To characterize the state of elements in compounds, the concept of the degree of oxidation has been introduced.
DEFINITION
The number of electrons displaced from an atom of a given element or to an atom of a given element in a compound is called oxidation state.
A positive oxidation state indicates the number of electrons that are displaced from a given atom, and a negative oxidation state indicates the number of electrons that are displaced towards a given atom.
From this definition it follows that in compounds with non-polar bonds, the oxidation state of the elements is zero. Molecules consisting of identical atoms (N 2 , H 2 , Cl 2) can serve as examples of such compounds.
The oxidation state of metals in the elementary state is zero, since the distribution of electron density in them is uniform.
In simple ionic compounds, the oxidation state of their constituent elements is equal to the electric charge, since during the formation of these compounds, an almost complete transfer of electrons from one atom to another occurs: Na +1 I -1, Mg +2 Cl -1 2, Al +3 F - 1 3 , Zr +4 Br -1 4 .
When determining the degree of oxidation of elements in compounds with polar covalent bonds, the values of their electronegativity are compared. Since, during the formation of a chemical bond, electrons are displaced to atoms of more electronegative elements, the latter have a negative oxidation state in compounds.
Highest oxidation state
For elements that exhibit different oxidation states in their compounds, there are concepts of higher (maximum positive) and lower (minimum negative) oxidation states. The highest oxidation state of a chemical element usually numerically coincides with the group number in the Periodic system of D. I. Mendeleev. The exceptions are fluorine (the oxidation state is -1, and the element is located in group VIIA), oxygen (the oxidation state is +2, and the element is located in group VIA), helium, neon, argon (the oxidation state is 0, and the elements are located in group VIII group), as well as elements of the cobalt and nickel subgroups (the oxidation state is +2, and the elements are located in group VIII), for which the highest oxidation state is expressed by a number whose value is lower than the number of the group to which they belong. The elements of the copper subgroup, on the contrary, have a higher oxidation state of more than one, although they belong to group I (the maximum positive oxidation state of copper and silver is +2, gold +3).
Examples of problem solving
EXAMPLE 1
- In hydrogen sulfide, the oxidation state of sulfur is (-2), and in a simple substance - sulfur - 0:
Change in the oxidation state of sulfur: -2 → 0, i.e. sixth answer.
- In a simple substance - sulfur - the oxidation state of sulfur is 0, and in SO 3 - (+6):
Change in the oxidation state of sulfur: 0 → +6, i.e. fourth answer.
- In sulfurous acid, the oxidation state of sulfur is (+4), and in a simple substance - sulfur - 0:
1×2 +x+ 3×(-2) =0;
Change in the oxidation state of sulfur: +4 → 0, i.e. third answer.
EXAMPLE 2
| Exercise | Valence III and oxidation state (-3) nitrogen shows in the compound: a) N 2 H 4; b) NH3; c) NH 4 Cl; d) N 2 O 5 |
| Solution | In order to give a correct answer to the question posed, we will alternately determine the valency and oxidation state of nitrogen in the proposed compounds. a) the valency of hydrogen is always equal to I. The total number of hydrogen valency units is 4 (1 × 4 = 4). Divide the value obtained by the number of nitrogen atoms in the molecule: 4/2 \u003d 2, therefore, the nitrogen valency is II. This answer is incorrect. b) the valency of hydrogen is always equal to I. The total number of hydrogen valence units is 3 (1 × 3 = 3). We divide the obtained value by the number of nitrogen atoms in the molecule: 3/1 \u003d 2, therefore, the nitrogen valency is III. The oxidation state of nitrogen in ammonia is (-3): This is the correct answer. |
| Answer | Option (b) |
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.
The degree of oxidation. Determination of the oxidation state of an atom of an element by the chemical formula of the compound. Compilation of the formula of the compound according to the known oxidation states of the atoms of the elements
The oxidation state of an element is the conditional charge of an atom in a substance, calculated with the assumption that it consists of ions. To determine the degree of oxidation of elements, it is necessary to remember certain rules:
1. The oxidation state can be positive, negative or zero. It is denoted by an Arabic numeral with a plus or minus sign above the element symbol.
2. When determining the oxidation states, they proceed from the electronegativity of the substance: the sum of the oxidation states of all atoms in the compound is zero.
3. If the compound is formed by atoms of one element (in a simple substance), then the oxidation state of these atoms is zero.
4. Atoms of some chemical elements are usually assigned oxidation states to steel. For example, the oxidation state of fluorine in compounds is always -1; lithium, sodium, potassium, rubidium and cesium +1; magnesium, calcium, strontium, barium and zinc +2, aluminum +3.
5. The oxidation state of hydrogen in most compounds is +1, and only in compounds with some metals is it equal to -1 (KH, BaH2).
6. The oxidation state of oxygen in most compounds is -2, and only in some compounds it is assigned an oxidation state of -1 (H2O2, Na2O2 or +2 (OF2).
7. Atoms of many chemical elements exhibit variable oxidation states.
8. The oxidation state of a metal atom in compounds is positive and numerically equal to its valency.
9. The maximum positive oxidation state of an element is usually equal to the group number in the periodic system in which the element is located.
10. The minimum oxidation state for metals is zero. For non-metals, in most cases, the lower negative oxidation state is equal to the difference between the group number and the number eight.
11. The oxidation state of an atom forms a simple ion (consists of one atom), equal to the charge of this ion.
Using the above rules, we determine the oxidation states of chemical elements in the composition of H2SO4. This is a complex substance consisting of three chemical elements - hydrogen H, sulfur S and oxygen O. We note the oxidation states of those elements for which they are constant. In our case, these are hydrogen H and oxygen O.
Let us determine the unknown oxidation state of sulfur. Let the oxidation state of sulfur in this compound be x.
Let's make equations by multiplying for each element its index by the oxidation state and equate the extracted amount to zero: 2 (+1) + x + 4 (-2) = 0
2 + X - 8 = 0
x = +8 - 2 = +6
Therefore, the oxidation state of sulfur is plus six.
In the following example, let's find out how you can write a formula for a compound with known oxidation states of the atoms of the elements. Let's make the formula of ferrum (III) oxide. The word "oxide" means that to the right of the symbol for iron, the symbol for oxygen should be written: FeO.
Note the oxidation states of chemical elements above their symbols. The oxidation state of iron is indicated in the name in brackets (III), therefore, it is equal to +3, the oxidation state of oxygen in oxides is -2.
Let's find the least common multiple for the numbers 3 and 2, this is 6. Divide the number 6 by 3, we get the number 2 - this is the index for iron. We divide the number 6 by 2, we get the number 3 - this is the index for oxygen.
In the following example, let's find out how to formulate a compound formula with known oxidation states of element atoms and ion charges. Let's make a formula of calcium orthophosphate. The word “orthophosphate” means that to the right of the Calcium symbol, the acid residue of orthophosphate acid should be written: CaPO4.
Note the oxidation state of calcium (rule number four) and the charge of the acid residue (according to the solubility table).
Let's find the least common multiple for the numbers 2 and 3, this is 6. Divide the number 6 by 2, we get the number 3 - this is the index for calcium. We divide the number 6 by 3, we get the number 2 - this is the index for the acid residue.
The oxidation state is the conditional charge of an atom in a molecule, it receives an atom as a result of the complete acceptance of electrons, it is calculated from the assumption that all bonds are ionic in nature. How to determine the degree of oxidation?
Determination of the degree of oxidation
There are charged particles, ions, whose positive charge is equal to the number of electrons received from one atom. The negative charge of an ion is equal to the number of electrons accepted by one atom of a chemical element. For example, the entry of such an element as Ca2 + means that the atoms of the elements have lost one, two or three elements. To find the composition of ionic compounds and compounds of molecules, we need to know how to determine the oxidation state of elements. The oxidation states are negative, positive and zero. If we take into account the number of atoms, then the algebraic oxidation state in the molecule is zero.
To determine the oxidation state of an element, you need to be guided by certain knowledge. For example, in metal compounds, the oxidation state is positive. And the highest oxidation state corresponds to the group number of the periodic system, where the element is located. In metals, oxidation states can be positive or negative. This will depend on the factor by which atom the metal is connected. For example, if it is connected to a metal atom, then the degree will be negative, but if it is connected to a non-metal, then the degree will be positive.
The negative highest oxidation state of the metal can be determined by subtracting the number of the group where the necessary element is located from the number eight. As a rule, it is equal to the number of electrons located on the outer layer. The number of these electrons also corresponds to the group number.
How to Calculate the Oxidation State
In most cases, the oxidation state of an atom of a particular element does not match the number of bonds that it forms, that is, it is not equal to the valence of this element. This can be clearly seen in the example of organic compounds.
Let me remind you that the valency of carbon in organic compounds is 4 (that is, it forms 4 bonds), but the oxidation state of carbon, for example, in methanol CH 3 OH is -2, in CO 2 +4, in CH4 -4, in formic acid HCOOH + 2. Valency is measured by the number of covalent chemical bonds, including those formed by the donor-acceptor mechanism.
When determining the oxidation state of atoms in molecules, an electronegative atom, when one electron pair is displaced in its direction, acquires a charge of -1, but if there are two electron pairs, then -2 will be a charge. The degree of oxidation is not affected by the bond between the same atoms. For example:
- The bond of C-C atoms is equal to their zero oxidation state.
- The C-H bond - here, carbon as the most electronegative atom will correspond to a charge of -1.
- The C-O bond, the charge of carbon, being less electronegative, will be +1.
Examples of determining the degree of oxidation
- In a molecule such as CH 3 Cl, there are three C-HC bonds). Thus, the oxidation state of the carbon atom in this compound will be equal to: -3 + 1 = -2.
- Let's find the oxidation state of carbon atoms in the acetaldehyde molecule Cˉ³H3-C¹O-H. In this compound, three C-H bonds will give a total charge on the C atom, which is (Cº+3e→Cˉ³)-3. The double bond C = O (here oxygen will take electrons from the carbon atom, because oxygen is more electronegative) gives a charge on the C atom, it is +2 (Cº-2e → C²), while the C-H bond has a charge of -1, which means the total the charge on atom C is: (2-1=1)+1.
- Now let's find the oxidation state in the ethanol molecule: Cˉ³H-Cˉ¹H2-OH. Here, three C-H bonds will give a total charge on the C atom, which is (Cº+3e→Cˉ³)-3. Two C-H bonds will give a charge on the C atom, which will be equal to -2, while the C→O bond will give a charge of +1, which means the total charge on the C atom: (-2+1=-1)-1.
Now you know how to determine the oxidation state of an element. If you have at least basic knowledge of chemistry, then this task will not be a problem for you.
In chemical processes, the main role is played by atoms and molecules, the properties of which determine the outcome of chemical reactions. One of the important characteristics of an atom is the oxidation number, which simplifies the method of taking into account the transfer of electrons in a particle. How to determine the oxidation state or the formal charge of a particle and what rules do you need to know for this?
Any chemical reaction is due to the interaction of atoms of various substances. The reaction process and its result depend on the characteristics of the smallest particles.
The term oxidation (oxidation) in chemistry means a reaction during which a group of atoms or one of them lose electrons or gain, in the case of acquisition, the reaction is called "reduction".
The oxidation state is a quantity that is measured quantitatively and characterizes the redistributed electrons during the reaction. Those. in the process of oxidation, the electrons in the atom decrease or increase, being redistributed among other interacting particles, and the level of oxidation shows exactly how they are reorganized. This concept is closely related to the electronegativity of particles - their ability to attract and repel free ions from themselves.
Determining the level of oxidation depends on the characteristics and properties of a particular substance, so the calculation procedure cannot be unambiguously called easy or complex, but its results help to conventionally record the processes of redox reactions. It should be understood that the obtained result of calculations is the result of taking into account the transfer of electrons and has no physical meaning, and is not the true charge of the nucleus.
It's important to know! Inorganic chemistry often uses the term valency instead of the oxidation state of elements, this is not a mistake, but it should be borne in mind that the second concept is more universal.
The concepts and rules for calculating the movement of electrons are the basis for classifying chemicals (nomenclature), describing their properties and compiling communication formulas. But most often this concept is used to describe and work with redox reactions.
Rules for determining the degree of oxidation
How to find out the degree of oxidation? When working with redox reactions, it is important to know that the formal charge of a particle will always be equal to the magnitude of the electron, expressed in numerical value. This feature is connected with the assumption that the electron pairs that form a bond are always completely shifted towards more negative particles. It should be understood that we are talking about ionic bonds, and in the case of a reaction at , electrons will be divided equally between identical particles.
The oxidation number can have both positive and negative values. The thing is that during the reaction, the atom must become neutral, and for this you need to either attach a certain number of electrons to the ion, if it is positive, or take them away if it is negative. To designate this concept, when writing formulas, an Arabic numeral with the corresponding sign is usually written above the designation of the element. For example, or etc.
You should know that the formal charge of metals will always be positive, and in most cases, you can use the periodic table to determine it. There are a number of features that must be taken into account in order to determine the indicators correctly.
Degree of oxidation:
Having remembered these features, it will be quite simple to determine the oxidation number of elements, regardless of the complexity and number of atomic levels.
Useful video: determining the degree of oxidation
The periodic table of Mendeleev contains almost all the necessary information for working with chemical elements. For example, schoolchildren use only it to describe chemical reactions. So, in order to determine the maximum positive and negative values of the oxidation number, it is necessary to check the designation of the chemical element in the table:
- The maximum positive is the number of the group in which the element is located.
- The maximum negative oxidation state is the difference between the maximum positive limit and the number 8.
Thus, it is enough to simply find out the extreme boundaries of the formal charge of an element. Such an action can be performed using calculations based on the periodic table.
It's important to know! One element can have several different oxidation indices at the same time.
There are two main ways to determine the level of oxidation, examples of which are presented below. The first of these is a method that requires knowledge and skills to apply the laws of chemistry. How to arrange oxidation states using this method?
The rule for determining oxidation states
For this you need:
- Determine whether a given substance is elemental and whether it is out of bond. If yes, then its oxidation number will be equal to 0, regardless of the composition of the substance (individual atoms or multilevel atomic compounds).
- Determine whether the substance in question consists of ions. If yes, then the degree of oxidation will be equal to their charge.
- If the substance in question is a metal, then look at the indicators of other substances in the formula and calculate the metal readings by arithmetic.
- If the entire compound has one charge (in fact, this is the sum of all the particles of the elements presented), then it is enough to determine the indicators of simple substances, then subtract them from the total amount and get the metal data.
- If the relationship is neutral, then the total must be zero.
For example, consider combining with an aluminum ion whose total charge is zero. The rules of chemistry confirm the fact that the Cl ion has an oxidation number of -1, and in this case there are three of them in the compound. So the Al ion must be +3 for the entire compound to be neutral.
This method is quite good, since the correctness of the solution can always be checked by adding all the oxidation levels together.
The second method can be applied without knowledge of chemical laws:
- Find particle data for which there are no strict rules and the exact number of their electrons is unknown (possible by elimination).
- Find out the indicators of all other particles and then from the total amount by subtracting find the desired particle.
Let us consider the second method using the Na2SO4 substance as an example, in which the sulfur atom S is not defined, it is only known that it is nonzero.
To find what all oxidation states are equal to:
- Find known elements, keeping traditional rules and exceptions in mind.
- Na ion = +1 and each oxygen = -2.
- Multiply the number of particles of each substance by their electrons and get the oxidation states of all atoms except one.
- Na2SO4 consists of 2 sodium and 4 oxygen, when multiplied it turns out: 2 X +1 \u003d 2 is the oxidizing number of all sodium particles and 4 X -2 \u003d -8 - oxygen.
- Add the results 2+(-8) = -6 - this is the total charge of the compound without a sulfur particle.
- Express the chemical notation as an equation: sum of known data + unknown number = total charge.
- Na2SO4 is represented as follows: -6 + S = 0, S = 0 + 6, S = 6.
Thus, to use the second method, it is enough to know the simple laws of arithmetic.
Table of oxidation
For ease of operation and calculation of oxidation indicators for each chemical, special tables are used, where all the data are recorded.
It looks like this:
Useful video: learning to determine the degree of oxidation by formulas
Conclusion
Finding the oxidation state for a chemical is a simple operation that requires only care and knowledge of the basic rules and exceptions. Knowing the exceptions and using special tables, this action will not take much time.
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