Thermodynamics I Flashcards
What is thermodynamics?
Thermodynamics summarises the properties of energy and its transformation from one form to another.
When studying the universe in thermodynamics, what two things can we split it into?
System and surroundings.
What is a system?
The subject that we are interested in.
What are surroundings?
The remainder of the universe outside the system.
What is an open system?
A system where energy and matter can be transferred with the surroundings.
What is a diathermic (closed) system?
A system where only energy can be transferred with the surroundings.
What is an adiabatic system?
A system where neither energy or matter can be transferred with the surroundings.
What are the two properties that the system depends on?
Extensive and intensive properties.
What are extensive properties?
They depend on the quantity of matter in the system, eg mass, volume.
What are intensive properties?
They are independent of the amount of matter present, eg temperature, density.
What is work?
Work is a motion against an opposing force.
When is work done?
Work is done when a force moves.
Do chemical changes do work?
Yes, they may release electrical or light energy as a result of doing work.
Most common form of work in Thermodynamics I?
pV work.
What happens for pV work to be done?
Work is done to increase the volume against the surrounding pressure.
What is energy in thermodynamics?
The capacity of a system to do work.
What is heat?
A means of transferring energy (process), it is not a form of energy!
What is internal energy?
The total KE due to motion of particles and PE associated with atoms within the molecules.
What scale is internal energy on?
The microscopic.
What is the symbol for internal energy?
U.
For a simple idea gas, all internal energy is what?
KE
What temperature is raised in an idea gas, what happens to U?
U increases as the KE of the particles increases.
What does +q show?
Heat going into the system (endo).
What does -q show?
Heat is given out by the system (exo).
What does +w show?
Work is done on the system (eg gas is compressed).
What does -w show?
Work is done by the system (eg gas expanding).
-w is also the same as…
…w’
(Ideal piston) what are the conditions for the gas to expand?
Pint>Pext
(Ideal piston) how much does the piston move?
dx
(Ideal piston) how much work is done for the expansion?
dw’
Due to dw’ = Fdx, F=PextA and Adx=dV, what is a formula for work done on a gas?
dw’=PextdV
(Ideal piston) if Pext=0 what can be said about the expansion of a gas?
There is no work done in expanding the gas.
(Ideal piston) if Pext=constant what can we do to find the work done?
Integrate dw’=PextdV.
First Law of Thermodynamics:
Energy can neither be created nor destroyed on transformed from one form to another.
First Law of Thermodynamics equation (U):
ΔU = Ub - Ua
Equation for ΔU with work and heat:
ΔU = q + w
q and w are what kind of functions?
Path functions.
What is a state function?
A function whose value depends only on the state of the material under consideration. It has the same value for a given state no matter how the state came about.
What is a path function?
A function whose value depends on the path which the system takes between two states.
Reversible change in U equation:
dU = dq + dw
Reversible change in U equation assuming only pV work:
dU = dq - PextdV
Can state and path functions be integrated?
Only state functions can be integrated as they are exact differentials.
What are the conditions for maximum work to be done?
External pressure should be as high as possible without exceeding the internal pressure.
Pext should be infinitesimally smaller than Pint.
Thermodynamic Equilibrium:
A system is in equilibrium with its surroundings if an infinitesimally small change in conditions in either direction will result in change of state in opposite directions.
Reversible processes are linked to equilibrium how?
If a system is at equilibrium with its surroundings then an infinitesimal change can take place causing a change in state so the system is reversible.
If a system is not in equilibrium can the process of expansion be thermodynamically reversible?
No it is irreversible.
Features of reversible processes:
Infinitely slow.
At equilibrium.
Do maximum work.
Features of irreversible processes:
Finite rate.
Not at equilibrium.
Do less than maximum work.
At constant volume can pV work be done?
No, pV work cannot be done, all heat absorbed goes into internal energy.
At constant volume, when we supply energy to an object, what is the relationship between the heat supplied and temperature rise?
dq = CdT
C is the heat capacity
For a process at constant volume, how is the heat and internal energy related to temperature?
dU = dq(V) = C(V)dT
What does the Equipartition of Energy state:
That the motion of an atom in any direction is equally likely and contributes an average of 1/2kt for each degree of freedom (Ideal Gas).
At constant pressure, some heat will do what?
Some will increase the internal energy and some will appear as the work of expansion.
For a fixed amount of heat, the change in internal energy is greater or less for a system at constant pressure compared to one at constant volume?
It is less as less energy remains in the system, the temperature rises less than a system with constant volume (heat capacity higher).
Equation for enthalpy linking internal energy and work of expansion:
H = U + pV
Small change in enthalpy equation:
dH = dU + pdV + Vdp
If a process only has pV work, what does the small change in enthalpy equation become?
dH = dq + Vdp
as dU=dq-pdV so sub in
Equation for enthalpy change at constant pressure:
dH = dq(p)
as at constant surrounding pressure, dp=0
What can the enthalpy change be identified as?
The same as the energy supplied as heat.
What can we measure about enthalpies?
We can only measure changes in enthalpies rather than enthalpies themselves.
What assumption can we make about C(p)?
C(p) is constant.
What is a more accurate form of C(p)?
C(p) = a + bT + c/(T^2)
where a, b and c are constants and depend on the substance.
What is Hess’s Law:
Enthalpy change for a process is the same no matter what pathway we take in going from initial to final state.
Explain Kirchhoff’s Equation:
See lecture notes, involves enthalpy changes of a reaction at a different temperature.
What does the second law allow us to do?
Predict whether a reaction is spontaneous.
Entropy is associated with what?
Disorder.
Work simulates what?
A uniform (ordered) motion of atoms.
Heat simulates what?
Disordered motion of atoms.
The thermodynamic definition of entropy is motivated by what?
The idea that change in entropy depends on how much energy is transferred as heat.
Definition of entropy equation:
dS = q(rev)/T
When is a process spontaneous?
When the entropy of the universe increases.
What do we do if a process is not reversible and we want to calculate the entropy change of the system?
We calculate the heat change to the system if the process took place reversibly.
In an isothermal process, what happens to dU?
dU=0
What is the enthalpy change of the surroundings due to heat exchange with the system?
H(surr,final) – H(surr,initial) = dH(surr) = q(rev,surr)
Why does a change in state only depend on the amount of heat, independent as to whether it was supplied reversibly?
Enthalpy is a state function.
dH(surr) = q(rev,surr) = q(surr) = –q(sys)
How can we use q(surr) = –q(sys) to find the entropy change of the universe in terms of the system?
dS(surr) = –q(sys)/T(surr)
dS(univ) = dS(sys) – q(sys)/T(surr)
Derive an expression involving Cp for the variation of entropy with temperature.
Check derivation.
How does entropy change at phase changes?
It changes by a certain value related to the ∆H of transformation.
State the Gibbs function and the Helmholtz function.
G = H - TS A = U - TS
What conditions is the Gibbs function valid for?
Constant pressure.
What conditions is the Helmholtz function valid for?
Constant volume.
Derive an expression relating G and the entropy of the universe.
Check derivation
Relate maximum work under reversible conditions and G.
Check derivation
Derive and stater the 3 master equations.
Check
Derive the molar Gibbs energy relation.
Check derivation
Derive the Gibbs-Helmholtz equation.
Check derivation
Derive an expression for the free energy of mixing of ideal gases.
Check derivation
Derive an expression relating G to K using chemical potentials.
Check derivation
Derive Van’t Hoff Isochore law.
Check derivation
Integrate the Van’t Hoff Isochore law.
Check
Using a statistical definition of entropy, derive the entropy of mixing and compare to the result from the Gibbs equation.
Check derivation
What is an Ellingham plot of?
Standard free energy, when 1 mole of gaseous O2 reacts with a pure element to form an oxide as a function of temperature, T.
3 main uses of an Ellingham diagram.
Determine the relative ease of reducing a metallic oxide.
Determine the partial pressure of O2 in equilibrium with a metal oxide at a given temperature.
Determine the ratio of carbon monoxide to carbon dioxide that will reduce a metal oxide at a given temperature.
What does position on an Ellingham diagram show?
The affinity of a metal for oxygen, lower means highly reactive and more difficult to reduce.
On an Ellingham diagram, a carbon reaction can reduce any metal oxide, when?
When it is below the metal oxide reaction.
Why is it more difficult to extract metals from sulfides using hydrogen or carbon?
They have a lower affinity for sulfur than they do for oxygen.
List the reactions with oxygen for removing a metal from its sulfide.
Check list
What equation is a Kellogg diagram built on?
∆G˚ = -RTlnK
Derive the phases rule.
Check derivation
Derive the Clapeyron equation
Check derivation
Consider the Clapeyron equation at solid->gas and liquid->gas transitions.
Check
Consider the Clapeyron equation for solid->liquid transitions.
Check
Derive the Clausius-Clapeyron equation.
Check derivation
Give expressions for Raoult’s law
Check
When is Raoult’s law valid?
For ideal solutions..
When is Henry’s Law valid?
For non-ideal solutions when solute is very diluted.
Give an expression for Henry’s law.
Check
Derive an expression for the free energy of mixing of a regular solution.
Check derivation.
What is the enthalpy change of mixing of an ideal solution.
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