EQUILIBRIUM

1. EQUILIBRIUM

2. How Chemical Reactions Occur • Collision model – molecules must collide in order for a reaction to occur • Rate depends on concentrations of reactants and temperature.

3. Conditions That Affect Reaction Rates • Concentration – increases rate because more molecules lead to more collisions • Temperature – increases rate • Why?

4. Conditions That Affect Reaction Rates • Activation energy – minimum energy required for a reaction to occur

5. Conditions That Affect Reaction Rates • Catalyst – a substance that speeds up a reaction without being consumed • Enzyme – catalyst in a biological system

6. Reactions • Homogeneous reaction – all reactants and products are in one phase • Gas • Solution • Heterogeneous reaction – reactants in two phases

7. Reactions

8. The Equilibrium Condition • Equilibrium – the exact balancing of two processes, one of which is the opposite of the other

9. The Equilibrium Condition • Chemical equilibrium – a dynamic state where the concentrations of all reactants and products remain constant

10. Although time continues to pass, the numbers of reactant and product molecules are the same as in (c). No further changes are seen as time continues to pass. The system has reached equilibrium. The reaction continues as time passes and more reactants are changed to products. The reaction begins to occur, and some products (H2 and CO2) are formed. Equal numbers of moles of H2O and CO are mixed in a closed container. Chemical Equilibrium: A Dynamic Condition

11. Chemical Equilibrium: A Dynamic Condition • Why does equilibrium occur?

12. The Equilibrium Constant: An Introduction • Law of chemical equilibrium • For a reaction of the type • aA + bB cC + dD • Equilibrium expression • Each set of equilibrium concentrations is called an equilibrium position.

13. Applications Involving the Equilibrium Constant • The Meaning of K • K > 1  the equilibrium position is far to the right • *Favors products • K < 1  the equilibrium position is far to the left • *Favors reactants

14. Heterogeneous equilibria – an equilibrium system where the products and reactants are not all in the same state *Their value is 1

15. Le Chatelier’s Principle • Le Chatelier’s Principle – when a change is imposed on a system at equilibrium the position of the equilibrium shifts in a direction that tends to reduce the effect of that change

16. Effect of a Change in Concentration

17. Effect of a Change in Concentration • When a reactant or product is added the system shifts away from that added component. • If a reactant or product is removed, the system shifts toward the removed component.

18. The system is initially at equilibrium. The piston is pushed in, decreasing the volume and increasing the pressure. The system shifts in the direction that consumes CO2 molecules, lowering the pressure again. • Effect of a Change in Volume

19. Effect of a Change in Volume • Decreasing the volume • The system shifts in the direction that gives the fewest number of gas molecules.

20. Effect of a Change in Volume • Increasing the volume • The system shifts in the direction that increases its pressure.

21. The value of K changes with temperature. We can use this to predict the direction of this change. • Exothermic reaction – (heat is a product) • Adding energy shifts the equilibrium to the left (away from the heat term). • Endothermic reaction – (heat is a reactant) • Adding energy shifts the equilibrium to the right (away from the heat term). • Effect of a Change in Temperature

22. The value of K for a system can be calculated from a known set of equilibrium concentrations. • Unknown equilibrium concentrations can be calculated if the value of K and the remaining equilibrium concentrations are known.