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Internal Energy . Internal energy is defined as the energy associated with the random, disordered motion of molecules.

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internal energy

Internal Energy

Internal energy is defined as the energy associated with the random, disordered motion of molecules.

It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale.

For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic .

But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second.

If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole.


U is the most common symbol used for internal energy.Related energy quantities which are particularly useful in chemical thermodynamics are enthalpy,Helmholtz free energy, and Gibbs free energy.

molecular of chemical energy
Molecular of chemical energy

Molecular of chemical energy can mean several things:

  • Chemical bonds are a source of energy,
  • the movement of molecules in space is kinetic energy, the vibrations and rotations of molecules is another soource of chemical energy.
  • All of these forms of chemical energy contribute in one way or another to chemical reactions.
chemical energy
Chemical Energy
  • The energy held in the covalent bonds between atoms in a molecule is called chemical energy. Every bond has a certain amount of energy. To break the bond requires energy -- in chemical language it is called endothermic. These broken bonds then join together to create new molecules, and in the process release heat -- chemists call this exothermic. If the total heat given out is more than the heat taken in then the whole reaction is called exothermic, and the chemicals get hot. The burning of methane in oxygen is an example of this. If the heat taken in is more than the heat given out then the whole reaction is endothermic and the chemicals get cold. Combining carbon and hydrogen to make methane is an example. We rarely meet such reactions in every day life. They happen in living cells, the energy being supplied by sunlight or some other source. ATP is the molecule used by life to carry chemical energy. The bond between two of its phosphate groups carries a lot of energy because both phosphates have negative electric charge.
energy and chemical reactions
Energy and Chemical Reactions
  • When matter undergoes transformations that change its chemical and physical properties then that transformation was brought about by a chemical reaction. On the other hand chemical reactions can only take place if there is sufficient energy to make the reaction proceed. Therefore energy is a prerequisite for chemical reactions.Energy can come in many forms e.g., heat, work, light, kinetic, potential, chemical etc.. Moreover, energy can itself transform among these various forms. For example a ball at the edge of a table has zero kinetic energy and positive potential energy. If the ball drops it will have zero portential energy and positive kinetic energy the instant it hits the floor. However the sum of the potential abnd kinetic energy is the same throughout the ball's dropping history. Therefore energy has neither been created or destroyed but has transformed from potential to kinetic energy.
energy units
Energy units

There are many other units for energy including electron volt (ev), erg, kjoule (kJ) etc.

endothermic and exothermic reactions
Endothermic and Exothermic Reactions
  • Enthalpy, Entropy, and Spontaneity
  • Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ÄS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ÄH < 0). In the lab, exothermic reactions produce heat or may even be explosive. There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ÄH).
examples of endothermic and exothermic processes

Examples of Endothermic and Exothermic Processes

Photosynthesis is an example of an endothermic chemical reaction.

In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5Cl2(s) = NaCl(s)


The Driving Force: Why do Reactions Take Place?

Classification of Energy:

Potential Energy: “ Energy in storage,

waiting to beused “

Kinetic Energy: “ Energy in motion,

can be harnessed

to do work “

  • Water at the top of the waterfall

has potential energy which is

waiting to be released

  • As the water falls over the edge

of the waterfall it is converted

into kinetic energy and is

available to do work


8 Al (s) + 3HClO4 (s) 4 Al2O3 (g) + 3HCl (g) + Energy

The Driving Force (cont)

  • The energy output from

chemical reactions is often

put to a variety of uses

  • Potential energy is stored

in chemical compounds

(the reactants)

  • Kinetic energy is released when the

reactants undergo reaction to form products



Energy + NH4NO3 (s) NH4NO3 (aq)

8 Al (s) + 3HClO4 (s) 4 Al2O3 (g) + 3HCl (g) + Energy

The Driving Force (cont)

Exothermic: “ A process that proceeds with the release of energy as heat.

Energy is released. Energy (heat) is a product “

Endothermic: “ A process that proceeds only with energy as heat from an

external source. Energy is absorbed.

Energy (heat) is a reactant “

Example: The oxidation of Aluminum metal when a rocket propellant ignites

  • This is an exothermic reaction. Energy is a product.

Example: The dissolution of ammonium nitrate in water

  • This is an endothermic reaction. Energy is a reactant.

The Driving Force (cont)

What will be the direction of a chemical reaction?

  • The natural direction of any reaction is in the direction
  • of lower energy and higher entropy (disorder)

Exothermic Reactions:

  • In most cases, a reaction will proceed when it is exothermic

CH4 + 2O2CO2 + 2H2O + Heat

An Exothermic


Heat as Product

The products have lower energy content

than the reactants (CH4 + O2 )


The Driving Force (cont)

Endothermic Reactions:

  • For an endothermic reaction to proceed:
    • Energy must be added to the reactants from some external source, or
    • An increase in entropy, disorder must take place

Energy added:

CaCO3 (s) + Heat CaO (s) + CO2 (g)

An Endothermic


Heat as Reactant

The products have higher energy content

than the reactant (CaCO3 )



6.5 The Driving Force (cont)

Endothermic Reactions:

Endothermic Reactions (cont):

Increase in Entropy (disorder):

  • This following reaction proceeds from an ordered state to a disordered state

Ba(OH)2•8H2O (s) + 2NH4SCN (s) Ba(SCN)2(aq) + 2NH3 (aq) + 10 H20 (l)

An Endothermic


Heat will be absorbed

from the surroundings

13 separate products are formed.

Entropy (disorder) increases



The Driving Force (cont)

First Law of Thermodynamics: “ Energy can be converted from one

form to another but cannot bedestroyed or created “

Also known as the Law of Conservation of Energy

  • The total energy of the universe is constant
  • Energy can be converted from one form to another
  • Second Law of Thermodynamics: “ The total entropy of the universe is constantly increasing “
  • Once energy is converted to entropy it is never again available for useful purposes
the driving force cont
The Driving Force (cont)

From a Chemical Equation (Reaction) View Point:

CH4 + 2O2→ CO2 + 2H2O + Heat

An Exothermic


Heat as Product

The products have lower energy content

than the reactants (CH4 + O2 )

  • A define, unique quantity of energy is release during this reaction (1st Law)
  • This energy is collected and dispersed into the random molecular motion of the surroundings where it is not available to do more work (2nd Law)
  • Energy is conserved in quantity but not in quality (2nd Law)
the driving force cont42
The Driving Force (cont)

Law of Conservation of Energy (cont):

  • Reversing the Reaction:
  • The Law of Conservation of Energy states that the same

amount of energy must be put back into the reaction system

when running the reaction in reverse

Heat +CO2 + 2H2O → CH4 + O2

An Endothermic


Heat as Reactant

The products have higher energy content

than the reactants (C02 + H2O )