How to tell how many electrons are lost or gained in a redox reaction

It is judged by the valency of the element in the substance before and after the chemical reaction.

When the elements are combined with each other, the ratio of the number of reactant atoms is always constant. For example, a sodium atom must be bound to a chlorine atom. One mg atom must be bound to two chlorine atoms.

If it were not for this number ratio, the anions and cations that make up ionic compounds and the outermost electron shells of atoms that make up the molecules of covalent compounds would not be stable structures. It is not possible to form stable compounds. And since atoms are the smallest particles that cannot be separated in a chemical reaction, when the elements are combined with each other to form a certain compound, there must be a certain simple integer ratio between the number of electrons outside the nucleus that changes between the elements.

The concept of valency comes from this, then the number of electrons outside the nucleus of an element that are mutually combined determines the valency of this element, and the valency is set to conveniently represent the number of atoms that are mutually combined. When learning valency, you should be aware of the rules for elemental valency in compounds.

In addition, it is stipulated that in elemental molecules, the valency of elements is zero, and the algebraic sum of positive and negative valencies of both ionic compounds and covalent compounds is zero.

The easiest way to do this is to mark the valency of the elements.

If the valency increases, electrons are lost, and when the valency decreases, electrons are gained.

The valency increases, electrons are lost, and an oxidation reaction occurs.

<> in the simplest redox reaction, electron gain and loss are easy to judge. Suppose 1molo2 and 2molh2 ignite in a closed container and react completely. As shown in the figure, write the equation, mark the valency of the species before and after the reaction, and then draw the two-line bridge.

The number of electrons gained and lost on a bridge is the number of electrons transferred – 4Na.

After understanding this truth, the electron gain and loss of all redox reactions will become easy. For example, complex reactions:

lz pushes the valency himself and sees if he can calculate the number of electron gains and losses. Answer: 2na 4na 20na

No matter how complex the equation is, the approach is the same. The valency is marked first, and then the valency change is marked with a double-line bridge to calculate the electron gain and loss. The above three are a bit difficult, just understand it, the key is the method. If you have any questions, please feel free to ask.

The first reaction is not trimmed and should be Cu+4Hno3(concentrated)==Cu(NO3)2+2NO2+2H2O

CU changes from 0 valence to +2, so it loses 2 electrons. , this sentence is true, but when considering the gain and loss of electrons of a reaction, we should also consider how many copper gains and losses there are, and in this reaction there is exactly one so that two electrons are lost. And n changes from +5 to +4, one electron is obtained, but there are two n valences in this reaction, so there should be two electrons.

The second reaction after 3*2 is a total of three oxygen molecules, each oxygen molecule contains two oxygen atoms, multiplied by a total of six oxygen atoms valence, so it becomes twelve valence.

Every n gets an electron, and the valency of two n changes so it's 2

The first equation is mismatched, and the coefficient before nitrogen dioxide should be 2, so 1*2=2, and the number of electrons obtained is 2

3*2 is the ,

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Ideas for doing questions:

In general, it mainly depends on the change of the valency of the element, how much the valency changes, and then multiplies the element.

Definition: Oxidation-reduction reaction (also known as redox reaction) is a type of reaction in which the oxidation number of an element changes before and after a chemical reaction.

Significance: 1. Combustion, respiration, photosynthesis, chemical batteries in production and life, metal smelting, rocket launching, etc. in nature are closely related to redox reactions.

2. The study of redox reaction is of great significance to the progress of mankind.

The number of transferred electrons in a redox reaction is equal to the product of the change in valency of an element and the number of atoms! Such as:

2h2+o2=2h2o

h changes from 0 to 1, and the change number is 1

The atomic number is 2*2=4

Then the number of electrons transferred is: 4*1=4

There is electron transfer when the valency changes before and after the reaction.

The number of changes in valency multiplied by the number of atoms is the number of electrons transferred, such as 4Fe+3O2=2Fe2O3

Fe from 0 valence - Fe + 3 valence, the valency increases by 3, that is, the loss of 3 electrons 4Fe reaction, that is, the loss of 3x4 = 12 electrons.

Redox reaction electron gain and loss are conserved.

The electrons lost by Fe must have been gained by O.

o From 0 valence - o-2 valence, 2 electrons are obtained.

6o gives 6x2=12 electrons.

The whole process is that Fe loses 12 electrons, these 12 electrons are transferred to O2, and the whole process transfers 12 electrons.

Abstract: Redox reaction is one of the backbone knowledge of high school chemistry, and the reproducibility rate of the college entrance examination over the years is almost 100, and the calculation of transfer electrons is the focus and hot spot of the college entrance examination proposition. The syllabus requires "an understanding of the essence of redox reactions as electron transfer, and an understanding of common redox reactions."

In recent years, the trend of examining the number of electron transfer in the chemistry test questions of the college entrance examination is mainly "judging how many molar electrons are transferred by a certain amount of a substance in a reaction" and "inferring how many molar electrons a certain amount of substance gains or loses to form the chemical formula of a certain substance". The main basis for answering such questions is the basic law of redox reactions.

The number of electrons gained and lost in the redox reaction mainly depends on how much the valency of the atom where the valency change occurs, how many electrons are gained and lost, and then multiplied by the coefficient to reflect the total number of electrons gained and lost. As shown in Fig.

Looking at the change of valency, the amount of gain and loss will rise and fall as much as the electron valence increases. Pay attention to the coefficient.

Look at the rise and fall of the valence, and the amount of gain and loss will rise and fall as much as the valency of the electron valence increases.