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Reaction energy

Endothermic reaction

Exothermic reaction

Although we will not show it here, kinetic molecular theory shows that the fraction of molecules with energygreater than E a at temperature T is proportional to E a R T . This means that the reaction rate and therefore also the rateconstant must be proportional to E a R T . Therefore we can write

k T A E a R T
where A is a proportionality constant. If we take the logarithm of both sides of , we find that
k T E a R T A
This equation matches the experimentallyobserved . We recall that a graph of k versus 1 T is observed to be linear. Now we can see that the slope of that graph is equal to E a R .

As a final note on , the constant A must have some physical significant. We have accounted for the probability ofcollision between two molecules and we have accounted for the energetic requirement for a successful reactive collision. We havenot accounted for the probability that a collision will have the appropriate orientation of reactant molecules during the collision.Moreover, not every collision which occurs with proper orientation and sufficient energy will actually result in a reaction. There areother random factors relating to the internal structure of each molecule at the instant of collision. The factor A takes account for all of these factors, and is essentially the probability that a collision with sufficient energy for reactionwill indeed lead to reaction. A is commonly called the frequency factor .

Observation 4: rate laws for more complicated reaction processes

Our collision model in the previous section accounts for the concentration and temperature dependence of thereaction rate, as expressed by the rate law. The concentration dependence arises from calculating the probability of the reactantmolecules being in the same vicinity at the same instant. Therefore, we should be able to predict the rate law for anyreaction by simply multiplying together the concentrations of all reactant molecules in the balanced stoichiometric equation. Theorder of the reaction should therefore be simply related to the stoichiometric coefficients in the reaction. However, shows that this is incorrect for many reactions.

Consider for example the apparently simple reaction

2 I Cl ( g ) + H 2 ( g ) 2 H Cl ( g ) + I 2 ( g )
Based on the collision model, we would assume that the reaction occurs by 2 I Cl molecules colliding with a single H 2 molecule. The probability for such a collision should be proportional to [ I Cl ] 2 [ H 2 ] . However, experimentally we observe (see ) that the rate law for this reaction is
Rate k [ I Cl ] [ H 2 ]
As a second example, consider the reaction
N O 2 ( g ) + C O ( g ) N O ( g ) + C O 2 ( g )
It would seem reasonable to assume that this reaction occurs as a single collision in which an oxygen atom isexchanged between the two molecules. However, the experimentally observed rate law for this reaction is
Rate k [ N O 2 ] 2
In this case, the [ C O ] concentration does not affect the rate of the reaction at all, and the [ N O 2 ] concentration is squared. These examples demonstrate that the rate law for a reaction cannot be predicted from the stoichiometriccoefficients and therefore that the collision model does not account for the rate of the reaction. There must be somethingseriously incomplete with the collision model.

Questions & Answers

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Mueller Reply
what is hormones?
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Source:  OpenStax, General chemistry ii. OpenStax CNX. Mar 25, 2005 Download for free at http://cnx.org/content/col10262/1.2
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