The reaction profile for a chemical reaction is shown in the diagram below. Which label corresponds to the activation energy?
First off, we are being asked to identify the activation energy in the diagram. 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, the reactants won’t be able to transform into the products and the reaction won’t proceed. To determine if 𝐴, 𝐵, or 𝐶 depicts the activation energy of the chemical reaction, let’s review what information a reaction profile provides about a chemical 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 potential energy stored in a compound, 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 reactants on the left to the products on the right. In this reaction profile, this is also indicated by the double-headed arrow at the bottom of the diagram labeled with the letter 𝐶, which means we can eliminate label 𝐶 because it is related to the reaction progress and is not the activation energy.
Going back to the potential energy positions of the reactants and the products, the potential energy level of the reactants in comparison to the potential energy level of the products indicates that the chemical reaction is an exothermic reaction. An exothermic reaction is defined as a chemical reaction where energy is released from the reacting chemicals to the surroundings. This can be denoted with a negative Δ𝐻, or negative enthalpy change value. The vertical double-headed arrow stretching from the reactants to the products, also denoted as label 𝐵 in the diagram, is known as the enthalpy change, or Δ𝐻, of the reaction and, as we demonstrated off to the side of the diagram, should be a negative Δ𝐻 to indicate the release of energy as the higher-potential-energy reactants were transformed into lower-potential-energy products.
The enthalpy change, or Δ𝐻, is defined as the difference in energy between the products and the reactants. Based on this information, the portion of the diagram indicated by label 𝐵 is not the answer because it is the enthalpy change of the reaction and not the activation energy. This means that the correct answer is the portion of the diagram depicted by label 𝐴. This corresponds with the definition listed for the activation energy in that it occurs prior to the product potential energy position, indicating it is an energy associated with reacting particles. And it demonstrates energy levels higher than the reactants, indicating it is the boost or increase of energy needed for reactant particles successfully colliding with one another to create new products.
Therefore, which label corresponds to the activation energy? Label 𝐴 corresponds to the activation energy.