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Unformatted text preview: III. LE CHATELIERS PRINCIPLE Le Chateliers Principle is a valuable qualitative tool for predicting how, when a system in a state of dynamic equilibrium is disturbed, the system will respond. A statement of Le Chateliers Principle is given below. When a system that is in a state of dynamic equilibrium is disturbed, the system will respond in a way that counteracts (undoes) the disturbance. This principle was used previously to predict an increase in the boiling point or a decrease in the freezing point when a solute is added to a solvent (colligative properties). This principle is also very useful when it comes to disturbing reactions that are in a state of dynamic equilibrium. The first type of disturbance considered here is to add heat or remove heat from the system. This is often equivalent to increasing or decreasing the temperature. When a reaction occurs, heat is either generated or consumed when one mole of reaction (reactants going to products) occurs. This heat is called the heat (enthalpy change) of the reaction ( H Rx ) and has units of kJ/mol. If heat is generated it is called an exothermic reaction ( H Rx <0) and if heat is consumed it is called an endothermic reaction ( H Rx >0). When a reaction is turned around the sign of H Rx changes so that if the forward reaction is exothermic the reverse reaction would be endothermic. Consider the reaction below where the forward reaction is an exothermic reaction....
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