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Ea. Potential Energy (Ep). Fraction of molecules. Path of reaction . Kinetic energy . kinetic vs. potential energy diagrams. Recall the Maxwell-Boltzman distribution (i.e. kinetic energy diagram).

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kinetic vs potential energy diagrams

Ea

Potential Energy (Ep)

Fraction of molecules

Path of reaction 

Kinetic energy 

kinetic vs. potential energy diagrams
  • Recall the Maxwell-Boltzman distribution (i.e. kinetic energy diagram)
  • The Ea is a critical point. To examine it more closely we can use a potential energy graph
  • The axes are not the same, thus the Ep graph is not a blow up of the Ek graph; however it does correspond to the part of the Ek graph that is circled
potential energy graph a closer look

Activated complex / transition state

Ea

Ep (Potential energy stored in chemical bonds)

H

Collision begins

Collision ends

Path of reaction

potential energy graph: a closer look

Reactants

Products

A2 & B2 rush together

molecules slow down Ep , Ek 

molecules speed up

Ep , Ek 

2AB molecules float apart

Ep + Ek =constant throughout

Overall Ep(reactants)>Ep(products)

Ek(reactants)<Ek(products)

ep graph important points

Ea forward

Ep

Ea reverse

 H is positive

Ep graph: Important points

Exothermic

Endothermic

  • Forward and reverse reactions are possible
  • Ea is the difference between Ep at transition state and initial or final Ep
  • The graph depicts an exothermic reaction. Endothermic reactions are also possible
  • H is the difference between initial and final Ep. It is -ve for exothermic,+ve for endothermic
the collision theory
The collision theory
  • Related to the Ep graph is the “collision theory” - the idea that for molecules to react they must meet with sufficient force
  • Factors that affect reaction rate can be explained via the collision theory:
  • Increased temperature causes molecules to move faster (increased number of collisions per unit time and greater kinetic energy)
  • Increased concentration means more collisions
  • Homogenous reactions occur faster because reacting molecules collide more frequently
  • Catalysts decrease Ea, decreasing the amount of kinetic energy needed to overcome Ea
catalysts

Fraction of molecules

potential energy

Kinetic energy 

Path of reaction

Catalysts
  • Recall, catalysts speed a reaction
  • This can be explained by the Ek or Ep graphs
  • In both, the catalyst works by lowering the Ea:
  • Catalysts speed forward and reverse reactions
  • However, most reactions favour the side that has the lowest potential energy (most stable)
  • Catalysts are heterogenous or homogenous
  • They provide a substrate (p. 768) for a reaction or they can bond temporarily to a molecule, increasing the odds of a favourable meeting
transition state lab purpose
Transition state lab: purpose

Purpose: 1) to visualize an activated complex, 2) to observe the influence of a catalyst

We will be examining the following reaction:

NaKC4H4O6(aq) + H2O2(aq)  CO2(g) + …

Procedure:

  • Turn hot plates immediately to medium heat
  • Get a 10 mL graduated cylinder, a 100 mL beaker, a test tube, and a rubber stopper.
  • Weigh1.7gNaKC4H4O6.Addtobeakeralong with 10 mL distilled H2O. Swirl to dissolve.
  • Add 4.5 mL of 10% H2O2 to beaker. Heat.
  • Get 5 mL of CoCl2 but don’t add it yet.
transition state lab procedure
Transition state lab: procedure

Procedure:

  • As soon as tiny bubbles start to form and rise, remove the beaker from the hot plate. Add the CoCl2 at this point.
  • Record your observations (in order to answer the questions). Clean up – wash everything down the drain, wipe off your lab bench.

Questions: answer on a separate sheet of paper

  • Look at the chemical equation that represents the reaction. What physical sign will there be when a reaction is occurring?
  • The products of the reaction are colourless. What colour are the reactants?
transition state lab conclusions
Transition state lab: conclusions

Questions: read 18.11 (pg. 767 – 769)

  • What was the catalyst in the lab? What colour was it? Is it homogenous or heterogeneous?
  • At the beginning of step 5, both reactants were present; why was there no reaction? (Illustrate with a Ek diagram).
  • Why is the reaction still slow after heat is added? (illustrate using the Ek diagram)
  • Was the catalyst a different colour at the end of the experiment than at the beginning?
  • What colour was the activated complex?
  • Illustrate the affect the catalyst had on the reaction (using both Ek and Ep diagrams)
answers

Fraction of molecules

Kinetic energy 

Answers
  • The production of CO2 (bubbling) is a physical sign that the reaction is occurring
  • The reactants are colourless
  • CoCl2 was the catalyst in the lab (pink, homogenous)
  • There was no reaction because the Ea was not reached (Illustrate with a Ek diagram).
  • The reaction still slow after heat is added because very few molecules exceed Ea.
answers10

Fraction of molecules

potential energy

Kinetic energy 

Path of reaction

Answers
  • The catalyst was the same colour at the end of the experiment (catalysts don’t change).
  • The activated complex was green
  • Illustrate the affect the catalyst had on the reaction (using both Ek and Ep diagrams)

For more lessons, visit www.chalkbored.com