"How to Write Chemical Equations Correctly" Chemical Equations PPT Courseware 3

"How to Write Chemical Equations Correctly" Chemical Equations PPT Courseware 3

Knowledge review:

Essence: The reactant molecules break apart and the atoms contained in them recombine to form new molecules.

Essence: New matter is generated

Quantitative relationship: The sum of the masses of the reactants is equal to the sum of the masses of the products. (i.e. comply with the law of conservation of mass)

Representation method: chemical equation

The meaning of chemical equations

Macroscopic view: 1. It means that the reactants are phosphorus and oxygen, the product is phosphorus pentoxide, and the reaction condition is ignition.

Microscopically: 2. It means that the number ratio of phosphorus atoms, oxygen molecules in the reactants and phosphorus pentoxide molecules in the product is 4:5:2.

Mass: 3. It means that every 124 parts by mass of phosphorus reacts with 160 parts by mass of oxygen to produce 284 parts by mass of phosphorus pentoxide.

How to write chemical equations correctly?

1. Principles of chemical equations

1. It must be based on objective facts, and must not be made up of imagination or fabrication of substances and chemical reactions that do not actually exist.

2. Comply with the law of conservation of mass. The types and numbers of atoms on both sides of the equal sign must be equal.

2. Steps to write chemical equations

⑴ "Write": write the chemical formulas of the reactants and products, connecting them with short lines in the middle.

⑵ "balance", balance the chemical equation, and change the short line into an equal sign. Steps of the least common multiple method: (Find the least common multiple of the atoms on both sides)

①Select an element: Choose an element that appears once on the left and right sides of the formula and has a large difference in the number of atoms or a large number.

② Find the least common multiple.

③ Find the stoichiometric number: least common multiple/number of atoms of the element in the chemical formula.

④ Balance the measurement numbers before other chemical formulas and change the dashes to equal signs.

⑤Check: Whether the number of atoms of each element is equal before and after the reaction.

Practice 1 Practice:

1. Balance the following chemical equations

(1) C + CO2 = CO

(2) H2+ O2=H2O

(3)CuO + C =Cu + CO2

(4) H2S + O2 = S + H2O

If there is gas in the product, a "↑" sign should be noted on the right side of the chemical formula of the gas substance; if there is a solid in the product of the reaction in a solution, a "↓" sign should be noted on the right side of the chemical formula of the solid substance.

Na2CO3 + 2HCl = 2NaCl + H2O+CO2

CuSO4 + 2NaOH=Na2SO4 + Cu(OH)2↓

However, if there are gases in the reactants and products, the gas products do not need to be marked with a "↑"; for reactions in solutions, if there are solids in the reactants and products, the solid products do not need to be marked with a "↓" Number.

S + O2 = SO2

Fe + CuSO4 = Cu + FeSO4

Odd couple method: FeS2+O2�Fe2O3+SO2

⑴ Select elements: Select elements that appear more often on the left and right sides of the formula and the total number of atoms on both sides is one odd and one even.

⑵Odd couple: Test the number of couples before chemical formulas with an odd number of atoms of the element.

⑶ Observe the balance of the atomic number of other elements, then balance the atomic number of this element, and finally change the short line to an equal sign.

⑷Check: Whether the number of atoms of each element is equal before and after the reaction

Observation method: observe and analyze the characteristics of chemical reactions CO+Fe2O3�Fe+CO2

⑴ Deducing the stoichiometry of each reactant and the stoichiometry of the product from a product with a relatively complex chemical formula

⑵ Based on the calculated stoichiometric numbers of the chemical formula, find the stoichiometric numbers of other chemical formulas until they are balanced.

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