Transcript Thermodynamics - Independent School District 196

Thermodynamics
What is thermochemistry?
• The study of the changes in heat energy
that accompany chemical reactions and
physical changes
How do we measure these
changes in heat energy?
• Both physical and chemical processes should
be conducted in a calorimeter, a device used
to measure the heat absorbed or released
• Calorimeters should have good isolative
properties to prevent heat from entering or
leaving when measuring the heat transfer of
the physical or chemical process
What types of heat transfer
will we measure?
• Endothermic --> when heat is absorbed
during a chemical or physical change
Examples include:
• fusion (melting) solid + ΔH  liquid
• sublimation solid + ΔH  gas (dry ice)
• vaporization liquid + ΔH gas
• baking bread
• photosynthesis
• Exothermic-->when heat is released during a
chemical or physical change
Examples include:
Solidification (freezing) liquid  solid + ΔH
Condensation gas  liquid + ΔH
Deposition gas  solid + ΔH (frost)
Rusting iron
Burning a candle
Mathematical equation used to
measure energy absorbed or
released during a temperature
change
Q = cpn∆T
Q = energy gained or lost
Cp = specific heat
n = amount (g)
∆T = change in temperature
How will this energy be
measured?
• The SI unit of energy is the Joule (J)
• The calorie(cal) is more commonly used
to measure food energy
• 1 cal = 4.18 J
What is specific heat (cp)?
• The amount of heat energy required to
raise the temperature of 1 gram of a
substance by 1 Celsius degree or Kelvin
• This value is dependant on:
-the substance
-the state the substance is in (solid, liquid
or gas)
Mathematical equation used to
measure energy absorbed or
released during a phase change
Q = Hxn
Q = energy gained or lost
Hx = heat of fusion, vaporization,
solidification or condensation
n = amount (grams or moles)
What is Hx?
• Heat of fusion (Hf) --> the amount of
energy required to melt 1 mole (or gram
depending on the units given) at it’s
melting point
• Heat of solidification (Hs) --> same as
Hf except a negative value because
energy needs to be removed to go from
a liquid to a solid
• Heat of vaporization (Hv) --> the
amount of energy required to vaporize 1
mole (or gram) at it’s boiling point
• Heat of condensation (Hc) --> same as
Hv except a negative value because
energy needs to be removed to go from
a gas to a liquid
Endo Versus Exo?
• An endothermic energy change will be
denoted with a positive value
• Ex. 468 J
• An exothermic energy change will be
denoted with a negative value
• Ex. -9548 cal
Simple Practice Problems
• How many joules of heat are given off
when 5.0 g of water cool from 75.0°C
to 25.0°C? (cp= 4.18 J/g°C)
• How many calories is this?
• How many joules of energy does it take
to melt 35 g of ice at 0°C?
(Hf = 333 J/g)
• How many calories are given off when
85 g of steam condenses to liquid
water? (Hv= 539.4 cal/g)
Practice Problem
• An unknown compound has the following properties:
• Hs = -5800 J/mol
• Hv = 54200 J/mol
• cp (liquid) = 185.3 J/mol•C
• cp (gas) = 42.9 J/mol •C
• mp = 5C
• bp = 15C
A. Be able to sketch a fp/bp graph for this substance,
labeling the axes and indicating the mp and bp)
B. Suppose 65000 J of heat are added to a 1.08 mol
solid sample of this substance at 5C. When all the
heat is absorbed by the substance, at what
temperature and state(s) will the substance be?
1 cal = 4.18 J
What is Equilibrium?
• A dynamic condition in which 2 opposing
changes occur at equal rates in a closed
system.
• Ex. A pool is at maximum capacity with 50
people and at equilibrium. If 2 people enter
the pool, how many people need to exit the
pool to keep it at equilibrium?