Understanding Chemical Kinetics: Factors Influencing Reaction Rates and Spontaneity

1. Homework Read pages: 533 - 538 Answer questions from packet on pages: 177 - 179

2. Chemical Kinetics Def: study of the rate at which chemical reactions are carried out

3. Collision theory Particles must undergo effective collisions in order to react and form new products Effective Collisions • Colliding particles must have sufficient energy • particles must have proper orientation (lined up)

4. Collision Theory (continued) • Activation energy – minimum amount of energy needed for a reaction to occur • Activated complex – temporary unstable intermediate particle (combination of reactants) • - has more PE than the initial reactants

5. Factors that Affect Reaction Rate • Increases the # of effective collisions 1. Nature of the reactants • a reactant that has few bonds to brake will react faster than one that has many bonds to break • rxns of aqueous compounds proceed quickly (dissolving breaks ionic bonds)

6. Factors that Affect Reaction Rate 2. Temperature • increase temp, increase KE • Collisions occur with more energy • Increase temp, increase rxn rate • Increase temp by 10 ⁰C, doubles the rxn rate

7. Factors that Affect Reaction Rate 3. Concentration of reactants • the greater the # of particles, the more effective collisions

8. Factors that Affect Reaction Rate 4. Pressure (gases only) • increase pressure, decrease volume • Increases concentration of gaseous particles

9. Factors that Affect Reaction Rate 5. Surface Area • increase surface area, greater opportunity for collisions to occur • more particles exposed to one another, rxn rate increases

10. Factors that Affect Reaction Rate 6. Catalyst • speeds up rxns by lowering the activation energy • catalyst remains unchanged after rxn

11. How does a catalyst increase the rate of a chemical reaction? Lowers the Activation energy for the reaction

12. Energy changes in Chemical Reaction • Chemical energy: Energy stored in chemical bonds Recall: Bonds formed – energy given off Bond broken – energy absorbed

13. Enthalpy (H) Def: The heat content of matter • Measures the PE stored in chem bonds • measured in kJ/mol

14. Heat of Reaction (DH) Def: amount of heat energy absorbed or released during a chemrxn DH = Heat of products – Heat of reactants Table I

15. Exothermic Reactions • Enthalpy of products is less than the Enthalpy of reactants DH = (-) • Energy is written as a product in chem equation C(s) + O2 → CO2(g) DH = -393.5 kJ C(s) + O2 → CO2(g) + 393.5 kJ

17. Endothermic Reactions • Enthalpy of products is greater than the Enthalpy of reactants DH = (+) • Energy is written as a reactant in chem equation 2C(s) + H2(g) → C2H2 DH = +227.4 kJ 2C(s) + H2(g) + 227.4 kJ → C2H2

19. The Driving Forces of Nature How do we know if a reaction will proceed spontaneously?

20. Spontaneous reactions • Occur without any added energy (naturally) • Spontaneity of a chemical reaction is dependent on 2 key factors: - Enthalpy - Entropy

21. Enthalpy • Nature tends toward more stable products • Reactions tend to proceed toward products that have lower enthalpies - This means exothermic reactions!

22. Entropy (S) Def: degree of randomness or disorder of matter - The greater the disorder of a system, the greater the entropy - Nature favors increasing entropy (DS = +)

23. Changes in Entropy (DS)

24. When does entropy increase? • moremoleculesproduced than present in reactants • phase changes: • Solid → Liquid • Liquid → Gas • Solid → Gas • temp increases

25. ***Chemical reactions have a tendency to be spontaneous if: proceed toward greater stability (exothermic) 2. increase in entropy