Video Transcript
This chemical equation describes the reaction of carbon monoxide and nitrogen dioxide gas molecules. CO gas plus NO2 gas react to produce CO2 gas plus NO gas. The reaction occurs as carbon monoxide and nitrogen dioxide molecules combine and form a transition-state complex. This information is described in the following graph. Which label represents the potential energy of the transition-state complex?
First off, while there seems to be a lot of information presented in this question, the focus is on which of the labels in the energy diagram depicts the energy of the transition-state complex for this reaction. To determine this, we need to understand what information an energy diagram provides.
A reaction profile or energy diagram demonstrates the changes in energy at different steps in a chemical reaction. Notice the 𝑥-axis represents the progress of the reaction, from reactants on the left, represented by the capital letter R, to products on the right, represented by the capital letter P. 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.
Referencing the chemical reaction provided, this energy diagram demonstrates the energy changes along the reaction pathway of the reactants, carbon monoxide and nitrogen dioxide, colliding and reacting with one another to form new products, carbon dioxide and nitrogen monoxide. In addition to depicting the energy of the reactants and products in a chemical reaction, an energy diagram also shows the energy of the activated complex, also referred to as the transition-state complex.
An activated complex or transition complex is defined as a high-energy chemical structure where chemical bonds in the reactants are being broken and new bonds are being formed. More specifically, the transition-state complex is formed at maximum energy or demonstrates the highest energy due to the instability of the configuration of atoms, which depicts reacting bonds being broken simultaneously as new bonds are being formed to make the products. We can draw this using dashed lines to indicate which bonds are breaking and which bonds are forming. Once the high-energy transition-state complex is formed, it should be followed by a decrease in energy as the reactant bonds are broken and new bonds are formed to make the products of the chemical reaction.
Looking at the energy diagram provided, the energy of the transition-state complex should occur in between the energy of the reactants and products along the reaction pathway and be the highest point of energy compared to the energy of the reactants and products. We will notice labels A, B, D, and E are all at a lower energy than label C, because labels A and B are associated with the energy of the reactants and labels D and E are associated with the energy of the products. The high energy level of label C corresponds with the description of the transition-state complex, making label C the correct answer.
Therefore, which label corresponds to the energy of the transition-state complex? The correct answer is label C.
