Heat
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What is heat?
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What do we know about temperature?
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What do we mean when we say something is hot?
What do we mean when we say something is cold?
Is “heat” the same as temperature?
It is a measure of internal energy.
It is a measure of closeness of thermal equilibrium.
What happens when two systems that are not
in thermal equilibrium come in contact?
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Energy will be transferred from the higher
temperature system to the lower energy system until
thermal equilibrium is reached – that is they have
the same temperature.
Heat
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What is heat?
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Heat (Q) is the “flow” or “transfer” of energy from
one system to another
Often referred to as “heat flow” or “heat transfer”
Requires that one system must be at a higher
temperature than the other
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Heat will only flow from the system with the higher
temperature to the system with the lower temperature
Heat will only flow from the system with the higher average
internal energy to the system with the lower average
internal energy
Total internal energy does not matter.
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Heat
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Units
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Since heat is the “flow” of energy from one system
to another, the standard SI unit is the joule J
“Traditional” unit is the calorie (cal)
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A kilocalorie (kcal) or Calorie (Cal) is 1000 cal
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1 cal is the heat needed to raise 1g of water from 14.5°C
to 15.5°C
Used to measure energy content of food
Conversion
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4.186 J = 1 cal,
4.186 x 103 J = 1 kcal
British Thermal Unit (Btu) 1 kcal = 3.97 Btu
Heat Capacity
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When the temperature of system changes, there has
been heat flow Q
Heat capacity connects heat flow to temperature change:
Q = CΔT
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Heat capacity C depends on material, and also on the
quantity of material present.
Eliminate quantity dependence by introducing specific
heat c and molar heat capacity c′:
Q = mcΔT
m = mass
Q = nc′ΔT
n = number of moles
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Specific Heats and Molar Heat Capacities
Measuring Heat Capacities
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Calorimeter
System is adiabatically
isolated from the environment
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Net Q = 0 from or into the device
Heat lost = heat gained
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Conservation of energy
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An axe head consisting of 1.8 kg of iron is left outdoors one cold winter’s night
and is brought indoors when the outside temperature is a brisk 240K. The room
into which it is brought is initially at a nice, comfortable 293K and 1.0 atm of
pressure. The volume of the room, which is well insulated, is 38 m3. Assuming
that the axe head comes to thermal equilibrium with the air in the room, by how
much is the temperature of the room lowered? (Ignore the thermal interaction
with furniture, walls, and so forth. Use cair = 0.172 cal/g-K and 28.8 g/mol for the
molecular weight of air.)
Latent Heat
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A phase change occurs when a solid melts to a
liquid, a liquid boils to a gas, a gas condenses to a
liquid, and a liquid freezes to a solid.
Each of these phase changes requires a certain
amount of heat, although the temperature does not
change.
If a solid becomes liquid, or vice versa, the amount
of heat per gram is the latent heat of fusion.
If a liquid becomes gas, or vice versa, the amount of
heat per gram is the latent heat of vaporization
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Latent Heat
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This diagram
shows how water
(in the form of
ice) behaves as
heat is steadily
added to it:
Latent Heat
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A glass is filled with 100 g of ice at 0.00°C and 200 g of water at 25.0°C. (a)
Characterize the content of the glass after equilibrium has been reached.
Neglect heat transfer to and from the environment. (b) Repeat your calculations
for 50.0 g of ice and 250 g of water.
First Law of Thermodynamics
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When temperature changes, internal energy has changed –
may happen through heat transfer or through mechanical work
First law is a statement of conservation of energy
Change in internal energy of system equals the difference
between the heat added to the system and the work done by the
system
ΔU = Q − W
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dU = dQ − dW
Differential form
Heat added +, heat lost -, work done by system +, work done on system –
Internal Energy U is a state property
Work W and heat Q are not
But work and heat are involved in thermodynamic processes that change
the state of the system
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