Balancing Equations

Law of Conservation of Mass States matter can neither be created nor destroyed. Thus, atoms are neither created nor destroyed, only rearranged in a chemical reaction. So…the number of a particular atom is the same on both sides of a chemical equation.

How Molecules are Symbolized Molecules may also have brackets to indicate numbers of atoms. Ex: Ca(OH) 2 Notice that the OH is a group The 2 refers to both H and O Cl 2 2Cl 2Cl 2 O H O H Ca

Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

How many? How many of each atom are in the following? a) NaOH b) Ca(OH) 2 c) 3Ca(OH) 2 Na = 1, O = 1, H = 1 Ca = 1, O = 2, H = 2 Ca = 3, O = 6, H = 6

Chemical Equations Their Job: Depict the kind of reactants and products and their relative amounts in a reaction.Their Job: Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s)4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) The letters (s), (g), and (l) are the physical states of compounds.The letters (s), (g), and (l) are the physical states of compounds.

Balancing Equations notWhen balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. –Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Steps to Balancing Equations 1.Write the correct chemical equation (if not already given to you). DO NOT TRY AND BALANCE IT YET! 2.Count the number of atoms of each element on the left side and compare to the number of atoms of each element on the right. 3.Determine where to place coefficients so that the number of atoms of each element is the same on the left and right. 4.Check to make sure the entire equation is balanced.

Some Helpful Hints LEARN THESE DIATOMIC MOLECULES: O 2 H 2 N 2 F 2 Cl 2 Br 2 I 2 Take one element at a time, working left to right. Leave O and H to the end. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again.