Chemical equation

Formula for chemical reaction of substance

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Chemical equation, also known as chemical reaction equation, is used chemical formula （ Organic chemistry Medium organic matter for general use skeleton symbol ）To express the chemical reaction of a substance.^[1]

The chemical equation not only shows the reactants, products and reaction conditions. At the same time, Stoichiometric number It represents the quantity relationship of reactants and products relative molecular mass Or relative atomic mass can also represent the mass relationship between substances, that is, the mass ratio between substances. For gaseous reactants and products, the volume ratio can also be obtained directly from the stoichiometric number.

Chinese name: Chemical equation
Foreign name: chemical equation

Discipline: Chemistry
Alias: Chemical reaction formula
Reaction conditions: heating 、 high temperature 、 light 、 electrolysis etc.

catalog

▪ Parity balancing
▪ Oxygen acquisition and deoxidation method
▪ Postulate one method
four Common reactions in junior high school
▪ oxygen
▪ water

Writing points

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Writing Steps

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For example, the chemical equation of thermal decomposition:

Step 1: Write the chemical formulas of reactants and products.

Step 2: Balance the chemical formula.

Step 3: indicate the reaction conditions and state of matter.

Step 4: Check whether the chemical equation is correct.^[2]

Reaction conditions

1. Write reaction conditions in chemical equation. Generally, mark Δ above the equal sign. The trim coefficient can be an integer or a fraction^{[5 ]} (Except that they can be reduced to each other).

2. It is unnecessary to write conditions for reactions that can be carried out under normal temperature and pressure; However, if there are specific conditions for the experiment in the title, the specific conditions (such as heating) shall be marked.

3. When a single condition is reflected, the condition shall be written above; If there are two or more conditions, those that cannot be written down on the top should be written down below; If there is both catalyst and other reaction conditions, the catalyst shall be written on it.

four Reversible reaction use Double Arrow express.^[2]

Arrows and equal signs

In the literature of professional field, whether inorganic Organic reaction The arrow "→" is used to connect reactants and products in the equation. The arrow numbers are also used in foreign chemistry textbooks, Shanghai secondary school chemistry curriculum reform textbooks (Shanghai Science and Technology Press, July 2006, edited by Yao Zipeng), 2011-2012 Jiangsu Education Press chemistry books, Shanghai college entrance examination chemistry volume and science comprehensive volume. However, in the middle school teaching in most provinces and cities, it is still recommended to use the equal sign when writing the inorganic chemical reaction equation according to the textbook, so as to better express the "generation" and“ mass conservation ”Meaning.^[2]

Secondary school stage in 2013 Inorganic reaction The equation always uses the equal sign to connect the reactants and products, Organic reaction Always use arrows to connect reactants and products. Yes Reversible reaction All are represented by reversible symbols.^[2]

Gas symbol and sediment symbol

1. The gas symbol "↑" and the precipitation symbol "↓" are the status symbols of the products in the chemical reaction. Only products can use "↑" or "↓" symbols, which are written on the right side of the corresponding chemical formula.

2. "↑" is only used when the product is in gaseous state under reaction conditions, such as:

However, two points should also be noted:

(1) If there is one or more gaseous substances in the reactants, the generated gaseous substances will not use "↑", such as:

(2) No matter what the state of the reactant is, as long as the product has only one substance and is a gas, "↑" is not needed.

3. The chemical reaction carried out in the solution with insoluble or insoluble substances is represented by "↓". For example:

4. If the reaction is not carried out in the solution, there will be no precipitation, and the number "↓" cannot be used. For example:

5. For the replacement reaction between metal and salt solution, the generated metal simple substance, because it is covered on the surface of metal and has no precipitation, should not use "↓", such as:

6. In the metathesis reaction, if the product is slightly soluble, it should be regarded as "precipitation" and marked with "↓". For example:

7. It must be based on objective facts. Even for the same chemical reaction, it must depend on the specific situation. If hydrogen sulfide reacts with sulfur dioxide gas to generate sulfur, no precipitation symbol is needed, namely:

When sulfur dioxide is introduced into hydrogen sulfuric acid, yellow turbidity appears in the solution, and the generated elemental sulfur should be marked with "↓", that is

Another example is to drop sodium sulfide solution into the silver chloride suspension, and the white precipitate turns to black, and the precipitate is transformed. The silver sulfide generated should be marked with "↓", that is:

Substance state

Thermochemical equation

The heating chemical equation needs to mark the state of each reactant and product, which can be divided into four categories: g (gas), l (liquid), s (solid) and aq (solution). Brackets are required outside the letter, such as H two O (l)、Na two CO three (aq)、CaCO three (s)。

Ordinary equations do not use the labels g, l, s, aq. In terms of products, when there is no gas in the reactants, the gas products should be marked with "↑"; Only in the solution, when the reactant contains no solid, the insoluble or solid products shall be marked with "↓". For example, chemical equation of electrolytic cell reaction:

Since the generated Cu is adsorbed on the electrode, it is not written“ ↓ ”。^[2]

steam

steam Generally, arrows are used, but occasionally there are exceptions（ Ammonium bicarbonate Thermal decomposition).

Different reactant states

If the reactant participates in the reaction in a special state, such as concentrated sulfuric acid, melting alumina , excessive and small amount, always mark the status with brackets after the corresponding reactants

<1> Different reactant concentrations lead to different products

For example, the chemical equation for the reaction of copper with concentrated nitric acid:

Chemical equation for the reaction of copper with dilute nitric acid:

<2> reactant Different amounts of, different products

For example, the chemical equation for the reaction of magnesium with excess carbon dioxide:

Chemical equation of magnesium reacting with a small amount of carbon dioxide:

<3> Reactant State of matter Different products

For example, chemical equation of electrolytic sodium chloride solution:

Chemical equation of electrolytic molten sodium chloride:

skill

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To correctly write the chemical formula of substances, first, we should be proficient in memorizing the element symbols; second, when writing the chemical formula of compounds, we should be proficient in memorizing the valence of common elements and atomic groups; third, writing the chemical formula of compounds should conform to the algebraic sum of positive and negative valence of each element and the principle of zero; fourth, we should master the general method of writing chemical formulas. In general, this is the essence of writing chemical formulas.^[3]

The general writing method of the chemical formula is:

(1) Chemical formula of simple substance

Division H two 、O two 、N two 、F two 、Cl two 、Br two And I two Seven simple substances are diatomic molecules, O three Is a triatomic molecule, P four Other simple substances (rare gases, metal simple substances and non-metallic solid simple substances, etc.) are expressed in their chemical formulas by element symbols except for tetraatomic molecules.

(2) Chemical formula of compound

When writing, the general rule is to read first, then write, and then read first write; Arrange the order and write the horn. The element or Atomic cluster It is written on the left, and the elements or atomic groups with negative valence are written on the right. The writing steps are: positioning, pricing, cross reduction and testing.

① Positioning: determining the order of elements or atomic clusters.

② Mark valence: mark the valence of each element or atomic group correctly.

③ Cross reduction: the absolute value of the reduction value is cross written at the lower right of the opposite element or atomic group, and the offer that can be reduced is the simplest number. Atom or Atomic cluster When the number of is 1, 1 is usually omitted. When there are two or more atomic groups, parentheses should be added.

④ Test: The chemical formula is correct when the sum of the total number of positive and negative prices is zero.

Writing Skills

1. Steps of writing chemical equation

Taking writing the chemical equation of carbon reacting with iron oxide at high temperature as an example, the steps are summarized as follows:

① According to the fact of the reaction (the reaction produces iron and carbon dioxide gas), write the chemical formula of the reactant on the left side of the formula and the chemical formula of the product on the right side of the formula. If there is more than one reactant or product, use plus Connect them ("+" between reactants means "reaction with...", and "+" between products means "reaction with..."), and draw a short line between the left and right of the formula. In this step, be sure to write the chemical formula of various substances correctly, otherwise, the formula is meaningless!

② According to the law of conservation of mass, the number of atoms of the same element on the left and right sides is equal by using the method of matching coefficient, and then the short line segment is changed to an equal sign (the equal sign means "generated..."). It should be noted that only appropriate coefficients can be selected during leveling, and the chemical formula cannot be changed.

③ The reaction conditions such as ignition, electrolysis, high temperature, heating (indicated by "△"), catalyst, etc. shall be indicated above and below the equal sign or arrow, and the state of the gas or precipitate in the product shall be indicated. If the product is gas, add "↑" to the right of its chemical formula (if there is gas in the reactant, the gaseous product will not be marked with "↑" symbol); If the product is precipitated, add "↓".

In a word, when writing chemical equations, we must conform to the objective laws of chemical reactions, and we cannot make assumptions. The writing procedure is generally as follows: write the chemical formula well - the coefficient should be balanced - the middle line should be equal - the conditions should be clear - the generated gas or sediment should be marked with arrows.

To write chemical equations well, one should also master and use the solubility table of acid, alkali and salt, the sequence of metal activity, the valence of common elements and atomic groups, and the skills to write chemical formulas of substances.

2. Analysis of common mistakes in writing chemical equations

The first is the "formula". Some students, while writing chemical equations, paid attention not to invent chemical equations out of thin air, but did not master the method of combining typical memory with type deduction, coupled with the lack of basic chemical knowledge, so they used a familiar chemical equation they remembered as a general formula, making a mistake of "formula".

The second is reverse reaction of scribbling. The reason for this kind of mistake is that junior high school students combine the chemical equation with Mathematical equation The formula is confused, and the chemical equation is written upside down at will. We know that mathematical equations can transposition , or will Equal sign The left and right ends are written in reverse; Chemical equation and mathematical equation are totally different concepts. Although some chemical reactions are reversible, most chemical reactions are not. As for some reactions, under the same or different conditions, they can go in the opposite direction. They are not necessarily reversible reactions, but have completely opposite meanings Reaction type 。

The third is the state and reaction conditions of the disorderly labeled products.

The fourth is to scribble the chemical formula.^[4] For example:

calculation

1. Calculation according to chemical equation

The chemical equation not only represents what substances participate in the reaction and what substances are generated after the reaction, but also represents the mass ratio of reactants and products. The calculation according to the chemical equation is based on the mass ratio between reactants and products. Therefore, given the mass of reactants, the mass of products can be calculated; Given the mass of the product, the mass of the reactant can be calculated; Given the mass of one reactant participating in the reaction, the mass of another reactant participating in the reaction can be calculated; Given the mass of one product, the mass of another product can be calculated.

Chemical calculation questions reflect the substance and its change law from the aspect of "quantity". Chemical calculation It includes two factors: chemistry and mathematics. Chemical knowledge is the basis of mathematical calculation, and mathematics is the tool of chemical calculation. According to the calculation of chemical equations, it is necessary to have a clear understanding of relevant chemical knowledge and master the chemical equations of relevant reactions. If the chemical formula in the chemical equation is written incorrectly or not balanced, the chemical calculation will inevitably lead to wrong results.

2. Calculation steps according to chemical equation

(1) Set unknown quantity according to the meaning of the question.

(2) Write the correct chemical equation according to the meaning of the question.

(3) Write the relative molecular weight or the sum of the relative molecular weight and the known and unknown quantities of the relevant substances. It is written below the corresponding chemical formula of the chemical equation.

(4) Column scale formula, solve.

(5) A concise answer.

3. Precautions for calculation according to chemical equation

(1) The format of problem solving must be standardized.

(2) According to the chemical equation, the calculation is based on Pure substance The mass of impure reactants or impure products must be converted into the mass of pure products before calculation.

(3) The unit must be used correctly.

4. Common errors in calculation according to chemical equation:

(1) Don't examine the questions carefully and answer the questions inappropriately.

(2) The element symbol or chemical formula is incorrectly written.

(3) The chemical equation is not balanced.

(4) Relative molecular weight calculation error.

(5) The unit is used incorrectly.

(6) Calculate the impure matter as pure matter.

Trim method

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Least common multiple method

A、 Find the atoms that have a large number of atoms and appear once on both sides of the reaction formula, and find its least common multiple.

B、 The coefficients of each molecule are derived.

For example:

Step 1: CuSO four +NaOH—Cu(OH) two +Na two SO four

Step 2: CuSO four +2NaOH—Cu(OH) two +Na two SO four (Trim)

Step 3: CuSO four +2NaOH=Cu(OH) two ↓+Na two SO four (Indicate the product state)

Observation method

Calculate the stoichiometric number of the chemical formula of each reactant and the stoichiometric number of the product from a product with complex chemical formula; According to the stoichiometric number of the chemical formula obtained, find out the stoichiometric number of other chemical formulas until balancing.

For example: Step 1 H two O(g)+Fe—Fe three O four +H two

Step 2: 4H two O(g)+3Fe—Fe three O four +H two

Step 3

Parity balancing

See which element appears most frequently on the left and right sides of the reaction chemical equation; Starting from the chemical formula with odd number of elements, match them to even number (i.e., the stoichiometric number is 2); The stoichiometric number obtained from it balances the stoichiometric number of other chemical formulas, so that the number of atoms on both sides is equal.

Example: trim

H two O(g)+Fe—Fe three O four +H two

Step 1: Trim Oxygen atom

4H two O(g)+Fe—Fe three O four +H two

Step 2: Trim hydrogen atom , iron atom

4H two O(g)+3Fe—Fe three O four +4H two

Step 3: chemical equation after balancing:

4H two O(g)+3Fe=Fe three O four +4H two (Reaction condition: heating)

Oxygen acquisition and deoxidation method

about Redox reaction First observe the number of oxygen lost by oxidant, then observe the number of oxygen gained by reductant, and then balance

For example: 3CO+Fe two O three =2Fe+3CO two

Oxidant ferric oxide Three oxygen is lost before and after reaction, reducing agent One oxygen is obtained before and after the carbon monoxide reaction, so three carbon monoxide are needed to take away the oxygen in iron oxide. Carbon monoxide and carbon dioxide are matched with 3, and iron is matched with 2

Postulate one method

(1) Scope of application: It is applicable to the reaction of compounds containing carbon and hydrogen (or compounds containing carbon, hydrogen and oxygen) with oxygen.

(2) Step: find a substance with complex chemical formula in the reactants or products, set its stoichiometric number as 1, and calculate the stoichiometric number of its related chemical formula based on this chemical formula.

Common reactions in junior high school

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oxygen

(1) Simple substance and oxygen Reaction of:（ Combination reaction ）

1. Magnesium burning in oxygen:

2. Iron burning in oxygen:

3. Copper heated in oxygen:

4. Hydrogen burning in oxygen:

5． phosphorus Combustion in oxygen (experiment to study air composition):

6. Sulfur burning in oxygen:

7. Full combustion of carbon in oxygen:

8. Incomplete combustion of carbon in oxygen:

(2) Reaction of oxygen with compounds:

9. Carbon monoxide burning in oxygen:

10. Methane burning in oxygen:

11． alcohol Burning in oxygen:

(3) Source of oxygen:

12. Lavoisier studies the composition of air

13. Heating potassium permanganate:

(Principle of oxygen production in laboratory 1)

14. Heating Potassium chlorate And manganese dioxide mixture:

(Principle of oxygen production in laboratory 2)

15． hydrogen peroxide stay manganese dioxide Decomposition reaction under the condition of catalyst:

(Principle of oxygen production in laboratory 3)

Photosynthesis of plants:

water

16. Decomposition of water under the action of direct current (research on water composition experiment):

17. Quick lime dissolved in water:

Carbon and carbon oxides

(1) Carbonaceous chemical property

18. Carbon in oxygen Moderate and full combustion:

19. Reduction of copper oxide by charcoal:

20. Reduction of iron oxide by coke:

(2) Three reactions occurred in the coal furnace: (several chemical reactions)

21. Bottom layer of coal furnace:

22. Middle layer of coal furnace:

23. Generation of blue flame at the upper part of coal furnace:

(3) Preparation and properties of carbon dioxide:

24. Reaction of marble with dilute hydrochloric acid (carbon dioxide produced in laboratory):

25. Carbonic acid decomposes due to instability:

26. Carbon dioxide is soluble in water:

27. High temperature calcined limestone (industrial carbon dioxide):

28. Reaction of lime water with carbon dioxide (identification of carbon dioxide):

29. Lime water reacts with sulfur dioxide (so if sulfur element participates in the reaction, the clarified lime water cannot identify carbon dioxide

(4) Nature of carbon monoxide:

30. Reduction of copper oxide by carbon monoxide:

31. Flammability of carbon monoxide:

Other reactions:

32． sodium carbonate Reaction with dilute hydrochloric acid (principle of chemical reaction fire extinguisher):

Metal

(1) Metal reacts with oxygen:

33. Magnesium burning in air:

34. Iron burning in pure oxygen:

35. Copper heated in air:

36. Aluminum forms aluminum oxide film in air:

(2) Metal element+acid → salt+hydrogen（ Displacement reaction ）

37. Zinc and dilute sulfuric acid (hydrogen production in laboratory)

38. Iron and dilute sulfuric acid

39. Magnesium and dilute sulfuric acid

40. Aluminum and dilute sulfuric acid

(3) Metal element+salt (solution) → new metal+new salt

41. Reaction of iron with copper sulfate solution:

42. Reaction of zinc and copper sulfate solution:

43. Copper reacts with mercuric nitrate solution:

44. Copper and silver nitrate Reaction:

45. Reaction of aluminum and copper sulfate solution:

(4) Smelting principle of iron:

46．

Common reactions in high school

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Basic reaction type	Reaction phenomenon	purpose
Combination reaction	Hydrogen burns in chlorine, giving off a pale flame, generating a lot of heat and white fog	Synthesis of hydrogen chloride to prepare hydrochloric acid.
Combination reaction	White smoke generation	—
Combination reaction	White smoke generation	—
Combination reaction	There is a yellow flame, which emits a lot of heat and produces white smoke.	—
Combination reaction	It burns violently to produce brown smoke.	—
Combination reaction	It burns violently to produce brown smoke.	—
Combination reaction	It burns violently to produce brown and yellow smoke.	—
Combination reaction	—	—
Combination reaction	—	—
Combination reaction	—	—
Displacement reaction	At the beginning, milky white precipitates are formed, and then the precipitates become light yellow	—
Redox reaction	The gas in the test tube fades, and irritating odor gas escapes from the test tube mouth	—
Displacement reaction	—	—
Combination reaction	Iron burns violently in pure oxygen and sparks all around, releasing a lot of heat and forming a black solid.	—
Combination reaction	Potassium burns in pure oxygen with light purple flame to produce yellow solid.	preparation Potassium superoxide 。
Combination reaction	—	—
Combination reaction	—	—
Combination reaction	After heating and melting, the solid turns black.	—
Combination reaction	After heating, it will react violently to emit white light and generate a lot of heat.	—
Combination reaction	The gas with pungent odor escapes	Industrial synthetic ammonia
Combination reaction	Magnesium burns in nitrogen, giving off dazzling white light, releasing a lot of heat, and producing a yellowish solid.	—
Combination reaction	Hydrogen is ignited in bromine vapor to release heat, and the reddish brown color gradually fades to produce white fog.	—

Memory method

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Master the writing essentials of chemical equation

Write reactants on the left, and generate on the right. Write the chemical formula and coefficient to balance, with an equal sign in the middle. Note the conditions clearly, generate precipitation gas, and indicate with an arrow.

Categorical memory

According to the memory of knowledge series or reaction types or material categories, the scattered knowledge can be summarized into a regular knowledge network centered on key materials.

Memory of connection experiment phenomenon

Chemical equations are boring and difficult to remember. How to make the learning of chemical terms lively and interesting can be linked to the memory of experimental phenomena. If sulfur burns in oxygen, the memory association is: burning sulfur into oxygen, the combustion becomes stronger, the flame is blue and purple, beautiful, and produces gas, which is really "choking". Phosphorus burns in oxygen, memory association: red phosphorus burns, but it is really lively, thick smoke billows, bright like a "bulb".

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