NC Standards. NC Standards Chm Infer the shift in equilibrium when a stress is applied to a chemical system (LeChatelier s).

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1 Equilibrium and Le Chatelier s Principle NC Standards Chm.3.1. Explain the conditions of a system at equilibrium. A. Define chemical equilibrium for reversible reactions. B. Distinguish between equal rates and equal concentrations. C. Explain equilibrium expressions for a given reaction. D. Evaluate equilibrium constants as a measure of the extent that the reaction precedes to completion. NC Standards Chm Infer the shift in equilibrium when a stress is applied to a chemical system (LeChatelier s). A. Determine the effects of stresses on systems at equilibrium. (Adding/ removing a reactant or product; adding/removing heat; increasing/decreasing pressure) B. Relate the shift that occurs in terms of the order/disorder of the system. 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 Chemical Equilibrium Reversible Reactions: A chemical reaction in which the products can react to re-form the reactants Chemical Equilibrium: When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged HgO(s) Hg(l) + O ( Arrows going both directions ( ) indicates equilibrium in a chemical equation 1

2 Fe 3O ( s) 3Fe( s) O( NOTE: Ms. Siddons will draw arrows correctly on board. Initially, there is only Fe 3 O and H in the reaction vessel. As it is heated, Fe and H O begin to form and the rate of the fwd rxnbegins to decrease (Figure 1 next page) When there is some Fe and H O, they begin to reform Fe 3 O and H, so the rate for the rev rxn begins to increase. (Figure 1 next page) Rate of Reaction Figure 1 Forward Rate Reverse Rate 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 next page) Rate of Reaction Forward Rate Reverse Rate Figure Reaction Coordinate (time) Equilibrium Concentrations N + 3H NH 3 At equilibrium, for a specific reaction, at a given temperature, there is a set concentration values of each reactant and product. Since these concentrations are the same for each reaction, at a set temperature, we can write an equilibrium constant, K eq According to the equation for the ammoniaproducing reaction, mol of ammonia should be produced when 1 mol of nitrogen and 3 mol of hydrogen react. However, when the reaction reaches a state of equilibrium, less than mol of ammonia are created.

3 The Law of Chemical Equilibrium In 186, chemists proposed and developed the law of chemical equilibrium, which states that at a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value. 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, K eq 13 K eq The Eq. Constant Consider a generalized reaction: For this reaction, ma + nb pc + qd K eq [ C] [ A] Notice that the coefficients become superscripts they raise the concentration to that power. p m [ D] [ B] q n HOWEVER! Pure liquids and solids are removed from the equilibrium constant expression because they have a constant concentration. So, for: Fe 3O ( s) 3Fe( s) O( The eq. constant is: [ Fe] [ H O] 3 K eq [ Fe3O ][ H ] [ H [ H O] ] What is K eq for this equation? H + I HI( [ HI] K eq [ H ][ I ] WARNING: Be Careful! The equilibrium constant expression is the ratioof the molar concentrations of the PRODUCTS tothe molar concentrations of the REACTANTS with each concentration raised to a power equal to its coefficient in the balanced chemical equation. In other words, the NUMERATOR PRODUCTS, and the DENOMINATOR REACTANTS 18 3

4 Interpreting the size of K eq Recall that a fraction with a numerator greater than its denominator has a value greater than 1. A fraction with a numerator less than its denominator has a value less than 1. For example, compare the ratios 5/1 and 1/5. Five is a larger number than one-fifth. Because the PRODUCT concentrations are in the numerator of the equilibrium expression, a numerically large K eq means that the equilibrium mixture contains more products than reactants. Similarly, a K eq less than 1 means that the equilibrium mixture contains more reactants than products. K eq >1: Products are favored at equilibrium K eq <1: Reactants are favored at equilibrium What does K eq mean?? If K eq ~1, then there are about the same amount of reactants and products at equilibrium. If K eq < 1, then there are more reactants than products at equilibrium. Therefore, the REVERSE rxn is favored. If K eq > 1, then there are more products than reactants at equilibrium. Therefore, the FORWARD rxn is favored. 19 LeChatelier s Principle Or How to Get Away from Stress When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress. Henry Le Chatelier 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. Le Chatelier Translated: When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you ve taken away. When you add something to a system at equilibrium, the system shifts in such a way as to use up what you ve added. What are the effects of each stress? Change in concentration system shifts to get back to the same ratio of reactants and products. N + 3H NH 3( Add more N? Equilibrium shifts to the right ( ). End with more NH 3 Add NH 3? Eq. shifts to the left ( ). End with more N and H. Take out NH 3? Eq. shifts. End with more NH 3.

5 3 SO + O SO + HEAT 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. 3H + N NH 3 Raise the pressure Eq. shifts Shift is towards smallest total number of moles of gas, so the eq. shifts away from moles (3 +1) towards 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. What about a catalyst? 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. LeChatelier Example #1 A closed container of ice and water at equilibrium. The temperature is raised. Ice + Energy Water the right to use up the added energy. LeChatelier Example # A closed container of N O and NO at equilibrium. NO is added to the container. N O ( + Energy NO ( the left to use up the added NO. LeChatelier Example #3 A closed container of water and its vapor at equilibrium. Vapor is removed from the system. water + Energy vapor the right to replace the vapor. 5

6 LeChatelier Example # A closed container of N O and NO at equilibrium. The pressure is increased. N O ( + Energy NO ( the left to lower the pressure, because there are fewer moles of gas on that side of the equation. 6