1. Balancing, States of Matter, and Writing
CHEMICAL REACTIONS
Balancing, States of Matter, and Writing

2. Evidence for A Chemical Reaction
Five Signs of a Chemical Change
Gas Given Off (Bubbles)
Color Change (BIGGIE!!!!!!)
Heat is Produced or Absorbed
Odor Change
Precipitate Forms (Solid is Made)

3. Chemical Equations
Chemical Equations are used to represent chemical reactions.
Reactants  Products
127. Law of Conservation of Matter (Mass) States: “Matter cannot be created or destroyed during a chemical reaction.”
We must balance chemical reactions (add coefficients) to comply with the law.

4. Chemical Equations (Cont)
Example:
CH4 + O2  CO2 + H2O
Methane + Oxygen  Carbon + Water
Dioxide
Reactants Products
1 – C 1 – C
4 – H 2 – H
2 – O 3 – O

5. Balancing
Chemical reactions change the groupings of the atoms, but DO NOT create or destroy atoms, so there must be the same number of each type of atom on each side of the arrow. We must use coefficients to Balance a Chemical Equation

6. Balancing Chemical Equations
Determine the correct formula for each reactant and product by supplying the subscripts to the formulas.
Once you write the correct formula, DO NOT change the subscripts.
Add coefficients in front of balanced formulas to give the same number and kind of atoms on both sides.
Use the lowest possible ratio of coefficients.

7. Example
CH4 + 2 O2  CO2 + 2 H2O
If I place a 2 in front of the O2 and the H2O, then I end up with:
Reactants Products
1 – C 1 – C
4 – H 4 – H
4 – O 4 – O

8. You Try this One
C2H4 + O2  CO2 + H2O
2 – C - 1
4 – H - 2
2 – O - 3

9. C2H O2  2CO H2O
2 – C - 2
4 – H - 4
6 – O - 6

10. Another Example
C3H6 + O2  CO2 + H2O
3 – C - 1
6 – H - 2
4 – O – 3

11. 2 C3H O2  6 CO H2O
6 – C - 6
12 – H - 12
18 – O – 18

12. You Try This One What would the coefficients be for this reaction:
PCl5 + H2O  H3PO4 + HCl
1,4,1,5

13. Physical States Symbols in parentheses after each formula: Example:
Solid (s)
Liquid (l)
Gas (g)
Aqueous (aq)
Aqueous means dissolved in water.
Example:
2 H2O2 (aq)  2 H2O (l) + O2 (g)

14. Words to watch for
States of matter – This is a symbol that is used to identify the state of matter for each substance in a reaction.
Gas, Liquid, Solid, Aqueous
Gas (g)
Liquid (l)
Solid (s) may also be called a Precipitant (ppt) or a crystal (c)
Aqueous (aq) means dissolved in water.