Understanding Chemical Reactions: The Role of Gibbs Free Energy and Activation Energy
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Introduction
When delving into the realm of chemical reactions, understanding the thermodynamics and kinetics underlying these processes is crucial. Among the key concepts in this area are Gibbs free energy and activation energy. These factors not only determine the feasibility of a reaction but also explain how enzymes facilitate these reactions. In this article, we will explore Gibbs free energy, activation energy, and how enzymes influence these parameters.
What is Gibbs Free Energy?
Gibbs free energy ( Delta G) is pivotal in assessing how much energy can be used in a chemical reaction. To understand Gibbs free energy better, consider a hypothetical reaction where reactants are transformed into products. If a reaction has not yet reached equilibrium, it can exhibit either negative or positive Gibbs free energy:
1. Negative Gibbs Free Energy (Exergonic Reactions)
- Definition: A negative Delta G signifies that the reaction is exergonic and spontaneous, meaning energy is released during the reaction.
- Example: Combustion reactions exemplify exergonic reactions. They release energy, and thus, have a negative Delta G.
2. Positive Gibbs Free Energy (Endergonic Reactions)
- Definition: A positive Delta G indicates that the reaction is endergonic and non-spontaneous, implying that energy input is required for the reaction to occur.
- Example: The synthesis of ATP in biological systems is an endergonic reaction where energy must be supplied.
How to Calculate Gibbs Free Energy
The formula for calculating Gibbs free energy is straightforward:
- ** Delta G = Free Energy of Products - Free Energy of Reactants** This calculation highlights that Gibbs free energy depends solely on the energies of the reactants and products. Notably, the pathway taken to reach from reactants to products (whether direct or through an enzyme substrate complex) does not alter the Gibbs free energy.
The Concept of Activation Energy
Activation energy (EA) is essential for understanding how quickly a reaction can proceed. It refers to the minimum energy required to convert reactants into products. Here’s a breakdown of activation energy:
Understanding Activation Energy
- Definition: Activation energy is the energy barrier that must be overcome for a reaction to occur.
- Visualization: Imagine a hill where the reactants start at a lower energy level, rise to the top (transition state), and descend to form products. The height of the hill represents the activation energy.
Transition State
The transition state is a brief, high-energy state that occurs during a reaction. It does not exist long enough to be studied in a stable form; instead, it represents a momentary peak in energy before products are formed.
Enzymes: Catalysts in Chemical Reactions
Enzymes are biological catalysts that significantly influence the speed of chemical reactions. It’s essential to understand how enzymes work concerning Gibbs free energy and activation energy:
1. Enzymes and Gibbs Free Energy
While enzymes catalyze reactions, they do not change the Gibbs free energy of the reaction. This means:
- The energy of the reactants and products remains constant whether an enzyme is present or not.
- The Delta G value is the same in catalyzed and uncatalyzed reactions.
2. Enzymes and Activation Energy
Enzymes specifically lower the activation energy, thereby facilitating quicker reactions. Here's how enzymes impact activation energy:
- Lowering the Transition State Energy: Enzymes stabilize the transition state, effectively lowering the hill (activation energy) the reactants need to overcome.
- Speeding Up Reactions: By reducing the activation energy, enzymes increase the rate at which equilibrium is achieved, allowing reactions to occur rapidly even if they are spontaneous.
Conclusion
Understanding Gibbs free energy and activation energy is fundamental in the study of chemical reactions and how enzymes interact with them. While Gibbs free energy determines the spontaneity of a reaction, activation energy dictates how quickly it can occur. Enzymes play a critical role by lowering activation energy, enabling efficient biochemical processes without altering the inherent stability of reactants or products. By comprehending these concepts, we can appreciate the intricate dance of biological reactions that sustain life.
so when we once understand the way that a chemical reaction takes place we usually study the thermodynamics and the
kinetics of that chemical reaction and that involves studying things like Gibbs free energy which involves enthalpy and
entropy as well as studying things like activation energy of that chemical reaction now because enzymes act on
chemical reactions if we are to actually understand how enzymes behave and act on those chemical reactions we also have to
study the Gibbs free energy and the activation energy of that chemical reaction so let's begin by discussing
Gibbs free energy then we'll look at activation energy and we'll finish off with how the enzyme actually effects
these two quantities so let's begin by supposing that we have the following hypothetical reaction so we have
reactants being transformed into products now we're going to assume that the reaction has not reached equilibrium
and what that basically means is the reaction can either have a negative Gibbs free energy or a positive Gibbs
free energy so what is Gibbs free energy well the Gibbs free energy loosely speaking describes how much energy can
be used in that chemical reaction so let's suppose we have the following graph so the y-axis is the energy value
and the x-axis is the reaction progress so these are the reactants here and the energy value of the reactant is
somewhere here now the products have a free energy value that is equal to somewhere here and notice that the
products have a low free energy than the reactants now to calculate mathematically the Gibbs free energy of
this reaction all we have to do is take the free energy of the products and subtract the free energy of the
reactants and that gives us the Gibbs free energy given by Delta G so this quantity here is how much energy is
going to be released in this reaction and it's basically how much we can use in some process now for this
particular case this reaction describes an exergonic reaction and exergonic reactions always have a negative Delta G
so a chemical reaction is said to be exergonic and spontaneous if the Delta G is negative and one example of a
spontaneous reaction in nature is combustion so combustion reactions are examples of exergonic reactions where
the Delta G value is negative now what about the opposite well if we read this reaction going backwards if this is the
reactant and this is the product that if we subtract a hi free energy from a low free energy we're going to get a
positive Delta G and the positive Delta G means the reaction is endergonic and non spontaneous and that means it will
not take place unless we input a certain amount of energy and one example of an endergonic reaction that is not
spontaneous is the synthesis of ATP molecules inside our body so to synthesize ATP we have to actually input
energy and the ATP molecules when they break down that is an exergonic reaction and energy is released and every time we
break down ATP molecules inside our body energy is released and we can use that energy to basically power different
types of processes that take place inside our body that require those ATP molecules so on the other hand a
chemical reaction is set to be endergonic and non spontaneous if the Delta G is positive and ATP synthesis is
an example of such an endergonic reaction so we can see that if we know what the Gibbs free energy value is of
some particular reaction we know whether or not that reaction is actually spontaneous now another
important fact that you have to know about this quantity Gibbs free energy is Gibbs free energy only depends on the
energy the free energy value of the products and the free energy value of the reactants so if we know what the
free energy of the products is and the free energy of the reactants all we have to do subtracted two to find that Gibbs
free energy so the pathway that we take when we go from the reactant to the products does not actually determine
does not change what the Gibbs free energy is it doesn't matter if we take pathway one two or three when we go from
the reactants to products Gibbs free energy will not actually change so if we for example compare a reaction that has
an enzyme and that same reaction that is on catalyzed does not have an enzyme the Gibbs free energy in those two reactions
will be exactly the same so a catalyzed and an uncannily reaction will have the same exact Gibbs free energy value and
that leads us to a very important point enzymes when they act on chemical reactions they do not affect the Gibbs
free energy value they do not change the energy of the reactants nor they actually change the energy of the
products and that's exactly why the difference namely the Delta G the Gibbs free energy will remain exactly the same
when an enzyme is used or when an enzyme is not used now the final thing I'd like to mention about Gibbs free energy is
in fact when Gibbs free energy is zero that reaction is said to have reached equilibrium and in that moment in time
the rate of the for reaction is equal to the rate of the reverse reaction so if the Gibbs free energy is zero the
reaction has achieved equilibrium and is said to be neither spontaneous nor non spontaneous in such a case the rate of
the four reaction going from reactants to products is equal to the rate of the reverse reaction going from products
back to reactants now let's move on to activation energy so what exactly is the activation energy well any reaction has
some activation energy and this is simply the amount of energy that we have the input for the reaction to take place
to convert the reactants to the products or in reverse now let's suppose we go from reactants to products in this case
our activation energy is simply this quantity here it's the difference between the energy of the molecule found
on this topmost portion of the hill and the energy of that reactant this is the Gibbs free energy given by Delta G with
the symbol on top or simply Delta e a where the a stands for activation now this topmost apex of the hill
describes the energy of the transition state of this chemical reaction and if you're if you recall from organic
chemistry the transition state is not something that exists for a very long time and that's because it has a very
high energy value as can be seen by the following diagram this apex has the highest energy value in that reaction
and that's precisely why the transition state does not exist for a very long time and in fact because it doesn't
exist for a very long time it's unstable and we can't actually study how the transition state looks like we can't
isolate it and we can't examine it because it it quickly converts into the products so the activation energy Delta
order to actually get it going now the activation energy describes how quickly a reaction actually takes place
so a reaction can be spontaneous it can have a negative Delta G value but it can take place very very slowly and
if a reaction takes place very slowly what that means is it has a very high activation energy so activation energy
is not the same thing as Gibbs free energy Gibbs free energy basically describes the difference between the
energy of the reactants and the products but activation energy describes how quickly a reaction actually takes place
so Gibbs free energy talks about where that equilibrium will be achieved while activation energy talks about how
quickly that equilibrium will actually be achieved and so once again as we'll see in more detail in a future lecture
the apex of this curve describes the energy of the transition state now what exactly does the enzyme do and how does
the enzyme affect the activation energy so we said previously that the enzyme does not change the Gibbs free energy of
the reaction it has no effect on the energy of the reactants and the products and so their difference the Delta G is
exactly the same it remains unchanged when the enzyme acts on that chemical reaction but the enzyme does have an
effect under activation energy in fact what the enzyme typically does is it actually lowers the energy of that
it makes this mountain smaller and so this height will be smaller and the Delta G the Gibbs free on the activation
energy of that reaction will become smaller and if we decrease the activation energy by essentially
stabilizing that transition state we will speed up the react because ultimately it's the activation
energy it's the energy barrier that determines the kinetics the speed and the rate of that chemical reaction so
enzymes do not affect the equilibrium they have no effect on the Gibbs free energy of that reaction the thermal free
energy of the reactants and the free energy of products remains unchanged in any catalyzed reaction however what the
enzymes do is they stabilize the transition state lower its energy they lower the energy of that transition
state and so they decrease the activation energy and that speeds up that chemical reaction so once again
enzymes when they act on chemical reactions they do not change the Gibbs free energy and that means they do not
increase or decrease how much products is formed at the end of that reaction but they basically allow equilibrium to