<< Chapter < Page Chapter >> Page >

As a cautionary note, the results from the previous Concept Development Study make it clear that not every rate law is as simple as the one for the reaction of O 3 with Cl. We will return to this topic in a later section.

Observation 2: temperature dependence of reaction rates

How is the effect of temperature change on the reaction rate reflected in the rate law equation? Recall that a rate law shows the dependence of the reaction rate on reactant concentrations, e.g. the rate of reaction of I 2 gas with H 2 as described in Equation (3) gas is given by the rate law in Equation (4).

H 2 (g) + I 2 (g) → 2HI(g)

Rate = k[H 2 ][I 2 ]

One possibility for the increase in the rate of this reaction when temperature is increased is that there is a slight dependence on temperature of the concentrations, since volumes do vary with temperature. However, this is insufficient to account for the dramatic changes in rate typically observed. Therefore, the temperature dependence of the reaction rate is primarily found in the rate constant, k . The rate constant of this reaction at each temperature can be found using the Method of Initial Rates, as discussed in the previous study, and we find in Table 1 that the rate constant increases dramatically as the temperature increases.

Rate constant for H 2 (g) + I 2 (g)
T(K) k (1/M∙sec)
667 6.8×10 -3
675 9.87×10 -3
700 3.00×10 -2
725 8.43×10 -2
750 2.21×10 -1
775 5.46×10 -1
800 1.27

As shown in Figure 1, the rate constant appears to increase exponentially with temperature. After a little experimentation with the data, we find in Figure 2 that there is a simple linear relationship between ln( k ) and 1/T.

From Figure 2, we can see that the data in Table 1 fit the equation

ln (k) = a 1 T + b

where a and b are constant for this reaction. It turns out that, for our purposes, all reactions have rate constants that fit Equation (5), but with different constants a and b for each reaction. Figure 2 is referred to as an Arrhenius plot, after Svante Arrhenius.

It is not obvious why this equation should work for every chemical reaction. When we remember that the temperature is related to energy, it would seem that exothermic reactions and endothermic reactions should have different temperature dependence. Since they do not, this suggests that temperature increases and corresponding energy increases are not connected to the overall energy change of a reaction. This will lead us to a model to account for the temperature dependence of the rate constant. We will develop this further later in the next section.

Observation 3: activation energy

Equation (5) is very convenient for describing the temperature dependence of a reaction. However, we would like some insight into why the temperature dependence should obey Equation (5), and in particular, into the meaning of the constants a and b in the equation.

As noted at the end of the last section, Equation (5) describes the rate constant temperature dependence regardless of whether the reaction is endothermic or exothermic. Therefore, whatever the energetic factor is that is required for the reaction to occur, it is not just that the reaction is endothermic. Exothermic reactions apparently require energy as well. It must be that all reactions, regardless of the overall change in energy, require energy to occur.

Questions & Answers

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
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.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Concept development studies in chemistry 2013. OpenStax CNX. Oct 07, 2013 Download for free at http://legacy.cnx.org/content/col11579/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Concept development studies in chemistry 2013' conversation and receive update notifications?