Chapter 11.1b Describing Chemical Reactions Chemistry

Balancing Chemical Equations An unbalanced equation does not tells us the quantity of the reactants needed to make the products. A complete description of the reaction has to have the kinds of atoms/compounds involved and the numbers of each needed to make the products. In order to have a balanced chemical equation we have to include coefficients - small whole numbers that are placed in front of the formulas in an equation in order to balance it. A balanced equation is when each side of the equation has the same number of atoms of each element and mass is conserved.

In order for products to be made the reactants have to have their bonds broken and new bonds formed. Atoms are not created or destroyed, they are just rearranged. This is important because it explains the law of conservation of mass. In any chemical change, the mass of the reactants is equal to the mass of the products. To write a balanced chemical equation, first write the skeleton equation. Then use coefficients to balance the equation so that is obeys the law of conservation of mass. In a balanced chemical equation you have to have the same number of atoms of each element.

Examples In the reaction of carbon and oxygen to produce carbon dioxide: C(s) + O2(g) → CO2 (g) This equation is balanced. We have one carbon atom on both sides of the equation and 2 oxygen atoms on both sides so it is balanced. There is no need to add coefficients. In this reaction it is different: H2(g) + O2(g) → H2O (l) this is not balanced. We need to add coefficients. We have 2 hydrogens on either side of the equation but we have 2 oxygens on one side and 1 oxygen on the other side. The balanced reaction would be: 2H2 + O2 → 2H2O