Video Transcript
The reaction profile for a chemical reaction is shown in the diagram below. Which label corresponds to the change in enthalpy, Δ𝐻, of the reaction.
The question is asking, which label in the diagram, A, B, or C, is the change in enthalpy denoted as Δ𝐻 for this chemical reaction? The enthalpy change of a reaction is defined as the difference in energy between the products and the reactants. To determine which of the three labels depicts the enthalpy change of this chemical reaction, let’s review what information is provided in a reaction profile.
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.
In this reaction profile, this is also indicated by the double-headed arrow at the bottom of the diagram labeled with the capital letter C, which means we can eliminate label C as a possible answer for this question because it is related to the reaction progress and is not the enthalpy change of the reaction. In addition to demonstrating the reaction progress, energy level diagrams can be used to show how the total energy has changed during a chemical reaction or the change in energy from the reactants to the products.
The vertical double-headed arrow stretching from the reactant energy level to the product energy level, denoted as label B in the diagram, is known as the enthalpy change or Δ𝐻 of the reaction. This is because the energy difference between the reactant energy level and the product energy level by definition is called the enthalpy change. Based on this information, the portion of the diagram indicated by label B is the answer to this question because label B depicts the change in energy that occurred between the reactants and products.
Even though we have determined the correct answer to this question is label B, let’s confirm this by discussing what label A depicts on this reaction profile. With the portion of the reaction pathway indicated by label A occurring before the product’s energy level is reached, it indicates it is an energy associated with reactant particles, and it demonstrates energy levels higher than the reactants, indicating it is the boost or increase of energy needed for reactant particles to successfully collide with one another to create new products. This means label A is the activation energy or 𝐸 a, which is defined as the minimum amount of energy required by reactant particles to collide and react with each other.
Without enough energy equal to the activation energy for the reaction at hand, the reactant won’t be able to transform into the products and the reaction won’t proceed. As a result, label A is not the correct answer for this question because the activation energy is not the same as the change of enthalpy for this reaction. Therefore, which label corresponds to the change in enthalpy of the reaction? The answer is label B corresponds to the enthalpy change of the reaction.
