Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions...

download Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration

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Transcript of Unit 9: Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions...

  • Unit 9: Chemical EquilibriumCollision theoryRates of reactionsCatalystsReversible reactionsChemical equilibriumLe Chateliers PrincipleConcentrationTemperatureVolumeCatalysts

  • A. Collision TheoryReaction rate depends on the collisions between reacting particles.Successful collisions occur if the particles...collide with each otherhave the correct orientation have enough kinetic energy to break bonds

  • ExothermicEndothermic

    Activation energyEnergy of reactionActivation energy: minimum energy required for a reaction to occur

  • A. Collision TheoryActivation Energydepends on reactantslow Ea = fast rxn rate

  • Chemical kinetics: the study of the rate (the speed) of a reactionRate of a chemical reaction depends on:

    16.2: Rates of Reactions3. TEMPERATURE (T) of reactants2. CONCENTRATION of reactants4. Presence/absence of a CATALYST1. SURFACE AREA

  • SURFACE AREASurface Areahigh SA = fast rxn ratemore opportunities for collisionsIncrease surface area byusing smaller particlesdissolving in water

  • Concentration:KMT (Kinetic-Molecular theory) states that increasing concentration of reactants results in more collisions.Effect of Concentration on RateMore collisions result in more reactions, increasing the rate of the reaction.

  • Temperature:Increasing T increases particle speed.Effect of Temperature on RateFaster reactants means more collisions have the activation energy, which increases the rate of the reaction.

  • A catalyst:A chemical that influences a reaction, but is not consumed in the reaction. (It can be recovered unchanged at the end of the reaction.)Lowers the activation energy of the reaction.Effect of Catalysts on RateActivation energyActivation energy with catalyst

  • 16.1: Reversible Reactions* Thus far, we have considered only one-way reactions: A + B C + DSome reactions are reversible:They go forward (to the right) : A + B C + Dand backwards (to the left) : A + B C + DWritten with a two-way arrow:A + B C + DBoiling & condensingFreezing & meltingRecharging a rechargeable batteryExamples:

  • Examples of irreversible reactions:

    Striking a match / burning paperDropping an eggCooking (destroys proteins)

  • For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established.Both the forward and backward reactions continue, but there is a balance of products un-reacting and reactants reacting. A + B C + D16.3: Chemical Equilibrium

  • * Le Chateliers Principle is about reducing stress a stress applied to a chemical equilibrium(1850 1936)Relax! Reduce stress brought on by chemical equilibrium with me, Henri Le Chatelier!

  • Le Chateliers Principle:When a stress is applied to a system (i.e. reactants and products) at equilibrium, the system responds to relieve the stress.The system shifts in the direction of the reaction that is favored by the stress.A stress is a change in:ConcentrationTemperatureVolume16.4: Le Chateliers Principle

  • Ex: Co(H2O)62+ + 4 Cl1- CoCl42- + 6 H2O(pink)(blue)

    StressResult16.5: Stress: Change ConcentrationAdd Cl1- Forward rxn favoredShifts forward to reduce extra Cl1-More CoCl42- will formAdd H2OBackward rxn favoredShifts backward to reduce extra H2O More Co(H2O)62+ will form

  • Ex: heat + Co(H2O)62+ + 4 Cl1- CoCl42- + 6 H2O(pink)(blue)

    This reaction is endothermic. For Le Chateliers principle, consider heat as a chemical.

    StressResult16.7: Stress: Change TemperatureDecrease T Backward rxn favored; shifts backward to replace lost heatMore Co(H2O)62+ will formIncrease TForward rxn favored; shifts forward to reduce extra heatMore CoCl42- will form

  • Ex: 1 N2 (g) + 3 H2(g) 2 NH3(g)

    (1 + 3 = 4 moles of gas) (2 moles of gas)

    Stress Result16.6: Stress: Change VolumeIncrease V Backward rxn favored; shifts backward to side with more moles of gas (to fill the larger volume with more molecules)Decrease VForward rxn favored; shifts forward to side with fewer moles of gas (reduces # of molecules packed into this smaller volume)

  • Ex: 2 H2O2 (aq) 2 H2O (l) + O2 (g)

    Since a catalyst increases the forward and backward rates equally, it will not shift the equilibrium.16.7: Catalysts & EquilibriumMnO2

  • Crystallization (backward rate) increasesDissolving (forward rate) decreasesEquilibrium is established:

    Forward rate = Backward rateEx: saturated salt solutionNaCl (s) Na+ (aq) + Cl- (aq)