1. Ch. 10 – Chemical Reactions
Intro to Reactions EQ: What are the properties and signs of a chemical reaction? Pg
Ch. 10 – Chemical Reactions

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

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

5. Chemical Equations

6. 2H2(g) + O2(g)  2H2O(g) Writing Equations How many? - coefficient
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. Writing Equations 2 Al (s) + 3 CuCl2 (aq)  3 Cu (s) + 2 AlCl3 (aq)
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? 2 Al
(s)
+ 3
CuCl2
(aq)
 3 Cu
(s)
+ 2
AlCl3
(aq)

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

9. Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
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 unit
of aqueous zinc chloride
and one
molecule of hydrogen gas.

10. Reaction Pathway
Shows the change in energy during a chemical reaction

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

12. 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”

13. Balancing Example Writing & Balancing.mpg
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

14. 2H2(l) + O2(l)  2H2O(g) + energy
Exothermic Reaction
reaction that releases energy
products have lower PE than reactants
energy released
2H2(l) + O2(l)  2H2O(g) + energy

15. Endothermic Reaction 2Al2O3 + energy  4Al + 3O2
reaction that absorbs energy
reactants have lower PE than products
energy absorbed
2Al2O3 + energy  4Al + 3O2