Chemical Kinetics

HomeworkRead pages: 533 - 538

Answer questions from packet on pages:177 - 179Chemical KineticsDef: study of the rate at which chemical reactions are carried outCollision theoryParticles must undergo effective collisions in order to react and form new products

Effective CollisionsColliding 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 reactantsFactors that Affect Reaction RateIncreases 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 Rate2. Temperature increase temp, increase KE Collisions occur with more energy Increase temp, increase rxn rate Increase temp by 10 C, doubles the rxn rateFactors that Affect Reaction Rate3. Concentration of reactants the greater the # of particles, the more effective collisions

Factors that Affect Reaction Rate4. Pressure (gases only) increase pressure, decrease volumeIncreases concentration of gaseous particlesFactors that Affect Reaction Rate5. Surface Area increase surface area, greater opportunity for collisions to occur more particles exposed to one another, rxn rate increasesFactors that Affect Reaction Rate6. Catalyst speeds up rxns by lowering the activation energy catalyst remains unchanged after rxnHow does a catalyst increase the rate of a chemical reaction?Lowers the Activation energy for the reactionEnergy changes in Chemical ReactionChemical energy:Energy stored in chemical bondsRecall: Bonds formed energy given offBond broken energy absorbedEnthalpy (H)Def: The heat content of matterMeasures the PE stored in chem bonds measured in kJ/molHeat of Reaction (DH)Def: amount of heat energy absorbed or released during a chem rxnDH = Heat of products Heat of reactantsTable IExothermic ReactionsEnthalpy of products is less than the Enthalpy of reactantsDH = (-)Energy is written as a product in chem equationC(s) + O2 CO2(g)DH = -393.5 kJ C(s) + O2 CO2(g) + 393.5 kJ

Endothermic ReactionsEnthalpy of products is greater than the Enthalpy of reactantsDH = (+)Energy is written as a reactant in chem equation2C(s) + H2(g) C2H2DH = +227.4 kJ 2C(s) + H2(g) + 227.4 kJ C2H2

The Driving Forces of NatureHow do we know if a reaction will proceed spontaneously?Spontaneous reactionsOccur without any added energy (naturally) Spontaneity of a chemical reaction is dependent on 2 key factors:- Enthalpy- EntropyEnthalpyNature tends toward more stable productsReactions 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 (DS = +)Changes in Entropy (DS)

When does entropy increase?more molecules produced than present in reactants phase changes:Solid LiquidLiquid GasSolid Gas temp increases***Chemical reactions have a tendency to be spontaneous if:proceed toward greater stability (exothermic)

2. increase in entropy