Final answer:
Chemical reactions can be categorized as endothermic or exothermic based on their energy transfer; endothermic reactions absorb energy and exothermic reactions release it. The given reactions and processes were categorized accordingly, with 2H₂ + O₂ forming water as exothermic, and N₂ + O₂ forming NO as endothermic. Boiling water, gasoline burning, and ice forming are endothermic, exothermic, and exothermic processes, respectively.
Step-by-step explanation:
Defining Exothermic and Endothermic Reactions
To categorize a reaction as endothermic or exothermic, one must consider whether the reaction absorbs or releases energy, respectively. For each of the given reactions and processes:
- a. The reaction 2H₂(g) + O₂(g) → 2H₂O(l) + 135 kcal releases energy in the form of heat (135 kcal), thus this is an exothermic reaction.
- b. The reaction N₂(g) + O₂(g) + 45 kcal → 2NO(g) requires an input of energy, indicated by the 45 kcal on the reactant side, making it an endothermic reaction.
- c. Boiling water absorbs heat, so it's an endothermic process.
- d. Gasoline burning releases heat and is an exothermic process.
- e. Ice forming on a pond releases heat into the surroundings, marking it as an exothermic process.
In case of the reaction mentioned for question (a), the number of gas molecules decreases from 3 to 2, indicating a decrease in the number of gas molecules.
Understanding Energy and Chemical Reactions
Chemical reactions involve the transfer of energy. Activation energy is required for all chemical reactions, serving as the energy barrier that reactants must overcome to transform into products. Whether a reaction is categorized as endothermic or exothermic can typically be determined by observing temperature changes or analyzing an energy diagram.