Chemical Equations
Chemists like to condense descriptions of chemical reactions into chemical equations. For example we can state iron combines with oxygen to yield iron(II) oxide. From this statement we can write an unbalanced equation which is oxymoronic but a commonly used term.
Fe(s) + O2(g) ----> Fe2O3(s)
When we balance the equation it becomes
4 Fe(s) + 3 O2(g) ----> 2 Fe2O3(s)
Before we go into how we balance an equation, lets talk about equations in general. The chemist uses the term equation in a different sense from the mathematical equation. In a mathematical equation, the equals sign indicates that the expression on the right side represents sonething identical with what is expressed on the left side of the equation.
Chemical equations are not meant to express identities, but instead the course of a chemical reaction, which produces new and different substances on the right from the original substances on the left. So the equals sign is not correct to use in a chemical equation and the word yields or produces replaces equals when translating a chemical equation into words. The chemist uses an arrow instead of the equals sign to indicate the direction in which the reaction proceeds. A plus sign between two formulas means that the substances are mixed.
The physical state of each substance in a chemical equation is often indicated parenthetically. We use the symbols (g), (l), (s), and (aq) for gas, liquid, solid, and aqueous, respectively. These are sometimes called phase labels. Three more terms used in connection with chemical equations are reactants, the substances on the left side of the equation, products, the substances on the right side of the equation, and finally the numbers in front of the formulas are called coefficients, where a blank is used instead of a one.
Balancing Chemical Equations
In order to write the chemical equation for a reaction, we must first determine by experiment those substances that are reactants and those that are products. Once we know the chemical formulas of the reactants and products, we can write the unbalanced chemical equation. We then balance the equation by determining the coefficients that provide equal numbers of each type of atom on each side of the equation. Generally whole-number coefficients are used, also we want to use the smallest set of coefficients that will balance the equation.
In balancing equations, it is important to understand the difference between a coefficient in front of a formula and a subscript in the formula. Changing a subscript in a formula changes the identity of the substance. Subscripts should never be changed in balancing an equation. As an example lets balance the equation for the combustion of ethane; C2H6(g) + O2(g) ---> CO2(g) + H2O(l)
Oxygen appears as the free element on the left, so we will leave it for last. To balance carbon, we place the coefficient 2 in front of CO2.
C2H6(g) + O2(g) ---> 2 CO2(g) + H2O(l) (not balanced)
To balance hydrogen we need the coefficient 3 in front of H2O.
C2H6(g) + O2(g) ---> 2 CO2(g) + 3 H2O(l) (not balanced)
Counting oxygen atoms, we find 2 on the left and 7 on the right. We can use the fractional coefficient 7/2 in front of O2, but generally we prefer whole-number coefficients. If we multiply each side of the equation by two, we will remove the fraction and achieve the following balanced equation.
2 C2H6(g) + 7 O2(g) ---> 4 CO2(g) + 6 H2O(l) (balanced)
The method we have used here is called balancing by inspection, which is essentially trial and error. We balance each kind of atom in succession, adjusting coefficients as necessary. Later we will show a more systematic method to balance more complex chemical equations.