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Equilibrium. The condition of a system in which competing influences are balanced. (Chemistry) The state of a reaction in which the rates of the forward and reverse reactions are the same. First, some definitions:. Eq. = Equilibrium Fwd., Rev., Rxn., Conc., Temp.
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Equilibrium The condition of a system in which competing influences are balanced. (Chemistry) The state of a reaction in which the rates of the forward and reverse reactions are the same.
First, some definitions: • Eq. = Equilibrium • Fwd., Rev., Rxn., Conc., Temp. • [A] = Concentration of A in mol/L or M
What do we know? • We know every rxn has a certain rate. • Some rxns are reversible – they spontaneously proceed from products to reactants. • Fwd rxn • Rev rxn
Initially, there is only Fe3O4 and H2 in the reaction vessel. • As it is heated, Fe and H2O begin to form and the rate of the fwd rxn begins to decrease (Figure 1 – next page) • When there is some Fe and H2O, they begin to reform Fe3O4 and H2, so the rate for the rev rxn begins to increase. (Figure 1 – next page)
Forward Rate Reverse Rate Figure 1 Rate of Reaction Reaction Coordinate (time)
Eventually, the forward rate decreases and the reverse rate increases so that they are equal. At this point, they are at equilibrium. (Figure 2 – next page)
Figure 2 Forward Rate Rate of Reaction Reverse Rate Reaction Coordinate (time)
Equilibrium Concentrations • At equilibrium, for a specific reaction, at a given temperature, there is a set concentration of each reactant and product. • Since these concentrations are the same for each reaction, at a set temperature, we can write an equilibrium constant, Keq
Keq – The Eq. Constant • Consider a generalized reaction: • For this reaction, • Notice that the coefficients become superscripts—they raise the concentration to that power.
HOWEVER! • Pure liquids and solids are removed from the equilibrium constant expression because they have a constant concentration. • So, for: • The eq. constant is:
What does Keq mean?? • If Keq = ~1, then there are about the same amount of reactants and products at equilibrium. • If Keq < 1, then there are more reactants than products at equilibrium. • Therefore, the REVERSE rxn is favored. • If Keq > 1, then there are more products than reactants at equilibrium. • Therefore, the FORWARD rxn is favored.
Review/Tutorial • We’ll review with an online tutorial, Equilibrium.
Le Chatelier’s Principle Or How to Get Away from Stress
Le Chatelier’s Principle • When a system at equilibrium is subjected to a stress, the equilibrium will tend to shift in the direction to relieve the stress. • What kind of “stress” could a chemical reaction be subjected to?? • Change in concentration • Change in temperature • Change in pressure • Change in pressure affects which state of matter? • Adding a catalyst? NO! Not a stress.
What are the effects of each stress? • Change in concentration – system shifts to get back to the same ratio of reactants and products. • Add more N2? • Equilibrium shifts to the right (). End with more NH3 • Add NH3? • Eq. shifts to the left (). End with more N2 and H2. • Take out NH3? • Eq. shifts . End with more NH3.
Change in temperature – system shifts eq. so that heat is equalized. • Raise the temperature • Eq. shifts to absorb the heat • Lower the temperature • Eq. shifts to produce more heat • NOTE: Lowering the temp. may shift the eq. in the direction of the products, but if the temp. is lowered TOO much, what happens to the rxn. rate? • SLOOOOOOOW!!!!
Change in Pressure – System shifts to equalize the number of moles of gas. Raise the pressure Eq. shifts Shift is towards smallest total number of moles of gas, so the eq. shifts away from 4 moles (3 +1) towards 2 moles. Lower the pressure Eq. shifts Shift equalizes pressure. NOTE: This is a famous reaction – the Haber process. It is extremely important in the agricultural industry to make fertilizers.
Change amount of Catalyst? • NO! This does not affect the position of the equilibrium. It speeds the reaction in BOTH directions, so that it reaches equilibrium faster.