In the process of chemical change, we must follow the law of conservation of mass, that is, the number of elements before and after the reaction is equal to the number of atoms. The common methods for balancing chemical equations are: (1) Least common multiple method: When balancing chemical equations, observe the number of complex elements before and after the reaction, and then balance them first. First, calculate the least common multiple of the atom of this element before and after the reaction, balance the atom by filling in the stoichiometric number before the chemical formula, and then observe the atomic numbers of other elements, so that the element types and atomic numbers of reactants and products in the chemical reaction are equal. For example, the balance method introduced in the textbook is the least common multiple method. In the reaction of P+O2 ―― P2O5, oxygen is mixed first: the least common multiple is 10, the stoichiometric numbers are 5 and 2, and P+5O2 ―― 2P2O5; Then balance the phosphorus atoms, 4p+5o2 = = 2p2o5. (2) Observation: a method of judging the stoichiometry in coordination by analyzing the chemical formula of a substance. For example, equilibrium Fe2O3+Co-Fe+CO2. In the reaction, each CO combines with one oxygen atom to generate CO2 molecules, and Fe2O3 provides three oxygen atoms at a time, so three CO molecules must accept these three oxygen atoms to generate three CO2 molecules, that is, Fe2O3+3co-Fe+3co2, and finally the planar Fe2O3+3co = = 2fe+3co2 is obtained. This equilibrium method determines the chemical properties of CO by observing and analyzing the number of oxygen atoms in the chemical formula of Fe2O3. (3) Odd-even method: The method of selecting the elements with odd-even number of atoms in the chemical formula before and after the reaction as the starting point of balancing, changing the odd number into even number, and then balancing the atoms of other elements is called odd-even method. For example, to balance the combustion equation of methane (CH4), the method of changing odd numbers into even numbers can be adopted: CH4+O2-H2O+CO2. Before the reaction, the oxygen atoms in O2 were even, but after the reaction, the oxygen atoms in H2O were odd. First, the number of oxygen atoms is changed from odd to even: CH4+O2-2H2O+CO2, and then the atomic balance of other elements is CH4+2O2. (4) Normalization method: find the key chemical formula in the chemical equation, set the stoichiometry before the chemical formula as 1, and then balance the stoichiometry before other chemical formulas according to the key chemical formula. If the measurement number is a fraction, then multiply each measurement number by the same integer to change the fraction into an integer. This equilibrium method of setting the measurement number of key chemical formula as 1 is called normalization method. For example, this method can be used to balance the combustion chemical equation of methanol (CH3OH): CH3OH+O2 ―― H2O+CO2. Obviously, the key to determine the amount of H2O and CO2 is the composition of methanol, so the metering number is 1, so the molecular number of H2O and CO2 generated after combustion can be obtained: ch3oh+O2-2h2o+CO2. Then balance the oxygen atoms: ch3oh+3/2o2 = = 2h2o+CO2, multiply each measurement by 2 and divide it into integers: 2ch3oh+3o2 = = 4h2o+2co2. It should be noted that no matter how to balance the chemical equation, only the stoichiometric number in front of the chemical formula can be changed, and the number in the lower right corner of the element in the chemical formula can never be changed. Because changing the number in the lower right corner of the element symbol means changing the composition of reactants and products, there may be substances that do not exist at all or reactants or products that have changed the original chemical changes, or there may be chemical changes that do not exist at all. (5) Valence method: This method aims at the redox equation. Remember here: "When the valence increases, the redox agent is lost." Corresponding to this is that "reducing oxidant is obtained by reducing valence". The specific usage is: (1) mark its valence on the elements with changing valence to distinguish who goes up and who goes down. (2) Connect the same elements with lines to find out whether the number of charges increases or decreases. (3) Find the least common multiple and multiply it by the number of increased or decreased charges respectively. (4) Balance: Write the least common multiple of each multiplication operation before each chemical formula (i.e. coefficient). And pay attention to whether these elements with valence changes are equal before and after chemical changes. Generally speaking, if it is not equal, it is an integer multiple. (5) Use observation method to balance other substances, such as water and generated insolubles.