Download Free energy and Equilibrium

 First
law of thermodynamics; In any process
the energy of the system plus surrounding
remains constant(principle of energy
conservation).
 Second law of thermodynamics; In all
processes the entropy(disorder or
randomness) of the system plus the
surroundings always increases. The 2nd Law
explains why chemical reactions tend to
favor a particular direction.
 These
laws apply to all chemical reactions .
 Free
energy (Gibb´s free energy) ; It is the
form of energy capable of doing work under
conditions of constant pressure and
temperature.
 It determines whether a given chemical
change is thermodynamically possible. Thus
if the free energy of the reactants is greater
than that of the products the reaction will
tend to take place spontaneously.
 Conversely, if the free energy of the products
exceeds that of the reactants, then the
reaction will not take place spontaneously,it
will need the input of energy.
 Each
compound involved in a chemical
reaction contains a certain amount of
potential energy, related to the kind and
number of its bonds.
 In reactions that occur spontaneously,the
products have less free energy than the
reactants, thus the reaction releases free
energy, which is then available to do work.
Such reactions are exergonic;
 the decline in free energy from reactants to
products is expressed as a negative value ,
ΔG is negative.
 Endergonic
reactions require an input of
energy thus they do not occur spontaneously,
and their ΔG values are positive. The
coupling of exergonic and endergonic
reactions through a shared intermediate is
absolutely central to the energy exchanges in
living systems.
 A great number of reactions that occur in nature are


reversible and do not proceed to completion . Infact in all
such cases, in the initial state, only the reactants are present
but as the reaction proceeds, the concentration of reactants
decreases and that of products increases. Finally a stage is
reached when no further change in concentration of the
reactants and products is observed and a state of chemical
equilibrium is attained.
In reversible reactions the products can react with one
another under suitable conditions to give back the reactants ,
In other words, in reversible reactions the reaction takes
place in both the forward and backward directions.
The tendency of a chemical reaction to go to completion
can be expressed as an equilibrium constant. For the
Reaction;


the equilibrium constant, Keq, is given by

where [Aeq] is the concentration of A, [Beq] the
concentration of B, and so on, when the system has
reached equilibrium.
A large value of Keq means the reaction tends to proceed
until the reactants have been almost completely converted
into the products.



On the other hand, the chemical reaction in which the
products formed do not combine to give the reactants are
known as irreversible reactions In case of irreversible
physical and chemical processes, the change occurs only in
one direction and the processes go to completion.
The sign of ΔG permits prediction of the behavior of a
proposed chemical reaction .
ΔG
Reaction Behavior
Negative
Proceeds spontaneously to the right
exogenic
Zero
Is at equilibrium
Positive
Will not proceed
endogenic

Keq and the direction of the reaction;

The magnitude of the equilibrium constant, K of a reaction indicates
how far a reaction can go in the direction as written. When K » 1

When the value of K is very high such as, 107-1015 or more, the
reaction proceeds to almost completion. In such reactions almost the
whole of the reacting substance gets converted into products

When the value of K is greater than one (but not too large), the
reaction in the forward direction is favoured more than the reaction
in the backward direction. In such cases, the equilibrium
concentration of products is higher than that of the reactants

When the value of K is equal to one, both the directions of the
reaction are almost equally favoured. In such cases, the equilibrium
concentration of the reactants and products are comparable.

When the value of K is smaller than one, the reaction in the
backward direction is favoured. In such cases, the equilibrium
concentration of reactants will be much higher than the
concentration of the products