Thermodynamics Flashcards
A system can do 2 types of work
- Expansion work
- Non-expansion work
Expansion Work
work from changing volume of a system
Non-expansion Work
work that doesn’t involve a change in volume of the system
Internal Energy (U)
The total store of energy in a system
*absolute energy cannot be calculated because it’s impossible to measure the energy of each atom, nuclei etc. –> we measure the change in internal energy , delta U
What are the two main types of expansion work?
- Expansion with constant external pressure (irreversible)
ex) gas expands by 0.5L against an opposing force with pressure of 1.2 atm - Expansion with changing external pressure (reversible)
ex) gas expands in a container which reduces the pressure of gas (isothermal expansion of gas occurs)
Irreversible Expansion and Reversible Expansion formula?
Irreversible: w = - Pex ΔV
Reversible: w = - nRT ln(Vf/Vi)
Expanding (reversibly/irreversibly) requires more work.
Expanding reversibly requires more work.
(Expanding a gas reversibly occurs in infinitesimally small, gradual steps, allowing the system to remain in equilibrium at each point –> the system is doing the maximum possible work against the external pressure at every step.
Irreversible expansion occurs suddenly –> less work is done, as the system does not remain in equilibrium and is not constantly pushing against a pressure that closely matches its own.)
Internal energy of a system can be changed by __ and __
work and heat
What is heat?
energy transferred as a result of a temperature difference
When is q>0 , q=heat
When heat enters the system, the system gains energy in the form of heat. Thus q > 0
Representative of endothermic process
What is U?
Internal Energy
When is q<0 , q=heat
When heat leaves the system, the system loses energy in the form of heat. Thus q < 0
Representative of exothermic process
What are the two boundaries between the system and surroundings?
Adiabatic and Diathermic
What is a adiabatic wall?
Does not allow transfer of energy as heat, even if temperature difference exists.
*adiabatic walls aren’t necessarily isolated. For example, the walls by be flexible and subject to expansion –> as long as energy is not transferred through HEAT, it’s considered adiabatic
What is a Diathermic wall?
Allows transfer of energy as heat.
