rd row. Temperature. What determines the rate of reactions? 2. Write down the kinetic equation for the reaction: 2H2 + O2 = 2H2O. Presence of catalysts or inhibitors. Problem solving. Catalysts and catalysis.

"The law of conservation of mass of substances" - 1673. The law of conservation of mass of substances. Index. The index shows the number of atoms in the formula unit of a substance. Like Boyle, the Russian scientist made experiments in sealed retorts. 1789 General secondary school No. 36 named after Kazybek bi. Robert Boyle. Coefficient. 5n2o. 1748 Chemical formula. Lesson objectives: Teaching - to experimentally prove the law of conservation of mass of substances.

"Radioactive transformations" - Milestones of history. No is the number of radioactive nuclei at the initial moment of time. t is the decay time. Law of radioactive decay. Experience. What is a half-life? T is the half-life. Rutherford research. Conclusion from the rules. The atoms of a radioactive substance are subject to spontaneous modifications. Prehistory of radioactivity research.

"Chemical reactions practical work" - PPG. H2 - Gas, colorless, odorless, lighter than air. 4) Black CuO turns red, H2O is formed on the walls of the test tube. Test tubes. 2) Pure H2 explodes with a dull pop, H2 with impurities - a barking sound. 3kcns+feci3=3kci+fe(cns)3 exchange. AI+HCI. Cu. Zn+H2SO4 = ZnSO4+H2 Substitution. Alcohol lamp. Observed signs of chemical reactions.

"Reactions" - Appearances of smell. Give a basic understanding of a chemical reaction. Gas release. Equipment: Solutions - hydrochloric acid and lime water, a piece of marble. Checking homework. Give examples of complex substances? The role of chemistry in human life. Sediment formation. The release or absorption of heat.

"Theory of electrolytic dissociation" - All simple substances, all oxides and n/r acids, bases and salts. Svante Arrhenius. Substances in solutions. Substances with ionic and covalent polar bonds. Theory of electrolytic dissociation (TED). II-nd provision of TED. Substances with a covalent bond: Orientation of water dipoles? hydration? ionization? dissociation.

In total there are 28 presentations in the topic

The well-known formula of the basis of life - water. Its molecule consists of two hydrogen atoms and one oxygen, which is written as H2O. If there is twice as much oxygen, then a completely different substance will turn out - H2O2. What is it and how will the resulting substance differ from its “relative” of water?

H2O2 - what is this substance?

Let's dwell on it in more detail. H2O2 is the formula for hydrogen peroxide, yes, the one used to treat scratches, white. Hydrogen peroxide H2O2 - scientific.

A 3% peroxide solution is used for disinfection. In pure or concentrated form, it causes chemical burns to the skin. A thirty percent peroxide solution is otherwise called perhydrol; it was previously used in hairdressing salons to bleach hair. The skin burned by him also becomes white.

Chemical properties of H2O2

Hydrogen peroxide is a colorless liquid with a "metallic" taste. It is a good solvent and is easily soluble in water, ether, alcohols.

Three and six percent peroxide solutions are usually prepared by diluting a thirty percent solution. When concentrated H2O2 is stored, the substance decomposes with the release of oxygen, so it should not be stored in tightly sealed containers in order to avoid an explosion. With a decrease in the concentration of peroxide, its stability increases. Also, to slow down the decomposition of H2O2, various substances can be added to it, for example, phosphoric or salicylic acid. To store solutions of strong concentration (more than 90 percent), sodium pyrophosphate is added to the peroxide, which stabilizes the state of the substance, and aluminum vessels are also used.

H2O2 in chemical reactions can be both an oxidizing agent and a reducing agent. More often, however, peroxide exhibits oxidizing properties. Peroxide is considered to be an acid, but a very weak one; salts of hydrogen peroxide are called peroxides.

as a method of obtaining oxygen

The decomposition reaction of H2O2 occurs when a substance is exposed to high temperature (more than 150 degrees Celsius). The result is water and oxygen.

Reaction formula - 2 H2O2 + t -> 2 H2O + O2

The oxidation state of H in H 2 O 2 and H 2 O \u003d +1.
The oxidation state of O: in H 2 O 2 \u003d -1, in H 2 O \u003d -2, in O 2 \u003d 0
2 O -1 - 2e -> O2 0

O -1 + e -> O -2
2 H2O2 = 2 H2O + O2

Decomposition of hydrogen peroxide can also occur at room temperature if a catalyst (a chemical that speeds up the reaction) is used.

In laboratories, one of the methods for obtaining oxygen, along with the decomposition of berthollet salt or potassium permanganate, is the reaction of peroxide decomposition. In this case, manganese (IV) oxide is used as a catalyst. Other substances that accelerate the decomposition of H2O2 are copper, platinum, sodium hydroxide.

The history of the discovery of peroxide

The first steps towards the discovery of peroxide were made in 1790 by the German Alexander Humboldt, when he discovered the transformation of barium oxide into peroxide when heated. That process was accompanied by the absorption of oxygen from the air. Twelve years later, the scientists Tenard and Gay-Lussac conducted an experiment on the combustion of alkali metals with an excess of oxygen, as a result of which sodium peroxide was obtained. But hydrogen peroxide was obtained later, only in 1818, when Louis Tenard studied the effect of acids on metals; for their stable interaction, a low amount of oxygen was needed. Conducting a confirmatory experiment with barium peroxide and sulfuric acid, the scientist added water, hydrogen chloride and ice to them. After a short time, Tenar found small solidified drops on the walls of the container with barium peroxide. It became clear that it was H2O2. Then they gave the resulting H2O2 the name "oxidized water". This was hydrogen peroxide - a colorless, odorless, hardly evaporable liquid that dissolves other substances well. The result of the interaction of H2O2 and H2O2 is a dissociation reaction, the peroxide is soluble in water.

An interesting fact is that the properties of the new substance were quickly discovered, allowing it to be used in restoration work. Tenard himself, using peroxide, restored the painting by Raphael, which had darkened with time.

Hydrogen peroxide in the 20th century

After a thorough study of the resulting substance, it began to be produced on an industrial scale. At the beginning of the twentieth century, an electrochemical technology for the production of peroxide was introduced, based on the electrolysis process. But the shelf life of the substance obtained by this method was small, about a couple of weeks. Pure peroxide is unstable, and for the most part it was produced in a thirty percent concentration for bleaching fabrics and in three or six percent for domestic use.

Scientists in Nazi Germany used peroxide to create a liquid fuel rocket engine that was used for defense purposes in World War II. As a result of the interaction of H2O2 and methanol / hydrazine, a powerful fuel was obtained, on which the aircraft reached speeds of more than 950 km / h.

Where is H2O2 used now?

• in medicine - for the treatment of wounds;
• in the pulp and paper industry, the bleaching properties of the substance are used;
• in the textile industry, natural and synthetic fabrics, furs, wool are bleached with peroxide;
• as rocket fuel or its oxidizer;
• in chemistry - to produce oxygen, as a foaming agent for the production of porous materials, as a catalyst or hydrogenating agent;
• for the production of disinfectants or cleaning agents, bleaches;
• for bleaching hair (this is an outdated method, since the hair is severely damaged by peroxide);

Hydrogen peroxide can be successfully used to solve various household problems. But only 3% hydrogen peroxide can be used for these purposes. Here are some ways:

• To clean surfaces, pour peroxide into a container with a spray bottle and spray on contaminated areas.
• To disinfect objects, they must be wiped with an undiluted solution of H2O2. This will help cleanse them of harmful microorganisms. Sponges for washing can be soaked in water with peroxide (proportion 1:1).
• To bleach fabrics when washing white things, add a glass of peroxide. You can also rinse white fabrics in water mixed with a glass of H2O2. This method restores whiteness, prevents fabrics from yellowing and helps remove stubborn stains.
• To combat mold and mildew, mix peroxide and water in a spray bottle in a ratio of 1:2. Spray the resulting mixture onto infected surfaces and clean them with a brush or sponge after 10 minutes.
• You can update the darkened grout in the tile by spraying peroxide on the desired areas. After 30 minutes, you need to carefully rub them with a stiff brush.
• To wash dishes, add half a glass of H2O2 to a full basin of water (or a sink with a closed drain). Cups and plates washed in such a solution will shine with cleanliness.
• To clean your toothbrush, you need to dip it in an undiluted 3% peroxide solution. Then rinse under strong running water. This method disinfects the hygiene item well.
• To disinfect purchased vegetables and fruits, spray a solution of 1 part peroxide and 1 part water on them, then rinse them thoroughly with water (can be cold).
• In the suburban area with the help of H2O2, you can fight plant diseases. You need to spray them with a peroxide solution or soak the seeds shortly before planting in 4.5 liters of water mixed with 30 ml of forty percent hydrogen peroxide.
• To revive aquarium fish, if they are poisoned by ammonia, suffocated when aeration is turned off, or for another reason, you can try placing them in water with hydrogen peroxide. It is necessary to mix 3% peroxide with water at the rate of 30 ml per 100 liters and place it in the resulting mixture of lifeless fish for 15-20 minutes. If they do not come to life during this time, then the remedy did not help.

Even as a result of vigorous shaking of a water bottle, a certain amount of peroxide is formed in it, since the water is saturated with oxygen during this action.

Fresh fruits and vegetables also contain H2O2 until they are cooked. During heating, boiling, roasting and other processes with an accompanying high temperature, a large amount of oxygen is destroyed. That is why cooked foods are considered not so useful, although some amount of vitamins remains in them. Freshly squeezed juices or oxygen cocktails served in sanatoriums are useful for the same reason - due to oxygen saturation, which gives the body new strength and cleanses it.

The dangers of peroxide when ingested

After the above, it may seem that peroxide can be specifically taken orally, and this will benefit the body. But that's not the case at all. In water or juices, the compound is found in minimal amounts and is closely related to other substances. Taking "unnatural" hydrogen peroxide inside (and all peroxide bought in a store or produced as a result of chemical experiments on your own cannot be considered natural in any way, besides, it has too high a concentration compared to natural) can lead to life-threatening and health-threatening consequences. To understand why, you need to turn to chemistry again.

As already mentioned, under certain conditions, hydrogen peroxide breaks down and releases oxygen, which is an active oxidizing agent. can occur when H2O2 collides with peroxidase, an intracellular enzyme. The use of peroxide for disinfection is based on its oxidizing properties. So, when a wound is treated with H2O2, the released oxygen destroys the living pathogenic microorganisms that have entered it. It has the same effect on other living cells. If you treat undamaged skin with peroxide and then wipe the treated area with alcohol, you will feel a burning sensation, which confirms the presence of microscopic damage after peroxide. But with the external use of peroxide at a low concentration, there will be no noticeable harm to the body.

Another thing, if you try to take it inside. That substance, which is capable of damaging even relatively thick skin from the outside, enters the mucous membranes of the digestive tract. That is, chemical mini-burns occur. Of course, the released oxidizing agent - oxygen - can also kill harmful microbes. But the same process will occur with the cells of the alimentary tract. If burns as a result of the action of an oxidizing agent are repeated, then atrophy of the mucous membranes is possible, and this is the first step towards cancer. The death of intestinal cells leads to the inability of the body to absorb nutrients, this explains, for example, weight loss and the disappearance of constipation in some people who practice peroxide "treatment".

Separately, it must be said about such a method of using peroxide as intravenous injections. Even if for some reason they were prescribed by a doctor (this can only be justified in case of blood poisoning, when there are no other suitable drugs available), then under medical supervision and with a strict calculation of dosages, there are still risks. But in such an extreme situation, it will be a chance for recovery. In no case should you prescribe yourself injections of hydrogen peroxide. H2O2 poses a great danger to blood cells - erythrocytes and platelets, as it destroys them when it enters the bloodstream. In addition, a deadly blockage of blood vessels by released oxygen can occur - a gas embolism.

Safety measures in handling H2O2

• Keep out of the reach of children and incapacitated persons. The lack of smell and pronounced taste makes peroxide especially dangerous for them, as large doses can be taken. If the solution is ingested, the consequences of use can be unpredictable. You must immediately consult a doctor.
• Peroxide solutions with a concentration of more than three percent cause burns if it comes into contact with the skin. The burn area should be washed with plenty of water.

• Do not allow the peroxide solution to get into the eyes, as their swelling, redness, irritation, and sometimes pain are formed. First aid before going to the doctor - plentiful rinsing of the eyes with water.
• Store the substance in such a way that it is clear that it is H2O2, that is, in a container with a sticker to avoid accidental misuse.
• Storage conditions that extend its life are a dark, dry, cool place.
• Do not mix hydrogen peroxide with any liquids other than pure water, including chlorinated tap water.
• All of the above applies not only to H2O2, but to all preparations containing it.

2Н2 + О2 ––> 2Н2О

the concentrations of hydrogen, oxygen and water vary to varying degrees: ΔС(Н2) = ΔС(Н2О) = 2 ΔС(О2).

The rate of a chemical reaction depends on many factors: the nature of the reactants, their concentration, temperature, the nature of the solvent, etc.

2.1.1 Kinetic equation of a chemical reaction. Reaction order.

One of the tasks facing chemical kinetics is to determine the composition of the reaction mixture (i.e., the concentrations of all reactants) at any time, for which it is necessary to know the dependence of the reaction rate on concentrations. In general, the greater the concentration of the reactants, the greater the rate of the chemical reaction. The basis of chemical kinetics is the so-called. basic postulate of chemical kinetics:

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants, taken to some extent.

i.e. for the reaction

aA + bB + dD + . ––> eE + .

can be written:

The coefficient of proportionality k is the rate constant of a chemical reaction. The rate constant is numerically equal to the reaction rate at concentrations of all reactants equal to 1 mol/L.

The dependence of the reaction rate on the concentrations of reactants is determined experimentally and is called the kinetic equation of a chemical reaction. It is obvious that in order to write the kinetic equation, it is necessary to experimentally determine the value of the rate constant and the exponents at the concentrations of the reactants. The exponent at the concentration of each of the reacting substances in the kinetic equation of a chemical reaction (in equation (II.4) respectively x, y and z) is a particular order of the reaction for this component. The sum of the exponents in the kinetic equation for a chemical reaction (x + y + z) represents the overall order of the reaction. It should be emphasized that the reaction order is determined only from experimental data and is not related to the stoichiometric coefficients of the reactants in the reaction equation. The stoichiometric reaction equation is a material balance equation and in no way can determine the nature of the course of this reaction in time.

In chemical kinetics, it is customary to classify reactions according to the overall order of the reaction. Let us consider the dependence of the concentration of reactants on time for irreversible (one-way) reactions of zero, first, and second orders.

2.1.2 Zero-order reactions

For zero-order reactions, the kinetic equation has the following form:

The rate of a zero-order reaction is constant in time and does not depend on the concentrations of the reactants; this is characteristic of many heterogeneous (occurring at the interface) reactions in the case when the rate of diffusion of reagents to the surface is less than the rate of their chemical transformation.

2.1.3 First order reactions

Let us consider the time dependence of the concentration of the initial substance A for the case of a first-order reaction A -–> B. First-order reactions are characterized by a kinetic equation of the form (II.6). We substitute expression (II.2) into it:

(II.7)

After integrating expression (II.7), we obtain:

We determine the integration constant g from the initial conditions: at time t = 0, the concentration С is equal to the initial concentration Сo. It follows from this that g = ln Co. We get:

Rice. 2.3 The dependence of the logarithm of concentration on time for reactions of the first order

Thus, the logarithm of the concentration for a first-order reaction depends linearly on time (Fig. 2.3) and the rate constant is numerically equal to the tangent of the slope of the straight line to the time axis.

From equation (II.9), it is easy to obtain an expression for the rate constant of a one-way first-order reaction:

Another kinetic characteristic of the reaction is the half-life t1 / 2 - the time during which the concentration of the starting substance is halved compared to the original. Let's express t1/2 for the first order reaction, considering that С = ½Сo:

(II.12)

As can be seen from the expression obtained, the half-life of the first-order reaction does not depend on the initial concentration of the starting material.

2.1.4 Second order reactions

For second-order reactions, the kinetic equation has the following form:

Let us consider the simplest case, when the kinetic equation has the form (II.14) or, what is the same, in the equation of the form (II.15) the concentrations of the initial substances are the same; equation (II.14) in this case can be rewritten as follows:

(II.16)

After separation of variables and integration, we get:

The integration constant g, as in the previous case, is determined from the initial conditions. We get:

Thus, for second-order reactions that have a kinetic equation of the form (II.14), a linear dependence of the reciprocal concentration on time is characteristic (Fig. 2.4) and the rate constant is equal to the tangent of the slope of the straight line to the time axis:

(II.20)

Rice. 2.4 Reciprocal concentration versus time for second order reactions

If the initial concentrations of the reactants Co,A and Co,B are different, then the reaction rate constant is found by integrating equation (II.21), in which CA and CB are the concentrations of the reactants at time t from the start of the reaction:

(II.21)

In this case, for the rate constant, we obtain the expression