Chemical equations represent molecular interactions and the way atoms are arranged in a reaction. Chemical formulas indicate the composition of a compound. Equations must be balanced, and stoichiometry is used to calculate product amounts. Additional notation can reveal more information about the reaction.
A chemical equation is a representation of molecular interactions in their relative quantities. In a chemical reaction, the way atoms are arranged with each other changes. The internal structures of the atoms are not affected, but energy, usually in the form of heat, is released or absorbed during a chemical reaction. The chemical composition of the reactants, or pre-reaction ingredients, is written on the left side of a chemical equation. The resulting products, or compounds, are written on the right-hand side.
The chemical composition of a compound is represented by a chemical formula. A chemical formula indicates how many atoms of each element are present in a molecule, or stable collections of atoms. For example, a water molecule has the chemical formula H2O because it has two hydrogen atoms and one oxygen atom. The chemical formulas of multiple molecules in an equation are separated by plus signs.
Although a large amount of matter can react, a chemical equation only affects the ratios of the molecules involved. The reactants and products in a chemical reaction can only react in specific proportions. This is because atoms are neither created nor destroyed; whatever atoms are present before the reaction must remain after. For this reason it is necessary to balance the chemical equations. A chemical equation shows a balanced reaction in its smallest possible amount.
You may need to add coefficients to the left side of chemical formulas to get a balanced equation. In many reactions, more than one molecule of a particular molecule is present, even in the basic example, or as small as possible. If two atoms of an element are present but are not in the same molecule, a coefficient of 2 is used instead of a subscript. For example, 2 H2O is two water molecules or 4 hydrogen atoms and 2 oxygen atoms.
The field of studying measurable relationships between chemical reactants and products is called stoichiometry. It is useful for calculating how much product can be created with a given amount of reactant. In real-world chemical reactions, it is generally impossible to turn 100% of the reactants into products. This is because matter is separated in space and reactants must be in contact with each other to react.
Additional notation in a chemical equation can reveal more information about the reaction. When atoms or molecules have an electric charge, for example, this number is superscripted with a plus or minus sign for a positive or negative charge. Other elements of the equation include the letters in parentheses, used to indicate whether a compound is solid, liquid, or gas. You can also place symbols between reactants and products to specify the rate of reaction and whether energy is released or absorbed.
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