AP Chem Unit 5

5.1 Reaction Rates / 5.5 Collision Theory

Chemical Kinetics: the area of chemistry concerned with the speed or rates, at which chemical reactions occur

Collision Theory: All reactions take place because of successful collisions between atoms or molecules. Successful collisions require sufficient energy and correct orientation

Collisions Theory Notes

• single replacement reaction

• high temp = more collisions

  • reactants move faster

• low temp = less collisions

  • reactants move slower

• reactant concentration increase = reactant percentage increase

• some nature of reactants cause faster reaction

• less collisions if less surface area of reactant

  • sugar cube v spoon of sugar

• adding inert gas does not impact rage

  • gas not part of reaction mechanisms

Factors That Affect Rates

• nature of reactants

• surface area of a solid

• concentration of reactants

• temperature at which the reaction takes place

• presence of catalyst

  • on both sides of equation

5.2 Reaction Rates

rate = k[A]^m[B]ⁿ

• k = rate constant

  • specific to experiment

  • get only from data

• [A] and [B] = concentration of …

• m and n = orders

  • bigger order = bigger impact

  • get from data only

5.4 Elementary Reactions

• an elementary reaction is a process in a chemical reaction that occurs in a single event of step

• an overall chemical ratio consists of one or more elementary steps

• cross out things that are products and reactants across each step to get overall equation

• slowest step determines rate

Rate Laws

• rate law for an overall reaction must be determined using experimental data

• in absence of data, rate law for an elementary reaction can be inferred from stoichiometry of the molecules participating in the collision

  • elementary reactions involving simultaneous collisions of three or more particles are rare so they are ignored

  • number of particles that collide during an elementary reaction is known as reaction molecularity

5.3 Concentration Changes Over Time

Determining Rate Law for a Reaction

• graph of your data for concentration versus time, make 3 graphs

  • [A] versus t

  • ln[A] versus t

  • 1/[A] versus t

• determine which graph is linear and determine order of reaction

  • [A] versus t = zero order

  • ln[A] versus t = first order

  • 1/[A] versus t = second order

• determine your constant

  • zero order: rate = k (k= - slope)

  • first order: rate = k[A] (k= - slope)

  • second order: rate = k[A]² (k = + slope)

Integrated Rate Laws

• differential rate laws relates the rate and concentrations

• integrated rate law relates amount and time

Cheat Sheet

5.7 / 5.8 Intro to Reaction Mechanism

Reminders

Chemical Reactions

• depicts the beginning and end of a reaction

• helpful for studying yields and quantities

• helps to study change in energy (enthalpy) and probability (entropy)

Collision Theory

• for a reaction to occur, a collision must take place with enough energy and in the correct orientation to cause a reaction to take place

Mechanism Overview

• reaction mechanism helps address what is happening at the molecular/atomic level and the concentration levels of each species (molecule)

• uses suggested elementary steps

• arrived at in a semi-empirical method

  • theoretical that is valid or stems from experiments

Identify Components in Reaction Mechanisms

Catalyst

• substance that increases the rate of a reaction by either stabilizing the transition state or forming a new transition state

• present at beginning and end

• can be in the rate law expression, but cancels out in final balanced equation

Intermediate

• temporary substance that is produced and consumed

• intermediate will not show up in the final rate law

• cancels out in the final balanced equation

Rate Law Mechanism

• in a multiple step mechanism, one step will be slower that the others

• the rate of a reaction can be no faster than the slowest step

Rate Law and Overall Stoichiometry

• cannot use stoic coefficients from balanced equation, overall equation to determine the rate

• can use stoich coefficients from an elementary reaction to determine the rate

• rate is dependent upon actual collisions

• rate is dependent upon actual collisions

• likelihood of five particles will collide simultaneously with sufficient energy and the correct orientation is minuscule

Elementary Reactions and Their Rate Laws

Unimolecular: consisting of or involving a single molecule

Bimolecular: consisting of or involving two molecules

Termolecular: consisting of or involving three molecules

• low probability

5.9 Pre-Equilbrilium Approximation

Intermediates in Rate Laws

• intermediates show up as a product and then are consumed

• in lab, cannot control concentration of intermediate

• catalysts are common in rate law bc we can control concentration

• intermediates should not be in the rate law since we can’t control concentration

• to get rid of intermediates from a rate law, we can use a fast equilibrium and the mathematical method of substation

Equilibrium Reaction

• reversible reactions where rate forward = rate backwards

• “dynamic equilibrium”

Steps to Success

1. write our rate law from slow step

   1. if intermediate is in rate law, continue to step 2

2. write our the forwards and backward rate for the fast equilibrium step

3. set the forward and backward rate equal to each other

4. solve from intermediate

5. sub intermediate into rate law from slow step