Chemguide Energy Profiles

8/10/2019 Chemguide Energy Profiles

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ENERGY PROFILES FOR SIMPLE REACTIONS

energy profiles for reactions are slightly different for

reactions involving an intermediate or just a transitionstate.

What is an energy profile?

If you have done any work involving activation energyor catalysis, you will have come across diagrams likethis:

This diagram shows that, overall, the reaction isexothermic. The products have a lower energy than thereactants, and so energy is released when the reaction

happens.

It also shows that the molecules have to possessenough energy (called activation energy) to get thereactants over what we think of as the "activation


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energy barrier".

In this example of a reaction profile, you can see that a

catalyst offers a route for the reaction to follow whichneeds less activation energy. That, of course, causesthe reaction to happen faster.

Diagrams like this are described as energy prof i les. Inthe diagram above, you can clearly see that you needan input of energy to get the reaction going. Once theactivation energy barrier has been passed, you can

also see that you get even more energy released, andso the reaction is overall exothermic.

If you had an endothermic reaction, a simple energyprofile for a non-catalysed reaction would look like this:

Unfortunately, for many reactions, the real shapes ofthe energy profiles are slightly different from these, andthe rest of this page explores some simple differences.


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What matters is whether the reaction goes via a singletransition state or an intermediate. We will look at thesetwo different cases in some detail.

Energy profiles for reactions which go via a singletransition state only

This is much easier to talk about with a real example.The equation below shows an organic chemistryreaction in which a bromine atom is being replaced byan OH group in an organic compound. The startingcompound is bromoethane, and the organic product isethanol.

During the reaction one of the lone pairs of electrons onthe negatively charged oxygen in the -OH group isattracted to the carbon atom with the bromine attached.

That's because the bromine is more electronegativethan carbon, and so the electron pair in the C-Br bondis slightly closer to the bromine. The carbon atombecomes slightly positively charged and the bromineslightly negative.

As the hydroxide ion approaches the slightly positive


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carbon, a new bond starts to be set up between theoxygen and the carbon. At the same time, the bondbetween the carbon and bromine starts to break as the

electrons in the bond are repelled towards the bromine.

At some point, the process is exactly half complete.The carbon atom now has the oxygen half-attached,the bromine half-attached, and the three other groupsstill there, of course.

And then the process completes:

Note: These diagrams have been simplifiedin various ways to make the process clearer.

For example, the true arrangement of the lonepairs of electrons around the oxygen in thefirst diagram has been simplified for clarity.The bromine also has 3 lone pairs as well asthe bonding pair, but they play no part. And, of


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course, the other groups attached to thecarbon have been left out in order toconcentrate on what is important.

The second diagram where the bonds are half-madeand half-broken is called the t ransi t ion s tate, and it isat this point that the energy of the system is at itsmaximum. This is what is at the top of the activationenergy barrier.

But the transition state is entirely unstable. Any tinychange in either direction will send it either forward tomake the products or back to the reactants again.Neither is there anything special about a transition stateexcept that it has this maximum energy. You can'tisolate it, even for a very short time.

The situation is entirely different if the reaction goesthrough an intermediate. Again, we'll look at a specific


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example.

Energy profiles for reactions which go via anintermediate

For reasons which you may well meet in the organicchemistry part of your course, a different organicbromine-containing compound reacts with hydroxideions in an entirely different way.

In this case, the organic compound ionises slightly in aslow reaction to produce an intermediate positiveorganic ion. This then goes on to react very rapidly withhydroxide ions.

Note: If you haven't come across the use ofcurly arrows in organic chemistry yet, all you

need to know for now is that they show themovement of a pair of electrons. In the firstequation, for example, the bonding pair ofelectrons in the C-Br bond moves entirely onto the bromine to make a bromide ion. In the


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second equation, a lone pair on the hydroxideion moves towards the positive carbon to forma covalent bond.

The big difference in this case is that the positivelycharged organic ion can actually be detected in themixture. It is very unstable, and soon reacts with ahydroxide ion (or picks up its bromide ion again). But,for however short a time, it does have a real presence

in the system. That shows itself in the energy profile.

The stability (however temporary and slight) of theintermediate is shown by the fact that there are small

activation barriers to its conversion either into theproducts or back into the reactants again.

Notice that the barrier on the product side of theintermediate is lower than that on the reactant side.


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That means that there is a greater chance of it findingthe extra bit of energy to convert into products. It wouldneed a greater amount of energy to convert back to the

reactants again.

I've labelled these peaks "ts1" and "ts2" - they bothrepresent transition states between the intermediateand either the reactants or the products. During eitherconversion, there will be some arrangement of theatoms which causes an energy maximum - that's all atransition state is.

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