Download Section 5.1

Chapter 5
Thermochemistry
Section 5.1
The Nature of Energy
Objectives
• Understand the nature of energy and the
forms it takes (kinetic and potential).
• Apply the units used to measure energy
• Demonstrate energy through work and heat
transfer
• Study energy changes in the system and
surroundings
Key Terms
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Thermodynamics
Thermochemistry
Kinetic energy
Potential energy
Joule
Calorie
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System
Surroundings
Work
Force
Heat
Energy
What is Thermodynamics?
• The study of energy and its
transformations
• Greek: “heat power”
Thermochemistry
• Portion of thermodynamics
• Relationships between chemical
reactions and energy changes
involving heat
Energy
• The capacity to do work or transfer heat
– Work: Energy used to cause an object
that has mass to move.
– Heat: Energy used to cause the
temperature of an object to rise.
Potential Energy
Energy an object possesses by virtue
of its position relative to other
objects
Ep = mgh
Potential Energy
• Arises when there is a force operating
on an object
• Force= push or pull
»Gravity
Kinetic Energy
Energy an object possesses by virtue
of its motion
1
KE =  mv2
2
Electrostatic Potential Energy
• Gravitational forces negligible for
submicroscopic objects in chemistry
• Eel arises from interactions between
charged particles
Eel = Q1Q2
d
= 8.99 x 109 J-m/C2
Chemical Energy
• Due to the potential energy
stored in the arrangements of the
atoms of the substance
Units of Energy
• The SI unit of energy is the joule (J)
kg m2
1 J = 1 
s2
Calorie
• An older, non-SI unit
• Amount of energy required to
raise the temperature of 1g of
water from 14.5 °C to 15.5 °C
1 cal = 4.184 J
System and Surroundings
• The system includes the
molecules we want to
study (here, the
hydrogen and oxygen
molecules).
• The surroundings are
everything else (here,
the cylinder and piston).
System and Surroundings
• System does not exchange matter
with surroundings
• Exchanges energy in the form of
work and heat
Work
• Energy used to move
an object over some
distance.
• w = F  d,
where w is work, F is
the force, and d is
the distance over
which the force is
exerted.
Heat
• Energy can also
be transferred
as heat.
• Heat flows from
warmer objects
to cooler
objects.
Transferal of Energy
a) The potential energy of this
ball of clay is increased when
it is moved from the ground
to the top of the wall.
Transferal of Energy
a) The potential energy of this
ball of clay is increased when
it is moved from the ground
to the top of the wall.
b) As the ball falls, its potential
energy is converted to kinetic
energy.
Transferal of Energy
a) The potential energy of this ball
of clay is increased when it is
moved from the ground to the top
of the wall.
b) As the ball falls, its potential
energy is converted to kinetic
energy.
c) When it hits the ground, its
kinetic energy falls to zero (since
it is no longer moving); some of
the energy does work on the ball,
the rest is dissipated as heat.
a.
1.
2.
3.
4.
frictional energy
electrical energy
kinetic energy
potential energy
a.
1.
2.
3.
4.
frictional energy
electrical energy
kinetic energy
potential energy
b.
1.
2.
3.
4.
potential energy
kinetic energy
heat energy
electrical energy
b.
1.
2.
3.
4.
potential energy
kinetic energy
heat energy
electrical energy
c.
1.
2.
3.
4.
transference
heat
transduction
work
c.
1.
2.
3.
4.
transference
heat
transduction
work
d.
1.
2.
3.
4.
work
transference
transduction
heat
d.
1.
2.
3.
4.
work
transference
transduction
heat