Transcript Thermochemistry

Thermochemistry
Chapter 6
Study of Energy Transfer in
Physical and Chemical
Processes
Energy
• Every chemical change has an
accompanying change of energy.
• Combustion of fuels
• Discharge of a battery
• Metabolism of foods
• Every physical change has an
accompanying change of energy.
• Phase changes
• Solution formation
Thermochemistry
• The study of energy and its
transformations is known as
THERMODYNAMICS.
• It studies the relationships between heat,
work and the energy content of a system.
• In this chapter we will examine an aspect
of thermodynamics that involves the
relationships between chemical reactions
and energy changes involving heat. This
portion of thermodynamics is called
THERMOCHEMISTRY.
The Nature of Energy
• The capacity to do work or to transfer
heat.
• Conserved.
• A state function.
• independent of the path, or how you get
from point A to B.
• Work is energy used to cause an object
with mass to move.
• Work = Force x distance
• Heat is energy used to cause the
temperature of an object to increase.
• The energy transferred as a result of
temperature difference.
Kinetic and Potential Energy
• Kinetic Energy = Energy of motion
Ek= ½ mv2
Mass (m)
Velocity (v)
• Potential Energy = Energy of position
Ep= mgh
Mass (m)
Gravitational constant = 9.8 m/s2 (g)
Height of the object relative to some reference
height (h)
Potential Energy in Chemistry
Gravity is an important kind of force for large objects,
but not in chemistry. (you will use it all the time in
physics, though)
Gravitational force is negligible in extremely small
particles such as atoms and molecules.
More important are the forces that arise from
electrical charges.
Electrostatic Potential Energy –the interactions
between charged particles.
Electrostatic Potential Energy
• Eel =
kQ1Q2
d
Q1and Q2represent the electrical charges on the two
interacting objects, typically the charge on an
electron 1.60 x 10 -19 Coulombs
d is the distance separating them
k is a proportionality constant, 8.99 x 109J-m/C2
Joule-meter per Coulomb squared
Electrostatic Potential Energy
• When Q1and Q2 have the same sign (both positive,
or negative), the two charges repel one another,
pushing them apart, and Eel is positive.
• When they have opposite signs, they attract one
another, pulling them toward each other, and Eel is
negative.
• The lower the energy of a system, the more stable
it is.
• Thus, the more strongly opposite charges interact,
the more stable the system.
Chemistry and Energy
• The chemical energy of substances
is due to the potential energy stored
in the arrangements of the atoms of
the substance.
• The energy possessed by a
substance because of its
temperature is associated with
kinetic energy of the molecules.
Units of Energy
• The SI unit for energy is the joule, J
in honor of James Joule, a British scientist who
investigated work and heat.
• Joule = 1 kg-m2/s2
• A mass of 2 kg moving at a speed of 1 m/s
possesses a kinetic energy of 1 joule
• Remember, Ek= ½ mv2
• A small unit, so kilojoules (kJ) is often used when
discussing the energies associated with chemical
reactions.
Calories
• A more familiar unit to us.
• The amount of energy needed to
raise the temperature of 1 grams of
water from 14.5 C to 15.5 C.
• 1 calorie = 4.184 J (exactly)
• 1 Calorie = 1000 calories = 1 kcal.
The Universe
• is divided into two halves.
the system and the surroundings.
• The system is the part you are concerned with.
• The surroundings are the rest.
• Exothermic reactions release energy to the
surroundings. (ΔH negative)
• Endothermic reactions absorb energy from the
surroundings. (ΔH positive)
Direction
Every energy measurement has three parts.
• 1. A unit ( Joules of calories).
• 2. A number how many.
• 3. A sign to tell direction.
negative -exothermic
positive-endothermic
Heat Flow
Reaction Progress
Same Rules for Heat and Work
• Heat given off is negative.
• Heat absorbed is positive.
• Work done by system on surroundings is
positive.
• Work done on system by surroundings is
negative.
• Thermodynamics-The study of energy and
the changes it undergoes.
First Law of Thermodynamics
• The energy of the universe is
constant.
• Energy may be transferred between
the system and its surroundings.
• Law of conservation of energy
Internal Energy
•
The sum of all the kinetic and potential energy of all its
components.
Represented by E
Generally don’t know the actual numerical value
We can measure the change in internal energy,
Delta E, DE= E final–E initial
Remember the three parts of DE,
•
1.
A number
•
2.
A unit (together these 2 give the magnitude of change)
•
3.
A sign that gives the direction
Signs of ΔE
• A positive value of ΔE results when
E final > E initial, indicating the system
has gained energy from its
surroundings.
• ENDOTHERMIC
• A negative value of ΔE is obtained
when E final < E initial, indicating the
system has lost energy to its
surroundings.
• EXOTHERMIC
Relating ΔE to heat and work
•
•
•
•
q = heat
w = work
ΔE = q + w
First Law of Thermodynamics in algebraic
terms
• It takes the system’s point of view to
decide signs.
• When heat is added to a system or work is
done on a system, its internal energy
increases.
Sign Conventions for q, w & ΔE
State Functions
• Reminder, Internal Energy is a state function
• A property of a system that is determined by
specifying the system's condition, or state (in
terms of temperature, pressure, location, etc.)
• The value of a state function depends only on the
present state of the system, not on the path the
system took to reach that state.
• ΔE only depends on the initial and final states of
the system, not on how the change occurs.
Analogy
• Suppose you are traveling between
Chicago and Denver. Chicago is 596 ft
above sea level; Denver is 5280 ft above
sea level.
• No matter what route you take, the altitude
change will be 4684 ft. The distance you
travel, however, will depend on your route.
• Altitude is a state function
• Distance is not a state function