Chemical thermodynamics

CHEMICAL THERMODYNAMICS

What is Thermodynamics ?
Thermo- = heat, temperature
-dynamics = flow, movement, work
Therefore: Chemical thermodynamics is the study of the dynamics of heat, energy transformations and the behavior
of systems in terms of the thermodynamic properties of a system (thermodynamic variables), e.g pressure,
volume, temperature, etc
The relationships between these thermodynamic variables can be deduced using the laws of thermodynamics.
What does thermodynamics Predict?The direction of changes
The position and nature of equilibrium
How equilibrium changes with temperature, pressure and the composition.

Thermodynamic Systems
A thermodynamic system is a region within defined boundaries that has been selected for study.

Types of Systems:
Open systems - allow both energy and mater to cross the boundary. eg. an open beaker.
Closed systems - allow energy, but not mater to cross the boundary. eg. an closed beaker
Isolated systems - allows neither energy nor mater to cross the boundary and are not influenced
in any way by the surroundings. eg. » a closed Dewer flask (vacuum flask).

State of a system
Thermodynamic State:
The condition of the system as characterized by the values of its properties.

Systems are defined by their properties (called state variables or state functions): eg
Temperature; Pressure; Amount of substance (mass or number of moles);Volume etc.


Chief criterion of a state function:
(a) the change in a state function depends only on the final and the initial state, and
(b) NOT on the method/process or pathway by which this change is made.
Boundary - A surface (real or imaginary) that separates the system from its surroundings.

surroundings
The surroundings include everything that is not within the system’s boundary
Universe = System + Surrounding

Thermodynamic Properties
Properties are either:
Extensive:
– depends on mass of the system (or the amount of substance present).
For example- total mass, volume, particle number and total energy

Intensive:
– do not depend on the mass of the system.
For example – temperature, pressure, density, specific volume (this is the volume per unit mass; v = V/M) or volume per mole (v = V/n), etc.

Thermodynamic Processes
A thermodynamic process is defined to be a process that leads to a change in the thermodynamic state of the system. Such a change can either be physical in nature, or chemical in nature,
A thermodynamic process has well defined beginning and ending points, namely, the initial and final states.
 Starting from the initial state, the system achieves the final state by going through a continuum of intermediate states (net changes are independent on intermediates)
The set of states connecting the initial and final states are said to constitute a thermodynamic path between the initial and final states.
reversible
– one in which the intermediate states are thermodynamic states
– the initial state could be reached starting from the final state by going through the same set of intermediate states in reverse.
irreversible
– one in which the intermediate states are not thermodynamic states

Thermodynamic Processes can also be:
– Isothermal ⇒ ΔT = 0
– Isobaric ⇒ ΔP = 0
– Adiabatic ⇒ Δq = 0
– Isochoric ⇒ ΔV = 0
Thermodynamic Equilibrium
 the condition of a system in which the quantities that specify its properties, such as pressure, temperature, etc., all remain unchanged. A system in thermodynamic equilibrium should satisfy the following strict requirements:
Mechanical equilibrium:
 requires that there are no unbalanced forces acting on any part of the system or on the system as a whole;
Thermal equilibrium:
 there are no temperature differences between the parts of the system or between the system and its surroundings;
Chemical equilibrium:
 no changes occur in the chemical composition of the system or parts of the system.

Equations of state (EOS):
– a formula describing the inter-relationships between various measurable properties of a system, P, T, V and n
general form for an EOS is: f(P,V,T, n) = 0
EOS for an ideal gas: PV = nRT
• Constant temp:
Pi Vi = Pf Vf ⇒ Boyle’s law
• Constant pressure:
Vi / Ti = Vf / Tf ⇒ Charles’ law


Mixtures of gases
The partial pressure:
The pressure that each gas would occupy if it occupied the same container alone at the same temperature.
PA = xA PT
Where xA is the mole fraction of the component A.
xA = nA/nT , nT = nA + nB + …….
whatever the composition of the mixture,
xA + xB + ... = 1

Dalton's law
the sum of the partial pressures is equal to the total pressure :
PA +PB + ... = (xA + xB + ... ) PT = PT
These relations are true for both real and perfect gases

REFERENCES

• P.W.Atkins and J.C.de Paul,physical chemistry for the life sciences.W.H.Freeman,New York(2005)
• J.Wisniak,The joule-Thomson coeffient for the pure gases and their mixtures.j.chem.educ 4,51 (1999)
• J.Walton,Three phases of matter.Oxford University press(1983)

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