December 12, 2019, 09:26:59 PM
Forum Rules: Read This Before Posting


Topic: rate of reactions  (Read 138 times)

0 Members and 1 Guest are viewing this topic.

Offline bennyh

  • New Member
  • **
  • Posts: 5
  • Mole Snacks: +0/-0
rate of reactions
« on: December 01, 2019, 10:42:15 AM »
hi guys , i'm trying to investigate the rate of reaction of some thermochemical reactions.

i'm searching a handbook or a way to find the reaction details  K (reaction rate coefficient) , A and Ea (Arrhenius constant and energy activation) and delta H (Enthalpy of formation)

thanks for any help


Offline Enthalpy

  • Chemist
  • Sr. Member
  • *
  • Posts: 3206
  • Mole Snacks: +278/-57
Re: rate of reactions
« Reply #1 on: December 02, 2019, 08:23:14 AM »
Hi, the easier part is the enthalpy of formation. One table is in the
CRC Handbook of Chemistry and Physics
more tables exist, including on the Web.

From the enthalpies of formation, you deduce the enthalpy of reaction. And because you can combine data for compounds to deduce the enthalpy for many more reactions, it makes sense to publish data for a set of compounds.

If your compound is not listed, and isn't published as a single data neither, you have some limited chances to estimate the enthalpy of formation, but only for some compounds very similar to measured ones, and with much judgement. Software estimates don't work.

==========

For reaction rates, including A and Ea, you need experimental data for every single reaction you're interested in, as no numerical deduction from the reactants and products is possible. This is one reason for scarce data. Prediction would also need to know each reaction step, and often it's done the other way: Ea serves as a proof against or element in favour of a proposed reaction mechanism.

Some groups try to compute Ea by software, simulating myriads of molecule collisions and reaction steps on big computers. They claim to have some success in explaining some observations. I would base no prediction on that.

For very simple reactions, some data exists at Nist, nice of them again
https://kinetics.nist.gov/kinetics/

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 2784
  • Mole Snacks: +443/-20
  • Gender: Male
  • A lover of spectroscopy and chocolate.
Re: rate of reactions
« Reply #2 on: December 02, 2019, 10:47:57 AM »
Some groups try to compute Ea by software, simulating myriads of molecule collisions and reaction steps on big computers. They claim to have some success in explaining some observations. I would base no prediction on that.
This works pretty well for reactions between simple molecules in the gas phase, by calculating transition state geometries and energies. Much harder in condensed phases where you have to worry about interaction energies with nearby molecules. The computation power increases rapidly.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Sponsored Links