Unit 8:
Chemical Reactions
Changes of Matter
• Physical Changes:
Demo
 do not change the composition of a
substance.
• melting, vaporizing, dissolving, size, etc.
• Chemical Changes:
A
B
 result in new substances.
• burning, rusting, decomposition, etc.
AB
4 Clues of Chemical Change
Demo:
Zn + H+  Zn2+ +
1) gas production Pb2+ + I– 
2) color change
Mg + O2 
3) energy transfer (temp change, light)
4) precipitate formation
(solid forms from solution)
• Chemical Change: new & different
substances formed.
Chemical Reaction
• Chemical Equation:
A+B
yields
AB
REACTANTS
PRODUCTS
present at the
start of the rxn
produced in the
rxn at the end
Law of Conservation of Mass
• In a chemical reaction, matter is
neither created nor destroyed.
• Atoms are rearranged.
4H
4H
2O
2O
Law of Conservation of Mass
mass of reactants = mass of products
?
Chemical Equations
Balancing Equations WHY?
mass is conserved (same before & after)
N2H4 + O2  N2 + H2O
N2H4 + O2  N2 + 2 H2O
Coefficient - # of units of each substance
Chemical Equations
H2 + O2  H2O
3 Balancing Steps
1. Count atoms on each side.
2. Add Coefficients to make # of atoms equal.
coefficient  subscript = # atoms
How many N’s in:
2 Fe(NO3)2
3. Check atom balance!!!
Chemical Equations
Aluminum and copper(II) chloride form
copper and aluminum chloride.
2 Al + 3 CuCl2  3 Cu + 2 AlCl3
2 1
Al
1 2
3 1
Cu
1 3
6 2
Cl
3 6
Quick Quiz!
1. Which of the following is a
chemical reaction?
A. melting of lead
B. dissolving sugar in water
C. rusting of iron (new substance)
D. crushing of stone
Quick Quiz.
2. Which of the following is NOT a
possible clue that a chemical
change is taking place?
A.
B.
C.
D.
a change of state
a change in color
production of a gas
formation of a precipitate
Quick Quiz.
3. Why do we have to balance
equations?
A.
B.
C.
D.
it’s so much fun!
it’s the law of the west
it’s the law of consternation of moose
it’s the law of conservation of mass
Quick Quiz.
4.
During any chemical change, the mass
of the final products is…
A. always equal to the mass of the
starting reactants.
B. always greater than the mass of the
starting reactants.
C. always less than the mass of the
starting reactants.
D. sometimes different than the mass
of the starting reactants.
Quick Quiz.
5. What is the coefficient of HCl when the
following reaction is balanced?
2__Al
Al ++ 6__HCl
HCl → __AlCl
2 AlCl33 ++ __H
3 H22
___Al___
1
1
___H___
1
2
___Cl___
3
1
A. 1
B. 2
C. 3
D. 6