In fact
chemical bond is a store of energy. Energy
is associated with formation and breaking of chemical bonds. As the number of
changing in chemical bond happened in the world, in all cases transformation of
energy
happened. We perform our every day works using the energy
of the changes which occur spontaneously. The amount of this energy is finite
in the world and is decreasing day by day. So it is necessary to think about
alternative source of energy. In the mean time, attempt is gone on to increase
the number of lighting, making solar panels using the power of sun. On the
other hand effort has been started to make use of nuclear power in our country
like developed countries.
Bond energy and change of
energy in chemical reactions
We have known
that, elements in a molecule joined together by mutual energy between them. The
energy by which the elements are joined together is chemical bonds. Besides this
molecules or ions of a compound comes closer by an energy named inter molecular
force composed of various energy and create a particular state as solid, liquid
and gaseous. The inter molecular force of ions or molecules of any solute, if
high forms solid, if low forms liquid and if more lower forms gaseous states.
That is inter molecular force differs at different states of the same compound.
For example, ice, water and water vapor is solid, liquid and gaseous states of
water, when water is heated water vapor is produced that means water converts from liquid to gaseous state by absorbing heat. Again on cooling of water that
means taking out heat from water, water converts to solid (ice). On the other
hand energy is also associated with bond formation. In different compound
molecules joined together with different bond energy. Now let us think, what will
happen, if the total energy of the products is less or greater than the total
energy of the reactants. If the energy of the product is lower, energy will be
evolved and if greater, energy will be absorbed. On the whole, in all chemical
reactions more or less amount of heat is either evolved or absorbed, although
we are not able to feel that. So it is clear that, as energy is associated with
the physical changes with solute, so as with the process of forming new
compounds by chemical reactions.
Endothermic and Exothermic Reactions
Now classify the
chemical reactions on the basis of heat change from the above discussion. On
the basis of heat change chemical reactions are of two kinds like, (1) Exothermic
Reactions and (2) Endothermic Reactions.
 |
| Exothermic reaction |
Exothermic Reactions: The chemical reactions in which heat
is evolved are called the exothermic reactions. For example, if wood, coal or
gas is burnt heat is evolved. Wood or coal is mainly carbon and other compounds
of carbon, which on burning reacts with atmospheric oxygen produce carbon-die-oxide
(CO2) and heat. When water is added to lime heat is produced. Lime
is calcium oxide (CaO), which reacts with water and form calcium hydroxide; Ca
(OH)2 and heat.
C(s) + O2(g) → CO2(g)
+ Heat
CaO(s) + H2O(l)
→ Ca(OH)2(aq) + Heat
Now explain
the phenomenon of evolving heat energy. In the first case the total stored energy
of reactants carbon and oxygen is greater than the total stored energy of
produced carbon-die-oxide. Similarly, in the second case the total stored
energy of the produced calcium hydroxide is less than the total stored energy
of reactants calcium oxide and water. It can be easily described that, the
excess amount of stored chemical energy of the reactant after consumption in
the formation of new compound is evolved as heat; that is Heat evolved = Total
energy of produced compound, (E2) – Total energy of reactants compound
(E1).
Heat of
reaction: The amount
of changed heat in a chemical reaction is called heat of reaction.
Heat of combustion: When 1 mole of
substance is burnt, the amount of heat evolved in the process is called the
heat of combustion.
 |
Endothermic reaction
|
Endothermic reactions: Chemical reactions in which heat is
absorbed are called endothermic reactions. In case of exothermic reactions we
generally feel the evolution of heat directly. But in the case of endothermic
reactions, we can feel in very fewer cases the absorption of heat. Now let us
try to realize the events where heat is absorbed. There is a half filled glass
of water at 60oC, add a piece of ice in it. Of course we can realize
that, within a short time the piece of ice will melt and temperature of water
will fall. Generally we use ice cubes to cool the soft drinks in this way. We
have known earlier that, water turns from liquid to solid (ice) if we remove
energy from water. So it is clear that, solid water (ice) will return to liquid
water, if we reinforce the removed energy (heat) to ice. Really the ice cube
dipped in glass turns to liquid water by absorbing heat energy from hot water
as a result temperature of hot water falls. So the ice cube converts to water
absorbing heat from the system. Similarly, there may take place absorption of
heat in performing chemical reactions. In this case sometimes we feel cool on
touching the containers used in performing chemical reactions. Again sometimes
no reaction occurs without applying heat from external sources. For example, in
the reaction of edible soda and lemon juice or vinegar absorption of heat
occurs. Edible soda is sodium-bi-carbonate (NaHCO3). On the other
hand there is citric acid in lemon juice and acetic acid in vinegar. Reaction
of sodium-bi-carbonate with acid forms carbon-die-oxide, salt and water. In the
reaction there occurs absorption of heat from the solution as a result we
observe the solution to become cool.
NaHCO3(aq) + CH3COOH(aq)
+ Heat (absorbed from solution) → CH3COO- Na+
(aq) + CO2(g) + H2O(l)
Or
NaHCO3(aq) + H+
(aq) (citric acid) + Heat (absorbed from solution) → Na+ salt
(sodium citrate) + CO2(g) + H2O(l)
Calculation of heat change
in chemical reactions
In performing
the chemical reaction breaking of old bonds and formation of new bonds take
place. Energy of different existing bonds in the chemical compounds is
different. Energy is needed to break the bond and energy is evolved in bond
formation. Calculation of the change of heat in a reaction is done using the
equation below by evaluating the total energy required for breaking the bonds
and total energy evolved in the formation of new bonds.
Change of heat
in a reaction = Total energy required for breaking the old bonds – Total energy
evolved in the formation of new bonds.
If the heat
change is negative the reaction is exothermic and if positive the reaction is endothermic.
Calculate the
heat change of the reaction below using the bond energy given in the chart.
Bond (kJ/ mole)
|
Bond energy
|
C-H
|
414
|
H-H
|
435
|
C-C
|
326
|
O-H
|
464
|
Cl-Cl
|
244
|
O=O
|
498
|
H-Cl
|
431
|
CH3CH3(s) + Cl2(g)
→ CH3CH2Cl(aq) + HCl(g)
In this
reaction 1 mole of C-H & 1 mole of Cl-Cl bonds is broken and 1 mole of C-Cl
& 1 mole of H-Cl bonds is formed. Total energy required for breaking 1 mole of C-H and 1 mole of Cl-Cl is = (414+244) kJ = 658 kJ.
Total energy
evolved in formation of 1 mole of C-Cl and 1 mole H-Cl is = (326+431) kJ = 757
kJ.
So the change
of heat in reaction (Delta H) = Total energy required for breaking the old
bonds - Total energy evolved in the formation of new bonds
= (658-757)
kJ
= -99 kJ.
Thus 99 kJ
heat is evolved in the reaction.
Energy diagram of a reaction
Absorption or
emission of heat in a chemical reaction can easily be understood by energy
diagram. Observe the energy diagram of exothermic and endothermic reactions in
fig-8.3 and 8.4. In case of exothermic reaction the total energy of the
reactants (E1) will be greater than the total energy of product (E2).
That means E1 > E2. The excess amount of chemical
energy of the reactant after consumption in the formation of products is
evolved as heat energy. On the other hand, the energy diagram of endothermic
reaction is opposite to the exothermic reaction.
 |
| Energy
diagram of exothermic reaction here, E1 > E2 |
In case of endothermic reaction the total energy of the reactants (E1)
will be less than the total energy of product (E2). That means E1
< E2. In this case, though the total energy of reactant is less
than the total energy of the product the required energy to occur the reaction
is obtained from the system. That is why; if endothermic reaction is occurred,
it is seen to decrease the temperature of reaction mixture or it is required to
apply heat.
 |
| Energy
diagram of endothermic reaction here, E1 < E2 |
Transformation of chemical
energy to heat, electricity and light energy
We see that, heat is
produced on burning of any substance. Light is produced as well as heat. Both
heat and light are energy, which spread as electromagnetic radiation around us.
Heat energy and light energy is produced on burning of wood, coal, plants,
paper, natural gas and petroleum etc. Both energies can be produced by burning
safety matches and candle (fig-Burning candle).
 |
Burning candle
|
Thus what is
the source of the energy of these substances? What is the meaning of burning of
any substance? We know, matter is a bundle of chemically bonded atoms. On the
other hand, burning is oxidation of molecules of any compound by oxygen. Thus
oxidation of any molecule of any compound means the creation of oxygen added
new compounds. Now consider the reaction given below- Carbon (C) of coal,
Methane (CH4) of natural gas and hydrogen molecules of hydrogen fuel
react with oxygen molecules and form carbon-die-oxide gas and water. Carbons
upon breaking bonds of themselves form carbon-oxygen (carbon-dioxide) bond. We
know that the energy requirements for the formation of all bonds or molecules
are not equal. Actually, the amount of energy consumed in formation of
molecules or ions on burning fuel is less than the stored energy in the
molecules of fuel. As a result excess energy spreads as electromagnetic
radiations which we see as light or heat.
C(g) + O2(g) → CO2(g)
+ Heat
CH4(g) + 2O2(g)
→ CO2(g) + 2H2O(g) + Heat
H2(g) + O2(g) →
H2O(g) + Heat
Electrical
energy is also produced by rotating the wheels of heat engine using the heat energy
evolved from burning of fuel. In our country almost all electrical energy is produced
by burning fuels. On the other hand, in hydrogen fuel cells (a kind of electrolysis
cell) not burning the hydrogen is converted directly to electricity by electrolysis.
Chemical energy is converted to electrical energy in different types of galvanic
cells such as – Daniel cell, dry cell and lead storage battery. Again the
electric energy produced in galvanic cells may convert to light energy such as,
lightening of torch by dry cell. In this way, converting chemical energy into
different energy is made usable to man.
Use of different energy
obtained from chemical energy
Efficiency of
work is energy. Heat is evolved on burning fuels and heat is a kind of energy.
Cooking is done by natural gas. Bricks, clay wares are burnt using heat energy.
Heat energy is used in industries for melting raw materials or to make them hot
in producing different substances. Huge amount of heat energy is used in
industries like Iron & steel and ceramic. Different fossil fuels like-
coal, petroleum and natural gas are burnt in heat engines as fuel and used in
running motor vehicles, ship, airplane, rail and other engine driven vehicles.
Water is lifted from depth by revolving the wheel of shallow engine by burning
petroleum. We enlighten our home by burning of kerosene and wax candle. On the
other hand, in modern age the most popular energy is electricity. We find the
use of electricity in everywhere. Though electricity can be used in various
ways, maximum electricity is produced in heat engines revolving the turbine by
burning of fossil fuels. Converting chemical energy into electrical energy by
electrochemical cells and battery, we use it for lighting and for operating
radio, TV and fan etc.
Appropriate uses of chemical
energy
Source of all
energy is sun. So, is the sun not indirect source of chemical energy? You have
known from life cycles, plants store energy in their body from sun light by
photosynthesis. Different bio-chemical compounds are produced in the plant’s
body from light energy and atmospheric carbon-die-oxide. Animals get energy
from plants. Plants and animal decompose in soil after death. These substances
under different processes for thousands of year are converted and stored in the
earth as petroleum, coal and natural gas. These are called fossil fuels. We get
these types of fuels in mines. Once upon a time these will be finished.
Question may arise, as plants and animals born and die regularly, why will
these fossil fuels be finished then? The answer is very easy. Because we are
using the fuels, which means consuming the fuels in such a rate that mine fuels
are not being stored in the earth at that rate. So it can be easily assumed
that, once upon a time reserve of these fuels will be finished. The subject is
appropriate and it is said that reserve of these fossil fuels will be finished
within the next one hundred years.
We have known
earlier that, mainly fossil fuels supply the maximum of our total demand of
fuels. Not only that, transformation of fossil fuels to other energy (i.e.
electricity) is expensive also. Moreover chemical energy particularly fossil
fuels cause different harms to environment. So, what will our role in using
chemical energy? In modern time it is impossible to survive without chemical
energy. But if we can ensure the limited use of chemical energy, demand on the
reserve will be certainly reduced. By this we can utilize the reserve of fuel
for a long time. Unfortunately we are misusing energy. We are keeping the
burner en-flamed, lighting lights, moving fan unnecessarily and using various decorative
lighting for entertainment and using engine vehicles in poor needs. Preventing these
misuses, we can ensure the long time use of fuels. On the other hand, we can
prevent the worse effect of the use of chemical energy on environment. Consciousness
about chemical energy use can help us to survive in the world for a long time.
Negative effect of use of
chemical energy
We have seen earlier
that, the mechanism of making use of chemical energy mainly production of heat
by burning the fuels with air (oxidation reaction). Though the mechanism of
using of chemical energy is different is case of fuel cell and for other cases
particularly in electrochemical cells and nuclear reactions. In fact maximum energy is produced by burning
fuels. Now the question is that, on burning of thousands ton of fuel, where
will go the produced equivalent amount of CO2 gas and another gases particularly
CO, SO2, NO and the unused fuel emits with smoke. Certainly they are
mixing with air. Note that, some CO2 of air is used in
photosynthesis process. But unfortunately in one side we are destroying plants
and on the other side increasing the use of fuel to meet the modern lifestyle.
This is why amount of CO2 is increasing usually in atmosphere day by
day. Although CO2 do not undergo any reaction with the other element
of air, but heat absorbing capacity of CO2 is high, that is CO2
can absorb heat and can trap the heat. Again CO2 gas stay nearer to
the surface of the earth as it is heavy in weight. That is why atmospheric
temperature is increasing day by day, which is called Global Warming.
The phenomenon of this temperature increase by CO2 gas is known as Green
House Effect and CO2 is called the green house gas. Due to the
global warming, ice melts to water in earth poles and cause unexpected flood
(Fig-below). On the other hand, other gases produced from the burning of fuels,
polluting air by different chemical reactions and destroying the balance of
elements in air and causing acid rain and photo chemical smog. Except this,
these gases directly react with ozone layer and decreasing the thickness of the
layer or causing hole to the layer. In fact ozone layer acts as a filter of sun
rays and prevents the various ultraviolet rays present in sunlight to enter
into the earth.
 |
Exhaust of carbon-die-oxide from industry (left) and due to the global
warming ice melts to water in earth pole (right)
|
End
