LeChâtelier's Principle

• When a stress is placed on a system at equilibrium, the reaction will shift to counteract the effect of the stress in order to return to equilibrium.

There are 4 variables which can be changed to effect a system at equilibrium

1. Concentration
2. Pressure
3. Volume
4. Temperature

Let's follow the effects of these variables on the Haber Process Reaction

• Haber Process - An industrial process used to make ammonia gas from nitrogen and hydrogen gas.

The negative H indicates that this reaction is exothermic, that is, heat is released in this reaction.

Concentration

1. If the concentration of a reactant or product is INCREASED, the reaction will shift toward the opposite side of where the increase occurs.  This causes the added substance to be used up.

     Add N₂ (reactant) shift RIGHT (toward products)

     Add H₂ (reactant) shift RIGHT (toward products)

   Add NH₃ (product) shift LEFT (toward reactants)

2. When the concentration of a reactant or product is DECREASED, the reaction will shift toward the same side as where the decrease occurs.  This causes more of that substance to be produced to replace what was removed.

   Remove N₂ (reactant) shift LEFT (toward reacants)

   Remove H₂ (reactant) shift LEFT (toward reactants)

   Remove NH₃ (product) shift RIGHT (toward products)

Pressure

1. If the pressure of a system is INCREASED, the reaction will shift toward the side with fewer moles of GAS.
2. If the pressure of a system is DECREASED, the reaction will shift toward the side with more moles of GAS.

   If the pressure of the system with the above reaction increases, the reaction will shift RIGHT.

   • There are 4 moles total of gas reactants (1 N₂ + 3 H₂) and there are 2 moles total of gas products (2 NH₃).  Since the products side has fewer moles of GAS, the reaction will shift toward the direction of the products.

   If the pressure of the system with the above reaction decreases, the reaction will shift LEFT.

   • The reactants side has more moles of gas, so the reaction will shift toward the direction of the reactants.
     
3. If the moles of gas reactants is equal to the moles of gas reactants, a change in pressure will have NO EFFECT on the equilibrium.

Volume

1. If the volume in which a gas reaction takes place is INCREASED, the reaction will shift toward the side with more moles of GAS.  According to Boyle's Law, the volume of a gas is inversely proportional to the pressure of a gas.  Therefore, increasing the volume has the same effect as decreasing the pressure.
2. If the volume in which a gas reaction takes place is DECREASED, the reaction will shift toward the side with fewer moles of GAS.  Again, according to Boyle's Law, decreasing the volume has the same effect as increasing the pressure.

   If the volume of the system with the above reaction increases, the reaction will shift LEFT.

   • There are 4 moles total of gas reactants (1 N₂ + 3 H₂) and there are 2 moles total of gas products (2 NH₃).  Since the reactants side has more moles of GAS, the reaction will shift toward the direction of the reactants.

   If the volume of the system with the above reaction decreases, the reaction will shift RIGHT.

   • The products side has fewer moles of GAS, so the reaction will shift towad the direction of the products.
     
3. If the moles of gas reactants is equal to the moles of gas reactants, a change in volume will have NO EFFECT on the equilibrium.

Temperature

The effect of temperature on a system at equilibrium depends upon whether the reaction is endothermic or exothermic.

Endothermic Reactions - (POSITIVEH)

• A process in which a system absorbs heat from its surroundings.

Since endothermic reactions absorb heat, this means that heat is added to the system.  In other words, heat can be thought of as a reactant.  Changes in temperature can be thought of as changes in the concentration of heat, so the effects are the same as the ones explained in the concentration section. 

1. If the temperature of an endothermic reaction is increased, the reaction will shift RIGHT.
2. If the temperature of an endothermic reaction is decreased, the reaction will shift LEFT.

Exothermic Reactions - (NEGATIVE H)

• A process in which a system releases heat to its surroundings.

Since exothermic reactions release heat, this means that heat is given off to the surroundings.  In other words, heat can be thought of as a product.  Again, changes in temperature can be thought of as changes in the concentration of heat.

1. If the temperature of an exothermic reaction is increased, the reaction will shift LEFT.
2. If the temperature of an exothermic reaction is decreased, the reaction will shift RIGHT.

This reaction can be re-written with heat as a product:

In this reaction, if the temperature is increased, the reaction will shift LEFT.  If the temperature is decreased, the reaction will shift RIGHT.

NOTE:  Some equilibrium constants (K_c and K_p) change with temperature.

Next:  "Solubility Equilibrium"