5.2 Basic Concept of Energy

The concept that we call energy was very slow to develop; it took more than a hundred years just to get people to agree on the definitions of many of the terms we use to describe energy and the interconversion between its various forms. But even now, most people have some difficulty in explaining what it is; somehow, the definition we all learned in elementary science (“the capacity to do work”) seems less than adequate to convey its meaning.

Although the term “energy” was not used in science prior to 1802, it had long been suggested that certain properties related to the motions of objects exhibit an endurance which is incorporated into the modern concept of “conservation of energy”. René Descartes (1596-1650) stated it explicitly:

When God created the world,  He “caused some of its parts to push others and to transfer their motions to others…” and thus “He conserves motion”.*

In the 17th Century, the great mathematician Gottfried Leibniz (1646-1716) suggested the distinction between vis viva (“live force”) and vis mortua (“dead force”), which later became known as kinetic energy (1829) and potential energy (1853).

Kinetic energy and potential energy

STEMonstrations: Kinetic and Potential Energy

Whatever energy may be, there are basically two kinds.

Kinetic energy is associated with the motion of an object, and its direct consequences are part of everyone’s daily experience; the faster the ball you catch in your hand, and the heavier it is, the more you feel it.  Quantitatively, a body with a mass m and moving at a velocity v possesses the kinetic energy mv2/2.

Problem Example 1

A rifle shoots a 4.25 g bullet at a velocity of 965 m s–1.  What is its kinetic energy?

Solution:  The only additional information you need here is that
1 J = 1 kg m2 s–2:

KE = ½ × (.00425 kg) (965 m s–1)2 = 1980 J

Potential energy is energy a body has by virtue of its location. But there is more: the body must be subject to a “restoring force” of some kind that tends to move it to a location of lower potential energy. Think of an arrow that is subjected to the force from a stretched bowstring; the more tightly the arrow is pulled back against the string, the more  potential energy it has.

More generally, the restoring force comes from what we call a force field— a gravitational, electrostatic, or magnetic field. We observe the consequences of gravitational potential energy all the time, such as when we walk, but seldom give it any thought.

If an object of mass m is raised off the floor to a height h, its potential energy increases by mgh, where g is a proportionality constant known as the acceleration of gravity; its value at the earth’s surface is 9.8 m s–2.

Problem Example 2

Find the change in potential energy of a 2.6 kg textbook that falls from the 66-cm height of a table top onto the floor.

Solution: PE =  m g h = (2.6 kg)(9.8 m s–2)(0.66 m) = 16.8 kg m2 s–2 = 16.8 J

Similarly, the potential energy of a particle having an electric charge q depends on its location in an electrostatic field.

“Chemical energy”

Electrostatic potential energy plays a major role in chemistry; the potential energies of electrons in the force field created by atomic nuclei lie at the heart of the chemical behavior of atoms and molecules. “Chemical energy” usually refers to the energy that is stored in the chemical bonds of molecules. These bonds form when electrons are able to respond to the force fields created by two or more atomic nuclei, so they can be regarded as manifestations of electrostatic potential energy.

In an exothermic chemical reaction, the electrons and nuclei within the reactants undergo rearrangment into products possessing lower energies, and the difference is released to the environment in the form of heat. 

Interconversion of potential and kinetic energy

Transitions between potential and kinetic energy are such an intimate part of our daily lives that we hardly give them a thought. It happens in walking as the body moves up and down.

Figure 5.1

Our bodies utilize the chemical energy in glucose to keep us warm and to move our muscles. In fact, life itself depends on the conversion of chemical energy to other forms.

Energy is conserved: it can neither be created nor destroyed. So when you go uphill, your kinetic energy is transformed into potential energy, which gets changed back into kinetic energy as you coast down the other side. And where did the kinetic energy you expended in peddling uphill come from? By conversion of some of the chemical potential energy in your breakfast cereal.

When you drop a book onto the floor, its potential energy is transformed into kinetic energy. When it strikes the floor, this transformation is complete. What happens to the energy then? The kinetic energy that at the moment of impact  was formerly situated exclusively in the moving book, now becomes shared between the book and the floor, and in the form of randomized thermal motions of the molecular units of which they are made; we can observe this effect as a rise in temperature.

Figure 5.3 Energy Conversions

Figure 5.2 Interconversion btween Potential & Kinetic Energy

Much of the potential energy of falling water can be captured by a water wheel or other device that transforms the kinetic energy of the exit water into kinetic energy. The output of a hydroelectric power is directly proportional to its height above the level of the generator turbines in the valley below. At this point, the kinetic energy of the exit water is transferred to that of the turbine, most of which (up to 90 percent in the largest installations) is then converted into electrical energy.

Will the temperature of the water at the bottom of a water fall be greater than that at the top? James Joule himself predicted that it would be. It has been calculated that at Niagra falls, that complete conversion of the potential energy of 1 kg of water at the top into kinetic energy when it hits the plunge pool 58 meters below will result in a temperature increase of about 0.14 C°. (But there are lots of complications. For example, some of the water breaks up into tiny droplets as it falls, and water evaporates from droplets quite rapidly, producing a cooling effect.)

Chemical energy can also be converted, at least partially, into electrical energy: this is what happens in a battery.  If a highly exothermic reaction also produces gaseous products, the latter may expand so rapidly that the result is an explosion — a net conversion of chemical energy into kinetic energy (including sound). 

                                                               Figure 5.4 Conversion of Chemical Energy into other Energy

Thermal energy

Kinetic energy is associated with motion, but in two different ways.  For a macroscopic object such as a book or a ball, or a parcel of flowing water, it is simply given by ½ mv2.

But as we mentioned above, when an object is dropped onto the floor, or when an exothermic chemical reaction heats surrounding matter, the kinetic energy gets dispersed into the molecular units in the environment.  This “microscopic” form of kinetic energy, unlike that of a speeding bullet, is completely random in the kinds of motions it exhibits and in its direction. We refer to this as “thermalized” kinetic energy, or more commonly simply as thermal energy. We observe the effects of this as a rise in the temperature of the surroundings. The temperature of a body is direct measure of the quantity of thermal energy is contains.

Conversion of thermal energy to heat is never completely recoverable

Once kinetic energy is thermalized, only a portion of it can be converted back into potential energy.  The remainder simply gets dispersed and diluted into the environment, and is effectively lost.

To summarize, then:

• Potential energy can be converted entirely into kinetic energy..
• Potential energy can also be converted, with varying degrees of efficiency,into electrical energy.
• The kinetic energy of macroscopic objects can be transferred between objects (barring the effects of friction).
• Once kinetic energy becomes thermalized, only a portion of it can be converted back into either potential energy or be concentrated back into the kinetic energy of a macroscopic. This limitation, which has nothing to do with technology but is a fundamental property of nature, is the subject of the second law of thermodynamics.
• A device that is intended to accomplish the partial transformation of thermal energy into organized kinetic energy is known as a heat engine.

How molecules take up thermal energy

As noted above, the heat capacity of a substance is a measure of how sensitively its temperature is affected by a change in heat content; the greater the heat capacity, the less effect a given flow of heat q will have on the temperature.

Remember: thermal energy is randomized translational kinetic energy. We also pointed out that temperature is a measure of the average kinetic energy due to translational motions of molecules. If vibrational or rotational motions are also active, these will also accept thermal energy and reduce the amount that goes into translational motions. Because the temperature depends only on the latter, the effect of the other kinds of motions will be to reduce the dependence of the internal energy on the temperature, thus raising the heat capacity of a substance.

Chemical reactions can give heat (thermal energy) to the surrounding during the process of the reactions and said to be exothermic chemical reactions. Also chemical reactions need to take in heat from the surrounding for these reactions to progress. Such chemical reactions are said to be endothermic chemical reactions.

In general, “exo” in Greek means outside and “endo” in Greek means inside.

Energy can be utilized from these chemical reactions changes in many applications in the industry and some other applications around us.

The videos below illustrate the exothermic versus endo thermic chemical reactions changes:

Endothermic and Exothermic Reactions

What Are Endothermic & Exothermic Reactions | Chemistry | FuseSchool

Exothermic and Endothermic Curves:

The chemical reactions changes of exothermic and endothermic reactions can be observed and followed through the curves of exothermic and endothermic reactions as shown below:

Exothermic Reaction                                                            Endothermic Reaction

Figure 5.6 Energy Diagram for Exo and Endothermic Reaction

Reference: https://commons.wikimedia.org/

The types of energies in the curves (potential energy and activation energy) will be discussed later. In general, in the exothermic chemical reaction or change, energy will be released (given away) and in the endothermic chemical reaction or change, energy will be absorbed (taken in).

Writing Exothermic and Endothermic Reactions Equations:

Exothermic and Endothermic chemical reactions equations can be written in three ways:

Examples:

1. Using the word “Heat”.

Exothermic Equation Example:

C(s)      +     O₂(g)      →    CO₂(g)   +  Heat

Endothermic Equation Example:

Heat   +    C(s)    +   H₂O(g)   →   H₂(g)   +   CO(g) 

2. Using a value instead of the “Heat”

Exothermic Equation Example:

C(s)      +     O₂(g)      →    CO₂(g)   +  393 kJ/mole

Endothermic Equation Example:

131 kJ/mole   +    C(s)    +   H₂O(g)   →   H₂(g)   +   CO(g) 

3. Using the “Enthalpy Symbol” with negative or positive values to replace the word heat

Exothermic Equation Example:

C(s)      +     O₂(g)      →    CO₂(g)       ∆ H = – 393 kJ/mole  [Exothermic: Negative Value]

Endothermic Equation Example:

Heat   +    C(s)    +   H₂O(g)   →   H₂(g)   +   CO(g)   ∆ H = + 131 kJ/mole   [Endothermic: Positive Value]

The term “Enthalpy” will be discussed later.

Exothermic and Endothermic applications examples from around us:

Cold (Ice) pack (endothermic chemical reaction)

Hot pack (exothermic chemical reaction)

Cold (Ice) pack (endothermic chemical reaction)

Figure 5.7 Cold Pack

Ref: www.openstax.org/

Ammonium sold is put inside a large sealed plastic bag. Inside this plastic bag there is another small bag that is sealed and has water in it. As soon as the water containing small bag is punched through, water comes out and start dissolving ammonium nitrate and heat is taken in from the surroundings and the plastic bag will be cold and the reaction is said to be endothermic.

Cold or ice pack is used to cool down pain due to injuries.

The endothermic chemical reaction is given below:

Heat    +     NH₄NO₃(s)    +    H₂O(l)      ↔    NH₄(aq)      +     NO₃^–(aq)      +      H₂O(l)      or

25.7 kJ/mole   +     NH₄NO₃(s)    +    H₂O(l)      ↔    NH₄(aq)      +     NO₃^–(aq)      +      H₂O(l)     or

NH₄NO₃(s)    +    H₂O(l)      ↔    NH₄(aq)      +     NO₃^–(aq)      +      H₂O(l)        ∆H  =   + 25.7 kJ/mole

Hot pack (exothermic chemical reaction)

Calcium chloride is put inside a large sealed plastic bag. Inside this plastic bag there is another small bag that is sealed and has water in it. As soon as the water containing small bag is punched through, water comes out and start dissolving Calcium chloride and heat is given away from the reaction system to the surroundings and the plastic bag will be warmed up and the reaction is said to be exothermic.

Hot pack is used to warm up the surrounding especially it is used in the camping in the winter times.

The exothermic chemical reaction is given below:

CaCl₂(s)   +  H₂O(l)     ↔     Ca²⁺(aq)     +    2  Cl^–(aq)      +    H₂O(l)    + Heat            or

CaCl₂(s)   +  H₂O(l)     ↔     Ca²⁺(aq)     +    2  Cl^–(aq)      +    H₂O(l)    +  82.8 kJ/mole    or

CaCl₂(s)   +  H₂O(l)     ↔     Ca²⁺(aq)     +    2  Cl^–(aq)      +    H₂O(l)         ∆H  =   – 82.8 kJ/mole

Figure 5.8 Hot Pack

Ref: www.openstax.org/

Reference: https://www.youtube.com/watch?v=leonysaMIRo

Heat, Temperature and Thermal Energy:

Heat:

A term is used to describe the transfer the thermal energy between molecules within a system. Heat measures how thermal energy flows, moves or transfer.

It is measured in different systems as follows:

English System: Heat measured in British Thermal Unit (BTU)

Metric System: Calories (cal)

International System (SI): Joules (J)

Note that 1 calorie = 4.184 Joules

Most of our heat in our solar system is taken from the sun (about 90%).

Temperature:

A tool is used to measure the heat. It is measured in different systems as follows:

English System: Fahrenheit (^oF)

Metric System: Centigrade Celsius (^oC)

International System (SI): Kelvin (K)

Figure 5.9 Different Scales of Temperature

Reference: https://commons.wikimedia.org/

Thermal Energy:

Itis the energy contained within a system and associated with the random motion of atoms and molecules in that system. Such energy is called kinetic energy.

The figure below illustrates the thermal energy concept:

Reference: https://slideplayer.com/slide/8214047/

Heat Transfer:

Heat transfers from hot to cold and never reverse.

The figure above shows this move:

Figure 5.10 Heat & Temperature

Reference: https://phet.colrado.edu

Hot matter expands while cold matter shrinks:

Figure 5.11 Kinetic energy & Temperature

Reference: https://manoa.hawaii.edu/exploringourfluidearth/physical/density-effects/density-temperature-and-salinity/weird-science-macroscopic-changes-liquid-water-volume

The heat transfers from hotter matter to colder matter and at the end of the transfer both matters will attain an equilibrium temperature which is the same for both matters.

The figure below illustrates this phenomena:

Figure 5.12 Thermal energy Transfer

Reference: https://legacy.etap.org/demo/chem5/instruction5tutor.html