Transcript Energy and Entropy

Energy Changes in Chemical
and Physical Processes 1
Energy
The ability to
– do WORK (an object is pushed or pulled for a
distance)
– CAUSE CHANGE
– MOVE MATTER
Involved in every natural process
Two general categories
Kinetic
– Energy of motion
– Depends on mass and velocity of object
KE=1/2 mv2
Potential
– Stored energy due to position or condition
Different forms
Mechanical
– Associated with the motion or position of an object
Thermal
– Total energy (kinetic and potential) of all the particles in an
object
Chemical
– Potential energy related to ability to bond chemically
Electrical
– Produced by moving electric charges
Electromagnetic
– Produced by the motion of charged particles within atoms
Nuclear
– Related to the forces within the nucleus of atoms
Conservation of Energy and Mass
Energy conversions
– One form of energy can be converted to any other
form
Law of conservation of energy
– Energy cannot be created or destroyed; the total
amount of energy is the same before and after every
process
– This is also known as the First Law of
Thermodynamics
Law of conservation of mass
– The total amount of mass is the same before and
after every process
Temperature
Kinetic Theory
– Matter is made up of particles that are always
in motion (have kinetic energy)
Temperature
– A measure of the average kinetic energy of
the particles in substance
– Changes when particles gain or lose energy
– 3 common temperature scales (F, C, K)
Temperature scales
Kelvin (K)
– SI unit
– Directly proportional to Kinetic energy of particles
Example- If you double the Kelvin temperature, you have
doubled the kinetic energy of the particles
– Absolute zero
0 K, the temperature at which the kinetic energy of particles
is zero and molecular motion ceases
– At normal atmospheric pressure, water freezes/melts
at 273K and boils/condenses at 373K
Celsius
– Commonly used in lab
– At normal atmospheric pressure, water
freezes/melts at 0C and boils/condenses at
100C
Fahrenheit
– Commonly used in everyday situations
– At normal atmospheric pressure, water
freezes/melts at 32F and boils/condenses at
212F
We can convert between temperature
scales
K=C + 273 or C= K – 273
C=5/9 (F-32) or F= 9/5 C + 32
Temperature and thermal energy
Thermal energy
– total energy of all the particles in an object
– Found by adding the energy of each particle
– Depends on the total number of particles in the sample
Temperature
– Measure of the average kinetic energy
– does not depend on the number of particles in the sample
Example- 100mL of boiling water has the same
temperature as 1000mL of boiling water, but the
1000mL of boiling water would have more thermal
energy
Temperature and heat
Heat (q)
– The movement of thermal energy from a
substance at a higher temperature to a
substance at a lower temperature
– Has to do with transfer of energy
– The “flow” of energy from one substance or
object to another
System vs Surroundings
In studying energy changes, we define
a system as the part of the universe on
which we focus our attention to study.
The surroundings include everything
else in the universe.
A few more terms…
Endothermic
–
–
–
–
A process that requires energy
energy is absorbed or gained by the system
Energy is lost by the surroundings
Usually evidenced by a decrease in temperature of
the surroundings
Exothermic
–
–
–
–
A process that releases energy
Energy is given off or emitted by the system
Energy is gained by the surroundings
Usually evidenced by an increase in the temperature
of the surroundings
Heat flow is defined from the point of view
of the system.
– In an endothermic process, heat flows into
the system from the surroundings
q has a positive value for the system
q has a negative value for the surroundings
– In an exothermic process, heat flows from the
system to the surroundings.
q has a negative value for the system
q has a positive value for the surroundings
Energy is conserved!
qsys = -qsurr
Potential Energy Diagram
Endothermic Reaction
a) Activation Energy for
forward reaction
b) Activation energy for
reverse reaction
c) ΔHrxn (enthalpy or
energy change for
the reaction)
•
Note that for an
endothermic rxn, ΔH
is always positive
Potential Energy Diagram
Exothermic Reaction
Note that for an exothermic
reaction, ΔH is always
negative. (reactants have more
potential energy than the
products); energy is released
into the surroundings
All reactions, even exothermic
reactions, require some initial
addition of energy. This energy
is required to reach the
unstable, high energy state
known as the activated
complex.
17.1
Units for Measuring
Heat Flow
Heat flow is measured in two common
units, the calorie and the joule.
1 cal = 4.184J
The energy in food is usually expressed in
Calories.
17.1
Heat Capacity and
Specific Heat
The heat capacity of an object depends
on both its mass and its chemical
composition.
–The amount of heat needed to increase the
temperature of an object exactly 1°C is the
heat capacity of that object.
17.1
The specific heat capacity, or simply the
specific heat, of a substance is the
amount of heat it takes to raise the
temperature of 1 g of the substance 1°C.
17.2
Calorimetry
Calorimetry is the precise measurement
of the heat flow into or out of a system for
chemical and physical processes.
17.2
In calorimetry, the heat released by the
system is equal to the heat absorbed by its
surroundings. Conversely, the heat
absorbed by a system is equal to the heat
released by its surroundings.
The heat content of a system at constant pressure is
the same as a property called the enthalpy (H) of the
system.
17.2
The insulated device used to measure the
absorption or release of heat in chemical or
physical processes is called a calorimeter.
In a simple constant-pressure calorimeter, a
thermometer records the temperature change
as chemicals react in water. The substances
reacting in solution constitute the system. The
water constitutes the surroundings.