Chemical energy

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 60^oC, 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

More topics about Chemistry…