Balancing Chemical Equations Created by: Mrs. Dube

Law of Conservation of Mass Law of Conservation of Mass states that in a chemical reaction atoms are neither created nor destroyed. Whatever reactants you start with, you will have the same number of atoms in the products All atoms present in the reactants are also present in the products

Chemical Equations Chemical equations represent how atoms are rearranged in a chemical reaction The atoms in the reactants are shown on the left side of the equation The atoms in the products are shown on the right side of the equation

In order to write a chemical equation, you must know the following information: the reactants and products the atomic symbols and chemical formulas of the reactants and products the direction of the chemical reaction The arrow shows the direction in which the reaction proceeds

Balancing Equations Chemical equations must follow the Law of Conservation of Mass The same number of atoms must appear on both sides of the equation You have to balance the equation to make the number of atoms equal on each side of the equation

Balancing Equations To balance a chemical equation, the first thing you have to do is count the number of atoms of each element shown in the equation Now check to see how many atoms of that element are shown on the product side of the equation

Balancing Equations You can balance chemical equations by changing a reactant’s or product’s coefficient A coefficient indicates how many molecules take part in the reaction (ex. 2H2O shows 2 water molecules) If there is no coefficient, then there is only one molecule of that type in the reaction

Using Coefficients To Balance Equations Only coefficients can be changed to balance chemical equations! Subscripts are part of the chemical formula for the reactants and products and cannot be changed to balance an equation Changing a subscript changes the substance represented by the formula

Using Coefficients To Balance Equations Balancing the equation: N2 + H2  NH 3 First, count the atoms of the reactants: N= 2 atoms; H=2 Next, count the atoms of the products: N=1; H=3 Use a coefficient to add atoms to one side of the equation (try 2 to start out) DO NOT CHANGE THE SUBSCRIPTS

Using Coefficients To Balance Equations Look at the second atom of the reactants and the second atom of the product and compare the number of atoms Add a coefficient to the second reactant to balance the number of atoms Now, count up all the atoms to make sure that the equation is balanced

Using Conservation Of Mass Balance the following equations: Hg + O2  HgO N2 + O2  NO2 Zn + HCl  ZnCl2 + H2 N2 + + H2  NH3 Why are coefficients changed but subscripts are not changed when balancing chemical equations?