The Law of Conservation of Mass: The total mass of the reactants must equal the total mass of the products Total mass of reactants = Total mass of products

This makes sense because atoms cannot be created or destroyed Atoms present at the beginning of the reaction must still be present after the reaction has taken place How?? Atoms rearrange themselves, bonding to new atoms to make a different product Fruit salad game example, Square dancing

So when you write a chemical equation… The number of atoms of each type must be the same on each side Equations must be balanced! reactants products

Writing Chemical Equations A CHEMICAL EQUATION is used to represent a chemical reaction E.g. oxygen and hydrogen in a balloon react to produce water There are 3 forms of chemical equations that we can use to represent a reaction

1. Word Equations hydrogen + oxygen water reactant reactant product and reacts to produce

2. Skeleton Equations H2 + O2 H2O

3. Balanced Chemical Equations 2H2 + O2 2H2O A COEFFICIENT is a number that is placed in front of a chemical formula in a balanced chemical equation

H2 + O2 H2O2 hydrogen peroxide The only way to balance a chemical equation is to change the coefficients. If you change a subscript, you will change the identity of the substance H2 + O2 H2O2 hydrogen peroxide

Tips for Balancing Equations Remember the diatomic molecules: H2, N2, F2, Cl2, Br2, I2 and O2 (Remember “HOFBrINCl”) Make sure your chemical formulas are correct Do a final check by counting atoms of each element

Example A Bunsen burner works when methane gas burns in oxygen to produce carbon dioxide and water. Write the balanced chemical equation for this reaction

Step 1: Write the word equation products Methane + oxygen  carbon dioxide + water reactants

Step 2: Write the skeleton equation Replace each name with its formula Methane + oxygen  carbon dioxide + water CH4 + O2  CO2 + H2O

Step 3: Count atoms Count the number of atoms of each type in reactants (left side) and products (right side) CH4 + O2  CO2 + H2O Type of Atom Reactants Products Balanced? C 1  H 4 2  O (2+1) = 3

Step 4: Balancing Multiply each compound by the appropriate coefficients to balance the number of atoms (do NOT change subscripts) Balance compounds first and elements last Balance hydrogen and oxygen last If a polyatomic ion appears in both a reactant and a product, think of it as a single unit Trial and error (be patient )

Step 4: Balancing CH4 + O2 -- CO2 + 2H2O Type of Atom Reactants Products Balanced? C 1  H 4 O 2 (2+2) = 4  Remember when you put a coefficient in front of a compound, ALL atoms are multiplied!!

Step 4: Balance CH4 + 2O2  CO2 + 2H2O Type of Atom Reactants Products Balanced? C 1  H 4 O

Human Balancing Act Sodium + Chlorine Gas  Table Salt Na (s) + Cl2 (g)  NaCl (s) 2 Na (s) + Cl2 (g)  2 NaCl (s)