The First Law Of Thermodynamics Flashcards
What’re the contributions to the energy of matter?
Kinetic and potential energy
What is the difference between an intensive and an extensive property?
Extensive property = a property that depends on the amount of substance in a sample.
Intensive property = a property that does NOT depends on the amount of substance in a sample.
What does reference state of an element mean?
It is an elements most stable form under the prevailing conditions
What is standard state referring to?
Any specified state at 1 bar
What is an open system?
Both matter and energy can be exchanged with surroundings
What is a closed system?
Energy, but not matter, can be exchanged with its surroundings
What is an isolated system?
Neither matter nor energy can be exchanged with its surroundings
What is a state function?
A state function is a physical property that depends on the present state of the system and is independent of the path by which that state was reached.
Heat and work are NOT state functions
What is the first law of thermodynamics?
That the internal energy of an isolated system is constant
Is internal energy a state function?
Yes
What equation relates to the first law?
∆U = w + q
What equation gives the work done when a system expands against an external opposing pressure?
W = -Pex ∆V
∆V because it is ∆Area x ∆distance = ∆Volume
What is free expansion?
It is expansion against zero external pressure,
Pex = 0
What does reversible mean in thermodynamics?
It means the system can be reversed by an infinitesimal change to a variable
What can be said about the expansion work done in a system which is in mechanical equilibrium with its surroundings?
It can be concluded that at all stages of the expansion, maximum expansion work is done
How do you derive the equation for the work done for a reversible isothermal expansion of an ideal gas?
W = -nRT ln (Vf/Vi)
What is ∆U for an ideal gas?
∆U = 0
How do work done and heat relate to each other for the isothermal expansion of an ideal gas?
q = -w
How do work done and heat relate to each other for the isothermal reversible expansion of an ideal gas?
Since q = -w
q = nRT ln (Vf/Vi)
When Vf > Vi in the isothermal reversible expansion of an ideal gas, what can be said about q?
If Vf > Vi, then ln Vf/Vi is positive and hence q > 0 as expected; heat flows into the system to make up for the energy lost as work.
The greater the ratio of the final and initial volumes, the greater the influx of energy as heat.
What is heat capacity by definition?
C = ∆q / ∆T
Or more generally, C = dq / dT
If heat capacity, C, is constant, what does q = ?
q = C∆T
What is Cs, Cm, Cp and Cv when talking about heat capacity?
Specific heat capacity, C / mass(g)
Molar heat capacity = C / n(mol)
Heat capacity at constant pressure
Heat capacity at constant volume
Is heat capacity intensive or extensive property of a system?
It is an extensive property which means that it depends on the amount of substance in the sample.
What is enthalpy by definition?
∆H = ∆U + ∆(p+v)
This applies to any system, not just ideal gases
What can be said about the change in internal energy that is free to expand or contract and energy supplied as heat to a system?
The change in internal energy of a system that is free to expand or contract IS NOT EQUAL TO the energy supplied as heat.
How do you find the enthalpy when considering an ideal gas?
∆Hm = ∆Um + R∆T
Sub pV = nRT into,
∆H = ∆U + ∆(pv)
How do you find enthalpy at constant pressure?
∆H = ∆U + P∆V
Is enthalpy a state function?
Yes
At constant pressure, no nonexpansion work, what does ∆H = ? WHY?
∆H = q
- ∆H = ∆U + p∆V = ∆U + Pex ∆V
∆U = w + q
W = -Pex ∆V
These expressions are subbed into eq. 1
To give:
∆H = (w + q) + Pex ∆V
∆H = (-Pex ∆V + q) + Pex ∆V = q
At constant volume, no nonexpansion work, what does ∆U = ? WHY?
∆U = qv
∆U = w + q
∆U = -Pex ∆V + q
As there is no change in volume, ∆V = 0 so work done = 0.
Hence ∆U = qv
What is a bomb calorimeter an example of?
A constant-volume calorimeter
How can temperature dependent heat capacity be calculated with change in internal energy and change in temperature?
Well, C = dq / dT
And dU = qv at constant volume, no non-expansion work
Hence Cv = dU / dT
This is hence heat capacity is temperature dependent
What equation gives heat capacity when it is independent of temperature?
Cv = ∆U / ∆T
What can you find using a bomb calorimeter at constant pressure?
It tells you how the enthalpy of a system changes as its temperature is raised at constant pressure.
How do you find temperature dependent heat capacity at constant pressure?
Well, C = dq / dT
And dH = qp at constant pressure
So Cp = dH / dT
What is always greater, enthalpy or internal energy?
Enthalpy is always greater, this difference increases with increasing temperature
What is heat capacity when independent of temperature at constant pressure?
Cp = ∆H / ∆T
What is always greater, Cp or Cv?
Cp > Cv since H = U + pV and Cpm = R + Cvm
Cp is always greater than Cv, because work is done while volume changes, using up some of the energy added. Therefore, more energy must be added to increase temperature by the same amount to make up for the work done
How does R relate to Cp and Cv for an ideal gas?
