Problem 107 - Introductory Chemistry Atoms First

Question

Indicate whether each reaction is endothermic or exothermic: (a) \(\mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{CH}_{4}+2 \mathrm{O}_{2} \Delta E_{\mathrm{rxn}}=+890 \mathrm{~kJ}\) (b) \(\mathrm{CH}_{4}+2 \mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O} \Delta E_{\mathrm{rxn}}=-890 \mathrm{~kJ}\) (c) \(\mathrm{S}+\mathrm{O}_{2} \rightarrow \mathrm{SO}_{2}+\) Heat (d) \(\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightarrow 2 \mathrm{NH}_{3} \Delta E_{\mathrm{rxn}}=-92 \mathrm{~kJ}\) (e) Heat \(+\mathrm{NH}_{4} \mathrm{NO}_{3} \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow} \mathrm{NH}_{4}^{+}(a q)+\mathrm{NO}_{3}^{-}(a q)\) (f) \(2 \mathrm{H}_{2}+\mathrm{O}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O} \Delta E_{\mathrm{rxn}}=-479 \mathrm{~kJ}\)

Step-by-Step Solution

Answer

(a) Endothermic (b) Exothermic (c) Exothermic (d) Exothermic (e) Endothermic (f) Exothermic

Step 1: Reaction (a)

The given reaction is: \(CO_2 + 2H_2O \rightarrow CH_4 + 2O_2\) The ΔE_rxn value is +890 kJ, which means energy is absorbed by the reaction. Therefore, this reaction is endothermic.

Step 2: Reaction (b)

The given reaction is: \(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\) The ΔE_rxn value is -890 kJ, which means energy is released by the reaction. Therefore, this reaction is exothermic.

Step 3: Reaction (c)

The given reaction is: \(S + O_2 \rightarrow SO_2 +\) Heat In this case, no ΔE_rxn value is given, but heat is being released in the reaction. Therefore, this reaction is exothermic.

Step 4: Reaction (d)

The given reaction is: \(N_2 + 3H_2 \rightarrow 2NH_3\) The ΔE_rxn value is -92 kJ, which means energy is released by the reaction. Therefore, this reaction is exothermic.

Step 5: Reaction (e)

The given reaction is: Heat \(+ NH_4 NO_3 \stackrel{H_2 O}{\longrightarrow} NH_4^+ (aq) + NO_3^− (aq)\) In this case, no ΔE_rxn value is given, but heat is being absorbed in the reaction. Therefore, this reaction is endothermic.

Step 6: Reaction (f)

The given reaction is: \(2H_2 + O_2 \rightarrow 2H_2O\) The ΔE_rxn value is -479 kJ, which means energy is released by the reaction. Therefore, this reaction is exothermic.

Key Concepts

Chemical Thermodynamics

Chemical thermodynamics is the field of chemistry that deals with energy transformations in chemical reactions. This concept focuses on understanding how and why energy changes occur during reactions.
Thermodynamics provides the tools to measure these changes and predict the behavior of substances.

• It involves studying laws that govern energy interactions in chemical processes.
• Key concepts include energy, entropy, and equilibrium.
• By analyzing these factors, chemists can predict whether a reaction will occur spontaneously and how it will proceed.

Understanding these principles helps decipher whether a reaction absorbs or releases energy, making it either endothermic or exothermic.

Energy Changes in Reactions

During chemical reactions, energy changes occur that can either absorb or release energy. This is a fundamental aspect of chemical reactions, affecting their progression and results.

• In endothermic reactions, energy is absorbed from the surroundings, indicated by a positive ΔE (e.g., +890 kJ in reaction a).
• Exothermic reactions release energy to the surroundings, shown by a negative ΔE (e.g., -890 kJ in reaction b).

For reactions where ΔE is not given explicitly, other indicators like 'heat' in the reaction equation suggest energy release, hence exothermic.
Recognizing these patterns aids in classifying and understanding chemical reactions.

Enthalpy Change

Enthalpy change, denoted as ΔH, is a measure of the total heat content change in a chemical reaction at constant pressure.
This is crucial for determining whether a reaction absorbs or releases heat.

• A positive ΔH indicates an endothermic process, where energy is taken in from surroundings.
• A negative ΔH signals an exothermic process, where energy is released.

In chemical equations, observing the sign of ΔH helps understand the thermodynamics of the reaction. Whether you see terms like "heat + reactants" or "products + heat," these provide hints about the enthalpy change.
Mastering this concept enhances the understanding of how energy is transferred during chemical reactions.