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An endothermic reaction is a chemical process that absorbs energy, specifically in the form of heat, from its surroundings. In this particular reaction between nitrogen gas (\(\mathrm{N}_2\)) and oxygen gas (\(\mathrm{O}_2\)), heat energy is required to break the strong bonds present in the diatomic molecules, forming nitrogen monoxide (\(\mathrm{NO}\)).
This absorption of heat is what makes the reaction endothermic. As a result, the reaction feels cooler to the touch, because it is drawing in heat from the environment. The enthalpy change, denoted as \(\Delta H\), for this reaction is \(+181.8 \, \mathrm{kJ}\), indicating that the products have a higher energy than the reactants. This positive \(\Delta H\) signifies that the reaction is absorbing energy. When describing reactions, the sign of \(\Delta H\) is crucial:

• Positive \(\Delta H\): Endothermic, heat absorbed.
• Negative \(\Delta H\): Exothermic, heat released.

Understanding whether a reaction is endothermic or exothermic helps predict how it will behave under different conditions.

The concept of moles is central to understanding chemical reactions, as it allows chemists to quantify substances. In this exercise, we needed to know how many moles of \( \mathrm{N}_2 \) are present when given a mass of 2.20 grams. To calculate the number of moles, you use the formula:
\[\text{Moles} = \frac{\text{Mass in grams}}{\text{Molar mass in g/mol}} \]
Given that the molar mass of \( \mathrm{N}_2 \) is approximately 28.02 g/mol, the calculation becomes:
\[\text{Moles of } \mathrm{N}_2 = \frac{2.20 \, \text{g}}{28.02 \, \text{g/mol}} \approx 0.0785 \mathrm{mol}\]
This step is crucial because chemical reactions are depicted in terms of moles. Understanding moles allows us to relate the mass of a substance to the amount (in moles) required or produced in a chemical reaction, which further enables calculations of energy changes, like enthalpy changes in this example.

Reaction stoichiometry refers to the quantitative relationships between reactants and products in a chemical reaction. It is based on the balanced chemical equation, which in this exercise is:
\( \mathrm{N}_2(g) + \mathrm{O}_2(g) \rightarrow 2 \mathrm{NO}(g) \)
This equation tells us that one mole of nitrogen gas reacts with one mole of oxygen gas to produce two moles of nitrogen monoxide. The stoichiometric coefficients (the numbers in front of the molecules) indicate the proportions in which the substances react and form.
Stoichiometry is essential when calculating enthalpy changes. In this scenario, the enthalpy change (\(\Delta H = +181.8 \, \mathrm{kJ}\)) is based on one mole of \(\mathrm{N}_2\) reacting. However, if you have a different amount of \(\mathrm{N}_2\), you must scale \(\Delta H\), using: \[\Delta H' = \text{Moles of } \mathrm{N}_2 \times \frac{\Delta H}{1 \, \text{mol}}\] This ensures that the calculated energy change corresponds correctly to the amount of substance involved in your reaction. Overall, understanding stoichiometry is vital for accurately predicting and calculating chemical reaction outcomes.