Understanding Equilibrium Calculations: Debunking Misconceptions

[Entanglement]

My solution for this question is Kc=1/10

As the ratio of both forward and backward rate constants never changes unless the temperature changes.

My teacher told me that the answer is 1/100 because the reaction's products and reactants are doubled, that seems senseless to me, I don't want to go in an argument with him until I make sure whether my answer is right or wrong,
so I need your help, thanks in advance

 

[Borek]

Your teacher is right.

Have you wrote Kc for both reactions? Name them Kc and Kc', and try to express one using the other. There is no need to guessing here, pure and simple arithmetic.

 

[Entanglement]

Your teacher is right.

Have you wrote Kc for both reactions? Name them Kc and Kc', and try to express one using the other. There is no need to guessing here, pure and simple arithmetic.

I did that, but I'm not convinced that it should be done, because when A,B and C is doubled the reaction should move toward C opposing the change so Kc remains constant

"I'm sure that you are right but I don't want to blindly accept that answer, I want to know why"

 

[amasider]

Here, I hope this helps! ;)
If you change the stoichiometric numbers, the equilibrium constant (the concentration quotient "Kc" in fact) also changes, because it's dependant on those stoichiometric numbers.

 

[Entanglement]

Here, I hope this helps! ;)
If you change the stoichiometric numbers, the equilibrium constant (the concentration quotient "Kc" in fact) also changes, because it's dependant on those stoichiometric numbers.

Q changes not Kc it's not the same thing

 

[amasider]

At equilibrium it's the same thing ;)

 

[Borek]

Q changes not Kc it's not the same thing

Kc value depends on the way reaction is written.

That's why it is convenient to follow the convention that properly balanced reaction equation uses set of the lowest possible integer coefficients, it removes ambiguity.

 

[Entanglement]

In a closed container if Iodine reacts with hydrogen to give hydrogen iodide, if we opened the container and doubled the amount of iodine, hydrogen, and hydrogen iodide, can we say that Kc Changes ??

 

[Borek]

You are confusing stoichiometric coefficients with concentrations.

Doubling concentrations doesn't change stoichiometric coefficients, doubling stoichiometric coefficients doesn't change concentrations.

 

[Entanglement]

aA + bB \rightarrow cC

If we say that a,b and c are the number of molecules
this means that cC is only produced when aA reacts with bB

2aA + 2bB \rightarrow 2cC

doesn't this equation give the same meaning as the first one ??

 

[Entanglement]

I don't understand the difference between the two equations

 

[Borek]

doesn't this equation give the same meaning as the first one ??

No, they are different.

Lets say we have a simple reaction

A <-> B

and in the mixture at equilibrium concentration of A is 2 M and concentration of B is 3 M.

For the reaction as written

K_c = \frac {<b>}{[A]}</b>

and Kc value is

K_c = \frac {[3]}{[2]} = 1.5

Now let's say we want to write the reaction as

2A <-> 2B

If so

K_c&#039; = \frac {<b>^2}{[A]^2}</b>

Our mixture was at equilibrium, so concentrations are what they are, however, now Kc' is

K_c&#039; = \frac {[3]^2}{[2]^2} = 2.25

Just because we decided to use a different reaction equation value of the equilibrium constant changes, and, obviously

K_c&#039; = K_c^2

That's just the way it is.

 

[Entanglement]

No, they are different.
Lets say we have a simple reaction
A B
and in the mixture at equilibrium concentration of A is 2 M and concentration of B is 3 M.
For the reaction as written
K_c = \frac {<b>}{[A]}</b>
and Kc value is
K_c = \frac {[3]}{[2]} = 1.5
Now let's say we want to write the reaction as
2A 2B
If so
K_c&#039; = \frac {<b>^2}{[A]^2}</b>
Our mixture was at equilibrium, so concentrations are what they are, however, now Kc' is
K_c&#039; = \frac {[3]^2}{[2]^2} = 2.25
Just because we decided to use a different reaction equation value of the equilibrium constant changes, and, obviously
K_c&#039; = K_c^2
That's just the way it is.

That means that the rate constant of the forward reaction of the second equation is square the rate constant of the forward reaction of the first equation and same thing with the rate constant of the backward reaction ?

 

[Borek]

In terms of kinetic theory of chemical equilibrium - yes. But please remember that the reaction kinetics is never guaranteed to follow the reaction equation, and that the kinetic theory of the equilibrium is not the only theory explaining how the equilibrium works and how it is reached.

 

[Entanglement]

Logically, I can't still visualize why 2A , AB react giving 2C faster than A , B

 

[Borek]

Because in reality they don't. What you see is just an artifact of the kinetic theory of equilibrium. It looks as if the reaction kinetic was higher, because the theory blindly assumes reaction kinetic can be calculated from the reaction equation. I told you it is not true, have you read the second phrase of my previous post?

 

[Entanglement]

Because in reality they don't. What you see is just an artifact of the kinetic theory of equilibrium. It looks as if the reaction kinetic was higher, because the theory blindly assumes reaction kinetic can be calculated from the reaction equation. I told you it is not true, have you read the second phrase of my previous post?

Theoretically speaking, the reaction is faster ?

 

[Borek]

No, real reaction speed (just like the equilibrium) doesn't depend on the way the reaction equation is written.

 

[Entanglement]

No, real reaction speed (just like the equilibrium) doesn't depend on the way the reaction equation is written.

Then all the calculations we do is just crap !

 

[Borek]

Then all the calculations we do is just crap !

You are either trolling, or just wasting my time:

please remember that the reaction kinetics is never guaranteed to follow the reaction equation

It looks as if the reaction kinetic was higher, because the theory blindly assumes reaction kinetic can be calculated from the reaction equation. I told you it is not true, have you read the second phrase of my previous post?

I told you twice what the problem is. As we know what the problem is, we DON'T calculate kinetics from the overall reaction equation. OTOH equilibrium calculations are perfectly right.

I can't stop you from doing crappy calculations, but you have been warned.