thermodynamics thermodynamics thermodynamics way to calculate if a reaction will occur way to...
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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 Tells direction a reaction should go Basis for estimated rates Farther from equilibrium, faster rate
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
Open system Exchanges with surroundings Mass, also heat and work
Closed system no exchange of matter between
surrounding and system, energy can be exchanged
Isolated system there is no interaction with
surroundings, no exchange of energy or matter
Steady state system Flux in = flux out There can be exchange, but no change
in total abundance
Parts of 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
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 @ 25º C A couple of state functions for this sytem are
amount of mass (1 g) and T (25º C) There are others we will learn about
Path independent E.g., state would be the same if you
condensed steam or melted ice For the values of the state functions, it
doesn’t matter how the state got there
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
If two systems are in thermal equilibrium No heat is exchanged between the
systems They have the same “temperature”
T is the newly defined state function How is temperature defined?
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 + a2TWhere V = volume
T = temperaturea1 & a2 = constants
Fig. Levine
T (ºC)
V (L)
• 1 mole of N2 at constant P• 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
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
Three forms of energy
Potential + Kinetic energy + internal energy
Minimum or rest energyHere only internal energy, U
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 (H)
PV = pressure * volume = work done on/by the system
Units – energy, e.g. J, kJ, cal etc. Extensive – i.e., additive.
H = U + PV
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
Extensive = units of energy/T, e.g. kJ/K
Entropy is a variable used to defined Gibbs free energy (G)
G used to determine equilibrium of reactions
Equilibrium Thermodynamics
Equilibrium occurs with a minimum of energy in system
Systems not in equilibrium move toward equilibrium through loss of energy
If system is at constant T and P, measure of energy of system is given by Gibbs free energy (G) G = f(H,S,T)
G and H units = kJ (kcal) S units = kJ/K (kcal/K) T is Kelvin scale (K)
G = H - TS
Equilibrium A, B, C, and D present
A + B ↔ C + D
Imagine some system with A, B, C, and D components:
Consider processes in system at constant T & P “Process” means system changes May be chemical reaction
Here D is change in state:
For all properties: G, H, T or S
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 Possible to calculate DG, and thus
determine (1) if reaction will occur spontaneously, and (2) which way reaction will go.
Non-equilibrium system:
Equilibrium system
A + B ↔ C + D DG = 0
A + B → C + D DG ≠ 0
A + B ← C + D DG ≠ 0
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
(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
(on board)