Thermal Concepts
Heat & Thermodynamics
Objectives
State that the temperature
determines the direction of thermal
energy transfer
State the relation between the Kelvin
and Celsius temperature scales
Thermal Energy (Heat) Transfer
State that the temperature
determines the direction of
thermal energy transfer
TEMPERATURE: A measurement
of how hot an object is. A
scalar quantity. If two objects
have the same temperature,
there will be no net heat flow
between them.
Heat energy moves from
‘hotter’ to “colder” bodies.
What happens when two objects have
the same temperature?
 When two bodies are at the same
temperature as each other, they are in
thermal equilibrium.
 This leads to the ‘Zeroth’ law of
thermodynamics’: If two bodies A and B
are separately in thermal equilibrium with
body C, they are also in thermal
equilibrium with each other and thus have
the same temperature.
“Cold” isn’t real
“Cold” is the word
assigned to a lack of
heat energy.
The Cosmic Microwave Background
2.72548±0.00057 K
State the relation between the Kelvin and
Celsius temperature scales
Temperature
The absolute temperature scale (measured in Kelvin and
with a minimum value of zero Kelvin) is used in physics
calculations.
Kelvin Temperature = Celsius Temperature + 273
(Where a change of one Kelvin equals a change of one
degree Celsius)
The absolute (Kelvin) temperature of a body is a
measure of the average amount of kinetic energy
that each of its molecules have.
(i.e. The KE of the particles is directly proportional to the
temperature)
Practice
1. Convert 25.0 °C to Kelvin.
2. Convert 375 K to degrees Celsius.
3. Convert −50 °C to Kelvin. (That's a minus 50.)
4. The boiling point of water on the Kelvin scale is
__________.
1. 298 K
2. 102 °C
3. 223 K
4. 373 K
5. 40.0 °C is what temperature on the Kelvin scale?
5. 313 K
6. -20.0 °C is what temperature on the Kelvin scale?
6. 253 K
7. 298 K is what temperature in degrees Celsius?
7. 25 °C
8. What is the temperature (give in both °C and K) at STP? 8. 0 °C or
273 K
9. Convert -145.0 °C to K
9. 128 K
10.Convert 20.0 °C to K
10.293 K
11.Convert 252 K to °C
11.-21 °C
Objectives
 State that the internal energy of a
substance is the total potential
energy and random kinetic energy
of the molecules of the substance.
 Explain and distinguish between the
macroscopic concepts of
temperature, internal energy and
thermal energy (heat).
Absolute Zero
The lowest temperature possible. -273.16°C or
zero kelvin (0K). Kelvin is the absolute
thermodynamic temperature scale. Particles have
no kinetic energy at 0 K.
INTERNAL ENERGY: The energy contained in an object
due to the random KE and PE of the molecules.
The kinetic energy of molecules
arises from their
random/translational/rotational
motion and the potential energy
arises from the forces between
the molecules.
State that the internal energy of a substance is
the total potential energy and random kinetic
energy of the molecules of the substance.
Potential Energy
Random Kinetic Energy
Explain and distinguish between the
macroscopic concepts of temperature,
internal energy and thermal energy (heat).
Thermal Energy
The term thermal energy refers to the
non-mechanical transfer of energy
between a system and its
surroundings.
In this respect it is just as incorrect to
refer to the “thermal energy in a
body” as it would be to refer to the
“work in a body”.
Summary
Write a paragraph comparing the
macroscopic concepts of temperature,
internal energy and thermal energy
(heat).
Objectives
 Define the mole and molar mass
 Define the Avogadro Constant
Define the mole and molar mass
MOLE: The amount of a substance that
contains the same number of particles as
there are atoms in 12g of carbon-12.
MOLAR MASS: The mass of 1 mole of a
substance. eg molar mass of water is 18g
Define the Avogadro Constant
The number of particles in a
mole. A=6.02 x 1023
Moles & the Avogadro Number
The number of molecules in a substance is
measured in ‘moles’. Just like ‘a dozen’ means
twelve of something, ‘one mole’ means 6.02 x 1023
molecules of something.
One mole of a substance contains the same number of
molecules as in 12 grams of Carbon-12.
The Avogadro number (NA) is equal to the number of
molecules in 12 grams of Carbon-12 (NA= 6.02 x 1023)
The molar mass (M) is the mass in grams of one mole of a
particular substance.
Note: the mass number of an element is equal to the molar
mass for atoms of an element.
E.g. One mole of Nitrogen atoms will have mass 14g.
Examples
1. The mass number of Helium is 4. How
many atoms are in 4kg of Helium?
A.Mass no. is 4 so molar mass is 4g.
4000 g / 4g = 1000 moles
1000 x NA = (1 x 103) x (6.02 x 1023)
= 6.02 x 1026 atoms
Examples
2. The molar mass of iron atoms is 56 g mol-1
and its density is 7.9 g cm-3.
a) Find the mass of one atom of Iron.
b) Calculate the number of iron atoms in one cubic
metre of iron.
Examples
2. The molar mass of iron atoms is 56 g mol-1 and its
density is 7.9 g cm-3.
a. If one mole of atoms has mass 56g then one
atom will have mass = 56 ∕ NA
= 56 ∕ (6.02 x 1023)
= 9.3 x 10-23g
b. Calculate the number of iron atoms in one cubic
metre of iron.
One cm3 has mass = 7.9 g
So one m3 has mass = 7.9 x (100x100x100) g =
7.9 x 106 g
So # of iron atoms per m3 = 7.9 x 106 / 9.3 x 10-23
= 8.5 x 1028 atoms
MOLES
The number of moles of a substance (n) can
be determined form the total mass (m)
divided by the molar mass (mass per mole,
M), usually both measured in grams:
n= m
M
Similarly if you knew the total number of
molecules (N), you could find the number of
moles (n) from… n = N
NA
Examples
Calculate the number of moles of oxygen atoms in
40g of oxygen.
Calculate the number of moles of oxygen
molecules (O2) in 40g of oxygen.
A sample of fluorine gas contains 20 x 1023 atoms
of fluorine. Calculate the number of moles present.
Examples
Calculate the number of moles of oxygen atoms in
40g of oxygen.
N = m / M = 40 / 16 = 2.5 moles
Calculate the number of moles of oxygen
molecules (O2) in 40g of oxygen.
N = m / M = 40 / 32 = 1.25 moles
A sample of fluorine gas contains 20 x 1023 atoms
of fluorine. Calculate the number of moles present.
N = N / NA = 20 x 1023 / 6.02 x 1023 = 3.3 moles
Your turn!
Complete the assigned
problems for whiteboards
tomorrow!