Thermo Flashcards
Define state of equilibrium
Thermodynamic state that ≠ change if external conditions are constant
When is a reaction reversible
If successive states differ only infinitesimally from equilibrium
∴ can only be between equilibrium states
(In practice external conditions should be varied v.slowly to achieve this ∵ allows system time to equilibrate under new conditions)
Zeroth law of thermodynamics
If 2 systems are in thermal equilibrium w/ 3rd = also in thermal equilibrium w/ each other
(• implies existence of an absolute scale of temp)
First law of thermodynamics
E ≠ created/destroyed - only transferring/changed between forms
What if V = +ve?
W = -ve
What is a state function?
Something that depends on the thermodynamic state of the system = P, E, T, V, entropy
What is a path function
Something that depends on how they were applied = heat, W
State Hess’ law
Heat evolved in a reaction = only determined by p & r ≠ depend on reaction path/intermediate reactions
Kirchoff’s law
Δ heat capacity during a reaction depends only on the heat capacities of p & r
Latent heat
Extra heat absorbed/released during a phase change
Adiabatic flame
(Theoretical) max temp to which the products can be heated
Why would a true flame temp be lower than the adiabatic flame temp?
1) incomplete combustion
2) heat transfer to the surroundings
3) dissociation of products
4) kinetic effects
What acc is the flame temp?
Temp of surroundings?
Adiabatic process
One in which no heat is transferred
(I.e. Q = 0)
Temp can still change since W
How does a heat engine work
Converts heat into W by bringing some substance from a high temp -> low temp
Carnot Cycle
Theoretical representation of one type of heat engine, w/ gas as the working substance
= most efficient possible heat engine
Explain the four components of a Carnot Cycle
1) reversible isothermal expansion of the gas @ hot temp
2) reversible adiabatic expansion of the gas w/cooling
3) reversible isothermal compression of the gas @ cold temp
4) reversible adiabatic compression of the gas w/heating
2nd law of thermodynamics
Entropy of an isolated system can never decrease
What are the implications of the 2nd law?
1) heat = only transferred spontaneously from higher T -> lower
2) an isolated system which converts W -> heat = possible : BUT not vice versa since heat ≠ W (contradicts 1st law)
How can the total entropy of the universe stay constant ?
If system and surroundings undergo reversible processes
What happens if a process is irreversible?
Increase of entropy of universe
• entropy is always created (no known way to destroy entropy)
What happens when 2 gases mix? + implication?
- they don’t unmix
- implies there’s entropy associated w/mix
What is Gibbs Paradox?
There’s no increase in entropy when identical gases
3rd law of thermodynamics
Entropy of a perfect crystal @ absolute zero = 0
What is a perfect crystal ?
A perfectly spatially ordered structure i.e. 1 possible configuration
State the 2 Carnot’s principles
1) thermal η of all reversible heat engines operating between the same 2 thermal reservoirs = same
2) thermal η of a reversible heat engine > η of an irreversible heat engine operating between the same thermal reservoirs
