Transcript 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 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
• 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
•
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
There are many other units for energy including electron volt (ev), erg, kjoule
(kJ) etc.
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
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
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)
8 Al (s) + 3HClO4 (s)
Kinetic energy is released when the
reactants undergo reaction to form products
4 Al2O3 (g)
+
3HCl (g) +
Energy
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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
8 Al (s) + 3HClO4 (s)
4 Al2O3 (g)
+
3HCl (g) +
Energy
This is an exothermic reaction. Energy is a product.
Example: The dissolution of ammonium nitrate in water
H2O
Energy + NH4NO3 (s)
NH4NO3 (aq)
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
+
2O2
CO2
The products have lower energy content
than the reactants (CH4 + O2 )
+
2H2O
+
Heat
An Exothermic
Reaction,
Heat as Product
The Driving Force (cont)
Endothermic Reactions:
 For an endothermic reaction to proceed:
1) Energy must be added to the reactants from some external source, or
2) An increase in entropy, disorder must take place
Energy added:
CaCO3 (s)
+
Heat
CaO (s)
+
CO2 (g)
An Endothermic
Reaction,
Heat as Reactant
The products have higher
energy content
than the reactant (CaCO3 )
continue….
6.5 The Driving Force (cont)
Endothermic Reactions
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)
An Endothermic
Reaction,
Heat will be absorbed
from the surroundings
Ba(SCN)2(aq) + 2NH3 (aq) + 10 H20 (l)
13 separate products are formed.
Entropy (disorder) increases
continue….
The Driving Force (cont)
First Law of Thermodynamics: “ Energy can be converted from one
form to another but cannot be destroyed 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)
From a Chemical Equation (Reaction) View Point:
CH4
+
2O2
→
CO2 +
The products have lower energy content
than the reactants (CH4 + O2 )
2H2O +
Heat
An Exothermic
Reaction,
Heat as Product
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)
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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 +
An Endothermic
Reaction,
Heat as Reactant
2H2O
→
CH4
+
O2
The products have higher energy content
than the reactants (C02 + H2O )