Nature of chemical reaction - Environmental-Chemistry

Nature of chemical reaction:
• The words such as grow, ripen, decay, and
burn describes chemical reactions that we
see everyday.
• Chemical reaction occur when substances go
through chemical changes to form new
substances. For an example.
Chemical equation:
Chem. equation uses symbols to represent a
chem. Reaction and shows the relationship
between the reactants and products of a
CH4 + 2O2 → CO2 + 2H2O + ENERGY
CH4 + 2O2 are called reactants that participate
in reaction, and reactants are always written
at left side of equation.
CO2 + 2H2O called products that is formed as a
result of reaction and products are always
written at right side of equation.
Chemical equation:
An arrow(→) is put between reactants and
products to indicate that products are
produced or yield from reactants.
The reactants and products contain same
type of atoms. Atoms cannot be created or
destroyed during chemical reaction.
A balanced equation tells you the mole ratio,
or proportion of reactants and products, in a
chemical reaction. It means relative amounts
of reactants and products.
Mole ratio can be converted to masses.
Balanced equation and mole ratio:
• Law of definite proportions: A compound
always contains the same elements in the
same proportions regardless of how the
compound is made or how much the
compound is formed.
Energy and chemical reactions:
• Chemical reactions are breaking of old bonds
from reactant-molecules and formation of
new bonds in product-molecules.
• Chemical reactions involve changes in
energy. Photosynthesis is an endothermic
• Energy is released (exothermic) during
formation of bonds and energy is required for
breaking the bonds endothermic)
• Energy is conserved in chemical reactions.
• The energy that is stored in form of chemical
bonds is called chemical energy.
Explain the types of reactions:
1. Combustion Reaction: A substance such as
wood, natural gas, or propane combines with
oxygen releasing a large amount of energy and
produces carbon dioxide and water.
CH4 + 2O2 ----- CO2 + 2H2O + energy
Carbon dioxide
C3H8 + 5 O2 ---- 3 CO2 + 4 H2O + energy
2. Exothermic Reaction: In this type of reaction,
generally energy in form of heat is released.
For example all combustion reactions are
“Meals ready to eat” packets and for fixing
“Clogged drain” process are also based on
exothermic reaction.
Chemical energy: reactants vs. products
• The hump in the middle of each graph
represents the energy required to start the
3. Endothermic Reaction: In this type of
reaction, generally energy in form of heat is
absorbed. For example,
• “instant cold pack” as a treatment for aching
muscles is endothermic reaction.
• Photosynthesis is an endothermic reaction in
which, reaction occur taking CO2 from air,
H2O from soil, green pigment chlorophyll
present in plants and light energy from sun is
absorbed to form sugar (stored in form of
product) and oxygen gas that is released into
the air.
light energy from sun is absorbed
6CO2 + 6H2O
C6H12O6 + 6O2
Define the following terms:
4. Dissolution Reaction: Dissolution reaction occurs
when an ionic compound dissolves in water to make an
ionic solution.
For example, cold pack reaction is also called dissolution
Insoluble: A term to describe a substance that does not
dissolve in water.
Precipitates: Solid particles are formed when two ionic
compounds are added to each other.
Free radical: An atom or a group of atoms that has one
unpaired electron.
Oxidation-Reduction Reaction: Any chemical change in
which one substance is oxidized (loses electrons) and
another substance is reduced (gains electrons)
This reaction is also called as Redox reaction
Explain the steps of balancing chemical equations
Example: Al + O2 ---- Al2O3
Calculate the number of atoms of each type on
both sides of equation.
Reactant side  Al = 1
Product side  Al = 2
Atoms are not equal on reactants and product side, therefore
equation is unbalanced.
To balance the equation, choose the correct
coefficient and place in front of any reactant and/or product
molecule to make the number of atoms equal on both sides.
First make Al-atoms equal.
4 Al + O2 --- 2 Al2O3
Now make oxygen atom equal.
4 Al + 3 O2 --- 2 Al2O3
Now you can check that Both “Al “ and Oxygen-“O” atoms are
equal on both sides of equation.
The equation is balanced.
Practice: Balance the following
1) Fe + O2
--- Fe2O3
2) P4 + O2 ---- P2O5
3) KOH + HCl ---- KCl + H2O
4) Ba(NO3) + NaI ---- BaI2 + Na (NO3)
5) C2H8 + O2 ----- CO2 + H2O
6) CaCO3 --- CaO + CO2
7) CuCl2 + Al --- Al Cl3 + Cu
8) H2O2 ----- H2O + O2
9) NH3 ----- N2 + H2
Synthesis Reaction:
gen. eq. A + B  AB
• The reaction in which two or more
substances combined to form a new
• When many Ethylene molecules combine
then polyethylene is produced. It is
synthesis or addition reaction.
• 2Na + Cl2 --- 2NaCl
• sodium metal + chlorine gas  table salt.
6. Decomposition Reaction:
AB -- A + B
• The reaction in which a substance –a bigger
molecule is broken apart into smaller
• Digestion and cracking oil are decomposition
• 2H2O --- H2 + O2
2NaHCO3 ----- Na2CO3 + CO2 + H2O
Sodium hydrogen carbonate  Sodium chloride
+ carbon dioxide gas + water
Single displacement Reaction
gen. Eq. AX + B  BX + A
• The reaction in which one element or radical
takes the place of another element or radical in
a compound.
• In general, a more reactive element will take
place of a less reactive one in singledisplacement reaction.
• Electroplating of silver or gold is single
displacement reaction.
• 3CuCl2 + 2Al  2AlCl3 + 3 Cu
Copper chloride solution + Aluminum ---
Aluminum chloride + Copper
Double displacement Reaction:
AX + BY  AY + BX
• The reaction in which a gas, a solid precipitate,
or a molecular compound is formed from the
apparent exchange of atoms or ions between
two compounds.
• Pb(NO3)2 + 2 KI  PbI2 + 2K(NO3)
• Lead nitrate + Potassium iodide -- Lead iodide
+ Potassium nitrate
• In general ions appear to be exchanged
between compounds in double displacement
Reaction rate and equilibrium:
• Enzymes: A molecule, ether protein or RNA, that
acts as catalyst in biochemical reactions.
• The reactant in reactions catalyzed by enzymes. For
enzymes to catalyze a reaction, the enzyme and
substrate must fit like lock and its key. For example,
Role of the enzyme
To break down starch
into smaller molecules
To break down long
into sugars molecules
nucleic acid
To build up DNA
chains in cell nuclei
To break down fat
into smaller molecules
To break down
proteins into amino acids
• Catalysts: A substance that changes the rate
of reaction without being consumed means
does not participate actively in chemical
reaction. N2 + 3H2 → 2NH3 Product Ammonia
is produced faster in presence of iron catalyst.
– Catalysts used in various industries to increase
rate of various chemical reaction and also to
increase percentage of products in less amount of
– Economically cost of reactions can be decreased
and more profit can be achieved.
• Inhibitors: The substances that slow reactions
are called inhibitors.
• Chemical equilibrium: Some chemical reactions,
however go in both directions, which results in an
equilibrium system.
• Reversible reaction: Reactants changes to product
and under same condition, products get
decomposed into reacants.
“A chemical equilibrium is a state in which a
reversible chemical reaction is proceeding in both
forward and reverse direction with equal rate at
given set of conditions.” Foe example,
CaCO3 ↔ CaO + CO2
In this reversible chemical reaction, if pressure,
temperature and concentration of any reactant and
product does not change then forward and reverse
reactions take place at the same rate.
The effects of change on equilibrium:
• Condition
• Temperature
Increasing temperature favors the
reaction that absorbs energy.
• Pressure
Increasing pressure favors the
reaction that produces fewer
molecules of gas.
• Concentration Increasing the concentration of one
substance favors the reaction
that produces less of that substance
• Ammonia is a chemical building block used to make
products such as fertilizers, dyes, plastics,
cosmetics, cleaning products, and fire retardants.
The Haber process, which is used to make ammonia
industrially, is N2(g) + 3H2(g) → 2NH3(g) + energy
• Le Chatelier’s principle: If a change is made
to a system in chemical equilibrium, the
equilibrium shifts to oppose the change until a
new equilibrium is reached automatically by
the system.
• Chatelier’s principle predicts changes in
• Le Chatelier’s principle can be used to control
• The percentage of yield can be increased in
reversible reactions using Le Chatelier’s