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

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)

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

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)

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

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

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

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

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

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

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

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

Heat of Reaction (  H) Def: amount of heat energy absorbed or released during a chem rxn  H = Heat of products – Heat of reactants Table I

Exothermic Reactions Enthalpy of products is less than the Enthalpy of reactants  H = (-) Energy is written as a product in chem equation C(s) + O 2 → CO 2 (g)  H = kJ C(s) + O 2 → CO 2 (g) kJ

Endothermic Reactions Enthalpy of products is greater than the Enthalpy of reactants  H = (+) Energy is written as a reactant in chem equation 2C(s) + H 2 (g) → C 2 H 2  H = kJ 2C(s) + H 2 (g) kJ → C 2 H 2

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

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

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

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 (  S = +)

Changes in Entropy (  S)

When does entropy increase? more molecules produced than present in reactants phase changes:-Solid → Liquid -Liquid → Gas -Solid → Gas temp increases

***Chemical reactions have a tendency to be spontaneous if: 1.proceed toward greater stability (exothermic) 2. increase in entropy