How to determine the possible oxidation states of elements. How to determine the oxidation state of an element

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When studying ionic and covalent polar chemical bonds, you got acquainted with complex substances consisting of two chemical elements. Such substances are called bi-pair (from Latin bi - “two”) or two-element.

Let us recall the typical binary compounds that we cited as an example to consider the mechanisms for the formation of ionic and covalent polar chemical bonds: NaHl - sodium chloride and HCl - hydrogen chloride. In the first case, the bond is ionic: the sodium atom transferred its outer electron to the chlorine atom and turned into an ion with a charge of -1. and the chlorine atom accepted an electron and turned into an ion with a charge of -1. Schematically, the process of transformation of atoms into ions can be depicted as follows:

In the HCl molecule, the bond is formed due to the pairing of unpaired outer electrons and the formation of a common electron pair of hydrogen and chlorine atoms.

It is more correct to represent the formation of a covalent bond in a hydrogen chloride molecule as an overlap of a one-electron s-cloud of a hydrogen atom with a one-electron p-cloud of a chlorine atom:

During chemical interaction, the common electron pair is shifted towards the more electronegative chlorine atom:

Such conditional charges are called oxidation state. When defining this concept, it is conditionally assumed that in covalent polar compounds, the binding electrons have completely transferred to a more electronegative atom, and therefore the compounds consist only of positively and negatively charged ions.

is the conditional charge of the atoms of a chemical element in a compound, calculated on the basis of the assumption that all compounds (both ionic and covalently polar) consist only of ions.

The oxidation state can have a negative, positive, or zero value, which is usually placed above the element symbol at the top, for example:

Those atoms that have received electrons from other atoms or to which common electron pairs are displaced, that is, atoms of more electronegative elements, have a negative value for the degree of oxidation. Fluorine always has an oxidation state of -1 in all compounds. Oxygen, the second most electronegative element after fluorine, almost always has an oxidation state of -2, except for compounds with fluorine, for example:

Those atoms that donate their electrons to other atoms or from which common electron pairs are drawn, that is, atoms of less electronegative elements, have a positive oxidation state. Metals always have a positive oxidation state. For metals of the main subgroups:

Group I in all compounds, the oxidation state is +1,
Group II is equal to +2. Group III - +3, for example:

In compounds, the total oxidation state is always zero. Knowing this and the oxidation state of one of the elements, you can always find the oxidation state of another element using the formula of a binary compound. For example, let's find the oxidation state of chlorine in the compound Cl2O2. Let's denote the oxidation state -2
oxygen: Cl2O2. Therefore, seven oxygen atoms will have a total negative charge (-2) 7 =14. Then the total charge of two chlorine atoms will be +14, and one chlorine atom:
(+14):2 = +7.

Similarly, knowing the oxidation states of the elements, one can formulate the formula of a compound, for example, aluminum carbide (a compound of aluminum and carbon). Let's write the signs of aluminum and carbon next to AlC, and first the sign of aluminum, since it is a metal. We determine the number of external electrons from the periodic table of elements: Al has 3 electrons, C has 4. An aluminum atom will give up its 3 external electrons to carbon and receive an oxidation state of +3, equal to the charge of the ion. The carbon atom, on the contrary, will take the 4 electrons missing to the "cherished eight" and will receive an oxidation state of -4.

Let's write these values in the formula: AlС, and find the least common multiple for them, it is equal to 12. Then we calculate the indices:

Knowing the oxidation states of elements is also necessary in order to be able to correctly name a chemical compound.

Names of binary compounds consist of two words - the names of the chemical elements that form them. The first word denotes the electronegative part of the compound - non-metal, its Latin name with the suffix -id is always in the nominative case. The second word denotes the electropositive part - a metal or a less electronegative element, its name is always in the genitive case. If the electropositive element exhibits different degrees of oxidation, then this is reflected in the name, indicating the degree of oxidation with a Roman numeral, which is placed at the end.

In order for chemists from different countries to understand each other, it was necessary to create a unified terminology and nomenclature of substances. The principles of chemical nomenclature were first developed by French chemists A. Lavoisier, A. Fourctua, L. Giton and C. Berthollet in 1785. At present, the International Union of Pure and Applied Chemistry (IUPAC) coordinates the activities of scientists from several countries and issues recommendations on the nomenclature of substances and terminology used in chemistry.

In school, chemistry is still one of the most difficult subjects, which, due to the fact that it hides many difficulties, causes in students (usually in the period from 8 to 9 classes) more hatred and indifference to study than interest. All this reduces the quality and quantity of knowledge on the subject, although many areas still require specialists in this field. Yes, sometimes there are even more difficult moments and incomprehensible rules in chemistry than it seems. One of the questions that concern most students is what is the oxidation state and how to determine the oxidation states of elements.

An important rule is the placement rule, algorithms

There is much talk here about compounds such as oxides. To begin with, every student must learn determination of oxides- These are complex compounds of two elements, they contain oxygen. Oxides are classified as binary compounds because oxygen is second in line in the algorithm. When determining the indicator, it is important to know the placement rules and calculate the algorithm.

Algorithms for Acid Oxides

Oxidation states - these are numerical expressions of the valency of the elements. For example, acid oxides are formed according to a certain algorithm: non-metals or metals come first (their valency is usually from 4 to 7), and then oxygen comes, as it should be, second in order, its valency is two. It is determined easily - according to the periodic table of chemical elements of Mendeleev. It is also important to know that the oxidation state of elements is an indicator that suggests either positive or negative number.

At the beginning of the algorithm, as a rule, a non-metal, and its oxidation state is positive. Non-metal oxygen in oxide compounds has a stable value, which is -2. To determine the correctness of the arrangement of all values, you need to multiply all the available numbers by the indices of one specific element, if the product, taking into account all the minuses and pluses, is 0, then the arrangement is reliable.

Arrangement in acids containing oxygen

Acids are complex substances, they are associated with some acidic residue and contain one or more hydrogen atoms. Here, to calculate the degree, skills in mathematics are required, since the indicators necessary for the calculation are digital. For hydrogen or a proton, it is always the same - +1. The negative oxygen ion has a negative oxidation state of -2.

After carrying out all these actions, you can determine the degree of oxidation and the central element of the formula. The expression for its calculation is a formula in the form of an equation. For example, for sulfuric acid, the equation will be with one unknown.

Basic terms in OVR

ORR is a reduction-oxidation reaction.

The oxidation state of any atom - characterizes the ability of this atom to attach or give electrons to other atoms of ions (or atoms);
It is customary to consider either charged atoms or uncharged ions as oxidizing agents;
The reducing agent in this case will be charged ions or, on the contrary, uncharged atoms that lose their electrons in the process of chemical interaction;
Oxidation is the donation of electrons.

How to arrange the oxidation state in salts

Salts are composed of one metal and one or more acid residues. The determination procedure is the same as in acid-containing acids.

The metal that directly forms a salt is located in the main subgroup, its degree will be equal to the number of its group, that is, it will always remain a stable, positive indicator.

As an example, consider the arrangement of oxidation states in sodium nitrate. Salt is formed using an element of the main subgroup of group 1, respectively, the oxidation state will be positive and equal to one. In nitrates, oxygen has the same value - -2. In order to get a numerical value, first an equation is drawn up with one unknown, taking into account all the minuses and pluses of the values: +1+X-6=0. By solving the equation, you can come to the fact that the numerical indicator is positive and equal to + 5. This is the indicator of nitrogen. An important key to calculate the degree of oxidation - table.

Arrangement rule in basic oxides

Oxides of typical metals in any compounds have a stable oxidation index, it is always no more than +1, or in other cases +2;
The digital indicator of the metal is calculated using the periodic table. If the element is contained in the main subgroup of group 1, then its value will be +1;
The value of oxides, taking into account their indices, after multiplication, summed up should be equal to zero, because the molecule in them is neutral, a particle devoid of charge;
Metals of the main subgroup of group 2 also have a stable positive indicator, which is +2.

Such a subject of the school curriculum as chemistry causes numerous difficulties for most modern schoolchildren, few people can determine the degree of oxidation in compounds. The greatest difficulties are for schoolchildren who study, that is, students of the main school (grades 8-9). Misunderstanding of the subject leads to the emergence of hostility among students to this subject.

Teachers identify a number of reasons for such a “dislike” of middle and high school students for chemistry: unwillingness to understand complex chemical terms, inability to use algorithms to consider a specific process, problems with mathematical knowledge. The Ministry of Education of the Russian Federation has made serious changes to the content of the subject. In addition, the number of hours for teaching chemistry was "cut down". This had a negative impact on the quality of knowledge in the subject, a decrease in interest in the study of the discipline.

What topics of the chemistry course are the most difficult for schoolchildren?

According to the new program, the course of the discipline "Chemistry" of the basic school includes several serious topics: the periodic table of elements of D. I. Mendeleev, classes of inorganic substances, ion exchange. The hardest thing is for eighth graders to determine the degree of oxidation of oxides.

Placement rules

First of all, students should know that oxides are complex two-element compounds that include oxygen. A prerequisite for a binary compound to belong to the class of oxides is the second position of oxygen in this compound.

Algorithm for Acid Oxides

To begin with, we note that the degrees are numerical expressions of the valency of elements. Acid oxides are formed by non-metals or metals with a valence of four to seven, the second in such oxides is necessarily oxygen.

In oxides, the valency of oxygen always corresponds to two; it can be determined from the periodic table of elements of D. I. Mendeleev. Such a typical non-metal as oxygen, being in the 6th group of the main subgroup of the periodic table, accepts two electrons in order to completely complete its external energy level. Non-metals in compounds with oxygen most often exhibit a higher valence, which corresponds to the number of the group itself. It is important to recall that the oxidation state of chemical elements is an indicator that implies a positive (negative) number.

The non-metal at the beginning of the formula has a positive oxidation state. Non-metal oxygen is stable in oxides, its index is -2. In order to check the reliability of the arrangement of values in acid oxides, you will have to multiply all the numbers you set by the indices of a particular element. Calculations are considered reliable if the total sum of all the pluses and minuses of the set degrees is 0.

Compilation of two-element formulas

The oxidation state of the atoms of the elements gives a chance to create and record compounds from two elements. When creating a formula, for starters, both symbols are written side by side, be sure to put oxygen second. Above each of the recorded signs, the values \u200b\u200bof the oxidation states are prescribed, then between the numbers found is the number that will be divisible by both digits without any remainder. This indicator must be divided separately by the numerical value of the degree of oxidation, obtaining indices for the first and second components of the two-element substance. The highest oxidation state is numerically equal to the value of the highest valency of a typical non-metal, identical to the group number where the non-metal stands in PS.

Algorithm for setting numerical values in basic oxides

Oxides of typical metals are considered to be such compounds. They in all compounds have an oxidation state index of no more than +1 or +2. In order to understand what the oxidation state of a metal will be, you can use the periodic table. For metals of the main subgroups of the first group, this parameter is always constant, it is similar to the group number, that is, +1.

Metals of the main subgroup of the second group are also characterized by a stable oxidation state, numerically +2. The oxidation states of oxides, taking into account their indices (numbers), should add up to zero, since the chemical molecule is considered to be a neutral, charge-free particle.

Arrangement of oxidation states in oxygen-containing acids

Acids are complex substances, consisting of one or more hydrogen atoms, which are associated with some kind of acid residue. Given that oxidation states are numbers, some math skills are required to calculate them. Such an indicator for hydrogen (proton) in acids is always stable, it is +1. Next, you can specify the oxidation state for the negative oxygen ion, it is also stable, -2.

Only after these actions, it is possible to calculate the degree of oxidation of the central component of the formula. As a specific sample, consider the determination of the oxidation state of elements in sulfuric acid H2SO4. Given that the molecule of this complex substance contains two hydrogen protons, 4 oxygen atoms, we obtain an expression of this form +2+X-8=0. In order for the sum to form zero, sulfur will have an oxidation state of +6

Arrangement of oxidation states in salts

Salts are complex compounds consisting of metal ions and one or more acid residues. The procedure for determining the oxidation states of each of the constituents in a complex salt is the same as in oxygen-containing acids. Given that the oxidation state of the elements is a numerical indicator, it is important to correctly indicate the oxidation state of the metal.

If the salt-forming metal is located in the main subgroup, its oxidation state will be stable, corresponds to the group number, is a positive value. If the salt contains a metal of a similar subgroup of PS, it is possible to show different metals by the acid residue. After the oxidation state of the metal is set, put (-2), then the oxidation state of the central element is calculated using the chemical equation.

As an example, consider the determination of the oxidation states of elements in (medium salt). NaNO3. The salt is formed by a metal of the main subgroup of group 1, therefore, the oxidation state of sodium will be +1. Oxygen in nitrates has an oxidation state of -2. To determine the numerical value of the degree of oxidation is the equation +1+X-6=0. Solving this equation, we get that X should be +5, this is

Basic terms in OVR

For the oxidative as well as the reduction process, there are special terms that students are required to learn.

The oxidation state of an atom is its direct ability to attach to itself (donate to others) electrons from some ions or atoms.

An oxidizing agent is considered to be neutral atoms or charged ions that acquire electrons during a chemical reaction.

The reducing agent will be uncharged atoms or charged ions, which in the process of chemical interaction lose their own electrons.

Oxidation is presented as a procedure for donating electrons.

Reduction is associated with the acceptance of additional electrons by an uncharged atom or ion.

The redox process is characterized by a reaction during which the oxidation state of an atom necessarily changes. This definition allows you to understand how you can determine whether the reaction is OVR.

OVR Parsing Rules

Using this algorithm, you can arrange the coefficients in any chemical reaction.

To characterize the redox ability of particles, such a concept as the degree of oxidation is important. OXIDATION STATE is the charge that an atom in a molecule or ion could have if all its bonds with other atoms were broken, and the common electron pairs left with more electronegative elements.

Unlike the real-life charges of ions, the oxidation state shows only the conditional charge of an atom in a molecule. It can be negative, positive or zero. For example, the oxidation state of atoms in simple substances is "0" (,
,,). In chemical compounds, atoms can have a constant oxidation state or a variable. For metals of the main subgroups I, II and III of groups of the Periodic system in chemical compounds, the oxidation state is usually constant and equal to Me +1, Me +2 and Me +3 (Li +, Ca +2, Al +3), respectively. The fluorine atom always has -1. Chlorine in compounds with metals always has -1. In the vast majority of compounds, oxygen has an oxidation state of -2 (except for peroxides, where its oxidation state is -1), and hydrogen +1 (except for metal hydrides, where its oxidation state is -1).

The algebraic sum of the oxidation states of all atoms in a neutral molecule is equal to zero, and in an ion it is equal to the charge of the ion. This relationship makes it possible to calculate the oxidation states of atoms in complex compounds.

In the sulfuric acid molecule H 2 SO 4, the hydrogen atom has an oxidation state of +1, and the oxygen atom is -2. Since there are two hydrogen atoms and four oxygen atoms, we have two "+" and eight "-". Six "+" are missing to neutrality. It is this number that is the oxidation state of sulfur -
. The potassium dichromate K 2 Cr 2 O 7 molecule consists of two potassium atoms, two chromium atoms and seven oxygen atoms. Potassium has an oxidation state of +1, oxygen has -2. So we have two "+" and fourteen "-". The remaining twelve "+" fall on two chromium atoms, each of which has an oxidation state of +6 (
).

Typical oxidizing and reducing agents

From the definition of reduction and oxidation processes, it follows that, in principle, simple and complex substances containing atoms that are not in the lowest oxidation state and therefore can lower their oxidation state can act as oxidizing agents. Similarly, simple and complex substances containing atoms that are not in the highest oxidation state and therefore can increase their oxidation state can act as reducing agents.

The strongest oxidizing agents are:

1) simple substances formed by atoms having a large electronegativity, i.e. typical non-metals located in the main subgroups of the sixth and seventh groups of the periodic system: F, O, Cl, S (respectively F 2 , O 2 , Cl 2 , S);

2) substances containing elements in higher and intermediate

positive oxidation states, including in the form of ions, both simple, elemental (Fe 3+) and oxygen-containing, oxoanions (permanganate ion - MnO 4 -);

3) peroxide compounds.

Specific substances used in practice as oxidizers are oxygen and ozone, chlorine, bromine, permanganates, dichromates, oxyacids of chlorine and their salts (for example,
,
,
), Nitric acid (
), concentrated sulfuric acid (
), manganese dioxide (
), hydrogen peroxide and metal peroxides (
,
).

The most powerful reducing agents are:

1) simple substances whose atoms have low electronegativity (“active metals”);

2) metal cations in low oxidation states (Fe 2+);

3) simple elemental anions, for example, sulfide ion S 2- ;

4) oxygen-containing anions (oxoanions) corresponding to the lowest positive oxidation states of the element (nitrite
, sulfite
).

Specific substances used in practice as reducing agents are, for example, alkali and alkaline earth metals, sulfides, sulfites, hydrogen halides (except HF), organic substances - alcohols, aldehydes, formaldehyde, glucose, oxalic acid, as well as hydrogen, carbon, monoxide carbon (
) and aluminum at high temperatures.

In principle, if a substance contains an element in an intermediate oxidation state, then these substances can exhibit both oxidizing and reducing properties. It all depends on

"partner" in the reaction: with a sufficiently strong oxidizing agent, it can react as a reducing agent, and with a sufficiently strong reducing agent, as an oxidizing agent. So, for example, the nitrite ion NO 2 - in an acidic environment acts as an oxidizing agent with respect to the ion I -:

2
+ 2+ 4HCl→ + 2
+ 4KCl + 2H 2 O