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Slide 1
Energa de activacin
UNACH-campus IV
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Energa en un sistema qumico mientras la reaccin procede
Una reaccin espontnea generalmente libera energa
los reactantes contienen ms energa que los productos.
La energa se entiende como energa qumica potencial
La diferencia entre energas potenciales se denomina entalpa de lareaccin
Energa potencial qumica de reactantes y productos. La diferencia es la entalpa de la reaccin.
http://www.science.uwaterloo.ca/~cchieh/cact/c123/eactivat.html
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Slide 3
Driving Force
La diferencia en la energa qumica potencial es la fuerza que impulsa
(driving force) a una reaccin qumica a efectuarse. Despus de la
reaccin, se libera energa. Los productos estn en un nivel energticoms estable que los reactantes.
http://www.science.uwaterloo.ca/~cchieh/cact/c123/eactivat.html
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Slide 4
Energa de activacin, Ea
Una mezcla de H2 y O2 no reaccionar a menos que su temperatura alcance
el punto de ignicin, a pesar de la gran cantidad de energa que se libera
durante una reaccin de oxidacin.
Este fenmeno se explica a travs del requerimiento de una energa de
activacin, Ea. La relacin entre Ea y la energa potencial qumica en una
reaccin es:
http://www.science.uwaterloo.ca/~cchieh/cact/c123/eactivat.html
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Si se aplica suficiente energa a la roca para empujarla sobre la colina
(barrera de energa de activacin), rodar espontaneamente lamontaa, liberando energa mientras se mueve a un estado ms bajo de
energa potencial. La tasa de rocas aventadas desde el risco depender
de la altura de la barrera de energa de activacin.
Barrera de la energa de activacin
http://www.ucdsb.on.ca/tiss/stretton/chem2/rate03.htm
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Slide 6
En cualquiera de los casos se requiere la Ea
Energa de Activacin
http://www.ucdsb.on.ca/tiss/stretton/chem2/rate03.htm
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Slide 7
1. Energa es la capacidad de hacer un trabajo, o calor o cualquier cosa
que se transforme en calor
2. La actividad celular esta basada en la energa proporcionada por el sol
o por enlaces qumicos
3. Los cambios en la energa estn gobernados por la primera y la
segunda ley de la termodinmica
4. La diferencia en la energa libre entre reactantes y productos determina
que una reaccin sea espontanea o que no lo sea.
5. Para que una reaccin se lleve a cabo se requiere energa de activacin
6. Las enzimas disminuyen la energa de activacin necesaria en rxs
biolgicas
7. Las enzimas interactuan con el sustrato
Conceptos bsicos
http://www.blc.arizona.edu/courses/181gh/rick/energy/default.html
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Slide 8
Primera Ley
La energa no se crea ni se destruye; las diferentes formas de energa son
interconvertibles
Leyes de la Termodinmica
http://www.blc.arizona.edu/courses/181gh/rick/energy/default.html
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Slide 9
Segunda Ley
En un sistema cerrado, la cantidad de desorden se mantiene estable o aumenta.
Entropa es una magnitud del desorden. La energa se convierte de maneraespontnea a formas menos ordenadas. Esto implica que la relacin entre molculasse hace ms desordenada.
Los procesos de ensamble biolgico como la formacin de membranas, plegamientode protenas, formacin de la doble hlice son dirigidos por entropa primeramente.
Leyes de la Termodinmica
http://www.blc.arizona.edu/courses/181gh/rick/energy/default.html
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Slide 10
En cualquier sistema, la energa total incluye la energa usable quepuede hacer un trabajo y la energa que se pierde en eldesorden:
Energa total = energa usable + energa inusable
Esta relacin se puede describir con termodinamicamente como:
H (Energa total ) = G ( energa libre) + TS (entropa)
Esta ecuacin se reordena para enfatizar la importancia de laenerga libre:
G = H - TS
El cambio en la energa libre durante el proceso de una reaccin auna tempertaura constante es definido en trminos de el
cambio en energa total y el cambio en entropa. La energalibre se mide en joules or kilocalorias
G = H - TS
Ecuacin para relacionar energa usable e inusable
http://www.blc.arizona.edu/courses/181gh/rick/energy/energy.html
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Slide 11
Only a small fraction of the collisions between reactant molecules convert the reactants into the
products of the reaction. This can be understood by turning, once again, to the reactionbetween ClNO2 and NO.
ClNO2(g) + NO(g) NO2(g) + ClNO(g)
In the course of this reaction, a chlorine atom is transferred from one nitrogen atom to another.In order for the reaction to occur, the nitrogen atom in NO must collide with the chlorineatom in ClNO2.
The Activation Energy of Chemical Reactions
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html
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Slide 12
Reaction won't occur if the oxygen end of the NO molecule collides with the chlorine atom on
ClNO2.
The Activation Energy of Chemical Reactions
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html
Nor will it occur if one of the oxygen atoms on ClNO2 collides with the nitrogen atom on NO
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Slide 13
But, before the reactants can be converted into products, the free energy of the system must
overcome the activation energy for the reaction, as shown in the figure below. The verticalaxis in this diagram represents the free energy of a pair of molecules as a chlorine atom istransferred from one to the other. The horizontal axis represents the the sequence ofinfinitesimally small changes that must occur to convert the reactants into the products ofthis reaction.
The Activation Energy of Chemical Reactions
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html
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Slide 14
The Activation Energy of Chemical Reactions
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html
To understand why reactions have an activation energy, consider what has to happen in order forClNO2 to react with NO. First, and foremost, these two molecules have to collide, therebyorganizing the system. Not only do they have to be brought together, they have to be held inexactly the right orientation relative to each other to ensure that reaction can occur. Both ofthese factors raise the free energy of the system by lowering the entropy. Some energy alsomust be invested to begin breaking the Cl-NO2 bond so that the Cl-NO bond can form.
NO and ClNO2 molecules that collide in the correct orientation, with enough kinetic energy toclimb the activation energy barrier, can react to form NO2 and ClNO. As the temperature ofthe system increases, the number of molecules that carry enough energy to react when theycollide also increases. The rate of reaction therefore increases with temperature. As a rule,the rate of a reaction doubles for every 10oC increase in the temperature of the system.
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Slide 15
The Activation Energy of Chemical Reactions
http://www chem vt edu/RVGS/ACT/notes/act energy html
NOT EVERY COLLISION LEADS TO REACTION!!!!!
It is possible to calculate the rate at which molecules collide with each other by using the kinetictheory.
Consider a mixture of CO and NO2 at 700 K and a concentration of 0.10 mol/L
every molecule would collide with about a billion other molecules in one second!
if every collision resulted in a reaction, then the whole mixture would be reacted in afraction of a second.
the actual reaction takes about 20 seconds.
In order for collisions to be effective, there must be considerable force in the colllisions. Theslower moving molecules do not have enough kinetic energy to react when theycollide...they bounce off one another and retain their identity.
Only those molecules moving at high speed have enough energy for collisions to result in areaction.
Every reaction requires a certain minimum energy for the reaction to occur--it is called activationenergy, Ea, and is expressed in kJ.
The CO + NO2 mixture from above has an Ea = 134 kJ. This means that the colliding moleculesmust have a total KE of 134 kJ/mol if they are to react.