PRINCIPLES OF CHEMICAL EQUILIBRIUM

1. PRINCIPLES OF CHEMICAL EQUILIBRIUM The Reaction Quotient, Q Altering Equilibrium Conditions: Le Châtelier’s Principle Equilibrium Calculations PRESENTED BY: Sherrylin C. Ompoc

2. THE REACTION QUOTIENT,Q:PREDICTING THE DIRECTION OF A NET REACTION The ability to predict the direction of net charge in establishing equilibrium is important to us for two reasons: • At times we do not need detailed equilibrium calculations. We may only need a qualitative description of the changes that occur in establishing equilibrium from a given set of initial conditions.

3. THE REACTION QUOTIENT,Q:PREDICTING THE DIRECTION OF A NET REACTION • In some equilibrium calculations it is helpful to determine the direction of net charge as a first step. Reaction Quotient, Qc [G]g [H]h … [A]a [B]b … Qc =

4. THE REACTION QUOTIENT,Q:PREDICTING THE DIRECTION OF A NET REACTION • If Qc=Kc , a reaction is at equilibrium. • If Qc<Kc,a net reaction proceeds from left to right ( the forward rxn). • If Qc>Kc ,a net reaction proceeds from right to left ( the reverse direction).

5. THE REACTION QUOTIENT,Q:PREDICTING THE DIRECTION OF A NET REACTION Qc=0 <Kc= Kc> Kc=infinity Reaction Proceeds: to the right to the left Initial Condition: Pure Reactants “left” of Equilibrium Equilibrium “right” of Equilibrium Pure Products

6. THE REACTION QUOTIENT,Q:PREDICTING THE DIRECTION OF A NET REACTION • See Example 16-5 Predicting the Direction of a Net Chemical Reaction in Establishing Equilibrium

7. Altering Equilibrium Conditions:LE CHÂTELIER’S PRINCIPLE LE CHÂTELIER’S PRINCIPLE • states that an action that tends to change the temperature,pressure, or concentrations of reactant in a system at equilibrium stimulates a response that partially offsets the change while a new equilibrium condition is established.

8. Altering Equilibrium Conditions:LE CHÂTELIER’S PRINCIPLE • Effect of Changing the amounts of Reacting Species on Equilibrium See Example 16-6 Applying Le Châtelier's Principle: Effect of Adding More of a Reactant to an Equilibrium Mixture • Effect of Changes in Pressure or Volume on Equilibrium

9. Altering Equilibrium Conditions:LE CHÂTELIER’S PRINCIPLE There are three ways we can change the pressure of a constant-temperature equilibrium mixture. • Add or remove a gaseous reactant or product from an equilibrium mixture. • Add an inert gas to the constant-volume reaction mixture. • Change the pressure by changing the volume of the system.

10. Altering Equilibrium Conditions:LE CHÂTELIER’S PRINCIPLE Effect of Changes in Pressure or Volume on Equilibrium • When the volume of an equilibrium mixture of gases is reduced, a net reaction occurs in the direction producing fewer moles of gases. When the volume is increased, a net reaction occurs in the direction producing more moles of gases. See Example 16-7 Applying Le Châtelier's Principle: The Effect of Changing Volume

11. Altering Equilibrium Conditions:LE CHÂTELIER’S PRINCIPLE • Effect of temperature on Equilibrium Raising the temperature of an equilibrium mixture shifts the equilibrium condition in the direction of the endothermic reaction. Lowering the temperature causes a shift in the direction of the exothermic reaction. See Example 16-8 Applying Le Châtelier's Principle: Effect of Temperature on Equilibrium

12. Altering Equilibrium Conditions:LE CHÂTELIER’S PRINCIPLE • Effect of a Catalyst on Equilibrium Note:The presence of a catalyst does not change the numerical value of the equilibrium constant. • The function of a catalyst is to change the mechanism of a reaction to one having a lower activation energy. • A catalyst has no effect on the condition of equilibrium in a reversible reaction.

13. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES SEE Example 16-9 Determining a Value of Kc from the Equilibrium Quantities of Substances Example 16-10 Determining a Value of Kc from Initial and Equilibrium Amounts of Substances: Relating Kc and Kp.

14. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES SEE Example 16-11 Determining Equilibrium Partial and Total Pressures from a Value of Kp. Example 16-12 Calculating Equilibrium Concentrations from Initial Conditions.

15. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES SEE Example 16-13 Using the Reaction Quotient, Qc in an Equilibrium Calculation.

16. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES Collection of “Comments” as the basic methodology of equilibrium calculations: • The quantities required in an equilibrium constant expression,Kc, are equilibrium concentrations in moles per liter, not simply equilibrium amounts in moles or masses in grams. You will find it helpful to organize all the equilibrium data and carefully label each item.

17. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES 2. The chemical reaction for a reversible reaction serves both to establish the form of the equilibrium constant expression and to provide the conversion factors (stoichiometric factors) to relate the equilibrium quantity of one species to those of the others. 3. For equilibria involving gases you can use either Kc andKp. In general, if the data given involve amount of substances and volumes, it is easier to work with Kc.

18. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES If data are given as partial pressures, then work with Kp. Whether working with Kc andKp or the relationship between them, you must always base these expressions on the chemical equation that is given, not on what you may have used in other situations.

19. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES 4. When using Kp expressions, look for relationships among partial pressures of the reactants. If you need to relate the total pressures of the reactants, you should be able to do this with some equations presented in Chapter 6 (e.g., Equations 6.15, 6.16, and 6.17)

20. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES • When you need to introduce an algebraic unknown, x, into an equilibrium calculation, • Introduce x into the ICE set-up in the row labeled “change”; • Decide which change to label as x, the amount of a reactant consumed or of a product formed. Usually, we base this on the species that has the smallest stoichiometric coefficient in the balanced chemical equation;

21. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES • Use stoichiometric factors to relate the other changes to x (i.e., 2x,3x,…); • Consider that equilibrium amounts = initial amounts + “change.” (If you have assigned the correct signs to the changes, then equilibrium amounts will also be correct.)

22. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES • It is sometimes helpful to compare the reaction quotient,Qc (or Qp), to the equilibrium constant, Kc (or Kp), to determine the direction of the net reaction. • In many equilibrium calculations-often those in aqueous solutions-you can work with molarities directly, without having to work with moles of reactants and solution volumes.

23. Equilibrium Calculations:SOME ILLUSTRATIVE EXAMPLES 8. Where possible, check your calculation, for instance by substituting calculated equilibrium concentrations into the reaction quotient, Qc (or Qp), to see if its numerical value is close to that of Kc (or Kp),

24. The End… Thank you for Listening God Bless Us All !!!