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Thermodynamics. 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

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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

Thermodynamic definitions

Thermodynamic definitions

  • 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.

Types of systems

Types of systems

  • 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

Within systems

Within Systems

  • 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

Thermodynamic properties

Thermodynamic Properties

  • 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

Thermodynamic laws

Thermodynamic Laws

  • Three laws – each derives a “new” state function

    • 0th law: yields temperature (T)

    • 1st law: yields enthalpy (H)

    • 2nd law: yields entropy (S)

Zeroth law

Zeroth law

  • If two systems are in thermal equilibrium

    • No heat is exchanged between the systems

    • They have the same temperature

Measurement of t

Measurement of T

  • 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

Fig levine

Fig. Levine

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

First law

First law

  • 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

Second law

Second Law

  • 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 thermodynamics

Equilibrium Thermodynamics

  • 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:


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


  • 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

Math review

Math Review

(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

Equilibrium activities chemical potentials

Equilibrium, activities, chemical potentials

(on board)

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