2.  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.

3.  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.

4.  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.

5.  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.

6.  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.

7.  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.

8.  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. Δ GReaction Behavior NegativeProceeds spontaneously to the right exogenic ZeroIs at equilibrium Positive Will not proceed endogenic

9.  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, 10 7 -10 15 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