1. Ch. 8 – Chemical Reactions
Intro to Reactions

2. A.Signs of a Chemical Reaction
Evolution of heat and light
Formation of a gas
Formation of a precipitate
Color change

3. B.Law of Conservation of Mass
mass is neither created nor destroyed in a chemical reaction
total mass stays the same
atoms can only rearrange
4 H
2 O
4 H
2 O
36 g
4 g
32 g

4. C. Chemical Equations
A+B  C+D
REACTANTS
PRODUCTS

5. C. Chemical Equations Pt
a catalyst is present (in this case, platinum)

6. 2H2(g) + O2(g)  2H2O(g) D. Writing Equations
Identify the substances involved.
Use symbols to show:
How many? - coefficient
Of what? - chemical formula
In what state? - physical state
Remember the diatomic elements.

7. D. Writing Equations 2 Al (s) + 3 CuCl2 (aq)  3 Cu (s) + 2 AlCl3 (aq)
Two atoms of aluminum react with three formula units of aqueous copper(II) chloride to produce three atoms of copper and two formula units of aqueous aluminum chloride.
How many?
Of what?
In what state? 2 Al
(s)
+ 3
CuCl2
(aq)
 3 Cu
(s)
+ 2
AlCl3
(aq)

8. E. Describing Equations
Describing Coefficients:
individual atom = “atom”
covalent substance = “molecule”
ionic substance = “formula unit”
3CO2 
2Mg 
4MgO 
3 molecules of carbon dioxide
2 atoms of magnesium
4 formula units of magnesium oxide

9. Subscripts vs. Coefficients

10. E. Describing Equations
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
How many?
Of what?
In what state?
One atom of solid zinc
reacts with
two molecules of aqueous hydrochloric acid
to produce
one f.u.
of aqueous zinc chloride
and one
molecule of hydrogen gas.

11. II. Balancing Equations

12. Coefficient  subscript = # of atoms
A. Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Note: you may not change a subscript
Coefficient  subscript = # of atoms
4. Reduce coefficients to lowest possible ratio, if necessary.
5. Double check atom balance!!!

13. B. Helpful Tips Balance one element at a time.
Update ALL atom counts after adding a coefficient.
If an element appears more than once per side, balance it last.
Balance polyatomic ions as single units.
“1 SO4” instead of “1 S” and “4 O”

14. C. Balancing Example 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 1
Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2
Al CuCl2  Cu AlCl3 Al Cu Cl 3 3 2
2 
 2
 6
3 
6 
 3

15. Balancing Equations
____C3H8(g) _____ O2(g) > _____CO2(g) _____ H2O(g)
____B4H10(g) _____ O2(g) > ___ B2O3(g) _____ H2O(g)

16. Balancing Equations Na3PO4 + Fe2O3 ----> Na2O + FePO4
Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate
Na3PO Fe2O > Na2O FePO4