Lesson 2.6 Heat Capacity
Objectives
This lesson deals with energy and the amount of energy
needed to increase the temperature or change the state of
a material. On completion of the lesson, you should be
able to discuss specific heat capacities, latent heats of
fusion and latent heats of vaporization. You should be
able to use specific heat capacity data to estimate
temperature or energy changes. You should also be able to
estimate the amount of energy needed to change a
particular amount of solid to liquid or liquid to gas.
Overview
When the temperature of a substance increases, the
particles that make up the material move faster.
Particles can vibrate, rotate and migrate. Generally,
migration only occurs in liquids and gases. If these
particles come into contact with a substance that has
slower moving particles, they collide and there is a
transfer of energy by conduction. Some particles do not
move as easily as others and the amount of energy needed
to increase the rate of movement of the particles is
called the heat capacity.
Energy will always flow
from a substance at a higher temperature to a substance
at a lower temperature. When the collisions between
particles and the particles absorbing energy change from
solid to liquid or liquid to gas, the temperature remains
constant until the change in phase is complete. The
quantity of energy needed to change a fixed amount of
solid to liquid at the same temperature is called the
latent heat of fusion. The quantity of energy needed to
change a fixed amount of liquid to vapor at the same
temperature is called the latent heat of vaporization
Steam is often used for
heating. This is due to its large heat of vaporization.
Steam burns can be serious because "wet heat"
transfers more efficiently than "dry heat".
Temperature &
Energy
When something is heated, its temperature either
increases or its state changes. An increase in energy
causes the particles that make up a substance move faster.
In liquids and gases, the particles can move around but
they also move by vibrating and rotating. In solids, the
particles only vibrate and rotate but when you touch a
hot solid, the interaction between the vibrating surface
particles and your hand results in a transfer in energy
that is painful. The collisions at the surface cause a
transfer in energy.
Heat Capacity
The particles of some materials do not move as easily as
others do. They need more energy to move the same way as
others do with less energy. The amount of energy needed
to increase the temperature of 1 kg of a substance by 1
ºC is its specific heat capacity. The specific heat
capacity of water is 4.18 kJ.kg –1.ºC –1
(kilo-joules per kilogram per ºC)
Specific Heat Capacities of Some
Common Substances
|
Substance
|
Specific Heat Capacity (kJ/kg.
°C)
|
Water (liquid)
|
4.183
|
Water (ice)
|
2.025
|
Aluminum(s)
|
0.890
|
Iron(s)
|
0.452
|
Mercury(l)
|
0.144
|
Carbon(s- graphite)
|
0.713
|
Sodium chloride(s)
|
0.859
|
Copper(s)
|
0.382
|
Fusion &
Vaporization
When the state of a material changes, energy is either
gained or lost. The amount of energy needed to change 1
kg of a solid to a liquid at the same temperature is its
latent heat of fusion.
The amount of energy
needed to change 1 kg of a liquid to vapor at the same
temperature is its latent heat of vaporization. When a
liquid changes to solid, it loses its latent heat of
fusion. Similarly, the energy lost by a vapor in going
from the vapor phase to the liquid phase is equal to its
latent heat of vaporization.
Latent Heat Of Fusion,
(Hf Or Lf)
The amount of energy needed to change 1 kg of a substance
from a solid to a liquid.
For water the heat of fusion = 333,000 J/kg
- Latent heat of
vaporization
- The amount of energy
needed to change 1 kg of a substance from a
liquid to a gas.
- for water, the heat
of vaporization = 2,280,000 J/kg
Latent heats for some
common substances:
...............................DHfus(kJ
kg-1) .............DHvap
(kJ kg-1)
Carbon tetrachloride
.......16.3 ...................................198
Ethanol
.........................279
...................................2123
Naphthalene
.................152
.....................................316
Steel
...........................277
...................................6,200
Water
..........................333
...................................2,280
H = m C D
t : Change in energy =
mass x specific heat capacity x change in temperature.
Where: H = change
in energy (J)
m= mass (kg)
C = specific heat
capacity (J / kg.ºC)
and D t =
change in temperature (.ºC)
Questions Specific heat capacity of water =
4180 J/kg.ºC
- Energy traveling from
one object to another object that is at a lower
temperature is called ……….
- If two objects are
placed together in contact with each other and
there is no net exchange of heat between them,
the objects are in a state of ……….
- The quantity of
energy needed to raise the temperature of one
kilogram of a particular material by 1ºC (or 1K)
is called the
……………… of the
material.
- The specific heat of
fusion of a particular material is the quantity
of energy needed to change 1 kilogram of the
substance from …….. to
……… at the same temperature.
- The specific heat of
vaporization of a particular material is the
quantity of energy needed to change 1 kilogram of
the substance from ………. to
………….. at the same
temperature.
- Why is it called the specific
heat of vaporization?
- A water balloon is
dropped from a height of 30 meters. If it retains
all of its energy on impact with the ground, how
much will the temperature of the water increase
by?
- A metal object with a
mass of 500 g and a temperature of 110ºC is
placed in 300 g of water with a temperature of 10ºC
in an insulated container. If the temperature of
the contents of the container reaches equilibrium
at 32ºC, what is the specific heat capacity of
the metal?
- If an electrically
heated insulated container is rated at 300 Watts,
how long will it take for 500 grams of ice (at 0ºC)
in the container to be converted to steam at 100ºC?
(Heat of fusion of water = 333000 J/kg, Heat of
vaporization = 2280000 J/kg)
|
