General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1 Chapter 9 Chemical Equilibrium 9.5 Changing Equilibrium Conditions: Le Châtelier’s Principle

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 2 Le Châtelier’s Principle Le Châtelier’s principle states that  any change in equilibrium conditions upsets the equilibrium of the system  a system at equilibrium under stress will shift to relieve the stress  there will be a change in the rate of the forward or reverse reaction to return the system to equilibrium

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 3 Adding Reactant For the reaction A + B C at equilibrium,  adding more A upsets the equilibrium  the rate of forward reaction increases to re-establish K c A + B C

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 4 Effect of Adding Reactant Consider the following reaction at equilibrium. H 2 (g) + F 2 (g) 2HF(g)  If more reactant (H 2 or F 2 ) is added, there is an increase in the number of collisions.  The rate of the forward reaction increases and forms more HF product until new equilibrium concentrations equal K c again.  The effect of adding a reactant shifts the equilibrium toward the products.

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 5 Effect of Adding Product Consider the following reaction at equilibrium. H 2 (g) + F 2 (g) 2HF(g)  When more HF is added, there is an increase in collisions of HF molecules.  The rate of the reverse reaction increases and forms more H 2 and F 2 reactants.  The effect of adding a product shifts the equilibrium toward the reactants.

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 6 Adding Reactant or Product The equilibrium shifts toward  products when H 2 (g) or F 2 (g) is added  reactants when HF(g) is added H 2 (g) + F 2 (g) 2HF(g) Add H 2 or F 2 Add HF

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 7 Effect of Removing Reactant Removing reactant, H 2 (g) or F 2 (g), from the following reaction at equilibrium: H 2 (g) + F 2 (g) 2HF(g)  decreases the collisions between reactants  decreases the rate of the forward reaction  shifts the equilibrium toward the reactants H 2 (g) + F 2 (g) 2HF(g) Remove H 2 or F 2

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 8 Effect of Removing Product Removing HF(g) from the following reaction at equilibrium: H 2 (g) + F 2 (g) 2HF(g)  decreases the number of collisions between products  decreases the rate of the reverse reaction  shifts equilibrium toward the products H 2 (g) + F 2 (g) 2HF(g) Remove HF

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 9 Concentration Changes and Equilibrium

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. Effect of a Catalyst Adding a catalyst  lowers the activation energy of the forward reaction  increases the rate of the forward reaction  lowers the activation energy of the reverse reaction  increases the rate of the reverse reaction  decreases the time to reach equilibrium  has no effect on the equilibrium 10

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 11 Learning Check Predict any shift in equilibrium for each of the following changes on the reaction NH 4 HS(s) H 2 S(g) + NH 3 (g) 1) to products 2) to reactants 3) no change A. H 2 S(g) is added. B. NH 4 HS(s) is added. C. NH 3 (g) is removed. D. A catalyst is added.

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 12 Solution Predict any shift in equilibrium for each of the following changes on the reaction NH 4 HS(s) H 2 S(g) + NH 3 (g) 1) to products 2) to reactants 3) no change A. 2 H 2 S(g) is added. B. 3 NH 4 HS(s) is added. C. 1 NH 3 (g) is removed. D. 3 A catalyst is added.

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 13 Effect of Decreasing the Volume When a reaction at equilibrium contains different numbers of moles of reactants than products, a decrease in volume  increases the concentrations (mole/L), upsetting the equilibrium  shifts the equilibrium toward the fewer number of moles N 2 (g) + 3H 2 (g) 2NH 3 (g) Decrease volume More molesFewer moles

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 14 Volume Decrease and Equilibrium A volume decrease  shifts the equilibrium toward the side (A) with the smaller number of moles

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 15 Effect of Increasing the Volume When a reaction at equilibrium contains different numbers of moles of reactants than products, an increase in volume  decreases the concentrations (mole/L), upsetting the equilibrium  shifts the equilibrium toward the greater number of moles N 2 (g) + 3H 2 (g) 2NH 3 (g) Increase volume More moles Fewer moles

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 16 Volume Increase and Equilibrium A volume increase  shifts the equilibrium toward the side (B and C) with the greater number of moles

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 17 Heat and Endothermic Reactions For an endothermic reaction at equilibrium,  a decrease in temperature (T) removes heat, and the equilibrium shifts toward the reactants  an increase in temperature adds heat, and the equilibrium shifts toward the products. CaCO 3 (s) kcal CaO(s) + CO 2 (g) Decrease T Increase T

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 18 Equilibrium Shift in Endothermic Reactions with Temperature

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 19 Heat and Exothermic Reactions For an exothermic reaction at equilibrium,  a decrease in temperature removes heat, and the equilibrium shifts toward the products  an increase in temperature adds heat, and the equilibrium shifts toward the reactants. N 2 (g) + 3H 2 (g) 2NH 3 (g) + 22 kcal Decrease T Increase T

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 20 Equilibrium Shifts with Temperature in Exothermic Reactions

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 21 Effects of Changes on Equilibrium

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 22 Learning Check Indicate if each change on a reaction at equilibrium shifts 2NO 2 (g) + heat 2NO(g) + O 2 (g) 1) towards products2) towards reactants 3) no change A. adding NO(g) B. adding N 2 (g) C. raising the temperature D. removing O 2 (g) E. increasing the volume

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 23 Solution Indicate if each change on a reaction at equilibrium shifts 2NO 2 (g) + heat 2NO(g) + O 2 (g) 1) towards products2) towards reactants 3) no change A. 2 adding NO(g) B. 1 adding NO 2 (g) C. 2 lowering the temperature D. 1 removing O 2 (g) E. 1 increasing the volume