Nitrogen oxide (NO) has been found to be a key component in many biological processes. It also can react with oxygen to give the brown gas \(\mathrm{NO}_{2}\). When one mole of NO reacts with oxygen, \(57.0 \mathrm{~kJ}\) of heat is evolved. (a) Write the thermochemical equation for the reaction between one mole of nitrogen oxide and oxygen. (b) Is the reaction exothermic or endothermic? (c) Draw an energy diagram showing the path of this reaction. (Figure \(8.4\) is an example of such an energy diagram.)

First, we need to write the balanced chemical equation for the reaction between nitrogen oxide (NO) and oxygen (O2), which forms nitrogen dioxide (NO2). The balanced equation is: NO + 1/2 O2 -> NO2 Now, add the heat evolved (-57.0 kJ) as a term in the equation: NO + 1/2 O2 -> NO2 + (-57.0 kJ) The thermochemical equation for this reaction is: NO + 1/2 O2 -> NO2, ΔH = -57.0 kJ/mol

The change in enthalpy (ΔH) for the reaction is negative (-57.0 kJ/mol). If the ΔH is negative, it means that heat is released during the reaction, making it an exothermic reaction.

An energy diagram shows the path of a chemical reaction in terms of the energy level of the reactants and products. In this case, the energy of the reactants (NO and O2) is higher than the energy of the product (NO2). To draw the energy diagram, follow these steps: 1. On the y-axis, mark the energy levels of the reactants and products, using the given energy change (-57.0 kJ/mol) as a reference. Since the reaction is exothermic, the energy level of the products will be lower than that of the reactants. 2. Label the axes. The x-axis represents the reaction progress, and the y-axis represents the energy level. 3. Draw a downward-sloping curve from the energy level of the reactants to the energy level of the products, with the minimum point of the curve representing the transition state (the highest energy point in the reaction). 4. Label the curve with the ΔH value (-57.0 kJ/mol) and indicate the direction of the reaction (from reactants to products). The energy diagram should resemble a downward-sloping curve, with the energy level of reactants higher than that of the products, indicating that the reaction is exothermic.