Chemical Equilibrium and Le Chatlier's Principle By: Elaina Foster-Allen

Discovery The law of mass action, was actually what led to the chemical equilibrium concept It was discovered in Egypt by Claude Berthollet.

What is Chemical Equilibrium Chemical equilibrium is the condition which occurs when the concentration of reactants and products participating in a chemical reaction exhibit no net change over time. This could Also be called "steady state reaction

Continued Chemical equilibrium applies to reactions that can occur in both directions. In a reaction such as: CH4(g) + H2O(g) CO(g) + 3H2(g)

What Happens The reaction happens both ways. After some products are created the products begin to react. This reaction causes reactants to form Even though the reactants are constantly forming products and vice-versa the amount of reactants and products does become steady.

The Equilibrium Expression The equilibrium expression for a chemical reaction may be expressed in terms of the concentration of the products and reactants. The concentration of liquid and solid does not change. jA + kB → lC + mD

Le Chatelier's Principal Henry-Louis Le Chatelier was a French chemist and engineer. In 1884 he proposed one of the central concepts of chemical equilibria. His Principal states A change in one of the variables that describe a system at equilibrium produces a shift in the position of the equilibrium that counteracts the effect of this change.

What does the principal describe? what happens to a system when something momentarily takes it away from equilibrium.

Three ways in which we can change the equilibrium: conditions of a chemical reaction at (1) changing the concentration of one of the components of the reaction (2) changing the pressure on the system (3) changing the temperature at which the reaction is run.

Example involving change of concentration 2NO(g) + O2(g) 2NO2(g) If you add more NO(g) the equilibrium shifts to the right producing more NO2(g) If you add more O2(g) the equilibrium shifts to the right producing more NO2(g) If you add more NO2(g) the equilibrium shifts to the left producing more NO(g) and O2(g)

Example involving pressure change 2SO2(g) + O2(g) 2SO3(g) an increase in pressure will cause the reaction to shift in the direction that reduces pressure, This is the side with that has the least amount of gas molecules. Therefore an increase in pressure will cause a shift to the right in result more product is produced A decrease in volume is one way of increasing pressure.

Example involving temperature change N2(g) + 3H2(g) 2NH kJ, The increase in temperature will cause a shift to the left because the reverse reaction uses the excess heat. An increase in forward reaction would produce more heat since the forward reaction is exothermic. Therefore the shift caused by a change in temperature depends upon whether the reaction is exothermic or endothermic.

References 1. l.htm l.htm 2. kshop4/4_5.html kshop4/4_5.html 3. mical-Equilibrium.htm mical-Equilibrium.htm 4. view/bp/ch16/lechat.html view/bp/ch16/lechat.html