Chemists use expressions, called chemical equations, to represent chemical reactions.
To write a chemical equation you need:
- know which substances are consumed (reagents) and which are formed (products);
- know the formulas of reagents and products;
- Always write the equation as follows: reagents => products
- when more than one reagent, or more than one product, participates in the reaction, the substance formulas will be separated by the "+" sign;
- if necessary, put numbers, called stoichiometric coefficients, before substance formulas so that the equation indicates the conservation of atoms. This procedure is called balancing or adjusting coefficients of an equation.
Using the rules above to represent water formation we have:
- reagents: hydrogen and oxygen;
- Product: water.
- Substances Formulas: Hydrogen: H2; oxygen: 02; water: H20.
- equation: H2 + 02 H2O.
Coefficient adjustment: The above expression indicates that a hydrogen molecule (formed by two atoms) reacts with an oxygen molecule (formed by two atoms) to form a water molecule (formed by two hydrogen atoms and one oxygen). We see, therefore, that the expression runs counter to the law of conservation of atoms (law of conservation of masses), because before the reaction there were two oxygen atoms and, after the reaction, there is only one. However, if some type of atom were to disappear, the mass of the reactants should be different from the mass of the products, which is not verified experimentally.
Like two oxygen atoms (in the form of molecule 02) interact, it is logical to assume that two water molecules are formed. But since two water molecules are made up of four hydrogen atoms, it will take two hydrogen molecules to provide this amount of atoms. Thus, the smallest number of molecules of each substance that should participate in the reaction is: hydrogen, two molecules; oxygen, a molecule; water, two molecules.
The chemical equation that represents the reaction is: 2H2 + 102 => 2H20
Which reads as follows: Two hydrogen molecules react with one oxygen molecule to form two water molecules.