Video Transcript
A chemical reaction is found to have a molar reaction enthalpy, Δ𝐻, of negative 80 kilojoules per mole and an activation energy of 25 kilojoules per mole. Which of the following reaction profiles represents this reaction?
A reaction profile or energy diagram demonstrates the changes in energy at different steps in a chemical reaction. Notice the 𝑦-axis shows the level or amount of energy associated with different parts of the chemical reaction, with energy increasing as one goes up the 𝑦-axis and energy decreasing as one goes down the 𝑦-axis. The 𝑥-axis represents the reaction progress, or the progress of the reaction, from the reactants on the left, represented by the capital letter R, to the products on the right, represented by the capital letter P. Energy level diagrams can be used to show how the total energy has changed during a chemical reaction, including whether the reaction is exothermic or endothermic.
An exothermic reaction is defined as a chemical reaction where energy is released from the reacting chemicals to the surroundings. What this means is that the reactants should be higher in energy and the products are positioned lower along the 𝑦-axis to depict them at a lower energy level due to energy being released. We see this in reaction profiles (B) and (C), and as such are reaction profiles of exothermic reactions.
In a reaction profile, the difference in energy between the products and the reactants is defined as the enthalpy change, where enthalpy is defined as the energy of a system. And the energy typically consists of chemical potential energy and thermal energy, otherwise known as heat. While the symbol for the term enthalpy is a capital letter 𝐻, the enthalpy change for a chemical reaction is denoted with a Δ𝐻 and more specifically for an exothermic reaction, a negative Δ𝐻 or negative enthalpy change value. Because reaction profiles (B) and (C) demonstrate an exothermic reaction, the Δ𝐻 is labeled as a negative Δ𝐻 or negative enthalpy change value.
In contrast, an endothermic reaction is defined as a chemical reaction where energy is absorbed by the reacting chemicals from the surroundings, which means the reactants should be lower in energy and the products should be positioned higher along the 𝑦-axis to depict them at a higher energy level due to energy being absorbed. We see this in reaction profiles (A) and (D). Therefore, they depict an endothermic reaction, which is denoted with a positive Δ𝐻 or positive enthalpy change value due to energy being absorbed or gained by the chemical reaction.
That being said, we are told that the molar reaction enthalpy, also known as the enthalpy change and denoted by the symbol Δ𝐻, is equal to negative 80 kilojoules per mole. As we mentioned, a negative enthalpy change or negative Δ𝐻 occurs when the reaction is exothermic. This means we can eliminate (A) and (D) because both reaction profiles indicate an endothermic reaction or a reaction with a positive Δ𝐻.
This leaves both (B) and (C) as potential answers because their reaction profiles depict an exothermic reaction with a negative Δ𝐻. One more piece of information we’ve been provided that we haven’t discussed yet is that the reaction profile has an activation energy of 25 kilojoules per mole. The activation energy, 𝐸 a, is defined as the minimum amount of energy required by reactant particles to collide and react with each other, otherwise known as the initial increase or boost of energy needed to get the reaction started.
Without enough energy equal to the activation energy for the reaction at hand, the reactants won’t have enough energy to transform into the products and the reaction won’t proceed. To determine if (B) or (C) depicts the reaction where 𝐸 a is equal to 25 kilojoules per mole and the Δ𝐻 is equal to negative 80 kilojoules per mole, let’s finish labeling the reaction profiles by drawing the 𝐸 a portion within their diagrams.
Since the reaction profiles are not drawn to scale with values along the 𝑦-axis, let’s use what we know about the relative heights of the 𝐸 a and Δ𝐻 on the desired reaction profile. The 𝐸 a being equal to 25 kilojoules per mole should demonstrate a shorter height in comparison to the enthalpy change being equal to negative 80 kilojoules per mole. In other words, they should not demonstrate equal heights in the desired reaction profile. Upon looking at reaction profile (C), we notice that 𝐸 a and Δ𝐻 heights are about equal. This means we can eliminate (C) as a possible answer because the 𝐸 a and Δ𝐻 should not be equal.
When we compare the 𝐸 a and Δ𝐻 of reaction profile (B), we see that the 𝐸 a is about a third of the height, or a third of the energy, that the Δ𝐻 is, or three 𝐸 a’s would make up almost one Δ𝐻. Therefore, which reaction profile represents the chemical reaction with an 𝐸 a of 25 kilojoules per mole and a Δ𝐻 of negative 80 kilojoules per mole? The answer is reaction profile (B).
