KEY2CHEM

Le Chatelier's Principle and Reaction Conditions

Le Chatelier’s principle can be used to predict the shift in equilibrium position when a chemical system at equilibrium is exposed to a stress (a change in concentration, pressure/volume, or temperature). Harnessing this shift can increase a desirable outcome, such as increasing the yield of a product.

Example 1.

The Haber-Bosch process is used industrially to synthesize ammonia ($$\require{mhchem}\ce{NH3}$$) from its elements: $$\require{mhchem}\ce{N2(g) + 3 H2(g) <=> 2 NH3(g)} \;\;\;\; \Delta H_{rxn} = -92\text{ kJ/mol}$$. Which set of conditions will result in the greatest yield of ammonia?

A. high pressure, low temperature

B. low pressure, high temperature

C. high pressure, high temperature

Solution

A. high pressure, low temperature

The goal to increase the yield of product is to favor the product side (right side) of the reaction. This means favoring the side with fewer moles of gas; this can be accomplished by increasing the reaction vessel volume, or lowering the reaction pressure. Since the reaction is exothermic, removing heat from the reaction (by lowering the temperature) will shift the equilibrium position toward the product side, increasing the yield of ammonia.

Example 2.

How can the reaction vessel volume be changed to increase the yield of products in the reaction $$\require{mhchem}\ce{CaCO3(s) <=> CaO(s) + CO2(g)}$$?

A. Increase volume

B. Decrease volume

C. No change, since there are solids

Solution

A. Increase volume

Increasing the volume corresponds to a decrease in pressure, which favors the side with the greater moles of gas (the products side).

Example 3.

Complex ions of cobalt have unique colors. $$\require{mhchem}\ce{CoCl4^{ \;2-}}$$ has a blue color while $$\require{mhchem}\ce{Co(H2O)6^{\;2+}}$$ has a pink color. In the following reaction $$\require{mhchem}\ce{CoCl4^{2-}(aq) + 6H2O(l) <=> Co(H2O)6^{2+}(aq) + 4 Cl^{-}(aq)}$$, what color will be primarily observed when excess $$\require{mhchem}\ce{Cl-}$$ is added to an equilibrium mixture?

A. pink

B. blue

C. purple

Solution

B. blue

Addition of $$\require{mhchem}\ce{Cl-}$$ shifts the equilibrium position toward the reactant side, meaning the blue color of  $$\require{mhchem}\ce{CoCl4^{2-}}$$predominates.