This is the final part of a four part series in the Energy Diagram Module.

Click on the following link to see earlier parts:

Part 1

Part 2

Part 3

 

For each of the following questions- choose the best answer among the four answer choices.

Scroll to the bottom to see the answer key.

 

Question1

The net amount of heat released from the formation of products in a reaction is represented by _______________.

A) Temperature

B) Activation Energy

C) Heat of Reaction

D) Entropy

 

Question 2

A transition state represents _______________.

A) The highest potential energy molecule of the mechanism

B) The lowest potential energy molecule of the mechanism

C) The a molecule that does not occur in the reaction

D) A new reactant introduced to the reaction.

 

Question 3

A high energy, sometimes isolatable, chemical species during the course of reactants to products is referred to as a _______________.

A) Product

B) Midpoint

C) Transition State

D) Intermediate

 

Question 4

Refer to the Gibbs Free Energy Equation

ΔG⁰ = ΔH⁰ – TΔS

Under which circumstance will DG⁰ _­NEVER be spontaneous. (Hint: spontaneous reactions have a negative ΔG⁰)

A) ΔH⁰ < 0 and ΔS < 0

 ΔH⁰ > 0 and ΔS < 0

C) ΔH⁰ > 0 and ΔS > 0

D) ΔH⁰ < 0 and ΔS > 0

 

 

 

ANSWER KEY:

1. C

2. A

3. C

4. B

This is part 3 of a four part series in the Energy Diagram Module. Stay tuned for Part 4!

 Click on the following links to see earlier parts:

Part 1

Part 2

Sometimes reactions are more complex than simply a transition state (Graph 3), which would represent a single step in the reaction mechanism.  You will soon see most reactions proceed in a multistep fashion.  In this case, reaction mechanisms often form lower energy and sometimes isolatable intermediates.   The reaction intermediate occurs between two transition states however its energy is still higher than either products or reactants.  Note that the activation energy between reactant and the intermediate (step 1, ΔG^‡₁) is greater than the activation energy between the intermediate and the products (step 2, ΔG^‡₂). Thus it can be said that step 1 is the rate-limiting step of the reaction, which is the highest energy barrier that must be overcome.

 

 

 

Graph 3

(click on image to enlarge)

 

This is part 2 of a four part series in the Energy Diagram Module. Stay tuned for the other parts!

To see part 1 click here.

In order to talk about energy of the reaction, a few key concepts are needed.

• If the products have less potential energy than the reactants, the reaction will release a net amount of energy (an exothermic reaction).
• If the products are higher in energy than the reactions, the reaction will consume a net amount of energy (an endothermic reaction).

But, in order to predict how well a reaction will progress, or how spontaneous the reaction will be, enthalpy is insufficient to make this estimation.  Therefore, we rely on the thermodynamic calculation of Gibbs Free Energy (ΔG⁰) which is represented by the equation;

ΔG⁰ = ΔH⁰ – TΔS

The components of Gibbs Free Energy are:

• Enthalpy, ΔH⁰ – The heat consumed or released by the reaction.
• Temperature, T – Temperature of the system.
• Entropy, ΔS.  The change in the degree of disorder.

The sign of the reaction indicates the release (negative) or absorption (positive) of heat during the reaction. Therefore, if ΔG⁰ is negative the reaction is always spontaneous.  Similarly, if ΔG⁰ is positive the reaction is never spontaneous.  When ΔG⁰ equals zero, the reaction is at equilibrium. In accounting for these additional thermodynamic properties using Gibbs Free Energy, different terms are used.

• If a reaction has a negative  ΔG⁰ , it is therefore spontaneous and is said to be exergonic.
• Conversely, a reaction that has a  positive  ΔG⁰  is not spontaneous is considered endergonic.

 

Graph 2

(click on photo to enlarge)

With the addition of the temperature variable in the Gibbs Free Energy equation, it is easy to see that an endergonic reactions can be driven forward simply by increasing the temperature of the reaction so that the term TΔS is more negative than ΔH⁰, thus making ΔG⁰ negative and making the reaction spontaneous!

This is part 1 of a four part series in the Energy Diagram Module.

In order to talk about reaction mechanisms, we have to first understand what makes up a reaction. It may help to think of a reaction as a landscape.  Let us look at the components in Graph 1 below.

• the x-axis as the reaction proceeds (the reaction coordinate).
• The potential energy of the molecules throughout the reaction along the y-axis.
• The shape of the line graph represents the pathway (or mechanism) by which the reactants must take to reach the products.
• Using the landscape analogy, reactants and products that are stable can be thought of as the “valleys”, which have some potential energy (refer to Graph 1).
• As reactants collide during the course of a reaction, they must go over a “hill” in order to each the product of the reaction.  This hill represents the energy needed to break the existing bonds in the reactions (the activation energy, ΔG^‡)
• At the top of the hill, some hybrid molecule (the transition state, [ ]‡) is formed which represents the simultaneous (often referred to as concerted) bond breaking and bond forming.  Because of the hybrid nature of the molecule, transition states are NOT isolatable.

The activation energy is usually provided by sum of the surrounding system and the collision of the reactant molecules. For reactions with high activation energy barriers, a Bunsen burner or heating element can be used to add energy.  Looking at Graph 2, as transition states form, the reaction can “roll” downhill both ways, backwards to reactants or forwards to products. As products begin to form, which as illustrated is much lower in energy than the reactants, the net release of energy (the heat of reaction, ΔH⁰) is released back to the surrounding system in the form of heat (enthalpy).

Graph 1

(click on photo to enlarge)

Graph 2

(click on photo to enlarge)

 

So you want to know the secret to getting an “A” in Organic chemistry? Let me tell you the fail-proof method of conquering what many deem the most difficult class in undergraduate studies.

Are you ready?

Well, here it is, here is the trick to pulling the Ace:

Studying

Not the answer you were expecting? I mean there’s got to be an easier way, right? Well sorry to disappoint you, but the only way you will see that “A” on your transcript next to Organic chemistry is by good old fashion studying. Of course just telling students to study is easier said than done. Organic chemistry is not something that most of us can inherently pick up and understand. Let me elaborate a little on this simple word, studying.

Here at StudyOrgo.com we teach a proven method that has our students scoring in the top of their class, however they still take the time to learn and comprehend the material a by studying.  We give them the secret sauce though, making it easier and their study time more efficient.

Organic chemistry is a lot of information. Probably more information than most students have been required to know for any class up to this point. It is not enough to just try to memorize all the reactions, names, and concepts. Many students who have tried this method end up not doing as well as they’d like. You cannot memorize Organic chemistry, you must understand the information, which you can accomplish with rigorous studying.

As a student, you must have a strong understanding of some very fundamental chemical phenomena. We make this easy using our Summary Guides that we call “Benny’s Notes”.

Just to name a few of the most repeating concepts in Organic chemistry, you must understand what acids and bases do (which protonates and which depronates?), which elements are electronegative, what are partial charges and dipoles, and one of the most important concepts, how an atoms electrons interact with a chemical system.

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At that point, it will not be memorization though, it will be complete understanding. You will become familiar with why certain atoms behave the way they do, and often times atoms behave the same way  in many reactions, so there is overlapping information.

This is the secret to studying Organic chemistry, you must understand the reactions, do not try to memorize them! You are now an enlightened individual and have the key to success in Organic chemistry. Now let’s talk a little bit about how you should study and what tools you can use to help. After every lecture, it is pertinent that you review your notes before the next lecture. The information will become much easier to digest in smaller bits than waiting until 4 lectures have passed to review the information. Your notes will highlight the main ideas that you as a student are responsible to know. Go over them, practice them, do not stop until you understand the information.

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