3 replies to this topic

[Manpreet]

Given a reaction set like

A + A -> A + A*

B + B -> B + B*

A* + B* -> C + D

For determining rate expression, should i use PSSH(pseudo steady state hypothesis) or RDS(rate determining step)?
Are both valid in this case? Even if they give different rate expressions?

[Francisco Angel]

Dear Manpreet:

You decide if to apply some of these approaches based on the relative values of the kinetics of each reaction. You assume, for example, that one or several reactions are fast, so the reactants' concentration satisfy the equilibrium constant value (pseudo steady state), whereas another reaction rate is determined from a kinetic expression (rate determining step).

Long story short, you need to have at hand and compare numerical values for kinetic and equilibrium expressions to see if some of these approximations are reasonable.

Best regards.

[Manpreet]

It actually was a question in my exam, where i had to get expression for rate equation relating concentrations of reactants.

Only information given was the reaction set, and rate expression was asked.

I used RDS, as it was faster to get the answer. but my answer was rejected, saying we can't use RDS here.

No explanation is given yet. So, i am waiting for the discussion session.

Is it that we only use RDS in adsorption/absorption kind of reactions, and not in reactions where active intermediates, like radicals, are formed?

Edited by Manpreet, 14 September 2016 - 02:29 AM.

[Francisco Angel]

Dear Manpreet:

Its really difficult to say what your professor expected answer was, as I understand, you apply some of this approaches when you know a certain reaction will achieve equilibrium fast, compared to the other simultaneous reactions.

As for your professor:
*He may had used one of these approaches in class, and expect you to do a similar process (biochemistry maybe?).

*He may had said explicitly that for example, in radical expressions the conditions for the approximation is not valid (again, based on relative reaction velocities, not because a radical is involved per se).

*etc

*etc

 

Without context, its difficult to say. Exams world is not the same as real world.

Best regards.