1. Chemical Equations and Reactions
Section 8.1

2. What is a chemical reaction?
Any change where a new chemical is formed.
Indications of a chemical reaction:
Change in temperature
Production of light/color change
Change in state

3. Changes in State
Not all changes in state indicate a chemical reaction. When water melts it is just a physical change.
When two liquids are mixed and a solid is formed that solid is called a precipitate.

4. What is a chemical equation?
Chemical equations represent chemical reactions
They:
Identify reactants and products
Contain the correct chemical formulas
Satisfy the Law of Conservation of Mass. Coefficients are used to tell how many of each chemical is present.

5. Types of chemical equations
Word Equations
methane + oxygen --> carbon dioxide + water
Formula Equations
CH4(g) + 2O2(g) --> CO2(g) + 2H2O(g)

6. State Symbols
State symbols - subscripted after each element or compound.
(s) - Solid
(l) - Liquid
(g) - Gas
(aq) - Aqueous. This means that a substance is dissolved in water.

7. Other Symbols
Yields sign - arrow that separates reactants from products.
Triangle above yields sign - reactants must be heated.
“Up” arrow- a gas is formed
“Down” arrow - a precipitate is formed
< > - reversible reaction

8. Law of Conservation of Mass
Matter cannot be created nor destroyed in a chemical reaction so we have to have the same number of each element on both sides of the equation.
This is done by balancing the equation with coefficients.
Zn(s) + 2HCl(aq) --> H2(g) + ZnCl2(aq)

9. Let’s balance some equations
The rusting of Iron:
Fe + O2 ----> Fe2O3
4 Fe + 3 O2 ----> 2 Fe2O3
Combustion of octane in an engine:
C8H O2 ----> H2O CO2
2 C8H O2 ----> 18 H2O + 16 CO2

10. The Diatomic Elements
There are seven diatomic elements. They are never found as individual atoms but are bonded as a pair of atoms:
H2, N2, O2, F2, Cl2, Br2, I2
You must memorize these!

11. Let’s write some equations
Calcium reacts with sulfur to produce solid calcium sulfide.
Ca (s) + S (s) ----> CaS (s)
Hydrogen reacts with fluorine to produce hydrogen fluoride gas.
H2 (g) + F2 (g) ----> 2 HF (g)

12. 2 Al(s) + 3 ZnCl2(aq) ----> 3 Zn(s) + 2 AlCl3(aq)
Here is another
Aluminum reacts with a solution of zinc chloride to produce zinc and aqueous aluminum chloride
2 Al(s) + 3 ZnCl2(aq) ----> 3 Zn(s) + 2 AlCl3(aq)