Question Video: Describing the Transfer of Energy in an Energy Profile Chemistry • 10th Grade

Illustrated in the reaction profile are the changes in chemical energy as reacting molecules are converted to products. Which of the following is the best description of this chemical reaction? [A] Energy is absorbed by the reaction, and the products are less stable than the reactants. [B] Energy is absorbed by the reaction, and the products are more stable than the reactants. [C] Energy is released by the reaction, and the products are less stable than the reactants. [D] Energy is released by the reaction, and the products are more stable than the reactants.

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Video Transcript

Illustrated in the reaction profile are the changes in chemical energy as reacting molecules are converted to products. Which of the following is the best description of this chemical reaction? (A) Energy is absorbed by the reaction, and the products are less stable than the reactants. (B) Energy is absorbed by the reaction, and the products are more stable than the reactants. (C) Energy is released by the reaction, and the products are less stable than the reactants. Or (D) energy is released by the reaction, and the products are more stable than the reactants.

This question asks us to interpret the reaction profile shown in the diagram. The 𝑦-axis represents the amount of chemical energy in the system. Higher up on the 𝑦-axis represents atoms and molecules with high energy, while lower down on the graph represents atoms and molecules with lower energy. The 𝑥-axis represents the progress of the reaction. As we move from left to right on the reaction profile, we will see how the energy levels change from the beginning of the reaction to the end of the reaction. More specifically, we can say that the plateau on the left of the graph represents the energy level of the reactants, while the plateau on the right of the graph represents the energy level of the products.

Looking at our answer choices, there are two key pieces of information we need to obtain before we can select the correct answer. First, is energy released or absorbed over the course of this reaction? And second, are the products of the reaction more stable or less stable than the reactants? To answer the first question, we should notice that there is a drop in the energy of the system over the course of the reaction. This means that energy leaves the system. In other words, energy is released. We call a reaction where energy is released an exothermic reaction.

The opposite situation where energy is absorbed over the course of the reaction would involve an upward curve in the reaction profile. The energy of the products would be higher up on the graph than the energy of the reactants. We call reactions like this endothermic reactions. But the reaction that we’re looking at here releases energy. We also need to figure out if the products are more or less stable than the reactants. Since they are lower down on the reaction profile, we know that the products are less energetic than the reactants. But how does energy relate to stability? Well, they are inversely related. The more energetic molecules toward the top of the reaction profile are less stable.

Meanwhile, the less energetic molecules toward the bottom of the reaction profile are more stable. Since the products are lower on the 𝑦-axis, they are less energetic and therefore more stable. It makes sense that something that is less energetic would be more stable. It’s less likely to spontaneously change, combine, break down, or release energy. So our second key piece of information is that the products are more stable than the reactants. This corresponds to answer choice (D). Energy is released by the reaction, and the products are more stable than the reactants. Choice (D) is the correct answer.

Looking at the reaction profile allows us to make claims about the energy and stability of the atoms and molecules involved in a reaction. So, which of the following is the best description of this chemical reaction? That’s choice (D). Energy is released by the reaction, and the products are more stable than the reactants.

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