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# Chemical equilibrium

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Grade 12 science summary presentation for chapter 8: chemical equilibrium

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### Chemical equilibrium

1. 1. 1 Everything Science www.everythingscience.co.za 8. Chemical equilibrium Chemistry Grade 12
2. 2. 2 Everything Science www.everythingscience.co.za Chemical equilibrium A reaction is reversible when reactants can react to form products, and products can react to form the reactants again. A reaction is in chemical equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. In an open system energy and matter can enter and leave the system. In a closed system energy can enter and leave the system, but matter cannot. In a closed system it is possible for reactions to be reversible and so to be in chemical equilibrium. There is a dynamic equilibrium in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. The amounts of reactants and products remain constant.
3. 3. 3 Everything Science www.everythingscience.co.za Equilibrium constant The equilibrium constant (Kc ) relates to the concentration of the reactants and products at equilibrium, and can be calculated using the following expression: Where [C] and [D] are the concentrations of the products and [A] and [B] are the concentrations of the reactants. a, b, c and d are the stoichiometric co-efficients of the reactants and products. We can use a RICE table to help us calculate Kc for any reaction. Kc= [C]c [D]d [ A]a [B]b
4. 4. 4 Everything Science www.everythingscience.co.za The meaning of Kc values A high Kc value means that the concentration of products at equilibrium is high and the reaction has a high yield of the products. A low Kc value means that the concentration of products at equilibrium is low and the reaction has a low yield of the products.
5. 5. 5 Everything Science www.everythingscience.co.za Le Chatelier's Principle Le Chatelier’s Principle states that if an external stress (change in pressure, temperature or concentration) is applied to a system in chemical equilibrium, the equilibrium will change in such a way as to reduce the effect of the stress. Concentration If the concentration of one of the reactants is increased or the concentration of one of the products is decreased the forward reaction is favoured and the equilibrium shifts to the right. If the concentration of one of the reactants is decreased or the concentration of one of the products is increased the reverse reaction is favoured and the equilibrium shifts to the left. Pressure If the pressure is increased the equilibrium will shift to favour a decrease in pressure. If the pressure is decreased the equilibrium will shift to favour an increase in pressure. Temperature If the temperature is increased the equilibrium will shift to favour the reaction which will reduce the temperature. The endothermic reaction is favoured. If the temperature is decreased the equilibrium will shift to favour the reaction which will increase the temperature. The exothermic reaction is favoured.
6. 6. 6 Everything Science www.everythingscience.co.za Factors that influence Kc Although a change in temperature, concentration or total pressure will affect the equilibrium position, only temperature will affect the equilibrium constant Kc . Concentration Changing the concentration of a reactant or product results in one of the reactions (forward or reverse) being favoured. This change in reaction rate minimises the effect of the change and restores the concentration ratio between reactants and products. There will just be more reactants and products. Kc will remain the same. Pressure Changing the pressure of the system will change the ratio between the reactant and product concentrations. The equilibrium then shifts to minimise the effect of the change and restores the ratio between reactant and product concentrations. Kc will remain the same. Catalyst Both the forward and reverse reactions rates are increased. Therefore the ratio between reactant and product concentrations will remain the same. Kc will remain the same. Temperature Changing the temperature will favour either the endothermic or exothermic reaction. The ratio between the concentration of the reactants and products will change. Kc will change.
7. 7. 7 Everything Science www.everythingscience.co.za Rate-time graphs A change in temperature or adding/removing a catalyst changes both the rate of the forward and reverse reactions. Adding a catalyst or removing a catalyst will change both rates equally while a change in temperature will affect the forward and reverse reactions to different extents. A change in pressure of the system or concentration of reactants/products will only affect one rate.
8. 8. 8 Everything Science www.everythingscience.co.za Concentration-time graphs A change in concentration of a reactant or product will cause a sharp increase or decrease in the rate of that compound. The concentrations of the other reactants and products will not change much. A change in temperature will affect the forward and reverse rates equally but in opposite directions. A change in pressure of the reaction would cause a sharp increase or decrease in all the reactants and products. The addition of a catalyst would increase both the forward and reverse reaction rates, meaning the equilibrium is reached faster.
9. 9. 9 Everything Science www.everythingscience.co.za For more practice see: everythingscience.co.za Shortcode: ESCNY
10. 10. 9 Everything Science www.everythingscience.co.za For more practice see: everythingscience.co.za Shortcode: ESCNY