The First Law of Thermodynamics Flashcards
What is Thermodynamics
Is the study of the way different types of energy interconver
Thermodynamics divides the world into two parts: they System and the Surroundings
What is the difference between the two
System: the part of the world we want to study
Surroundings: the rest of the world outside the system
We can only measure energy as
A change in the energy of a system
This can be achieved by observing its transfer to and from the surroundings
What is an open system
A system in which both matter and energy can be exchanged with the surroundings
For example: a reaction taking place in a beaker is an open system
What is a closed system?
A system that contains fixed amount of matter but allows the exchange of energy
For example: a sealed reaction vessels holds all the content in but heat can be conducted
What is an isolated system?
A system where there can be no exchange of energy or matter with the surroundings
For example: a sealed, well-insulated vaccum flask
What is energy
Energy can be thought of as the capacity to do work
What is work
Work is done when moving against a force
What is the equation for Work Done
w = Fs
w = work done (J)
F = Force (N)
s = distance (m)
What is Internal Energy (U)
The total capacity of a system to do work
What is an ideal gas
There are no forces between the molecules
How do internal energy and ideal gases related
- Ideal gases have no intermolecular potential energy
- The internal energy of an ideal gas is purely kinetic and therefore depend only on temperature
- The internal energy of an ideal gas is independent of volume
What is the First Law of thermodynamics
Energy can neither be created nor destroyed BUT is just transferred from one form into another
What happens to the internal energy of a system if work is doen
Internal energy decreases
What happens to the internal energy of a system, if work is done to it
Its internal energy increases
How does Work and Heat related to an isolated system
No work can be done on or by an isolated system
No heat flows to or from and isolated system
How can the internal energy of a system be changed
By transferring energy to or from the system as heat
Describe the movement of heat
Heat is energy which flows from high-temperature objects to low-temperature ones
What is thermal equilibrium
Objects at the same temperature are in thermal equilibrium, meaning there is no net heat flow between them
(Zeroth law of thermodynamics)
What is Molar Heat Capacity (C) of a substance
What is its units
- is a measure of how much heat must be supplied to raise its temperature by 1K
- Units = J mol⁻¹ K⁻¹
What is the equation used to work out the amount of heat supplied (J) relating to the Molar Heat Capacity (J mol⁻¹ K⁻¹)
q = nCΔT
q = amount of heat supplied (J)
n = amount of substance (mol)
C = molar heat capacity ( J mol⁻¹ K⁻¹)
ΔT = change in temp (K)
For gases, the value of heat capacity depend on wheather the measurements are carried out under which two conditions
- Constant pressure (Cp)
- Constant Volume (Cv)
The heat capacity of a particular substance is related to the number of degrees of freedom possessed by its molecules
How?
As the molecules become more complex there are more degrees of freedom which can absorb the heat
Hence have a higher Heat Capacity (C)
A system can do two kinds of work: expansion and non-expansion
What is the difference
Expansion: work involved a change in the volume of the system, such as the work done by a reaction with gaseous products
Non-expansion: work does not involved a change in volume, such as the electrical work done by the reaction in a battery
Imagine a gas confined to a cylinder by a disc, where the surroundings exert a constant pressure
If the pressure inside the cylinder (pi) builds up to become greater than that of the surrondings (pex), what will happen
- The disc will be push forward
- Work is done moving the disc through a displacement z
What is the equation for Force (F)
Force = pressure x area
If the equation for work done it W= Fs
Using the imaginary cylinder from before, how would you work out work done, from this displacement Z
(Hint: Pi = pressure initial, Vi = volume inital, Pex = external pressure)
w = Fz
= Pex x Area x z
= Pex (Vf - Vi)
Using the equation from before for working out the Work Done by the imaginary cylinder, how would the work done on the gas by the surrounding be work out?
It has the same value but opposite sign
w = -Pex (Vf - Vi)
What is Reversible Isothermal Expansion (An expanded gas becomes a compressed gas)
What is the Work Done equal to
- For an ideal gas expanding reversibly at constant temperature (isothermally)
- The Work done corresponds to the area under a Boyle’s law curve
- This is larger than irreversible
How does Reversible Isothermal Expansion link to the second law of Thermodynamics
Second Law: A process must occur reversibly to do the masimum possible work
The Work done for a Reversible Isothermal Expansion is great than irreversible, shown by the blue space under these graphs
Reversible Isothermal Expansion can only occur…
For an ideal gas
Taking both Work and Heat into account gives a mathematical statement for the first law being:
ΔU = q + w
ΔU = internal energy
q = Heat (J)
w = Work done
For ΔU = q + w
w is postive when … and negative when…
w is positive if work is done on the system
and negative if work is done by the system
The internal energy of an ideal gas is independent of volume, since there are no forces between molecules, and therefore only depends on temperature
If temperature is kep constant (isothermal process) then the internal energy
Is constant
IF ΔT = 0 THEN ΔU = 0
For a system at constant volume, there is no expansion work. Therefore heat released/absorbed by the system contributes only to the internal energy change
This makes internal energy a
state function
What is a state function
Are quantities such as pressure, volume and temperature, which depend only on the state of the system
If the system is changed from one state to another, the change in a state function is independent of how the change was brought about
What is a path function
Are quantities like work and heat, which depend on the path between states
Path functions depend on how the change was brought about
