What is a Chemical Reaction?

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 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:

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

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.

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

What is a Chemical Reaction?