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Thermodynamics

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- Thermodynamics
- Way to calculate if a reaction will occur

- Kinetics
- Way to determine the rate of reactions

- Thermodynamic equilibrium rarely attained:
- Biological processes – work against thermo
- Kinetic inhibitions

- Thermodynamics very useful
- Good approximation of reactions
- Tells direction a reaction should go
- Basis for estimated rates
- Farther from equilibrium, faster rate

- System – part of universe selected for study
- Surroundings (Environment) – everything outside the system
- Universe – system plus surroundings
- Boundary – separates system and surroundings
- Real or imagined
- Boundary conditions – solutions to Diff Eq.

- Open system
- Exchanges with surroundings
- Mass, also heat and work

- Closed system
- no exchange of matter between with surrounding and system, energy can be exchanged

- Isolated system
- there is no interaction with surroundings, either energy or matter possible

- Steady state system
- Flux in = flux out
- There can be exchange, but no change in total abundance

- Phase – physically and chemically homogeneous region
- Example: saturated solution of NaCl

- Species – chemical entity (ion, molecule, solid phase, etc.)
- E.g. NaCl (solid) + H20 (liquid)
- Also Na+, Cl-, OH-, H+, NaClo, others

- Components
- Minimum number of chemical entities required to define compositions of all species
- Many different possibilities
- Na+, Cl-, H+, OH-
- NaCl – H2O

- Characteristics of components:
- Every species can be written as a product of reactions involving only the components
- No component can be written as a product of a reaction involving only the other components

- Extensive
- Depends on amount of material
- E.g., moles, mass, energy, heat, entropy
- Additive

- Intensive
- Don’t depend on amount of material
- Concentrations, density, T, heat capacity
- Can’t be added

- State function
- a property of a system which has a specific value for each state (e.g., condition)
- E.g., 1 g water @ 25 C
- Variables are amount of mass (1 g) and T (25 C)

- Path independent
- E.g., state would be the same if you condensed steam or melted ice

- a property of a system which has a specific value for each state (e.g., condition)

- Three laws – each derives a “new” state function
- 0th law: yields temperature (T)
- 1st law: yields enthalpy (H)
- 2nd law: yields entropy (S)

- If two systems are in thermal equilibrium
- No heat is exchanged between the systems
- They have the same temperature

- Centigrade
- 100 divisions between melting and boiling point of water

- Kelvin - Based on Charles law
- At constant P and m, there is a linear relationship between volume of gas and T
- Size of unit is same as centigrade

V = a1 + a2J

Where V = volume

J = temperature

a1 & a2 = constants

V (L)

T (ºC)

Experimental results

- extrapolation of results show intercept T @ V = 0 is about -273ºC

- Kelvin scale based on triple point of water

- defined as being 273.16 K

- Change in the internal energy of a system is the sum of the heat added (q) and amount of work done (w) on system
- Energy conserved

- Three types of energy
- Kinetic and potential – physically defined
- Internal – chemically defined

- Internal energy (U)
- Molecular rotation, translation, vibration and electrical energy
- Potential energy of interactions of molecules
- Relativistic rest-mass energy

- In thermo, a system at rest
- Kinetic and potential energy = 0
- Thermodynamics considers only changes in internal energy

- New state function – Enthalpy
- PV = work done on/by the system

H = U + PV

- A system cannot undergo a cyclic process that extracts heat from a heat reservoir and also performs an equivalent amount of work on the surroundings
- i.e., it is impossible to build a machine that converts heat to work with 100% efficiency

- New state function
- Entropy = S

- Entropy is variable in definition of Gibbs free energy (G)
- G used to determine equilibrium of reactions

- Equilibrium occurs with a minimum of energy in system
- Systems not in equilibrium move toward equilibrium through loss of energy

Potential + Kinetic energy

Minimum or rest energy

- If system is at constant T and P, measure of energy of system is given by G
- G = f(H,S, T)
- G and H units = kJ/mol (kcal/mol)
- S units = kJ/mol.K (kcal/mol.K)
- T is Kelvin scale (K)

G = H - TS

Equilibrium A, B, C, and D present

- Consider processes in system at constant T & P
- Means system changes
- May be chemical reaction

- Here D is change in state:

DG =DH - TDS

D = State2 – State1

- When system moves toward equilibrium:
- may release heat, e.g. DH < 0
- entropy may increase, e.g. DS > 0
- Both may happen

- Thus:
- DG < 0 for spontaneous reaction
- G2 < G1; DG = G2 – G1 < 0

- DG = 0 for process at equilibrium

- DG < 0 for spontaneous reaction

- G is an extensive state variable
- It depends on the amount of material

- The amount of G in a system is divided among components
- Need to know how G changes for each component

- First look at what variables control G
- What is G a function of?
- Want to know how G changes if all (or any) other variable change

- Change = calculus

(on board)

- If system is in thermal and mechanical equilibrium:
- G = f(P, T, n1, n2, n3…)

- Then total differential:
(on board)

- Infinitesimal change in G caused by infinitesimal change in P, T, n1, n2, n3…
- These are values we need to know to know DG

- Last term defined by Gibbs as chemical potential (m)
(on board)

- m is the amount that G changes (per mole) with addition of new component
- Intensive property (G extensive)
- Doesn’t depend on mass of system
- For one component system m = G/n

- For system at equilibrium, m of all components are identical

(on board)