1. Chemical Reactions

2. Chemistry Joke
Oxygen: I went out with potassium last night!
How’d it go?
OK!!

3. Introduction
Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken.
Chemical reactions involve changes in matter—the making of new materials with new properties accompanied by energy changes.
Chemical Reactions are described using shorthand called chemical equations.

4. Four Indicators of a Chemical Reaction
1. Change in color
2. Release of a gas
3. Formation of a precipitate
4. Change in energy—of light or heat

5. But…
The only absolute proof of a chemical reaction is that a new substance has been produced!

6. A+B  C+D Chemical Equations REACTANTS PRODUCTS
Left of Arrow: Substances that existed before the reaction took place.
Right of Arrow: Substances that are produced in the reaction.

7. Three Types of Equations…
1.) Word Equations
Propane + Oxygen 
Carbon Dioxide + Water + Energy
Can see the products and the reactants
2.) Formula Equations
C3H8 + O2  CO2 + H2O + energy
Can see the formulas & how many atoms of each element are reacting
3.) Formula Equations w/ physical state indicated
H2(g) + O2(g)  H2O(l) + energy
Indicates state of matter in reaction

8. Symbols Used in a Chemical Equation
, yields
, reacts with
MnO2
catalyst was used (catalyst written above the arrow)

9. In a Chemical Equation…
Symbols are used to show:
How many? – Coefficient
Of what? – Chemical formula
Individual elements – atoms
Covalently bonded elements – molecules
Ionic compounds – formula units or just units
In what state? – physical state
(s) solid
(g) gas
(l) liquid
(aq) aqueous – dissolved in water

10. Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
For example…
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
How many?
Of what?
In what state?
One atom of solid zinc (reacts with)
2 molecules of aqueous hydrochloric acid
(to produce)
One unit of aqueous zinc chloride and
One molecule of hydrogen gas

11. O2 (g) + 2 H2 (g) → 2 H2O (l) Try one backwards…
One molecule of oxygen gas
Reacts with
Two molecules of hydrogen gas
To produce
Two molecules of liquid water.
Write the equation.
O2 (g) + 2 H2 (g) → 2 H2O (l)

12. Try another one… Al 2 (s) + 3 CuCl2 (aq)  3 Cu + 2 AlCl3 How many?
Two atoms of aluminum react with three units of aqueous copper (II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride.
How many?
Of what?
In what state? Al 2
(s)
+ 3
CuCl2
(aq)
 3 Cu
+ 2
AlCl3

13. In a Chemical Reaction…
Mass is neither created nor destroyed—Law of the Conservation of Mass and Energy
Total mass stays the same
Atoms are only rearranged, not changed into different ones
4 H
2 O
4 H
2 O
36 g
4 g
32 g

14. So…how does the conservation of mass affect chemical equations?
The types and amount of atoms on the reactant side must be the same as on the product side.
BALANCED.
The equation must be
C + O2 → CO2
1 atom of carbon plus 2 atoms of oxygen yields 1 atom of carbon and 2 atoms of oxygen.

15. Coefficient  subscript = # atoms
Balancing Equations
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient  subscript = # atoms
4. Reduce coefficients to lowest possible ratio.
5. Double check atom balance!!!

16. Helpful Guidelines Balance one element at a time.
Update ALL atom counts after adding a coefficient.
If elements appear more than once per side, balance them last.
Balance polyatomic ions as single units.
“1 SO4” instead of “1 S” and “4 O”

17. More Helpful Guidelines
Balance diatomic molecules last.
Remember: change coefficients only—not subscripts.
Go in this order:
Metals
Nonmetals except H & O
Polyatomics H O

18. Balancing Example Al + CuCl2  Cu + AlCl3 Al Cu Cl 1 1 2 3 2 3 3 2 2 /
Aluminum and copper(II)chloride form copper and aluminum chloride.
Al CuCl2  Cu AlCl3 Al Cu Cl 2 3 3 2
2 /
/ 2
3 /
/ 3
6 /
/ 6

19. Try these examples… C2H4 + O2  CO2 + H2O AgNO3 + Cu Cu(NO3)2 + Ag
Fe(OH)3  Fe2O3 + H2O
2 Fe(OH)3  Fe2O3 + 3 H2O

20. Try these examples… NH3 + O2  NO + H2O NH3 + O2  NO2 + H2O

21. Sick Chemistry Humor Chemistry Joke … From way down in my cranium
This prediction I will make: …
That if you eat uranium
You’ll get atomic ache!