Chemical Equilibrium

1. Chemical Equilibrium Chapter 18

2. Chemical Equilibrium • Section 18-1 • Pp. 589 - 591

3. Equilibrium is… • Equilibrium is not static • Opposing processes occur at the same time and at the same rate • Rate of forward reaction = Rate of reverse reaction • For example 1: water at 0.0°C • At 0.0, some liquid H2O is freezing and some ice (solid) H2O is melting. Their rates are equal, so equilibrium • For example 2: adding sugar molecules into water • At equilibrium, the rate of sugar molecules in solution crystallizing equals the rate of sugar crystals dissolving

4. Reversible Reactions • Every reaction can proceed: • Reactants  Products OR • Reactants  Products • Def. – chem rxn in which products can react to re-form reactants • 2HCl  H2 (g) + Cl2 (g) • Here most HCl is decomposing into hydrogen gas & chlorine gas • However, some H2 and Cl2 are synthesizing into HCl • At equilibrium, these rates are equal

5. Conclusion • A reversible chemical reaction is in chemical equilibrium when rate of forward reaction = rate of reverse reaction • The concentrations of the reactants and products are static • [Reactants] DOES NOT = [Products] !!! • Usually see double arrows to indicate reversibility:

6. Equilibrium lies to the right? • Some reactions “favor” the formation of products • At equilibrium: higher concentrations of Products than of Reactants • [Products] > [Reactants] • Sometimes the forward arrow will be longer than the reverse arrow, to indicate “product favored” • Some reactions “favor” the formation of reactants • At equilibrium: [Products] < [Reactants] • Sometimes the reverse arrow will be longer than the forward arrow, to indicate “reactant favored”

7. Equilibrium Constant • Given the general reaction: nA + mB  xC + yD • Initially, there is only A & B but no C or D, so the forward reaction rate is at its maximum. • Over time, C + D accumulate, so forward rate slows & reverse rxn rate increases • Eventually the two rates become equal to each other • Equilibrium !!!

9. Equilibrium Constant • After equilibrium is reached, the individual concentrations of A, B, C, and D undergo no further change if conditions remain the same. • A ratio of their concentrations should also remain constant. • The equilibrium constant is designated by the letter K.

10. The constant K is independent of the initial concentrations. • K is dependent on the temperature of the system. The Equilibrium Constant • The numerical value of K for a particular equilibrium system is obtained experimentally.

11. If the value of K is small, the reactants are favored. • A large value of K indicates that the products are favored. • Only the concentrations of substances that can actually change are included in K. • Pure solids and liquids are omitted because their concentrations cannot change.

12. Determining Keq for Reaction at Chemical Equilibrium

13. Shifting Equilibrium • Section 18.2 • Pp. 598 - 601

14. Le Chatelier’s Principle • A system is happily at equilibrium, then a change in concentration, pressure, or temperature occurs • What will happen to the equilibrium? • Def. – if a system in equilibrium is stressed, then the equilibrium is shifted in a way to relieve that stress • 3 main Stressors: pressure, concentration, and temperature

15. Pressure Change • N2(g) + 3H2(g)  2 NH3(g) • What does the (g) stand for? • How many moles of gas on the left? On the right? • So if there was a pressure INCREASE: • Causes an increase in gas concentration • Which side will experience the biggest change in gas pressure (concentration)? • How would the system relieve the stress? • Shifts toward side with fewer moles!! • Pressure increase = shift to side with fewer moles

16. Pressure Change (Page 2) • What is pressure was DECREASED? • Shifted to side with more moles of gas • INVERSE relationship between pressure change and side with number of moles • INCREASE pressure = shift to side with FEWER moles • DECREASE pressure = shift to side with MORE moles

17. Pressure change & Keq • Even though changes in pressure may shift the equilibrium position, they do not affect the value of the equilibrium constant. • Increasing pressure by adding a gas that is not a reactant or a product cannot affect the equilibrium position of the reaction system.

18. Concentration Change • If one side INCREASES concentration, then the system will shift the reaction to the opposite side • If one side DECREASES concentration, then the system will shift the reaction to its side • If we increase the H2 concentration, what will happen? • Will N2 increase or decrease? What about NH3? • If we decrease the N2 concentration, what will happen? • Will H2 increase or decrease? What about NH3?

19. Concentration & Keq • Changes in concentration have no effect on the value of the equilibrium constant. • Such changes have an equal effect on the numerator and the denominator of the chemical equilibrium expression.

20. Temperature Change • Reactions are either exothermic or endothermic • If endothermic, then energy is a REACTANT • If exothermic, then energy is a PRODUCT • So if a reaction is ENDOTHERMIC, then increasing temperature shifts equilibrium to products • Increase TEMP  Shift to PRODUCTS • Decrease TEMP  Shift to REACTANTS

21. Temperature Change • If a reaction is exothermic, then heat is a PRODUCT • In an EXOTHERMIC rxn, when temp is INCREASED, then the rxn shifts to REACTANTS • In an EXOTHERMIC rxn, when temp is DECREASED, then the rxn shifts to PRODUCTS • The value of the equilibrium constant (Keq) for a given system is affected by the temperature.

23. Haber Process • Artificial production of ammonia N2(g) + 3H2(g)  2 NH3(g) +92 kJ • In this reaction, we want to produce as maximum amount of NH3 (Ammonia) • There are how many moles of reactants? Products? • So how do we create the maximum product concentration by changing the pressure?

24. Haber Process (Page 2) N2(g) + 3H2(g)  2 NH3(g) +92 kJ • Temperature • Is the reaction endothermic or exothermic? • Is heat a reactant or product? • So what should be done to maximize products? • Concentration • How would me maximize products by altering concentrations?

25. Last Example • Bicarbonate Buffer in mammalian blood H2CO3  H+ + HCO3- • What happens when pH decreases? What happens to the [H+]? So what happens to the equilibrium? • What about the [H2CO3] & [HCO3-] • What happens when pH increases? What happens to the [H+]? So what happens to the equilibrium? • What about the [H2CO3] & [HCO3-]