Question

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.)

Short answer

Answer: The reaction between nitrogen oxide and oxygen is exothermic, as the change in enthalpy (ΔH) is negative (-57.0 kJ/mol).

Step-by-step solution

Write the thermochemical equation

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

Determine if the reaction is exothermic or endothermic

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.

Draw an energy diagram of the 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.

Key concepts

Exothermic Reaction

In chemical reactions, heat can either be absorbed or released. When a reaction releases heat, it's known as an exothermic reaction. A good example is the reaction between nitrogen oxide (NO) and oxygen (O₂) to produce nitrogen dioxide (NO₂). Here, 57.0 kJ of heat is evolved for each mole of NO reacting.

For exothermic reactions:

• The enthalpy change (ΔH) is negative.
• Heat is given off to the surroundings.
• Such reactions often feel warm to the touch.

Understanding whether a reaction is exothermic helps in predicting temperature changes and energy requirements. This is crucial in fields like chemistry and engineering, where control of heat is vital.

Energy Diagram

An energy diagram visually represents the energy changes during a chemical reaction. For the reaction of NO and O₂ to form NO₂, the diagram illustrates the exothermic nature.

Here's how to interpret the energy diagram:

• Y-axis: Represents the energy level.
• X-axis: Shows the progress of the reaction over time.

The energy of the reactants (NO and O₂) is initially higher. As the reaction progresses, the energy decreases, ending lower with the products (NO₂).

• The downward slope in the diagram indicates energy release.
• The curve's minimum represents the transition state, the highest energy point.

These diagrams help visualize energy shifts and understand the energy efficiency of reactions.

Enthalpy Change

Enthalpy change (ΔH) measures the heat change during a reaction at constant pressure. It helps in understanding the energy dynamics of chemical reactions.

For the given reaction, ΔH is -57.0 kJ/mol, indicating an exothermic reaction. Here's what to note:

• A negative ΔH signifies that heat is released.
• A positive ΔH would mean heat absorption, marking an endothermic reaction.

Enthalpy change calculations are crucial for determining reaction spontaneity and energy needs. In this reaction, the negative ΔH shows that forming NO₂ releases energy to the surroundings, making it advantageous for energy-saving processes.