Learn what Le Chatelier’s Principle is and how it can be applied to predict the effect of a change in conditions on chemical equilibrium.
Le Chatelier’s Principle is the name given to the principle in which a change in a chemical system prompts an opposing reaction. In chemistry, this principle was discovered independently by Henry Louis Le Chatelier and Karl Ferdinand Braun, so it is sometimes called the Le Chatelier-Braun Principle. The principle can be stated as follows:
If the temperature, concentration, volume, or partial pressure of a chemical system at equilibrium changes, then the equilibrium shifts to compensate for the change.
The more general form of the principle applies to other disciplines. Homeostasis and Lenz’s law are examples. Le Chatelier’s Principle is known by the same name when applied to economic equilibrium.
Le Chatelier’s Principle is used to predict how a change in pressure, volume, concentration, or temperature will affect chemical equilibrium. Knowing the impact on equilibrium allows chemists to manipulate the chemical reaction. For example, a chemist might apply Le Chatelier’s Principle to maximize yield from a reaction.
Fischer esterification can be a time-consuming process, requiring days for a reaction to reach equilibrium. In this article, researchers developed a way to hasten this process by using a specially designed microwave to heat the reaction quickly and evenly and at an increased pressure. In order to test the efficiency of the device, they synthesized 2-ethylhexyl benzoate from benzoic acid and 2-ethylhexanol as shown below.
Sulfuric acid as well as para-toluene sulfonic acid (PTSA) were used to catalyze the reaction. In order to shift the reaction towards the products, a large excess of 2-ethylhexanol was used and the water produced was constantly removed. One of the disadvantages of Fischer esterification is that dehydration can also occur, resulting in unwanted ether and alkene products. Because of this, the temperature and catalyst concentration must be carefully monitored. The researchers were able to show that microwave heating causes no ill effects on the reaction and reduces the time required to a matter of minutes while still producing a high level of the desired product.
A Breathalyzer makes use of the fact that alcohols (in this case ethanol) oxidize into carboxylic acids. It uses the strong oxidizing agent Potassium dichromate in a yellow solution of sulfuric acid, under the presence of a Silver Nitrate catalyst, to complete the reaction quickly. As ethanol oxidizes and the Potassium dichromate reacts, the chromate ion changes from Cr (VI) to Cr (III). This causes the color intensity of the yellow solution to decrease, and a spectrophotometer in the breathalyzer compares the absorbance of this solution with that of an unreacted solution
Using Beer’s law, the spectrophotometer can relate concentration to absorbance levels of the chromium ion. The amount of alcohol present is proportional to the stoichiometric coefficients. An actual breathalyzer only needs to detect 25 micrograms of ethanol to give a reading 0.10 Blood Alcohol Level.
