thermodynamic lecture 1 Flashcards
what is work?
is the mode of transfer of energy that achieves or utilises uniform motion of matter in the surroundings
what is heat ?
is the mode of transfer of energy that achieves or utilises disorderly motion in the surroundings
what is The Thermodynamic Universe consist from ?
1- System = the part of the world in which we have an interest
2- Surroundings = the part of the world from which we observe the system; surroundings are assumed to be infinite, remain
at constant temperature, pressure, volume – no matter what
the system does
3-Universe = System + Surroundings
what is open system?
can exchange energy and matter with the surroundings (changes in composition possible)
what is close system ?
can exchange energy but not matter with its surroundings
what is isolated system ?
can exchange neither energy nor matter with its surroundings
example of Some spontaneous changes are not associated with any change of internal energy at all ?
for example the isothermal expansion of a perfect gas
No change in internal energy
A non-spontaneous change always requires work ?
- A gas can be compressed
- A cool object can be heated up with electric current
- Water can be dissociated by electrolysis
A non-spontaneous change always requires work such as ?
- A gas can be compressed
- A cool object can be heated up with electric current
- Water can be dissociated by electrolysis
what is entropy
Entropy (S) is the function that permits us to predict whether a process is spontaneous or not.
what is entropy ?
Entropy (S) is the function that permits us to predict whether a process is spontaneous or not.
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During any spontaneous process, the entropy of the universe increases.
Energy leaves system→w and q ______
negative
Energy enters system→w and q ______
positive
Expansion Work becomes zero when the external ex pressure (Pex) is zero.
The greater the external pressure the greater the work done by the
system for a given volume change
When the external pressure exceeds the internal pressure then a
compression of the gas takes place, and w _______
becomes positive.
Maximum work is obtained when the external pressure is only infinitesimally less than the pressure of the gas in the system at all stages of the expansion
= mechanical equilibrium)
-A system that remains in mechanical equilibrium with its surroundings at all stages of an expansion does maximum expansion work.
when ideal gas occurs ?
Adjusting the external pressure to always match internal pressure leads to perfect gas behaviour
what is Reversible Process ?
A reversible process can be reversed by an infinitesimally small variation of one variable (e.g., pressure).
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reversible, isothermal expansion work of a perfect gas
Area under p=nRT/V curve is ____
Reversible isothermal expansion work
reversible =give max work expansion
Irreversible expansion work is always ________ in magnitude than the expansion work associated with the reversible process
lower
why the heat capacity also depends on whether the sample is maintained at constant volume during heating, or at constant pressure. ?
For the pressure to remain constant the gas must expand. This means that the gas loses energy as expansion work to its surroundings. So additional heat is required to compensate the energy loss from expansion work.
when you add heat to the system you get _____
negative in work
what is the internal energy (U) ?
The energy contained in the system (J)
U=q+w
When q=-w, U=0 ?
when the amount of heat add is the same goes out = the same in isolated system
what is 1st law of thermodynamics
The internal energy of an isolated system is constant
Heat q transferred to the system from the surroundings is positive.
Work w done by the system expanding against an external force is negative so
w = −pdV
so the first law will be :
ΔU = q + w
Heat q transferred to the system from the surroundings is positive.
Work w done by the system expanding against an external force is negative so
w = −pdV
so the first law will be :
ΔU = q + w
Heat q transferred to the system from the surroundings is positive.
Work w done by the system expanding against an external force is negative so
w = +pdV
so the first law will be :
ΔU = q - w
When the volume of a system is constant then no work can be exchanged with the surroundings (w = 0). The internal energy change becomes. U = qV
at constant volume
When the volume of a system is constant then no work can be exchanged with the surroundings (w = 0). The internal energy change becomes. U = qV(indicate at at constant volume)
at constant volume
what is uses of bomb calorimeter ?
1-used to determined heat released in a solution
2- measure temperature rise for combustion of test material
3-Determine calorimeter constant (heat capacity) in kJ/°C
what is Enthalpy ?
Most chemical reactions occur in vessels open to the atmosphere and under constant (atmospheric) pressure
Example of Enthalpy ?
decomposition of carbonates produces CO2 gas that must push back the surrounding atmosphere
At constant pressure, the enthalpy change is equal to the amount of heat exchanged.
dH= dq(p)
How do (i) the internal energy and (ii) the enthalpy of a perfect gas change during an isothermal expansion? Why?
nternal Energy and the 1st Law
• We have seen that heat and work are just different ways of transferring energy into or out of a system. This is called the equivalence of heat and work, a principle first formulated by James Joule in the 19th century.
• The energy contained in the system is called the internal energy, U [unit: J].
• In practice we cannot measure the total internal energy of a sample, because it
includes all the energies of all sub-atomic particles, i.e., nuclei, electrons, etc.
• We can only measure changes in internal energy:
U = w + q
Isothermal expansion of a perfect gas:
q = −w
The internal energy of a sample of perfect gas at a given temperature is
U = 0 independent of the volume it occupies.
Isolated system: no exchange of heat or work with surroundings.Hence the internal energy cannot change. This leads to the 1st law of thermodynamics:
The internal energy of an isolated system is constant
H = U + (pV) pV = nRT
H = U + (nRT)
(ii) Definition of enthalpy:
Perfect gas law:
Hence:
n and T are constant during an isothermal expansion, so (nRT) = 0 Since U is also zero (see above) it follows that
H = 0 as well!
