CHEMICAL
REACTIONS
Recognizing a chemical reaction:
When a chemical reaction occurs, we frequently observe at least
one of the following:
1.Change in color: Cr2O7 2+orange + HSO3 - colorless → Cr3+green solution.
2.Formation of a gas: HCl + Na2CO3 → CO2 gas bubbles.
3.Formation of a solid: CaCl2 solution + K2 CO3 solution → CaCO3 ↓ white solid
4.Release or absorption of heat: HCl + NaOH →
heat .
Note: We cannot always assume that a chemical reaction has occurred when heat
is absorbed or released. Many substances release or absorb heat when they are
mixed together, even though no reaction has taken place.
CLASSES OF CHEMICAL REACTIONS
1. Combination reactions
2. Decomposition reaction
3. Single replacement reaction
4. Double replacement reaction (metathesis reaction):
5. Oxidation – reduction reactions (redox reaction)
6. Reactions in which heat is absorbed or released
COMBINATION REACTIONS:
In a combination reaction, two or more simple substances
combine to produce a more complex substance.
DECOMPOSITION REACTIONS:
This is just the reverse of combination, a complex substance is
broken down into a number of simpler substances.
SINGLE REPLACEMENT REACTIONS:
During a replacement reaction one atom or ion in a compound is
replaced by another.
The activity series:
The following table consists of metallic elements (plus
hydrogen) arranged in such a way that any element in the table will displace ions of
the elements below it from aqueous solutions of their salts.
The activity series (or electrochemical or electromotive series)
Lithium
These metals displace hydrogen
Potassium
from water
Barium
Calcium
Sodium
Magnesium
These metals displace hydrogen
A specific metal will Aluminum
from acids
displace any metal ion Zinc
that appears below it. Iron
Nickel
Tin
Lead
Hydrogen
Copper
These metals do not react with
Mercury
acids (or water)
Silver
Gold
Thus, as we have seen in the previous example, iron is able to
displace copper from an aqueous solution of copper(II) sulfate
since iron is above copper in the electrochemical series.
The reverse reaction does not occur
Metals very high in the activity series:
The metals that are very high in the activity series can not only
displace hydrogen from an acid, but they can also displace
hydrogen from water.
For example, sodium metal reacts
violently with water to give hydrogen gas and a solution of
sodium hydroxide:
The halogen replacement series:
A series similar to the activity series exists for the halogens.
The halogen replacement series
A specific halogen will displace Fluorine
any halide ion that appears Chlorine
below it
Bromine
Iodine
An element in this series can replace the ions of any element below it from
aqueous solutions of its salts.
Double replacement reactions (metathesis reaction):
This may be best thought of as being a change of partners, the cation
from compound A joins with the anion of compound B, while the
cation from compound B joins with the anion from compound A. The
overall effect of such a reaction is that the positive ions exchange their
negative partners.
The metathesis reactions that involve the formation of covalently bonded
water molecules are given the special name of
NEUTRALIZATION
REACTION. A neutralization reaction involves the reaction of an acid with
a base to produce a salt plus water.
Arrhenius definitions of acids and bases: An acid is a substance that gives
H+(aq) ions when it is dissolved in water. A base is a substance that gives OH(aq) ions
when it is dissolved in water.
First let us consider the reaction between nitric acid (a strong acid) and
potassium hydroxide (a strong base). The products are potassium nitrate
and water.
HNO3 (aq) +
KOH(aq)
→ KNO3(aq) + H2O(ℓ)
Oxidation – Reduction reactions (redox reaction)
This type of reaction in aqueous solution involves an exchange of
electrons between two species. One species loses electrons and
is said to be oxidized. The other species, which gains electrons,
is reduced.
REACTIONS in which HEAT is ABSORBED or RELEASED:
Reactions which accomplished by the release of energy in the
form of heat are called Exothermic Reactions, whereas reactions
that absorbs heat from the surroundings are called Endothermic
Reactions.