1. 17-2 Describing Chemical Reactions

2. CHEMICAL EQUATION H2 + O2 H2O
a shorter, easier way to show chemical reactions, using
symbols, not words, for the reactants and the products
hydrogen molecules react with oxygen molecules to form water molecules
H O H2O
reactant + reactant “YIELDS” products

3. Law of Conservation of Mass
total mass of reactants must equal total mass of products
matter is NOT destroyed or created in a chemical reaction =

4. Chemical Formulas & Counting Atoms
ELEMENTS – represented by a one or two-letter symbol (letter)
C Na H
COMPOUNDS– represented by a chemical formula (word) which
uses subscripts to show the ratio of elements in the compound
H2O2 CO2 C3H8O
HOW MANY?
coefficient in front of the chemical formula tells “how many” atoms or molecules
3H2O2 4CO2 2C3H8O

5. Balancing Chemical Equations
Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H)
(note: only time a subscript can & must be changed)
H O     H2O
Count the number of atoms that are present in the basic equation and record
Pick an element that is not balanced on both sides of the equation ( H & O last)
Add a coefficient in front of the chemical formula to adjust the count and record the new values
Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation
(change coefficient, then change counts)

6. Balancing Chemical Equations
Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H)
(note: only time a subscript can & must be changed)
Mg O     MgO
Count the number of atoms that are present in the basic equation and record
Pick an element that is not balanced on both sides of the equation ( H & O last)
C Cl CCl4
Add a coefficient in front of the chemical formula to adjust the count and record the new values
Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation
(change coefficient, then change counts)

7. Building Equations (basic equation is given)
Mg O > MgO
C Cl > CCl4

8. Building w/Marshmallows
Al O Al2O3
Note: add like elements on the same side of the equation
SO O SO3

9. Balancing Chemical Equations
Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H)
(note: only time a subscript can & must be changed)
Al O Al2O3
Count the number of atoms that are present in the basic equation and record
Pick an element that is not balanced on both sides of the equation ( H & O last)
SO O SO3
Add a coefficient in front of the chemical formula to adjust the count and record the new values
Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation
(change coefficient, then change counts)

10. Classifying Chemical Equations
SYNTHESIS – two or more substances combine to make a more complex substance
AB C ABC (synthesize means to put together)
DECOMPOSTION - a complex substance breaks down into simpler substances
ABC AB C (decompose means to break down)
REPLACEMENT – two elements in two different compounds trade places
AB CD AC BD (replace means to find a new partner)
2 SO O H2O H2SO4
Synthesis of Acid Rain
2 H2O H2O O2
Decomposition of Hydrogen Peroxide
2 CuO C Cu CO2
Isolating Copper by Replacement

11. Synthesis, Decomposition, or Replacement