What is a Chemical Reaction? Two atoms can be mixed together to create a new compound (sodium & chlorine) Can also mix two compounds to create new compounds (baking soda & vinegar) New substances are produced in each case with properties different from the original substances.

Dalton’s Atomic Theory All matter is made up of small particles called atoms Atoms cannot be created, destroyed, or divided into smaller particles All atoms of the same element are identical in mass and size, but different in mass and size from atoms of other elements John Dalton 1766 - 1844

Conservation of Mass Law of conservation of mass states: The total mass of the reacting substances (the reactants) is always equal to the total mass of the resulting substances (the products). Matter is neither created or destroyed in a chemical reaction. Atoms in the reactants are present in the products.

Writing Chemical Equations Lets look at how water is formed. First we can look at the word equation. Hydrogen + Oxygen  Water Reactants on the left side of the arrow Products on the right side of the arrow Plus sign means “reacts with” Arrow means “produce”

The Hindenburg Zeppelin Hydrogen + Oxygen  Water Video on File

Diatomic Molecules Hydrogen - H2 Chlorine - Cl2 Oxygen - O2 Atoms can share electrons with the same atom. These molecules have two of the same atoms joined by a covalent bond. Since there are two of the same atoms the word diatomic is used. (“di” meaning two) Seven elements are diatomic: Hydrogen - H2 Chlorine - Cl2 Oxygen - O2 Bromine - Br2 Nitrogen - N2 Iodine - I2 Fluorine - F2

Skeleton Equations For water the skeleton equation is: H2 + O2  H2O According to Conservation of Mass the reactants and products must have the same number of atoms. We must balance the chemical equation. This is done by adding coefficients in front of the compounds.

Balancing the Reaction Important: Subscripts CANNOT be changed!!!!!! Hydrogen is balanced so you must balance the oxygen by adding another water molecule H2 + O2  2H2O Now hydrogen is not balanced so we must add another hydrogen molecule 2H2 + O2  2H2O

Check The Atoms Add up all the atoms of both reactants and products to make sure the amounts are the same. 2H2 + O2  2H2O Reactants 2 H2 Molecules 4 H atoms 1 O2 Molecule 2 O atoms Products 2 H2O molecules 4 H atoms 2 O atoms

Tips for Balancing Adjust co-efficients, not chemical formulas (subscripts). Balance metals first Add co-efficients to individual elements last If a polyatomic ion appears in in both reactant and product, treat it as a single unit. Balance oxygen and hydrogen last. Don’t forget diatomic molecules. Count your atoms as a check

Let’s Try Some Together Mg + O2  MgO AgI + Na2S  Ag2S + NaI NaOH + H2SO4  Na2SO4 + H2O CaCl2 + Na3PO4  Ca3(PO4)2 + NaCl FeS + O2  Fe2O3 + SO2

Your Turn 1. H2 + O2  H2O 2. S8 + O2  SO3 3. HgO  Hg + O2 4. Zn + HCl  ZnCl2 + H2 5. Na + H2O  NaOH + H2

Chemical States The state of the chemicals is important to the chemical reaction. State can be added by putting an abbreviation in parentheses after each chemical. State Abbreviation Example Solid (s) NaCl(s) Liquid (l) H2O(l) Gas (g) H2(g), O2(g) Aqueous (aq) NaCl(aq) (in water)

Water: Final Balanced Equation Hydrogen and Oxygen are both gases Water is a liquid Final balanced equation with the states of all components is: 2H2(g) + O2(g)  2H2O(l) You will not be required to give the states for an equation

Resources - Links Khan Academy Balancing Equations pHET Balancing Applet Balancing Complex Equations with the Table Method Video

Resources – Table Example Step 1: Start with an unbalanced equation

Resources – Table Example Step 2: Draw boxes around all the chemical formulas

Resources – Table Example Step 3: Make an element inventory Before = Reactant After = Product

Resources – Table Example Step 4a: Balance the Equation and the Inventory Before = Reactant After = Product

Resources – Table Example Step 4b: Final Steps… Before = Reactant After = Product

Resources – Extra Practice The following are Word Based Balancing Problems Page 137: Q14 Page139: Q22, Q23, Q26

Answers - Let’s Try Some Together 2Mg + O2  2MgO 2AgI + Na2S  Ag2S + 2NaI 2NaOH + H2SO4  Na2SO4 + 2H2O 3CaCl2 + 2Na3PO4  Ca3(PO4)2 + 6NaCl 4FeS + 7O2  2Fe2O3 + 4SO2

Answers - Your Turn 1. 2H2 + O2  2H2O 2. S8 + 12O2  8SO3 3. 2HgO  2Hg + O2 4. Zn + 2HCl  ZnCl2 + H2 5. 2Na + 2H2O  2NaOH + H2