Transcript 16.1 Energy

Thermochemistry and
Energy
Water freezing and boiling at the same
time, really?!
O Thermochemistry is the study of energy
changes that occur during chemical
reactions and phase changes (changes of
state)
The Nature of Energy
O Energy is the ability to do work or produce
heat
O Heat (q or Q) is energy; flows due to
temperature differences (always hot to cold)
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Law of Conservation of
Energy
O Energy cannot be created nor destroyed;
only converted into different types.
Kinetic vs. Potential Energy
O Two main types of energy – kinetic and
potential
O Kinetic – energy of motion
O Potential – energy due to position or energy
stored in chemical bonds
O Chemical potential energy - the energy stored in a
substance because of its composition
O Example: gasoline
Kinetic vs. Potential Energy
O Two main types of energy—kinetic and
potential
O Kinetic—energy of motion
O Potential—energy due to position or energy
stored in chemical bonds
O Chemical potential every—the energy stored
in a substance because of its composition
O Example: gasoline
Temperature vs. Heat
O Temperature is a measure of the motion in
a sample
O Temperature is a measurement of heat.
O Heat is the total energy of molecular
motion, dependent upon amount, size, and
type of particles. Heat is energy.
Units of Heat
O calorie - the amount of heat required to raise
the temperature of one gram of pure water by
one degree Celsius
O Calorie – nutritional calorie;
O 1 Calorie = 1000 calories = 1 kilocalories
(kcal)
O Joule – SI unit of heat
O 1 calorie = 4.184 J
Converting Energy Units
Calorie/calorie/kilocalorie
1 C = 1000 cal = 1 kcal
calorie/Joule
1 cal = 4.184 J
Example 1
A cereal has 155 nutritional Calories per serving.
How many calories, kilocalories and Joules is
this?
Example 2
A person on a diet consumed 1350 Calories in one
day. How many calories, kilocalories and Joules is
this?
System and Surroundings
O Universe = system + surroundings
O System – the specific part of the universe
you wish to study. In chemistry this is your
chemical reaction/physical process
O Surroundings – Everything else in the
universe
O When heat is transferred it can flow in or
out of the system
Endothermic vs. Exothermic
O An Exothermic process is one that
releases heat to its surroundings (feels
warm)
O Energyproducts < Energyreactants
O An Endothermic process is one that
absorbs heat from the surroundings (feels
cold)
O Energyproducts > Energyreactants
Exothermic Process
Eproducts < Ereactants
Potential
Endothermic Process
Eproducts > Ereactants
Potential
Q and heat flow
O Exothermic process, heat is released,
q is negative (-)
O Endothermic process, heat is absorbed,
q is positive (+)
Specific Heat = Cs
O Specific heat of a substance is the amount of
heat required to raise the temperature of one
gram of that substance by one degree Celsius.
O Unit for specific heat is J/goC
O Each substance has a different specific heat
O Water = 4.184 J/goC
O Gold = 0.129 J/goC
O Copper = 0.386 J/goC
O The lower the specific heat the lower the amount
energy is required to raise its temperature.
Calculating Heat Released and
Absorbed
O Q = mCs(T)
O Q = Heat (Joules or kilojoules)
O m is mass (gram)
O T is temperature change (°C) Tfinal -Tinitial
O Cs is specific heat at a constant pressure (J/g°C)
Example 3
If the temperature of 56.6g of ethanol increases from
45.0°C to 80.0°C, how much heat has been
absorbed by the ethanol? Specific heat of ethanol
=2.44J/g°C
Example 4
A 4.00g sample of a substance was heated from
274K to 314K and absorbed 32J of heat, what is
the specific heat of the substance?
Example 5
If 98000J of energy are added to 6200g of water,
what will the change in temperature of the water
be? Specific heat of water is 4.184 J/g°C