1. SECTION 1 CHEMICAL EQUATIONS Unit 4 Chemical Reactions

2. Section 1 - Chemical Equations A useful description of a chemical reaction tells you the substances present before and after the reaction. The substances that undergo changes are called reactants (location: left side of formula). The new substances formed as a result of that chemical change are called products (location: right side of formula. Products Reactants Products

3. How to read a Chemical Equation You read the plus sign (+) as ‘and’ and you read the arrow ( ) as ‘yields’ or ‘makes’ For example: Carbon + OxygenCarbon dioxide (carbon and oxygen yields carbon dioxide) or C + O 2 CO 2

4. Law of Conservation of Mass The law states that mass is neither created nor destroyed in a chemical reaction. In other words, the reactants and products have the same mass before and after the reaction. In order to show that mass is conserved during a chemical reaction, a chemical equation must be balanced.

5. Balancing Chemical Equations In order to balance an equation you change the coefficients (the numbers that appear before the formulas). If there is no number shown, it is understood to be 1 (just like in algebra). You CANNOT change the subscripts in an equation, it would change the identity of the reactant or product. For example: H 2 0 cannot be changed to H 2 O 2 (water)(hydrogen peroxide)

6. Steps to balance an equation 1) Count the number of atoms of each element on both sides of the equation. Identify which atoms do not match. N 2 H 4 + O 2 N 2 + H 2 O N = 2 H = 4 H = 2 O = 2 O = 1 The hydrogen and oxygen do not match, so…..

7. Steps to balance an equation 2) Change one or more coefficients until the equation is balanced. N 2 H 4 + O 2 N 2 + 2 H 2 ON = 2 H = 4H = 4 (2 x 2) O = 2O = 2 (1 x 2) So by placing a coefficient of 2 in front of the water, the equation now indicates that there are 2 molecules of water produced in the reaction, and both sides are balanced.

8. Balancing Equations There are no magic formulas for balancing an equation, it is trial and error, so always work in pencil. You need to think in math terms, because you are doing simple multiplication to make the number of atoms on both sides of the equation the same. Practice makes perfect…..well, better anyway.

9. Chemical Formulas to Learn H = Hydrogen N = Nitrogen O = Oxygen He = Helium F = Fluorine C = Carbon Fe = Iron I = Iodine Ca = Calcium Mg = Magnesium

10. Balancing Equations Now Let’s Practice Writing and Balancing Equations