Physical Chemistry 2 Flashcards
What is thermodynamics?
The study of energy transformations
What is energy and work?
Work - a form of energy which can transfer in and out of a system, stored in the organized motion of molecules.
It is done when an object is moved against as opposing force.
Energy - capacity to do work. Energy of a system can be changed by work and heat.
How is work done calculated / defined?
W = Fx
By definition;
dW = -Fdz
and pressure, P = F/A
So by substitution,
dW = -PAdz
and Adz has units of volume (x³)
Therefore dW = -PdV
Which can then be integrated
What’s a diathermic and adiabatic system?
Diathermic - energy can transfer in and out as heat
Adiabatic - no heat can be transferred in and out
What’s internal energy?
Total energy of a system, as a function of state, and is an extensive property (dependent on the amount of matter).
For an adiabatic process, what is internal energy equal to?
Overall energy balance:
ΔU = Q + W
However ΔQ is 0 as there is no transfer of heat so:
ΔU = W
For an isobaric process, what is internal energy equal to?
Overall energy balance:
ΔU = Q + W
Both still exist for this process (although ΔP is 0)
What is an isochoric process?
One with a constant volume.
ΔV = 0
For an isochoric process, what’s internal energy equal to?
Overall energy balance:
ΔU = Q + W
However, ΔV = 0 (and W=PΔV)
Therefore:
ΔU = Q
For an isothermal process, what is internal energy equal to?
Temperature is fixed (ΔT = 0) and it’s energy stays constant so ΔU = 0
Therefore:
Q = -W
For an cyclic process, what is internal energy equal to?
Certain processes have the same initial and final states, represented by a closed path and ΔU = 0.
Therefore:
Q = -W
What’s a reversible and irreversible process?
Reversible - the system is never more than infinitesimally far from equilibrium and an infinitesimal change in external conditions can reverse the process
Irreversible - cannot be reversed by an infinitesimal change in external conditions
How can work by an ideal gas be calculated?
Using pV = nRT and W = pΔV
How is heat capacity at constant volume defined/calculated?
Cᵥ = (dU/dT)ᵥ = Q v
->
dU = CᵥdT
For a certain range of T where Cv is constant
No work is done
What’s molar heat capacity and specific heat capacity (how is it found)?
Molar: C vm = Cv / n
Specific heat is in terms of mass
How’s enthalpy found?
H = U + pV
= internal energy + work
How is enthalpy found for an isothermal process?
Change in U = 0.
Therefore ΔH = pV = ΔnRT
If no chemical reaction and no change in the # moles, ΔH = 0.
Otherwise ΔH = RTΔn
What’s the relation between Cp and Cv?
Cp - Cv = nR
What changes occur when a gas expands adiabatically?
Work is done since there’s a change in pV
Internal energy will fall
Temperature will decrease
Microscopic Ek of molecules will fall
What’s the standard enthalpy of transition, Δ trs H’?
Standard enthalpy change accompanying a phase change.
Enthalpy is a state function, so ∆H differs only in sign for forward and reverse processes.
Phase changes include: mixing, solution, atomisation, combustion etc.
What’s standard reaction enthalpy, ΔrH?
Change in enthalpy when reactants in standard states change to products in standard states
Enthalpies of mixing and separation are usually ignored.
How is standard reaction enthalpy found?
= enthalpy of products - enthalpy of reactants
Enthalpy of formation often used
What’s the standard enthalpy of formation?
What’s their reference state?
Changein enthalpy for formation of compound from its constituent elements in their reference states, which is the most stable state of an element at a certain temperature and 1 bar.
What’s Kirchhoff’s law?
ΔᵣH(T2) = ΔᵣH(T1) + ∫(ΔᵣCp)dt
Where the quantity, ΔᵣCp, is the difference in heat capacities between reactants and products under standard conditions, weighted by stoichiometric coefficients.
ΔᵣCp = ΣvCp * [products] - ΣvCp * [reactants]
(where v is stoichiometric coefficients)
