How to balance the chemical equation in junior high school

1. In general, find a relatively single element, that is, it appears in a substance before the reaction, and appears in a substance after the reaction, if there is an odd number and an even number, it will be flattened according to the odd number, which is the front of the odd number *2 If there is still a fraction in the result *4

2. Observation method, junior high chemical reaction and decomposition reaction are generally directly visible, ensuring that there are a few atoms in front and a few atoms in the back.

The balancing of the equation is fundamentally because it is an equation, the meaning of the equation is that the left and right sides should be equal, and there is generally nothing difficult in junior high school, mainly because the number of atoms on the left and right sides is equal.

For example, if you see that there are 2 o atoms on the left and 3 o atoms on the right, then the least common multiple is 6, then the o on the left is multiplied by 3, the right is multiplied by 2, and so on. After trimming, check to see if all the atoms are equal in number.

a) Least common multiple method.

This method is suitable for common chemical equations that are not too difficult. For example, in this reaction formula, the number of oxygen atoms on the right is 2, and the number of oxygen atoms on the left is 3, then the least common multiple is 6, so the coefficient before kclo3 should be matched with 2, and the coefficient before O2 should be matched with 3, and the formula becomes: 2kclo3 KCl+3O2, since the number of potassium atoms and chlorine atoms on the left becomes 2, then the coefficient 2 before KCL, ** is changed to equal sign, indicating the condition is:

2kclo3==2kcl+3o2↑

2) Odd-even equalization.

This method is suitable for multiple occurrences of an element on both sides of a chemical equation where the total number of atoms of the element on both sides is odd and even, for example: C2H2+O2 CO2+H2O, and the balance of this equation starts with the oxygen atom with the highest number of occurrences. There are 2 oxygen atoms in O2, and the total number of oxygen atoms should be even, regardless of the number of coefficients before the chemical formula.

Therefore, the coefficient of H2O on the right should be matched with 2 (if other molecular coefficients are introduced, the fraction can be matched with 4), from which it is inferred that the first 2 of C2H2, the formula becomes: 2C2H2+O2 CO2+2H2O, from which it can be seen that the coefficient before CO2 should be 4, and the final element O2 is 5

2c2h2+5o2==4co2+2h2o

c) Observational balancing.

Sometimes there will be a substance with a more complex chemical formula in the equation, we can deduce the coefficients of other chemical formulas through this complex molecule, for example: Fe + H2O Fe3O4 + H2, Fe3O4 chemical formula is more complex, obviously, Fe 3 in Fe ** in the elemental Fe, O comes from H2O, then Fe is preceded by 3, H2O is preceded by 4, then the formula is: 3Fe + 4H2O Fe3O4 + H2 From this, the H2 coefficient is 4, and the conditions are written, ** can be changed to an equal sign:

3fe+4h2o==fe3o4+4h2

1. The easiest trimming method is direct trimming.

2. Another complex chemical equation should be balanced by the valence rise.

Like the formula Z Fe3O4+4Co=3Fe+4CO2 you can just memorize the stoichiometric numbers 1,4,3,4. For some equations, when you can't see how to match them at first glance, you can write the stoichiometric numbers as fractions on scratch paper, and then reduce them.

Junior high school chemistry is very simple, and when you are proficient in it at the end, you don't have to think about how to balance it in your own head, and you will write it naturally. Study hard

The general reaction can be observed and the redox reaction can be conserved by electron gain and loss.

The method of balancing chemical equations in junior high schools is pending coefficients method, odd-even balancing method, normalization method, observation method, least common multiple method, etc., and the specific methods of balancing chemical equations in junior high school are shared next.

The principle is: before and after the chemical reaction, "two constants" - the type of elements does not change, and the number of atoms does not change. For more complex chemical equations, this method can be used by first adding the alphabetical measurement number before each chemical formula, then listing the equivalent relations according to conservation, that is, the system of equations, and finally solving the equation to obtain the solution (taking a certain letter as a constant).

1) Find the key chemical formula in the chemical equation, set the number before the chemical formula as 1, and then balance the stoichiometric number before the other chemical formula according to the key chemical formula.

2) If there are measured numbers as fractions, multiply each measured number by the same integer to turn the fraction into an integer.

1) Estimating the stoichiometric number of the chemical formula of each reactant and the stoichiometric number of the product from a product with a complex chemical formula.

2) According to the stoichiometric number of the obtained chemical formula, find the stoichiometric number of other chemical formulas until the trim.

1) Find the atom with a large number of atoms and appear once on each side of the reaction formula, and find its least common multiple.

2) The coefficients of each molecule are introduced.

1) Find the element with the highest number of atoms at the left and right ends of the chemical equation and find their least common multiple.

2) Divide this least common multiple by the original number of atoms on the left and right sides, and the quotient is the stoichiometric number of their chemical formula.

3) According to the stoichiometric number of the determined chemical formula of the substance, derive and find the stoichiometric number of the chemical formula until the chemical equation is balanced.

Trim method for junior high school chemical equations:

1. Zero-price method

Usage: It is impossible to determine that each element in the valency of a substance with valence is zero valence by conventional methods, and then calculate the rise and fall value of each element, and make the valence rise and fall value of the elements equal, and finally use the observation method to balance the stoichiometric numbers of other substances.

Example 1] Trial Match Fe3C + Hno3 Fe(No3)3 + No2 + CO2+ H2O.

Analysis: The specific valency of Fe and C cannot be determined according to the conventional valency analysis of the complex substance Fe3C, and the valence of each element constituting the substance can be zero, and then balanced according to the valence rise and fall method. Fe3C Fe(NO3)3 and CO2 increased by 13 valence, and HNo3 NO2 decreased by 13 valence (except for Fe and C, only N varied).

Fe3C + 22Hno3 3Fe(NO3)3 + 13NO2+ CO2 + 11H2O.

2. Average price method

Usage: When the atoms of the same element in the same reactant appear twice and the valence states are different, they can be treated equally, that is, assuming that their valency is the same, the average price is marked according to the principle of zero valency algebra in the compound, and if the equation appears diatomic molecule, the number of atoms should be increased by 2 times.

Example 2] Test match Nh4NO3 Hno3+ N2+ H2O.

Analysis: The average valency of N in NH4NO3 is +1 valence, and the valency of the elements is as follows

NH4NO3 HNO3: +1 +5 liters 4 1 valence.

NH4NO3 N2: +1 0 1 2 Valence.

Easy to get 5NH4NO3 2NHO3 + 4N2 + 9H2O

3. The overall pricing method

Usage: When there are several atoms or clusters of atoms of an element (mostly seen in organic reaction trimming) in a compound, it can be treated as a whole and priced as a whole according to the principle of algebraic sum of the valency of the elements in the compound.

Example 3: Trial Match S + Ca(OH)2 CasX + Cas2O3+ H2O

Analysis: The average valency of N in NH4NO3 is +1 valence (-3 valence in NH4, +5 valence in NO3), and the valency of the elements is as follows

NH4NO3 HNO3: +1 +5 liters 4 1 valence.

NH4NO3 N2: +1 0 minus 2 2 valence.

It is easy to obtain 2(x+1)s + 3ca(oh)2 2casx + cas2o3 + 3h2o

Chemistry equations in junior high school are stumped for many people, and they need to be memorized and calculated. Therefore, some people explore the quick solution method of chemistry equations in junior high school, in order to help improve the speed of students' solutions. The following is the "Trimming Method for Junior High School Chemical Equations" compiled by me for your reference only, welcome to read it.

Trim method for junior high school chemical equations:

1. Fractional balancing method: the "universal balancing method" of chemical reaction equations with single nucleus and spike participating in the reaction or element generation, that is, first leveling the atoms of the "elemental" elements in the chemical equation, and finally adding appropriate fractions to balance the atoms of the "elemental" elements.

2. Least common multiple method: find the elements that appear once on the left and right sides and have a large change in atomic number; Find the least common multiple of the number of atoms of the element; Divide the least common multiple by the number of atoms to obtain the number of quantities of the corresponding substance.

3. Determine the "1" method: start with the chemical formula with complex composition or atomic number, and make the measurement number "1"; Derive other chemical formula measurements.

4. Odd-even balancing method: an element appears more often in the formula, and the total number of atoms at each end is one odd and one change ruler. The element is selected as the starting point for the trim, the odd number is changed to the smallest even number (i.e. multiplied by 2), and then the metrology of the other chemical formulas is determined.

Further reading: How to improve your chemistry scores in junior high school

1. Listen carefully in class, record the key points of the teacher's lectures, and organize and review class notes in time after class.

2. Understand and recite common chemical formulas and chemical reaction principles every day.

3. Understand the extracurricular knowledge of related chemistry and expand the knowledge of Tongzhan.

4. Carefully complete and analyze the after-class exercises in the textbook, as well as the classic example problems taught by the teacher during class.

5. Do more practice questions, carefully analyze the test papers after doing them, find out the weak points, focus on the exercises, and selectively answer the knowledge points that you have mastered.

6. Prepare a mistake book, sort out the mistakes in each exam and practice on the error book, analyze the reasons for the errors, and avoid making mistakes again in the future study.

Is chemistry hard to learn in junior high school?

Middle school chemistry is very simple. It's much simpler than junior high school physics. Of course, junior high school physics is not difficult. There are very few knowledge points that need to be understood in junior high school chemistry, and most of them need to be memorized. Therefore, junior high school chemistry has the liberal arts in science.

The following is a junior high school chemical equation balancing method and some tips::

1. Zero-price method.

Second, the balancing of the disproportionation reaction.

The so-called chemical reaction means that in a chemical reaction, the same substance acts as both an oxidant and a reducing agent. At this time, we only need to divide the oxidant for balancing, and then press the reducing agent for balancing, and then add the coefficients obtained twice to get the coefficients of the substance. Then the coefficients of other substances are balanced to the same number of atoms.

3. Reverse trimming.

Reverse trimming is mainly based on the product that is balanced. It is mainly applicable to the fact that the valency of the same element in the product changes more. Therefore, when carrying out the matching, it is necessary to observe the specific situation of the reactants and products and choose the appropriate trimming method for the balancing of the chemical equation, and the reverse method is not used very often, but it plays an immeasurable role in the balancing of the specific and complex chemical reaction equation.

Fourth, the conservation method of the number of atoms.

This method is similar to the solution equation in mathematics, with different unknowns to represent the coefficients of each chemical formula in the chemical equation, and then according to the law of conservation of mass, the type of each atom before and after the reaction is unchanged, and its number is equal, and then list the mathematical equations, through the way of the system of equations, one of the unknowns is one, and find the value of the other unknowns, you can know the coefficients of each chemical formula in the original chemical equation.

5. Ionic electron method.

The ion electron method is mainly aimed at the ionic equation of redox, which is more convenient as the content of improvement in junior high school, and its main large-scale application is mainly in the high school chemistry part, but if the students in the junior high school part of this part of the content are studied in depth, it still plays a more positive role in the balancing of the chemical equation.

It is mainly based on the total number of electrons obtained by the oxidant and the reducing agent to carry out the balancing of the chemical equation, for most students, its difficulty is greater, and students who want to improve their ability can make self-breakthroughs, and it is recommended to study selectively.

Sixth, the step-by-step balancing method.

The main application scope of the distribution trim method is the decomposition reaction, but its specific description is mainly when the reaction is carried out with an acidic solution as the medium, because the chemical properties of the acid determine the formation of water molecules, so the number of oxygen atoms can be determined by this. In the specific trimming, its speed is still relatively fast, and it is recommended that students who review must master this type of trimming method.