Chemical Rxn Rates

1. Chemical Rxn Rates

2. Chemical Kinetics The area of chemistry that concerns reaction rates. Key Idea: Molecules must collide to react. However, only a small fraction of collisions produces a reaction. Why?

3. Collision Model Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy). Orientation of reactants must allow formation of new bonds.

4. Reaction Rate • Speed at which a chemical reaction takes place • Reaction rate depends on the collisions between reacting particles. • Determined by measuring the change in concentration of a reactant or product per unit of time

5. Factors Affecting Reaction Rates • Temperature: Measure of average kinetic energy of the molecules in a substance • Concentration: amount of molecules present in a unit volume • Catalyst: an agent that speeds up the rate of chemical rxns w/out being permanently changed or used up • Surface area: area of the surface

6. Temperature • Molecules at higher temp. have higher KE and move fastermore collisionsfaster rxn rate • A 10°C increase in Tempdouble the rate of rxn • Temp Collisions Rxn rate

7. Surface Area • high SA = fast rxn rate • more opportunities for collisions • Increase surface area by… • using smaller particles • dissolving in water • SA Collisions Rxn Rate

8. Concentration • Increasing concentration  increases chance of effective collisions  faster rxn rate • Conc Collisions Rxn Rate

9. Heat of Reaction The amount of heat released or absorbed during a chemical reaction. Endothermic: Reactions in which energy is absorbed as the reaction proceeds. Exothermic: Reactions in which energy is released as the reaction proceeds.

10. Endothermic Reactions

11. Exothermic Reactions

12. Catalysts • Provides a shorter pathway for the reactants to follow as they change into products (like a tunnel cut through a mountain) • Lowers Activation Energy • Speeds up rxn rate without being consumed in the rxn • + Catalysts Activation E Rxn Rate ENZYMES

13. Ea Activation Energy (Ea) • depends on reactants • minimum energy required for a reaction to occur • low Ea = fast rxn rate

15. Endothermic Reaction witha Catalyst

16. Exothermic Reaction with a Catalyst

17. Chemical Equilibrium Reversible Reactions: A chemical reaction in which the products can react to re-form the reactants Chemical Equilibrium: When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged 2HgO(s)  2Hg(l) + O2(g) Arrows going both directions ( ) indicates equilibrium in a chemical equation

18. LeChatelier’s Principle When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress.

19. Le Chatelier Translated: When you take something away from a system at equilibrium, the system shifts in such a way as to replacewhat you’ve taken away. When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added.

20. Equilibrium Shift • Consider the following reaction N2 + 3H2 2NH3 What is the effect of increasing the amount of ammonia? (which way would the equilibrium shift?)

21. Equilibrium shift • Consider the following reaction H2O2  H2O + O2 What is the effect of decreasing the amount of H2O2 ? (which way would the equilibrium shift?)

22. An increase in temp. speeds the rxn rate because the • A) distance between molecules increases • B) distance between molecules decreases • C) # of collisions increases • D) # of collisions decreases As the # of collisions between reactants increases, the • A) Reaction stops • B) rate of rxn decreases • C) rate of rxn increases • D) rate of rxn remains the same