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

[sylwesh98]

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!

 

[Borek]

What is the expression for the reaction velocity in this particular case?

 

[sylwesh98]

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

 

[Borek]

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.

 

[sylwesh98]

(-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

 

[Borek]

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.

 

[sylwesh98]

Sir can u please give me one example of those sort of equations please!
So that I can remember them!

 

[Borek]

Google is your friend, try to search for "rate equation".

 

[sylwesh98]

OK sir
Do u mean?
Rate =[a]*k

 

[sylwesh98]

Do u mean that rate constant is related to pressure?

 

[Borek]

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.

 

[sylwesh98]

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

 

[Borek]

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.

 

[sylwesh98]

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?

 

[epenguin]

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.

(-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.