[INeedSerotonin]

Consider the reaction of conversion of A to C, represented by the following chemical equation:

A ----------> B ----------> C
      slow             fast



The letters A, B, and C represent the different involved structures, and x, y, and z indicate the relative energy.

It is correct to say that

A) A and B represent the reagents of the direct reaction;
B) the activation energy of the direct reaction is given by z - y, while the activation energy of the inverse reaction is y - x;
C) the conversion of A to C occurs in two steps;
D) the rate of heat flow involved in the reaction is obtained by the sum of energy represented by y + x;
E) B represents the activated complex.

I don't understand why couldn't (A) and (B) be the answer. Could you guys please explain it to me?

Thank you

[mjc123]

B is an intermediate, not a starting material.
What is the activation energy of the reverse reaction? What is the barrier C has to get over to go to A?
(By the way, in English we talk of "forward" and "reverse" reactions, not "direct " and "inverse". "Direct reaction" might imply a reaction in which A goes directly to C without involving B as an intermediate.)

[INeedSerotonin]

B is an intermediate, not a starting material.

But isn't B a reagent to C? So intermediates are not considered to be reagents of the forward reaction?

What is the activation energy of the reverse reaction? What is the barrier C has to get over to go to A?
(By the way, in English we talk of "forward" and "reverse" reactions, not "direct " and "inverse". "Direct reaction" might imply a reaction in which A goes directly to C without involving B as an intermediate.)

Thanks; I didn't know the correct terminology.

I was thinking that it was y - x, but now I think that it is not. I think that it should be something like this:



So perhaps the answer is "w", which is unknown?

Or perhaps, still, the energy to make C was z - x, so to do all the reverse would require the same z - x?

[mjc123]

So intermediates are not considered to be reagents of the forward reaction?

No, they are not.

Why are you drawing something going downwards? If W was lower than A and C, they would both go to W.
Do you know the definition of activation energy?
Suppose this was a relief map and you were proposing to walk from A to C (or C to A) over the mountains.
How high would you have to climb starting from A? Starting from C?

[INeedSerotonin]

Why are you drawing something going downwards? If W was lower than A and C, they would both go to W.
Do you know the definition of activation energy?

I made that curve going downwards because I thought that the reverse reaction should be completely opposite to the forward reaction. The reagents need to have their bonds broken, receiving energy, and the products must have their bonds attached, releasing energy. But indeed now I see it doesn't make much sense. To undo it, the products need to receive energy, so the curve should be upwards.

I learnt that activation energy is the energy needed to start a reaction. So it is the vertix of the enthalpy curve minus the starting point. According to this idea, the activation energy of C --> A should be the vertix of the curve (z) minus the starting point (x)? What confused me is that there are two vertices.

Suppose this was a relief map and you were proposing to walk from A to C (or C to A) over the mountains.
How high would you have to climb starting from A? Starting from C?

Well, I'm currently terrible at Geography  but A is on "y", and C is on "x", so perhaps the distance should be the absolute value of (y - x)? Still, that would be the energy needed for C to gain to become A again, and not the activation energy, right?

But if it were the activation energy, then wouldn't that proposition be correct? It says "the activation energy of the direct reaction is given by z - y, while the activation energy of the inverse reaction is y - x". Since the correct option is (C), then something in this proposition must be wrong.

[mjc123]

I learnt that activation energy is the energy needed to start a reaction

That is not a precise enough definition

So it is the vertix of the enthalpy curve minus the starting point.

Yes

According to this idea, the activation energy of C --> A should be the vertix of the curve (z) minus the starting point (x)?

Correct

What confused me is that there are two vertices.

You need the highest one. A molecule that had enough energy to get over the first barrier to B, but not enough to get over the second to A, would not accomplish the reaction.

A is on "y", and C is on "x", so perhaps the distance should be the absolute value of (y - x)? Still, that would be the energy needed for C to gain to become A again, and not the activation energy, right?

Right. It is common to confuse the enthalpy change of the reaction (difference between products and reactants) with the activation energy (difference between reactants and transition state - the maximum of the curve).

[INeedSerotonin]

Things are starting to make sense to me! Thank you!