Intro to Physical Chemistry Flashcards
There are two types of Mechanical Energy
What are they?
Kinetic and Potential Energy
Molecules possess both of them
What is Kinetic Energy
The energy possessed by molecules because of their motion
If a molecules is moving through space, we say it has
Translational motion
How can we calculate kinetic energy
Where:
m= mass of a molecule
v= speed
Molecule with more than one atom (most) have which types of kinetic energy
Rotational and Vibrational Kinetic energy
Energy of small molecules is quantised (quantum mechanics)
What does this mean in term of the motion of molecules
Each kind of motion is associated with a set of discrete energy levels
Each type of molecular motion is associated with a set of these levels
What is Translational Energy?
The movement of the molecule as a whole through space
i.e., along a place in the coordinate system (x, y and z axis)
What is Rotational Energy?
When the molecules rotate about its axis
What is Vibrational Energy?
When there is a change in the bond lengths and bond angles on molecules
What is Electronic energy?
Rather than motion these are associated with the electrons in the atoms/molecules
Increases in energy level from which types of energy
Lowest: Translational
Rotational
Vibrational
Highest: Electronic
What is the separation between levels in Transitional Energy
The separation between levels is 10¯⁴¹ J
The spacing of the translational energy level is so small that it can be treated as continuous
What is the Seperation between the energy levels in rotational energy
The seperation between levels is 10¯²³ J
Rotational energy levels become further apart as the energy level increases
What is the seperation between energy levels in Vibrational energy
The seperatation between levels is 10¯²⁰
Vibrational energy levels are constantly spaced
What is the energy level seperation for Electronic energy levels
The seperation between level is 10¯¹⁸
Electronic energy levels become closer together as the energy increases
These are usually more widely spaced than the vibrational levels
How can different energies interact based on energy levels
May rotational energies fit between the ground and lowest excited vibration energy state
While many vibrational levels fit between the ground and lowest excited electron states
These features have impacts on the study of molecular energy levels by spectroscopy
How can molecules exchange energy
Molecules which are moving collide with another and exchange energy during these collisions
We do not know the particular energy of each individula molecule
But, what do we know?
We do know the average number of molecules in a given energy level
This quantity is known as the population
What is the equation for the Boltzmann distribution
ni = population of molecules in an excited state
n₀ = population of molecules in the ground state
Ei = energy of the excited state
KB = Boltzmann constant = 1.381 x 10¯²³ J K¯¹
T = Temperature (K)
When two molecules are widely seperated, how does this affect the potential energy
The attractive contribution to the potential energy dominates
Therefore the overall potential energy is negative (although the sign is just a convention)
When two molecules are close together, how does this affect the potential energy
The repulsive contribution to the potential energy dominated
The overall potential energy is therefore positive
Somewhere between these positive and negative values for potential energy there’s a minimul in the overall potential energy
This corresponds to what
The equilibrium seperation between the molecules
What is the ideal gas equation
p = pressure (Pa or N m¯²)
V = volume (m³)
n = moles of gas
R = gas constant (8.314 J mol¯¹ K¯¹)
T = absolute temperature (K)
By keeping the amount of gas and one of the 3 variables that define our surface: pressure, volume or temperature constant, we can see that the different gas laws correspond to cross sections through the surface
What are these 3 laws
- Boyle’s law (constant temperature)
- Gay-Lussac’s law (constant volume)
- Charles’s law (constant pressure
What is Charles’s law
At constant pressure the ideal gas equation reduces to Charles’s law which states that the volume is proportional to temperature
When this a ploted on a graph and extrapolated to zero volume, we find that the volume of an ideal gas reach zero at -273.15°C, regardless of the pressure - absolute zero
This is where all molecular motion ceases
Using the assumptions of the kinetic theory, we can derive a relationship between the macroscopic pressure and volume of an ideal gas and the microscopic properties of it’s molecules (root mean square speed)
What is this equation
p = pressure (Pa)
V = volume (m³)
n = number of moles
M = molar mass of molecules (kg mol¯¹)
c = root mean square speed (√v²)
Substituting the ideal gas equation allows equations to be derived that give the root mean square speed of the gas molecule in terms of either molar or molecular quantities
What are these equations
Increasing the temperature will do what to probable speeds and the range
Increase the most probable speed in the distribution and the range of speeds which are observed
Lower peaks, further to the right
Increasing the molecular mass will do what to probable speeds and the range
Descreases the most probable speed and range of speeds which are observed in the distribution
High peak far to the left
