Aligned to Classic Chemistry Advanced Placement Laboratory
Le Chatelier’s Principle is a qualitative rule, which allows the prediction of the effect of temperature, pressure and concentration changes on chemical reactions. The principle states: A chemical system at equilibrium when stressed by external forces will adjust in such a way as to minimize that stress. For example when a system is subjected to increased pressure it adjusts so that it will occupy less volume. This offsets the pressure increase. If ice is placed under an increased pressure, it melts because the water obtained from a given mass of ice occupies less volume. In the formation of ammonia (the Haber process) from hydrogen and nitrogen, the product of the reaction (NH3) occupies less volume than the two uncombined gases. The increase in pressure favors the production of ammonia.
This experiment is divided into three separate reactions demonstrating how different types of stress effect equilibrium. Students are asked to predict the outcome of each situation and then prove or disprove their predictions.
1 X 25ml Bromothymol blue pH indicator solution 0.04%
1 X 10g Potassium thiocyanate crystal
1 X 25ml Ferric nitrate solution 0.2M
1 X 200ml Potassium thiocyanate solution 0.002M
1 X 10g Sodium phosphate, dibasic
1 X 25ml Concentrated hydrochloric acid, 36%
2 X 25ml Sodium hydroxide solution 0.1N
1 X 50g Sodium chloride