Le Châtelier's Principle
Source: Laboratory of Dr. Lynne O'Connell — Boston College
When the conditions of a system at equilibrium are altered, the system responds in such a way as to maintain the equilibrium. In 1888, Henri-Lewis Le Châtelier described this phenomenon in a principle that states, "When a change in temperature, pressure, or concentration disturbs a system in chemical equilibrium, the change will be counteracted by an alteration in the equilibrium composition."
This experiment demonstrates Le Châtelier's principle at work in a reversible reaction between iron(III) ion and thiocyanate ion, which produces iron(III) thiocyante ion:
Fe3+(aq) + SCN- (aq) 
FeSCN2+ (aq)
The concentration of one of the ions is altered either by directly adding a quantity of one ion to the solution or by selectively removing an ion from the solution through formation of an insoluble salt. Observations of color changes indicate whether the equilibrium has shifted to favor formation of the products or the reactants. In addition, the effect of a temperature change on the solution at equilibrium can be observed, which leads to the ability to conclude whether the reaction is exothermic or endothermic.
1. Preparation of the Iron(III) Thiocyanate Equilibrium Solutions
- Place 1 drop of 1 M Fe(NO3)3 solution in a test tube and dilute with 2 mL of water. Place 1 drop of 1 M KSCN in another test tube and dilute with 2 mL of water. These two test tubes serve as controls to compare against the other test tubes.
- Place 1 drop of 1 M Fe(NO3)3 solution in a test tube.
- Add 1 drop of 1 M KSCN to the test tube.
- Add 16 mL of water to the te
Observations of the initial solutions and the mixture of the two solutions can be seen in Table 1.
Observations of the equilibrium mixtures upon addition of various reagents can be seen in Table 2.
Observation when the temperature is changed: In test tube 7, the solution turns more orange in color (less red, more yellow) when heated.
Le Châtelier's principle is at work in human bodies. Oxygen is transported from the lungs to muscle and other tissues by a protein called hemoglobin (Hb) that is found in the blood. The oxygen molecule binds to this protein in a reversible reaction that can be described by an equilibrium equation:
Hb + 4 O2 Hb(O2)4
In the lungs, the partial pressure
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