Describe how to use le chatelier's principle to predict the possible ways a chemical system can respond to changes.

According to Le Chatelier's Principle," when a system at equilibrium is subjected to any external stress by changing concentration, pressure or temperature, it will modify itself in such a way so as to minimize the effect of that change made in it".

Change in Concentration:
Suppose following reaction at equilibrium,

                                             A  +  B    ⇆    C

When the concentration of reactants (A or B) is increased the equilibrium will shift in forward direction and will produce more product in order to cancel out the effect of extra reactant added. Or, if product (C) is added then it will respond in opposite direction.

Change in Pressure:
Suppose following reaction at equilibrium,

                                             A  +  B    ⇆    C

If pressure is applied on this equilibrium, the equilibrium will shift in forward direction because there  is a decrease in volume at product side (1 mole as compare to reactants which are 2 moles). And the opposite case will be observed if the pressure is decreases.

Change in Temperature:
Suppose following reaction at equilibrium,

                                             A  +  B    ⇆    C  + Heat

This is an exothermic reaction (heat is evolved) and the heat produced is in the product side. When the temperature is increased the equilibrium will shift in the backward direction. And decrease in temperature will shift the equilibrium in forward direction.

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.

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