CHAPTER 9
CHEMICAL REACTIONS AND
EQUATIONS
Law of conservation of
Matter
Mass is conserved in chemical
reactions
Mass of products = mass of
reactants
2H2 + O2
→
4 amu + 32 amu =
2H2O
2 (18 amu)
The atoms of each element
are also conserved.
2H2 + O2 →
2H2O
4 H atoms, 2 O atoms → 4 H atoms
and 2 O atoms
BALANCING CHEMICAL EQUATIONS
When you are given an equation to balance
the chemical formulas will be correct. DO
NOT CHANGE THESE.
H2 +
O2
→
H2O
Note (diatomic elements are marked with a
green X on the periodic chart at the front of
the room.)
Balance equations by placing coefficients in
front of the formulas.
Balance only one element (or polyatomic ion)
at a time until the entire equation is balanced.
Complete balancing worksheet
CLASSIFYING CHEMICAL
REACTIONS
Combination reaction (also called
synthesis)
A
+
B
→
AB
Identified by two reactants only one
product
Na
+
Cl2
→
NaCl
CO2 + H2O → H2CO3
Decomposition Reaction
Exact opposite of combination reaction
One compound is broken down
“decomposed” into two.
Often energy has to be added to make
this happen.
AB
→
A +
B
H2CO3 →
CO2 + H2O
Single Replacement
Reactions
One element kicks another element out
of a compound and takes its place.
A + BX → AX + B
A more reactive metal will replace a
less reactive metal or hydrogen.
(See metal activity series page 295)
A more reactive nonmetal will replace
a less reactive nonmetal (compare
electronegativity)
Double Replacement
Reaction
Compounds with positive and negative
groups exchange partners.
AX + BY → AY + BX
CaCO3 + HCl → CaCl2 + H2CO3
The 4 ions of the starting materials are all
dissolved in water and no reaction will take
place unless:
– A molecular compound is formed (like water)
– A precipitate is formed (check the solubility of
possible products)
– A gas that leaves the solution is formed
When will a reaction not
take place?
A metal will not react with a metal.
A noble gas will not react with
anything.
A less reactive element will not
replace a more reactive element.