CEM 142 1st Edition Exam #3 Study GuideRate of Reactions- How fast a reaction goes depends on the activation energy- Acid-base reactions are fasto Does not mean the reaction will go to completion- Slow reactions:o Rusting—oxidation of iron—goes to completion if left- The position of equilibrium DOES NOT effect the rate of reaction- Rate of a reaction is the change in concentration per timeo Rates are not usually constant- Increase in energy  collisions  reactionso #Of collisions=amounts of moleculeso *Not every collision leads to a reaction* Concentration of reactants determines whether # of collisions increase or decrease- Rate of forward reaction decreases due to fewer molecules to collide- Rate of reverse reaction increases due to more molecules produced to collideo When rate of forward reaction=rate of reverse reaction equilibrium-EQUAL CONCENTRATION DOES NOT MEAN EQUAL RATESKinetics…- Information about rates and how they change with conditions (temperature change/concentration change) tells us about reaction mechanism- Reaction mechanism: the pathway that leads from reactants to productso series of events (bond breaking or bond formation) and intermediatesformedRates and concentrations- Rate depends on concentrations of reactants- More moleculesmore collisionsfaster ratemore reactions happen- Rate is proportional to reactionsRate=k[reactants]Effects of Temperature- Rates increase with temperature- Some reactions need an initial “spark” to make them go (H2& O2)o Temperature is proportional to kinetic energy—molecules collide withmore energy—this increases rate- Activation energy is the “initial” spark that effects the rate of the reaction- Transition State: the highest point on the energy diagramo X-axis=progress of reaction AB, not time!- G is now used rather than Ho Exergonic reaction, not exothermic- You always want the hump of the activation energy to be smaller for a faster reaction!Equilibrium Constant (K) *rate constant is lower case k*- Is expressed with concentrations (kc) or partial pressure (kp)o Either for solutions or gases (leaves liquids and solid OUT, adding or removing will not change the equilibrium)- K will be larger with a strong acid- K tells you where the equilibrium position is for a reactiono Tells us in which direction the reaction will occur- A large K=more productso Small K=more reactantsLe Chatelier’s Principle- If stress (change in concentration, pressure, temp.) is applied to a system at equilibrium, the system will move to counteract the stress and establish a new equilibrium- Effect of concentration changes:o The reaction will shift away from the side that you add more substance to Increasing reactants=shifting right Increasing products=shifting leftNetworked reactions- Buffer: contains both an acid and its conjugate baseo The acid part (HA) will react with any added strong baseo The base part (A-) will react with any added strong acido Can be prepared by mixing an acid with its conjugate base or by titrating a weak acid with a strong base (or a weak base with a strong acid)- Exchanging a strong acid for a weak acid—pH remains the same- Adding a base means it will react with the acid causing it to decrease and produce more of the conjugate base where the concentration of the conjugatebase increases- Buffer capacity: amount of acid or base that the buffer can absorb without a large change in pHo Depends on concentration of buffer*more acidic=more electronegative=more of a negative conjugate base is=more resonance