| Predicting a chemical reaction |
| In 1888, a French Chemist, Le Chatelier stated a qualitative rule, which enables one to predict the effect of temperature, pressure and concentration for equilibrium chemical reactions. |
| The principle states that whenever a system in equilibrium is subjected to changes in temperature, pressure and concentration, the equilibrium shifts in such a direction so as to nullify or cancel the change made. (This is also known as Le Chaletier's principle) |
| This helps the chemist to predict the favorable conditions for carrying out the reaction. |
| Effect of temperature: If forward reaction is exothermic, the backward reaction is endothermic. An exothermic reaction is favored by decreasing the temperature but at very low temps the rate may become very low. So, an optimum temperature is achieved. |
| Effect of pressure: This is valid for only gaseous phase reactions. |
| At constant temperature, a reaction, which is accompanied by a decrease in volume, will be favored by increasing the pressure. |
| For example |
 |
| At constant temperature, the reverse reaction, which is accompanied by an increase in volume, will be favored by lowering of the pressure. |
| Effect of concentration: Yield of the product increases by increasing the concentration of any one of the reacting substances. Similarly the decomposition of the product is favored by increasing the concentration of anyone of the products. |
| Previous | Next |
