Pressure Equilibrium 2a+b: Why is Velocity x27?

Click For Summary

Discussion Overview

The discussion revolves around the relationship between pressure changes and reaction velocity in the context of the equilibrium reaction 2a + b ⇌ product. Participants explore how an increase in pressure affects the rate of the forward reaction, with specific focus on the claim that the velocity increases by a factor of 27 when pressure is tripled.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant initially claims that increasing pressure threefold results in a ninefold increase in reaction velocity, while another asserts it should be 27 times.
  • Several participants inquire about the expression for reaction velocity and rate equations, indicating a lack of clarity on the topic.
  • There is a suggestion that the rate law is related to the stoichiometry of the reaction, with one participant attempting to derive the relationship between pressure and rate.
  • Another participant points out that the rate equation can be expressed in terms of partial pressures, leading to confusion about how this relates to the increase in pressure.
  • A later reply acknowledges a misunderstanding regarding the contributions of both reactants to the rate, suggesting that the rate is proportional to the cube of the pressure when both reactants are considered.
  • One participant emphasizes that the relationship between reaction rate and concentration is not universally defined and depends on the specific reaction mechanism.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between pressure and reaction velocity, with no consensus reached on the correct factor by which the velocity increases. The discussion remains unresolved regarding the general applicability of the rate law and the specific conditions under which the claims are made.

Contextual Notes

There are limitations in the discussion regarding assumptions about the reaction mechanism and the specific conditions under which the rate laws apply. Participants have not established a clear framework for how pressure influences the reaction rate in this case.

sylwesh98
Messages
42
Reaction score
0
2a+b⇌product
if pressure is increased 3 times of the initial pressure the velocity of the forward reaction will be of___________ the previous velocity?
Actually i got 9 times but the answer is given 27 times
Can anyone please give the hint about it?
Thanks in advance!
 
Physics news on Phys.org
What is the expression for the reaction velocity in this particular case?
 
I think there no particular expression for this case but if there is please mention !
I don't even have any clue how to solve this sum!
Please help
 
What is general expression for the reaction velocity?

This is one of the first things taught when talking about kinetics, I don't believe you don't have it in the book or it wasn't shown during the lecture.
 
(-da/dt)*1/2=-db/DT
Sir this is the equation of rate law for that equation
But how come it is related to the increase in pressure
 
That's OK, unfortunately, that's not what I had on mind.

Have you heard about rate equations? They combine (elementary) reaction equation with the reaction rate.
 
Sir can u please give me one example of those sort of equations please!
So that I can remember them!
 
Google is your friend, try to search for "rate equation".
 
OK sir
Do u mean?
Rate =[a]*k
 
  • #10
Do u mean that rate constant is related to pressure?
 
  • #11
sylwesh98 said:
OK sir
Do u mean?
Rate =[a]*k

Close. Please note it should reflect the reaction equation.

Do u mean that rate constant is related to pressure?

Rate equation can be expressed using either concentrations or partial pressures of the substances involved in the elementary reaction.

Don't sir me, and don't use textspeak (that is, no "u" for "you" and so on). This is described in the forum rules.
 
Last edited:
  • #12
I still don't get it it can be written in the form of partial pressure also but how come it is a actually related to the increase in the pressure
Partial pressure =mole fraction of the solute * total pressure applied
 
  • #13
sylwesh98 said:
Partial pressure =mole fraction of the solute * total pressure applied

Mole fraction is pressure independent (no idea why you list it as "mole fraction of the solute" - there is no solvent and solute in the gas phase). When the total pressure goes up, partial pressure goes up as well.
 
  • #14
OK I got it
Actually I was doing wrong with this equation
I was doing wrong with this step!
All the time I was considering only the partial pressure of a and thinking that the rate is the square of the pressure applied
Actually I forgot to consider b also and then it will become rate is proportional to the cube of the pressure applied
So if pressure is increased over 3 times then the rate of the reaction increases by 27 times!
AM I correct?
 
  • #15
As far as we know there are some known unknowns here. This cannot reasonably be a legitimate question - you either were given some more information or you should have been. There is no general law for the dependence of an overall reaction rate on concentration, it depends on the mechanism. If rate is proportional to a2b then you should be told, but then the conclusion is fairly trivial.

sylwesh98 said:
(-da/dt)*1/2=-db/DT
Sir this is the equation of rate law for that equation
But how come it is related to the increase in pressure

This is not a rate law, this is restating the stoichiometry of your first equation: two molecules of a react with one of b, therefore two of a disappear for every one of b in the reaction.

You probably need to quote the whole question and where it comes from.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
3K
Replies
5
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
2
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 4 ·
Replies
4
Views
8K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 8 ·
Replies
8
Views
3K