Chemical Reactions
SCH3U - Unit 2
Chemical Change
Chemical Change = any change in which
a new substance is formed
Evidence of Chemical Change:
 Change in colour
 Change in odour
 Formation of gas/solid
 Release/absorption of heat
Collision-Reaction Theory

A theory stating that chemical reactions involve
collisions and rearrangements of atoms or
groups of atoms and that the outcome of
collisions depends on the energy and
orientation of the collisions

No reaction occurs if:
Molecules don’t have enough energy
 Molecules don’t collide in the right orientation

Orientation

The molecules must be in a certain 3-D arrangement to
allow a reaction e.g. CH2=CH2 + HCl -> CH3CH2Cl
Energy Required



A reaction doesn’t occur unless the particles collide with
a certain minimum energy called the activation
energy of the reaction
Activation energy is the minimum energy required before
a reaction can occur.You can show this on an energy
profile for the reaction. For a simple over-all exothermic
reaction, the energy profile looks like this:
Activation Energy Profile
Chemical Equations

Chemical Equation = a representation of a
chemical reaction that indicates the:
 Chemical formulas
 Relative number of entities
 States of matter of the reactants and products
Reactants  Products
Chemical Equations
In general:
 Reactant A + Reactant B  Product C

Reactant = is a chemical that is used up in a
chemical reaction

Product = is a product that is created during
a chemical reaction.
Chemical Formulas

A chemical formula uses subscripts to indicates
the number of atoms in a compound

Example: H2O



Has 2 atoms of H
And 1 atom of O
Example: C6H12O6



Has 6 atoms of C
Has 12 atoms of H
And 6 atoms of O
Relative # of Entities

Coefficient = a whole number indicating the
ratio of molecules of each substance involved
in a chemical reaction

The large number on the left side of a molecule’s
formula

Example: Mg +

Example:
2 Cl
 MgCl2
6 K + N2  2 K 3 N
State of Matter
Solid
 Liquid
 Gas
 Solution

=
=
=
=
 Example:
(s)
(l)
(g)
(aq)
6 K(s) + N2(g)  2 K3N(s)
Catalysts



A catalyst is a substance which speeds up a reaction, but
is chemically unchanged at the end of the reaction .e.g
conc. H2SO4 in many different reactions
Adding a catalyst has exactly this effect on activation
energy. A catalyst provides an alternative route for the
reaction. That alternative route has a lower activation
energy
Draw a standard energy profile and then draw a new line
to represent the inclusion of a catalyst
5 Types of Chemical Reactions
1.
Combustion
2.
Synthesis
3.
Decomposition
4.
Single Displacement
5.
Double Displacement
5 Types of Chemical Reactions
Generalizations:
Combustion: AB + oxygen  oxides of A & B + heat
Synthesis: A + B  C
Decomposition: AB  A + B
Single Displacement: A + BC  AC + B
Double Displacement: AB + CD  AD + CB
Combustion Reactions

Combustion Reaction: the reaction of a
substance with oxygen, producing oxides and
energy

Also know as burning

For a combustion reaction to occur 3 things
must be present:
1.
2.
3.
Fuel
Oxygen
Heat
Combustion Reactions

C
Type of Reaction: Synthesis
Example C + O2
C + O O

O C O
C
C
O O
C
CC
O O
C C C C C C CC
C
General:
A + B  AB
Synthesis Reaction Characteristics



Two or more substances (elements or compounds)
react to form ONE product. Combination of
smaller atoms/molecules into larger molecules.
Usually exothermic (energy is produced)
Can occur naturally or by an initial application of
energy (heat, flame, UV light, use of catalyst)
Predicting Products of Synthesis
Reactions







Metal + oxygen → metal oxide (basic oxide)
EX. 2Mg(s) + O2(g) → 2MgO(s)
Nonmetal + oxygen → nonmetallic oxide (acidic oxide)
EX. C(s) + O2(g) → CO2(g)
Metal oxide + water → metallic hydroxide (base)
EX. MgO(s) + H2O(l) → Mg(OH)2(s)
Nonmetallic oxide + water → acid
EX. CO2(g) + H2O(l) → ; H2CO3(aq)
Metal + nonmetal → salt
EX. 2 Na(s) + Cl2(g) → 2NaCl(s)
A few nonmetals combine with each other.
EX. 2P(s) + 3Cl2(g) → 2PCl3(g)
These two reactions should be remembered:
N2(g) + 3H2(g) → 2NH3(g)
NH3(g) + H2O(l) → NH4OH(aq)
Type of Reaction: Decomposition
Example: NaCl
Cl Na

General:
Cl
+
Na
AB  A + B
Type of Reaction: Decomposition
Example 2HgO
O Hg

O Hg
General:
Hg
+
O O
Hg
AB  A + B
Decomposition Reaction Characteristics




ONE reactant produces two or more products.
Splitting of large molecules into elements or
smaller molecules.
Usually endothermic (requires energy)
Can require energy in the form of heat,
electricity, catalyst, UV light
*some decomposition rxns occur at room
temperature
Predicting Products of Decomposition
Reactions






Metallic carbonates, when heated, form metallic oxides and CO2(g).
EX. CaCO3(s) → CaO(s) + CO2(g)
Most metallic hydroxides, when heated, decompose into metallic oxides
and water.
EX. Ca(OH)2(s) → CaO(s) + H2O(g)
Metallic chlorates, when heated, decompose into metallic chlorides and
oxygen.
EX. 2KClO3(s) → 2KCl(s) + 3O2(g)
Some acids, when heated, decompose into nonmetallic oxides and water.
EX. H2SO4 → H2O(l) + SO3(g)
Some oxides, when heated, decompose.
EX. 2HgO(s) → 2Hg(l) + O2(g)
Some decomposition reactions are produced by electricity.
EX. 2H2O(l) → 2H2(g) + O2(g)
EX. 2NaCl(l) → 2Na(s) + Cl2(g)
Type of Reaction: Single Displacement
Example: Zn + CuCl2
Cu
Cl
+
Cl
General:
Zn

Zn
Cl
+
Cu
Cl
AB + C  AC + B
Type of Reaction: Double Displacement
Example: MgO + CaS
Mg
+
O
General:
Ca
S

Mg
S
+
Ca
O
AB + CD  AD + CB
Chemical Reactions
combustion: AB + oxygen  oxides of A & B
synthesis: A + B  C
decomposition: AB  A + B
single displacement: A + BC  AC + B
double displacement: AB + CD  AD + CB