Transcript Thermochemistry

An Introduction into
Thermodynamics
Advanced Chemistry
Ms. Grobsky
What is Energy?
 Energy is the capacity to do work
•
Thermal energy is the energy associated with the random motion of
atoms and molecules
•
Kinetic energy is the energy due to motion
•
Potential energy is the energy available by virtue of an object’s
position
•
Chemical energy is the energy stored within the bonds of chemical
substances
• Type of potential energy
UNITS OF ENERGY
• S.I. unit of energy is the joule (J)
• Heat and work (energy
measured in joules
in
transit)
also
• 1 kJ (kilojoule) = 103 J
• Calorie (cal)
• 1 cal is the energy needed to raise the
temperature of 1 g of water by 1oC
• 1 cal = 4.184 J
• A food Calorie is actually a kilocalorie!
So, What is Work?
• Work (w) is the transfer of energy that takes
place when an object is moved against an opposing
force
• A system does work when it expands against an
external pressure
Car engine
Petroleum burns & produces
gases which push out pistons
in the engine and transfer
energy to the wheels of car
Energy Changes in Chemical
Reactions - HEAT
• HEAT (q) is thermal energy that transfers from one
object to another when the two things are at
different temperatures and in some kind of contact
• Kettle heats on a gas flame
• Cup of tea cools down (loses energy as heat)
• Thermal motion or random molecular motion
(temperature) is increased by heat energy
• Heat stimulates thermal motion
Temperature = Heat
• When dealing with energy, the world is divided into
a system and its surroundings
SURROUNDINGS
SYSTEM
Exchange:
Open
Closed
Isolated
Mass & Energy
Energy
Nothing
6.2
Exothermic versus Endothermic Reactions
Exothermic process is any process that gives off heat – transfers
thermal energy from the system to the surroundings.
2H2 (g) + O2 (g)
H2O (g)
2H2O (l) + energy
H2O (l) + energy
Endothermic process is any process in which heat has to be supplied to
the system from the surroundings.
energy + 2HgO (s)
energy + H2O (s)
2Hg (l) + O2 (g)
H2O (l)
Thermodynamics
Study of the relationships among heat,
work, temperature, and energy
Focuses on energy
There are 3 laws of Thermodynamics
First Law of Thermodynamics is also
known as Law of Conservation of
Energy
The total energy of the universe is
constant and can neither be created nor
destroyed
It can only be transformed
Total Internal Energy in General
Terms
Total Energy of System =
E
=
Kinetic Energy
EK
+
+
Potential Energy
EP
Kinetic energy & potential energy are interchangeable
by definition of Law of Conservation of Energy
Ball thrown upwards
slows & loses kinetic
energy but gains
potential energy
The reverse happens
as it falls back to
the ground
Total Internal Energy in
Chemistry Terms
 The internal energy, E or U, of a sample
is the sum of all the kinetic and potential
energies of all the atoms and molecules in
a sample
 It is the total energy of all the atoms and
molecules in a sample
More on INTERNAL ENERGY
(E)
Internal energy changes when energy enters or leaves
a system
E = Efinal - Einitial
E
change in the internal
energy
• Signs (+/-) will tell you if energy is
entering or leaving a system
• + indicates energy enters a system
• - indicates energy leaves a system
Heat and work are two equivalent ways of changing
the internal energy of a system
Change in
internal
energy
E
=
Energy
supplied to
system as
heat
= q (heat)
+
+
Energy
supplied to
system as
work
w (work)
 Sign conventions for q and w
 Heat is transferred from the surroundings to the system
 q is +
 Work is done on the system by the surroundings
 w is +
 Heat is transferred from the system to the surroundings
 q is  Work is done on the surroundings by the system
 w is -