Atoms and Molecules

Thermodynamics

Thermodynamics is used to describe heat and energy changes and chemical equilibria.
It allows the prediction of equilibria positions, but NOT the rates of reaction or change.

The First Law of Thermodynamics can be expressed in everyday terms:

You can't win
You do not get anything for nothing
There is no such thing as a free lunch

It is the law of conservation of energy:
                                            'Energy can neither be created nor destroyed'
The energy in an isolated system is conserved
    Energy is the capacity to do work
    System is a chemical reaction, a natural process, a cell, the earth, etc.
    Heat energy can do work by changing a temperature or pressure.
    H is the ENTHALPY; the heat content
    DH is the CHANGE IN ENTHALPY; the heat lost or gained
    DH is negative when heat is released by the system; exothermic
    DH is positive when heat is absorbed by the system; endothermic

In a sequence of reactions:         DH_overall = SDH
        A = B             DH₁                     e.g.         C + ½O₂ CO
        B = C             DH₂                     e.g.         CO + ½O₂ CO₂
sum A = C             DH₁ + DH₂                             C + O₂ CO₂
But DH does not tell us if the process will go: e.g.
desk burning;       wood + O₂ CO₂ + H₂O          DH is negative, heat is given out
melting of ice;                   ice water                     DH is positive, heat is absorbed

The Second Law of Thermodynamics can be expressed in everyday terms:

You can't even break even
The house always wins
The universe is becoming more chaotic
The randomness of the universe always increases
A perpetual motion machine cannot be built

No process for converting heat into energy is 100% efficient
    S is ENTROPY; a measure of disorder
    DS is ENTROPY; the change in disorder
    DS is positive when disorder increases
If there is no change in energy in a process, there must be an increase in entropy,
e.g. gases mixing.

When two systems are combined, the entropy is greater than the sum of the parts.
Entropy on its own does not tell whether a process will go

                             2H₂ + O₂ 2H₂O       clearly goes with negative entropy change
  (3 molecules of a mixture 2 molecule of the same)

The Third Law of Thermodynamics can be expressed in everyday terms:
Follows from the First and Second Law

You can't stay out of the game
The First and Second Laws cannot be got around

Absolute zero cannot be reached (S = 0 when T = 0 for a perfect crystal)
    G is the GIBBS FREE ENERGY
    DG is the change in the free energy
Free energy can do work at constant temperature and pressure
In living systems (constant temperature and pressure)

DG = DH - TDS

DG is the maximum work obtainable from a process
DG is negative when the system is able to go; exergonic
DG is positive when the system is unable to go; endergonic
DG is zero when the system is at equilibrium

Every reaction has a characteristic DG under defined conditions
Under standard conditions (1 M reactants and products, 298 K, 101 kPa), this is the standard Free Energy change, DG. Where pH = 7, this is DG°'
For

A + B = C + D

where R = gas constant (8.31 J mol^-1 K^-1)

Given DG is zero when the system is at equilibrium
At equilibrium, DG = 0
therefore             DG°'= -RTLn (K_eq°')

In a sequence of reactions: DG_overall = SDG

             A + B = C + D         DG₁
             C + E = F                 DG₂
sum          A + B + E = D + F         DG₁ + DG₂

So long as DG is negative the reaction will go from left to right

                                                                                                        DG°' kJ mol^-1
  Glucose + phosphate = Glucose-6-phosphate + H₂O             +13.8
                       ATP + H₂O = ADP + phosphate                           - 30.5
Glucose + ATP = ADP + Glucose-6-phosphate                         - 16.7

DG depends on the concentration of the reactants and products as well as the temperature and DG°'. The criteria of spontaneity depend on DG not DG°'.

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This page was last updated by Martin Chaplin
on 10 February, 2005