A chemical equation is an “equation of chemistry.” From the ratios shown by the coefficients, we can calculate the amounts of substances that react.
Basic Knowledge
In a chemical equation, reactants are written on the left and products on the right, and the types and numbers of atoms on both sides are made equal (law of conservation of mass). Example:2H2 + O2 → 2H2O
This equation is read as follows: “2 H2 molecules and 1 O2 molecule (= 2 mol and 1 mol) react to form 2 H2O molecules (= 2 mol).”
Coefficients represent the ratio of amounts of substance (mol). Using this, we can find the amount of one substance from the amount of another. For gases, the same ratio also applies to the volume ratio at the same temperature and pressure.
Chemical equation (An equation expressing the substances before and after a reaction using chemical formulas)
Coefficient (The number before each substance. Represents the ratio of amounts of substance. 1 is omitted)
Law of conservation of mass (The total mass of substances does not change before and after a reaction. Lavoisier)
Reactants and products (Left side = reactants; right side = products)
Quantitative relationship (Coefficient ratio = amount-of-substance ratio = volume ratio for gases)
Further Exploration (Background and Significance)
The coefficients in a reaction equation represent the number ratio in which atoms are rearranged. Because atoms do not disappear or newly appear before and after a reaction, it is necessary to balance the number of atoms on both sides by balancing the coefficients.
Representative reaction equations:
・Combustion of methane: CH4 + 2O2 → CO2 + 2H2O
・Decomposition of calcium carbonate: CaCO3 → CaO + CO2
・Combustion of iron: 3Fe + 2O2 → Fe3O4
Once a chemical equation can be written, its coefficients can be used as mol ratios to convert among mass, volume, and number of molecules. This is a central technique in chemical calculations.
- Make the number of atoms on both sides of a chemical equation equal
- Coefficient = ratio of amounts of substance (mol)
- For gases, coefficient ratio = volume ratio (at the same temperature and pressure)
- Do not write coefficient 1
- Combustion example:
CH4 + 2O2 → CO2 + 2H2O - Decomposition example:
2H2O2 → 2H2O + O2 - Law of conservation of mass (Lavoisier)
Points of Caution (Common Confusions)
① Coefficient ratio = amount-of-substance ratio, but it is not the mass ratio. Multiply by molar masses before comparing masses. ② Gas volume ratios apply only to gases at the same temperature and pressure. They cannot be applied to solids or liquids. ③ Distinguish subscripts (the 2 in H2O) from coefficients (the 2 before 2H2O). Never change subscripts arbitrarily. ④ Write coefficients as the simplest whole-number ratio (do not use fractions or multiples).
Practice
- State the coefficients in
__ H2 + __ O2 → __ H2O. - When 1 mol of methane CH4 undergoes complete combustion, state the amount of substance of O2 required and the amount of substance of CO2 produced.
- When 11.2 L of H2 reacts at standard conditions, how many mol of H2O are produced?