Counting Atoms and Balancing Chemical Equations

Identifying Compounds Each new element is identified by a capital letter Each new element is identified by a capital letter Example: H 2 SO 4 Example: H 2 SO 4 The elements in Sulfuric Acid The elements in Sulfuric Acid Hydrogen Hydrogen Sulfur Sulfur Oxygen Oxygen

Subscripts C 12 H 22 O 11 C 12 H 22 O 11 There are 12 atoms of Carbon There are 12 atoms of Carbon There are 22 atoms of Hydrogen There are 22 atoms of Hydrogen There are 11 atoms of Oxygen There are 11 atoms of Oxygen If there is not a subscript listed, it is understood to be 1. If there is not a subscript listed, it is understood to be 1. Example: NaCl Example: NaCl There is one atom of Sodium There is one atom of Sodium There is one atom of Chlorine There is one atom of Chlorine

You Practice! NaHCO 3 Sodium – 1 Hydrogen – 1 Carbon – 1 Oxygen -3 HCl Hydrogen – 1 Chlorine - 1

There are times you will see a compound with parenthesis. Pb(NO 3 ) 2 The 2 after the parenthesis indicates there are two sets of the parenthesis. Pb(NO 3 ) (NO 3 ) So, in counting the atoms, you would have the following: Lead – 1Oxygen - 6 Nitrogen -2

You Practice!! (NH 4 ) 3 PO 4 Nitrogen – 3 Hydrogen – 12 Phosphorus – 1 Oxygen – 4 Mg(OH) 2 Magnesium – 1 Oxygen – 2 Hydrogen – 2

Coefficient 2H 2 SO 4 This means there are 2 compounds of Sulfuric Acid. Think: H 2 SO 4 H 2 SO 4 Counting the atoms: Hydrogen – 4 Sulfur – 2 Oxygen - 8

You Practice!! 3H 3 PO 4 Hydrogen – 9 Phosphorus – 3 Oxygen H 2 O Hydrogen – 4 Oxygen - 2

Got It????

Chemical Equations Chemical equations express what is happening in a chemical reaction using symbols. 2K + Cl 2 2KCl Reactant Product yields

Law of Conservation of Mass In a chemical reaction, matter cannot be created or destroyed. It can only be changed. Therefore the Reactants must equal the Products for the equation to be balanced

What does that mean?? The reactants MUST contain the same elements and the same number of each element as the product. BUT they CAN be in different compounds.

HUH????? H 2 + O 2 H 2 O ReactantsProducts H = 2H = 2 O = 2O = 1 This cannot happen. An oxygen atom cannot be destroyed.

So what do we do? The chemical equation must be written according to the law. AGAIN --- HUH??? H 2 + O 2 H 2 O is the production of water. Well, how it actually happens in nature is this…. 2H 2 + O 2 2H 2 O Reactants: Products: H = 4 H = 4 O = 2O = 2

Do I Need to Know How to Balance? NOT YET. You need to be able to recognize if the chemical reaction follows the law of conservation of mass. Na + O 2 Na 2 O Mg + 2HCl MgCl 2 + H 2 NOPE YUP!!

You Practice!!! HgO Hg + O 2 Hg = Hg = O =O = N 2 + 3H 2 2NH 3 Zn + 2HCl ZnCl 2 + H 2 NOPE YUP

More Practice K + Br 2 2KBr 2Fe + O 2 6Fe 2 O 3 2Na + 2H 2 O2NaOH + H 2 NOPE YUP

Got It???

Balancing Chemical Equations H 2 + O 2 H 2 O Reactants: H = 2 Products: H = 2 O = 2 O = 1 O = 2 O = 1 What do we multiply by 1 to get 2?

Keep Going…. H 2 + O 2 2H 2 O Reactants: H = 2 Products: H = 4 O = 2 O = 2 O = 2 O = 2 Now the Oxygen is balanced but Hydrogen is not, so what do we multiply by 2 to get 4?

Almost There!!! 2H 2 + O 2 2H 2 O Reactants: H = 4 Products: H = 4 O = 2 O = 2 O = 2 O = 2 Now it is balanced!!! Congratulations, you have now learned the hardest thing we will do all year!!