Energy Profile

Have you ever looked in the mirror and turned your head to the side, in order to see your profile? You probably noticed the outline of your nose, your jawline, and maybe a side shot of your hairdo. An energy profile is kind of the same idea, you change your perspective in order to see things more simply by reducing the dimension.  

Energy Profile Energy Profile

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Table of contents
    • This article is about the energy profile diagram.
    • We will learn the basics of the diagram and see what each segment represents.
    • We will look at how the energy difference between products and reactants affects how the diagram is structured.
    • Then we will see how catalysts change the energy profile.
    • Lastly, we will look at the energy profile diagram for multistep reactions.

    The Energy Profile of a Reaction

    A chemical reaction involves a transfer of energy as reactants are converted into products. We illustrate this idea using an energy profile diagram.

    An energy profile diagram shows the theoretical "energy pathway" of a reaction as it progresses from reactants to products.

    An energy pathway is a road that a reaction travels to get from one place (the reactants) to its destination (the products). The type of energy that this pathway tracks is potential energy.

    Potential energy is the energy stored within bonds; It is "stored" energy. The higher the potential energy of a species, the more reactive and unstable it is.

    A system always wants to be as stable and low energy as possible, which is why high potential energy species are very reactive and unstable. Another way to think of it is that the potential energy is how much energy is required to hold a molecule together. As we move along our reaction road, there is always a "hill" we have to clear, called the activation energy.

    Activation energy is the minimum energy needed for a reaction to proceed. How large this energy is, is dependent on the difference in energy between the reactants and products. The higher in energy the products relative to their reactants, the higher the activation energy.

    Even when the reactants are higher in energy, there is still a hill to clear. That is because it requires energy to break chemical bonds. The creation of bonds is what releases energy. The shape of an energy diagram will always be a hill for this reason, but the "steepness" of the hill and the length of the "downhill" portion is dependent on the energy of the reactants and products. In this article, we will be looking at the different kinds of energy diagrams and learning how to interpret them.

    Energy Profile Diagram for Exothermic and Endothermic Reaction

    An energy profile shows how the energy of a system changes as the reaction progresses. It is synonymous with an energy diagram/energy profile diagram.

    As stated previously, the energy profile diagram follows the change in potential energy. The y-axis is the potential energy and the x-axis is the reaction coordinate/reaction progress. This represents the progress of the reaction from reactants to products. The reactants are labeled on the left and the products are labeled on the right. To get a clearer picture, we will be walking through the different types of energy diagrams.

    By looking at an energy diagram, we can determine whether a reaction is exothermic or endothermic.

    An exothermic reaction is a reaction where there is a net release of energy. This means that the products have a lower potential energy than the reactants. An endothermic reaction is a reaction where there is a net gain of energy. The products are higher in energy than the reactants, so the system requires an overall gain of energy to get there

    The diagrams for these types of reactions are shown below:

    Energy Profile Endothermic and Exothermic energy diagram  Energy Profile Diagram for Exothermic and Endothermic Reaction StudySmarterIn an exothermic reaction (left) the reaction has a smaller activation energy since the system is going to a more stable state. In an endothermic reaction (right) more energy is required since the system is going to a less stable state. StudySmarter Original

    Let's break down the different pieces here. The first is the activation energy (EA). The "hill" is much higher for an endothermic reaction since this reaction is thermodynamically unfavored (i.e. the system is becoming less stable), whereas it is much lower for the thermodynamically favored exothermic reaction. The activation energy is measured from the energy level of the reactants to the "peak" of the curve.

    The second piece is the difference in potential energy (ΔE). If ΔE>0, the reaction is endothermic since the potential energy of the system is increasing, while a negative ΔE is exothermic for the opposite reason. We measure this change where Einitial is the energy of the reactants and Efinal is the energy of the products.

    The difference in energy between the reactions can also be shown as a change in enthalpy (ΔH). Enthalpy is the part of the potential energy that can be converted into heat energy. The signs for the change in enthalpy is the same as for the change in potential energy. This also makes sense, since an exothermic reaction (literally meaning "outside heat") is releasing heat, so its heat energy is decreasing (ΔH < 0). However, an endothermic reaction ("inside heat") is gaining heat, so its heat energy is increasing (ΔH > 0). The last important section is the transition state.

    The transition state (also called the activated complex) is the species that exists at the "peak" of the energy diagram, in between when the reactants are present and the products are formed. It does not exist for very long.

    The transition state shows the moment of impact, where the reactants are coming together. For the general equation:$$XY + Z \rightarrow XZ + Y$$The transition state looks like this:

    $$[XY-Z]$$

    Then,

    $$XY + Z \rightarrow [XY-Z] \rightarrow XZ + Y$$

    The reaction is climbing up that hill so that the reactants can react to produce products. This "peak" is when they have finally gotten enough energy to cause a reaction. The energy then decreases because this is the point where the bonds are being formed, which releases energy. The energy also decreases because the transition state breaks apart into products.

    Catalyst energy profile

    Another type of energy profile is the one for reactions with a catalyst.

    A catalyst is a species that speeds up a reaction. While it is used in the reaction, it is never consumed by it.

    A catalyst speeds up a reaction by lowering the activation energy necessary for the reaction to occur. It does this by providing the reaction with an alternate reaction pathway (i.e. the reactants react with the catalyst to get to the same products). The energy of the reactants and products doesn't change, it is the pathway energy that does.

    Another example of a catalyst is an enzyme. An enzyme is a biological catalyst that works slightly differently from other catalysts. Enzymes will bind to the reactant (called a substrate) which forms an enzyme-substrate complex, this acts as our transition state. After binding, the enzyme will then release a new product and the enzyme will go on unchanged. An enzyme is like a mold that our substrate (think of a lump of clay) is placed in, so when it is removed from the mold, it has become something new. Like with other catalysts, enzymes lower the activation energy by providing another reaction pathway.

    Energy Profile Diagram for Decomposition of Hydrogen Peroxide

    To get a better idea of a catalyzed reaction, let's look at the decomposition of hydrogen peroxide.

    Energy Profile Catalyzed versus uncatalyzed energy diagram Energy Profile Diagram for Decomposition of Hydrogen Peroxide StudySmarterThecatalysts for the reaction create an alternate pathway so that the activation energy is lower. StudySmarter Original

    The two things you'll notice about the catalyzed reaction diagram are that there are two humps instead of one, and there are 2 extra species present (Br and H ions). The two humps mean that there is an extra step in the reaction. Catalysts can lower the activation energy by providing an alternate pathway. Think of it like taking a detour to get to your destination faster. There are two catalysts present in this reaction mechanism, and they are the Br and H ions. These ions react with the initial reactant (hydrogen peroxide) to end up with the same products as without the catalysts. Like with taking a detour, the place you are leaving, and your destination doesn't change, but your route does.

    Energy Profile Diagram for Multistep Reactions

    The last type of energy profile we will cover is for multistep reactions. These are reactions that, like the same suggests, have several steps that proceed in order. Think of it like knocking down dominoes: As one falls, the other is knocked over. The catalyst reaction we saw before is an example of a multistep reaction.

    Let's look at another multistep reaction mechanism:

    $$NO + NO \rightarrow N_2O_2$$ $$N_2O_2 + H_2 \rightarrow H_2O + N_2O$$ $$N_2O + H_2 \rightarrow N_2 + H_2O$$ $$\text{Net reaction:}\,2NO + 2H_2 \rightarrow N_2 + 2H_2O$$

    Species like N2O2 and N2O are called intermediates. These are species that are both formed and consumed during the reaction mechanism, so they aren't shown in the net reaction. These are different from transition states, since transition states are the in-between period during a single reaction, while intermediates are actual products for that respective intermediate step. Let's look at the energy diagram for this mechanism:

    Energy Profile Multistep reaction energy profile Energy profile diagram for multistep reactions StudySmarterEach step has its own activation energy, the largest activation energy is the one that dictates the activation energy for the whole reaction. StudySmarter Original

    Each step has its own activation energy and transition state. The dips in the curve show where a product has been formed and is labeled. The overall speed of the reaction is dependent on the step with the largest activation energy (here it is step 2). This step is called the rate-determining step. Picture the reaction like as the flow of traffic: it can only go as fast as the slowest car in front, even if the cars before it can go faster.

    To learn more about the rate-determining step, browse through our explanation on "Multistep reactions"!

    Energy Profile - Key takeaways

    • An energy profile diagram shows the theoretical "energy pathway" of a reaction as it progresses from reactants to products. It shows the change in potential energy (energy of chemical bonds)
    • Activation energy is the minimum energy needed for a reaction to proceed. How large this energy is dependent on the difference in energy between the reactants and products. The higher in energy the products relative to their reactants, the higher the activation energy.
    • In an exothermic reaction, the energy of the reactants is greater than that of the products, while an endothermic reaction is the opposite. The activation energy of an exothermic reaction is smaller for this reason
    • The transition state (also called the activated complex) is the species that exists at the "peak" of a reaction between when the reactants are present and the products are formed. It does not exist for very long.
    • In a catalyst reaction, the activation energy is lowered since the catalyst provides an alternate reaction pathway.
    • In a multistep reaction, each step has its own activation energy and transition state.
    Frequently Asked Questions about Energy Profile

    How to draw energy profile?

    To draw on energy profile diagram, you need to know three things: the potential energy of the reactants, the potential energy of the products, and the activation energy. You draw the diagram by drawing a curve from the potential energy of the reactants, to the activation energy (the curve's peak), and lastly to the potential energy of the reactants. The x-axis is the reaction progress, so the reactants are on the right and the products are on the left.

    how do enzymes affect the energy profile?

    Enzymes provide an alternate pathway for the reaction. Because of this, the diagram will have a new activation energy and may have more "peaks" that represent transition states that occur during this alternate pathway.

    what is an energy profile?

    An energy profile diagram shows the theoretical "energy pathway" of a reaction as it progresses from reactants to products.

    How to calculate delta h from potential energy profiles?

    The delta h (enthalpy change) is the vertical distance between the reactants and products.

    How to calculate reactant energy for a reaction profile?

    If you look at an energy profile, the placement of the reactants on the vertical axis (potential energy), tells us the reactant energy. If you don't have a reaction profile, the reactant energy is called the enthalpy of formation and can be looked up in data tables.

    Test your knowledge with multiple choice flashcards

    How does a catalyst lower the activation energy of a reaction?

    True or False: Intermediates are the same as the transition state

    Which curve represents the uncatalyzed energy profile?

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