Transition State

8/3/2019 Transition State

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kinetic vs. potential energy diagrams Recall the Maxwell-Boltzman distribution (i.e.

kinetic energy diagram)

Kinetic energy p

Ea

The Ea is a critical point. To examine it moreclosely we can use a potential energy graph

Path of reaction p

The axes are not the same, thus the Ep graphis not a blow up of the Ek graph; however itdoes correspond to the part of the Ek graph

that is circled


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potential energy graph: a closer look

Collisionbegins

molecules

speed up

Ep q, Ek o

Activated complex /

transition stateReactants

Products

Path of reaction

Collision ends

molecules

slow down

Ep o, Ek q

A2 & B2rush

together

2AB

molecules

float apart

Overall Ep(reactants)>Ep(products)

Ek(reactants)


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Ep graph: Important points

Forward and reverse reactions are possible Ea is the difference between Ep at transition

state and initial or final Ep

(H is the difference between initial and finalEp. It is -ve for exothermic,+ve for endothermic

Exothermic Endothermic

Eaforward Ea

reverse

The graph depicts an exothermic reaction.Endothermic reactions are also possible

( H ispositive


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The collision theory Related to the Ep graph is the collision theory

- the idea that for molecules to react they mustmeet with sufficient force

Factors that affect reaction rate can beexplained via the collision theory:

Increased temperature causes molecules tomove faster (increased number of collisionsper unit time and greater kinetic energy)

Increased concentration means more collisions Homogenous reactions occur faster becausereacting molecules collide more frequently

Catalysts decrease Ea, decreasing the amount

of kinetic energy needed to overcome Ea


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Catalysts Recall, catalysts speed a reaction

This can be explained by theE

k orE

p 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

Fraction of

molecules

Kinetic energyp

potential

energy

Path of reaction


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Transition state lab: purposePurpose: 1) to visualize an activated complex, 2)

to observe the influence of a catalystWe will be examining the following reaction:

NaKC4H4O6(aq) + H2O2(aq) p CO2(g) +

Procedure:

1. Turn hot plates immediately to medium heat

2. Get a 10 mL graduated cylinder, a 100 mL

beaker, a test tube, and a rubber stopper.

3. Weigh 1.7 g NaKC4H4O6. Add to beaker alongwith 10 mL distilled H2O. Swirl to dissolve.

4. Add 4.5 mL of 10% H2O2 to beaker. Heat.

5. Get 5 mL of CoCl2 but dont add it yet.


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Transition state lab: procedureProcedure:

6. As soon as tiny bubbles start to form andrise, remove the beaker from the hot plate.Add the CoCl2 at this point.

7. Record your observations (in order to answer

the questions). Clean up wash everythingdown the drain, wipe off your lab bench.

Questions: answer on a separate sheet of paper

1. Look at the chemical equation thatrepresents the reaction. What physical signwill there be when a reaction is occurring?

2. The products of the reaction are colourless.

What colour are the reactants?


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Transition state lab: conclusionsQuestions: read 18.11 (pg. 767 769)

3. What was the catalyst in the lab? What colourwas it? Is it homogenous or heterogeneous?

4. At the beginning of step 5, both reactantswere present; why was there no reaction?

(Illustrate with a Ek diagram).5. Why is the reaction still slow after heat is

added? (illustrate using the Ek diagram)

6. Was the catalyst a different colour at the endof the experiment than at the beginning?7. What colour was the activated complex?8. Illustrate the affect the catalyst had on the

reaction (using both Ek and Ep diagrams)


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Answers1. The production of CO2 (bubbling) is a

physical sign that the reaction is occurring2. The reactants are colourless3. CoCl2 was the catalyst in the lab (pink,

homogenous)

4. There was no reaction because the Ea wasnot reached (Illustrate with a Ek diagram).

5. The reaction still slow after heat is added

because very few molecules exceedE

a.Fraction of

molecules

Kinetic energyp


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Answers6. The catalyst was the same colour at the end

of the experiment (catalysts dont change).7. The activated complex was green8. Illustrate the affect the catalyst had on the

reaction (using both Ek and Ep diagrams)

Fraction of

molecules

Kinetic energyp

potential

energy

Path of reaction

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Transition State