[Dan]

This leads me to one last question. I think it is best to give an example:

- { d[H2]  / dt  }
 - { d[H2]  / dt  } = k[H2],
d[H2] / [H2] = -kdt
In[H2] – In[H2]0 = -kt

Why is it that doing this with first-order reactions gives a log, but for zero and 2nd orders it would be [H2] = -kt + [H2]0  and [H2]-1 = kt + [A]0-1 respectively?

The only answer I can give to that is: because that's what you get when you integrate it. It's a result of the calculus. Sorry I can't help more with that one.

Maybe this will help:

https://www.khanacademy.org/math/integral-calculus/indefinite-definite-integrals/indefinite_integrals/v/antiderivative-of-x-1
https://www.khanacademy.org/math/differential-calculus/taking-derivatives/der_common_functions/v/proof-d-dx-ln-x-1-x

[backjames24]

It is a shame what happened to your new building, one of my choices was Nottingham.

In a book I was reading it said that

k = Ae –(EA/RT)  or [ In k = In A – (EA/RT) ] can be linked to rate law,

what is the link to the In [a/(a-x)] = k’t from the document I sent?

[Dan]

Yes, the fire was a blow for the department.

In a book I was reading it said that

k = Ae –(EA/RT)  or [ In k = In A – (EA/RT) ] can be linked to rate law,

This won't help you here.

Do you understand how to integrate the second equation to get to the third equation?

what is the link to the In [a/(a-x)] = k’t from the document I sent?

k' is just a new constant. Since k₂ and b are constant, we can combine them to make a new constant k' = k₂b

[backjames24]

k' is just a new constant. Since k₂ and b are constant, we can combine them to make a new constant k' = k₂b

I see Dan,
What is happening with the building now?

So looking at the last equation we got of ln [ a / (a-x) ] =k 't , we can do [a / (a − x)] vs time where the gradient will be k'. How can we find the half-life for this or the so called relaxation time? Looking at some problems on the internet now, I am now able to understand. From the rate values the order in respect to each reactant

[Dan]

I see Dan,
What is happening with the building now?

It will be rebuilt. There is a thread here about it.

So looking at the last equation we got of ln [ a / (a-x) ] =k 't , we can do [a / (a − x)] vs time where the gradient will be k'.

Almost, a plot of ln[a/(a-x)] vs t will hve gradient k'

How can we find the half-life for this

Hint: After the half life time has elapsed, what is the relationship between a and x?

[backjames24]

I see Dan,
What is happening with the building now?

It will be rebuilt. There is a thread here about it.

So looking at the last equation we got of ln [ a / (a-x) ] =k 't , we can do [a / (a − x)] vs time where the gradient will be k'.

Almost, a plot of ln[a/(a-x)] vs t will hve gradient k'

How can we find the half-life for this

Hint: After the half life time has elapsed, what is the relationship between a and x?

(a-1/2x) (b-x)

[Dan]

Please show working/explanation, I don't understand what you did there.

[backjames24]

Please show working/explanation, I don't understand what you did there.

I was thinking that since

[S2O82-] = (what you started with) - (what has reacted)
              = a - x

then first half life would be a-0.5x, then a-0.25x etc

[Dan]

OK. Be careful, x is a varying parameter, not a constant value. a is a constant value, let's work with that.

So if half of the persulfate reacts, and you started with a, how much iodine has been produced (in terms of a)?

In other words: what is the value of x, in terms of a, at t_1/2?

[backjames24]

OK. Be careful, x is a varying parameter, not a constant value. a is a constant value, let's work with that.

So if half of the persulfate reacts, and you started with a, how much iodine has been produced (in terms of a)?

In other words: what is the value of x, in terms of a, at t_1/2?

1/2 a? so 1/2 of the elemental iodine has formed. by following the procedure in experiments in physical chemistry the starch is blue-black --> colourless when added at the end to stop it going lumpy. Why is it this way around , I would of thought the starch would form complex with the iodine at endpoint . Sorry if I am asking basic questions, I have catching up to do

[Dan]

OK. Be careful, x is a varying parameter, not a constant value. a is a constant value, let's work with that.

So if half of the persulfate reacts, and you started with a, how much iodine has been produced (in terms of a)?

In other words: what is the value of x, in terms of a, at t_1/2?

1/2 a?

Yes, when t = t_1/2, x = 0.5a

Now calculate t_1/2 using the appropriate equation.

by following the procedure in experiments in physical chemistry the starch is blue-black --> colourless when added at the end to stop it going lumpy. Why is it this way around , I would of thought the starch would form complex with the iodine at endpoint . Sorry if I am asking basic questions, I have catching up to do

Let's finish the half life question first.

[backjames24]

OK. Be careful, x is a varying parameter, not a constant value. a is a constant value, let's work with that.

So if half of the persulfate reacts, and you started with a, how much iodine has been produced (in terms of a)?

In other words: what is the value of x, in terms of a, at t_1/2?

1/2 a?

Yes, when t = t_1/2, x = 0.5a

Now calculate t_1/2 using the appropriate equation.

by following the procedure in experiments in physical chemistry the starch is blue-black --> colourless when added at the end to stop it going lumpy. Why is it this way around , I would of thought the starch would form complex with the iodine at endpoint . Sorry if I am asking basic questions, I have catching up to do

Let's finish the half life question first.

What equation do you speak of? I attempted the problem with 1/ k[A]₀ which i got from the integrated 2nd order law and got lost.

[Dan]

The equation is written in your first post.

I am having great difficulty steering you in the right direction, because it's not clear to me what your logic is. Why and what are you doing with 1/k[A]₀?

Please:

1. State exactly what you are trying to do.
2. Show what you are doing and explain the logic behind it.

[backjames24]

The equation is written in your first post.

I am having great difficulty steering you in the right direction, because it's not clear to me what your logic is. Why and what are you doing with 1/k[A]₀?

Please:

1. State exactly what you are trying to do.
2. Show what you are doing and explain the logic behind it.

Forget about that Dan,

[Dan]

I agree, well done.