14.3 LeChatelier Predicting Direction of a Chemical Reaction

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1 14.3 LeChatelier Predicting Direction of a Chemical Reaction Two equal-weight boys at equilibrium on a teeter totter. The boy at the right is given a five-pound weight thereby disturbing the equilibrium. The boy on the left scoots farther back on the teeter totter to restore the equilibrium. Dr. Fred Omega Garces Chemistry 201 Miramar College 1 LeChatlier and Chemical Direction

2 Quotient: Review For reactions not yet at equilibrium, the Law of Mass action yield information in terms of the Reaction Quotient. Consider the following chemical process not at equilibrium. aa + bb D rr + pp A reaction quotient expression can be written: Q = [R]r [P] p [A] a [B] b Where the numerical value of Q will determine the direction the reaction will proceed. Q < K eq Reaction shifts to g Right Q > K eq Reaction shifts to f Left 2 LeChatlier and Chemical Direction

3 Reaction Quotient: Direction of Reaction Consider, T =532 C, K c = 0.19 what direction will the reaction proceed if the initial concentration is change to- N 2 (g) + 3H 2 (g) D 2NH 3 (g) i) 0.30 M 0.20 M 0.10 M Q = [ 0.10M] 2 = [ 0.30M] [ 0.20M ] 3 3 LeChatlier and Chemical Direction

4 LeChatelier Principle: A Review Teeter Totter At Equilibrium Stress applied Self Adjust Re-establish Equilibrium 5 LeChatlier and Chemical Direction

5 Equilibrium: Stress / Relief on Reactant Altering Chemical Concentrations Stress on Reactant, Rxn shift right Relief on Reactant, Rxn shift left 6 LeChatlier and Chemical Direction

6 Equilibrium: Stress / Relief on Product Altering Chemical Concentrations Stress on Product, Rxn shift left Relief on Product, Rxn shift right 7 LeChatlier and Chemical Direction

7 Exothermic Heats of Solution Exothermic Process. Energy is a product Heating a solution in which the ΔH soln is exothermic Energy is a product, ΔH soln ( - ), results in a shift of the reaction to the left, or more solute precipitating out of solution. 8 LeChatlier and Chemical Direction

8 Endothermic Heats of Solution Endothermic Process. Energy is a reactant Heating a solution in which the ΔH soln is endothermic, Energy is a reactant, ΔH soln ( + ), results in a Shift of the reaction to the right, or more solute dissolving in solution. 9 LeChatlier and Chemical Direction

9 Solubilities of Solids Vs. Temperature Solubility of several ionic solid as a function of temperature. Most salts have positive heats of solution. When the salt solution is heated, more solute dissolves. Some salts have negative enthalpy of solution, (exothermic process) i.e., Ce 2 (SO 4 ) 3. When these salt solutions are heated, the solute becomes less soluble. Other example: Mg(OH) 2 and Starch 10 LeChatlier and Chemical Direction

10 2ii) Temperature & Solubility: Gases Temperature - (Gas) Consider the extent in which O 2 or CO 2 dissolves in water. What are the conditions which will increase the solubility of gas in water. As the temperature increase, both solute and solvent will be moving [Solute] D [Solute] gas Solution faster, the gas solute however will now have enough energy to leave the liquid interface Is this an exothermic or endothermic process? 11 LeChatlier and Chemical Direction

11 Gas solute; Exothermic ΔH soln As the temperature increase, both solute and solvent will be move faster. The gas solute however will now have enough energy to leave the liquid interface because IMF can be overcome 12 LeChatlier and Chemical Direction

12 Disaster: (1700 dead) from Gas Solubility Lake Nyos in Cameroon, the site of a natural disaster. In 1986 a huge bubble of CO 2 escaped from the lake and asphyxiated more than 1700 people. In the African nation of Cameroon in 1986 a huge bubble of CO 2 gas escaped from Lake Nyos and moved down a river valley at 20 m/s (about 45 mph). Because CO 2 is denser than air, it hugged the ground and displaced the air in its path. More than 1700 people suffocated. The CO 2 came from springs of carbonated groundwater at the bottom of the lake. Because the lake is so deep, the CO 2 mixed little with the upper layers of water, and the bottom layer became supersaturated with CO 2. When this delicate situation was changed, perhaps because of an earth-quake or landslide, the CO 2 came out of the lake water just like it does when a can of soda is opened. 13 LeChatlier and Chemical Direction

13 The effect of a Change in Temperature in terms of Reaction Quotient and LeChatelier In an endothermic process, energy is a reactant and an increase in temperature results in a shift of the reaction to the right. When the temperature is decreased the reaction shifts to the left. In an exothermic process, energy is a product and an increase in temperature results in a shift of the reaction to the left. When the temperature is decreased the reaction shifts to the right. The question is raised, why does the reaction adjust itself, when according to the Mass Action Expression, the concentrations of chemicals are not altered with temperature changes? What causes the Mass Action Expression not to equal K eq(new) ( k eq(old) ). Consider an Endothermic reaction: E + R D P: K eq = [Prod] / [React]. If the temperature is raised, the reaction shifts to the right, (Q < K eq ) A shift to the right means that [Prod] = h (will raised) and [React] = i (will lowers). This will only occur if the [Prod] / [React] (or Q) is now less than K new the temperature. In other words when the temperature is change (increase T), the equilibrium constant changes, the current [P] / [R] ratio is still equal to the old K eq which is now Q (the reaction quotient) and the reaction shifts to re-establish equilibrium that is to attain the new K eq. 14 LeChatlier and Chemical Direction

14 Temperature Effect (increase) & Reaction Quotient Endothermic Rxn: Increase in Temperature Exothermic Rxn: Increase in Temperature K eq (old Temp) = Q Direction Reaction K eq Direction Reaction K eq (old Temp) = Q When the temperature is raised for an endothermic reaction, the K eq constant changes. Because the current concentrations yields a reaction quotient less than K eq (new) the reaction must shift to the New Temperature When the temperature is raised for an exothermic reaction, the K eq constant changes. Because the current concentrations yields a reaction quotient greater than K eq (new) the reaction must shift to the left.) 15 LeChatlier and Chemical Direction

15 Temperature Effect (decrease) & Reaction Quotient Exothermic Rxn: Decrease in Temperature Endothermic Rxn: Decrease in Temperature K eq (old Temp) = Q Direction Reaction K eq Direction Reaction K eq (old Temp) = Q When the temperature is lowered for an exothermic reaction, the K eq constant changes. Because the current concentrations yields a reaction quotient lower than K eq (new) the reaction must shift to the New Temperature When the temperature is lowered for an endothermic reaction, the K eq constant changes. Because the current concentrations yields a reaction quotient greater than K eq (new) the reaction must shift to the left. 16 LeChatlier and Chemical Direction

16 Temperature Effect on [CoCl 4 ] 2- D [Co(H 2 O)] Consider the reaction: ΔH < 0 [CoCl 4 ] H 2 O D [Co(H 2 O)] + 4Cl - blue pink In this experiment when the solution was placed in cold water, the solution turned vivid pink. The pink color indicates a shift to the right for the reaction shown above. This will only occur if the new K eq at the instant the temperature is altered, is now higher than the old K eq, (which is now called Q). Therefore the reactant concentration decreases, the product concentration increases as the reaction adjust itself so that the Mass action equals K eq : Q g K eq K eq (old Temp) = Q Direction Reaction K eq (new Temp) 17 LeChatlier and Chemical Direction

17 3i) Pressure on Solubility: Solids / Liquid Pressure - (Solid and Liquid) The solubility of solids and liquids are hardly affected by pressure. Solids and liquids are already very close to each other. An increase in pressure will not affect solubility. 18 LeChatlier and Chemical Direction

18 3 ii) Pressure on Solubility: Gas Pressure - (Gas) Solubility of gas is greatly affected by pressure Henry s Law: P A = K H X A Gas solutes are very sensitive to pressure. Because gas particles are separated by void space, an increase in pressure will increase the solubility of the gas. Divers must be careful when diving to great depths because the potential of dissolved N 2 gas in the blood will lead to the bends. 19 LeChatlier and Chemical Direction

19 Pressure Affect: Teeter Totter Analogy [Solute] Pressure Sensitive D [Solute] gas Solution In utilizing LeChatelier Principle to determine the direction of solubility for a gaseous solute with variation in pressure, the first thing that must be establish is which side is more sensitive to pressure. In our case the gas is more sensitive than the solution. 20 LeChatlier and Chemical Direction

20 The effect of a Change Volume of reaction vessel in terms of Reaction Quotient and LeChatelier For a chemical reaction, LeChatelier Principle can be verified in terms of the Reaction Quotient. Consider the reaction: N 2 + 3H 2 D 2NH 3 Let [H 2 ] = M, [N 2 ] = M and [NH 3 ] = M for a 1-L vessel at equilibrium. In which direction will the reaction shift if the reaction vessel is decreased by half such that V = V o /2. The concentrations will now double for all specie at the instant the volume is decreased. Molarity = mol / L, [H 2 ] = M, [N 2 ] =.0804 M and [NH 3 ] = M. Plugging in to the mass action expression and solving for Q, In the reaction quotient equation Q < K eq which means that in order to regain equilibrium, the product must increase and the reactant decrease, a shift to the right in the overall reaction. 21 LeChatlier and Chemical Direction

21 Summary of Pressure, Temperature Affect on Solubility ΔH (s, l or g) Temp Direction Solubility (+) Endothermic h g Prod h increase (+) Endothermic i f React i decrease (-) Exothermic h f React i decrease (-) Exothermic i g Prod h increase Pressure Direction Solubility Gas solute h g Prod h increase Gas solute i f React i decrease 23 LeChatlier and Chemical Direction

22 Example Summary Consider the following system at equilibrium: 6H 2 O (g) + 6CO 2 (g) D 2 C 6 H 12 O 6 (s) + 6O 2 (g) Complete the following table. Indicate changes in moles and concentrations by entering I, D, N, or? in the table. (I = increase, D = decrease, N = no change,? = insufficient information to determine) 24 LeChatlier and Chemical Direction