4.2 TYPES OF REACTIONS: SYNTHESIS AND DECOMPOSITION
A. SYNTHESIS REACTIONS (A + B → AB)
 Also known as combination or formation reactions
 Two or more reactants (elements/compounds) combine to form a new substance
1)
3 Fe(s) + O2(g) → Fe2O3(s)
2)
H2(g) + Cl2(g)  2 HCl(g)
3)
2 K(s) + Cl2(g) → 2 KCl(s)
*For these reactions it is useful to think of these elements as forming
ions to determine your product (although this doesn’t apply for Case
2)
4)
SO3(g) + H2O(l) → H2SO4(aq) (Acid Rain Formation)
5)
CaO(s) + H2O(l) → Ca(OH)2(aq)
The reaction between non-metals is often hard to determine, because it depends on the
conditions of the reaction. For example, the amount of oxygen present allows this
reaction to occur in either one of two ways:
C(s) + O2(g) → CO2(g)
2 C(s) + O2(g) → 2 CO(g)
Overall, predicting the product of a synthesis reaction:
 is difficult
 can only list possible outcomes (stable octet)
 To know for sure, need to perform the reaction, then analyse the product
B. DECOMPOSITION REACTIONS (AB → A + B)
 A compound breaks down into elements/compounds
 A decomposition reaction is the opposite of a synthesis reaction
 It is safe to assume that most decomposition reactions require an input of energy,
although some are spontaneous.
1)
Decomposition of Potassium Chloride
2 KCl(l)
electricity
2 K(g) + Cl2(g)
Decomposition of Water
2 H2O(l)
electricity
2 H2(g) + O2(g)
2)
Decomposition of Ammonium Nitrate (Heated)
NH4NO3(aq)
heat
N2O(g) + 2 H2O(g)
*Predicting the products of molecular compounds and ionic compounds with polyatomic
ions are difficult, because it is possible for there to be multiple products.
HOMEWORK: Read Pages 156-160 and Complete Questions 1-4, 6, 8