Energy and equilibrium Reversible reactions and Dynamic Equilibrium Reversible reactions In some chemical reactions, the products of the reaction can react to produce the original reactants. These reactions are called reversible reactions and can be represented: A + B C + D The direction of the reversible reaction can be changed by changing the conditions, such as the temperature, e.g. Ammonium chloride ammonia + hydrogen chloride If a reversible reaction is exothermic in one direction it is endothermic in the opposite direction. The same amount of energy is transferred in either direction, e.g. Hydrated copper sulfate anhydrous copper sulfate + water (HT ONLY) The effect of changing conditions on equilibrium The relative amounts of reactants and products at equilibrium depend on the conditions of the reaction. Le Chatelier’s Principle states that “if a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract that change.” The effect of changing concentration If the concentration of a reactant is increased, the system is no longer at equilibrium, and according to Le Chatelier’s principle, the system will respond by reducing the concentration of reactants again; more products will be made until equilibrium is reached again. If the concentration of a product is decreased, the system is no longer at equilibrium, and according to Le Chatelier’s principle, the system will respond by increasing the amount of product; more reactants will react until equilibrium is reached again. heat cool Pressure and equilibrium forward reaction produces more molecules of gas….. forward reaction produces fewer molecules of gas….. …an increase in pressure decreases the amount of product formed. …an increase in pressure increases the amount of product formed. …a decrease in pressure increases the amount of product formed. …a decrease in pressure decreases the amount of product formed. Equilibrium When a reversible reaction happens in a sealed container, the reactants and products cannot escape. An equilibrium occurs when the forward and backwards reactions happen at exactly the same rate. Overall there is no change in the amount of the reactants or products. In the picture, the boy is running up the escalator at the same speed as the escalator is moving down, and overall there is no change in his position on the stairs: this is a model of a dynamic equilibrium. Energy and equilibrium if the forward reaction is exothermic… if the forward reaction is endothermic… …an increase in temperature decreases the amount of products formed. …an increase in temperature increases the amount of products formed. …a decrease in temperature increases the amount of products formed. …a decrease in temperature decreases the amount of products formed. Key words: reversible reaction, equilibrium, conditions, exothermic, endothermic, reactant, product