Multiple Choice
If you were to touch a flask in which an exothermic reaction is occurring, what would you most likely feel?
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8. Thermochemistry
Endothermic & Exothermic Reactions
Multiple Choice
Which of the following equations represents energy being absorbed as a bond is broken?
A
CH_4(g) + 2O_2(g) ightarrow CO_2(g) + 2H_2O(g) ext{; } riangle H < 0
B
NaCl(s) ightarrow Na^+(aq) + Cl^-(aq) ext{; } riangle H < 0
C
2H(g) ightarrow H_2(g) ext{; } riangle H < 0
D
H_2(g) ightarrow 2H(g) ext{; } riangle H > 0
Verified step by step guidance1
Understand that breaking chemical bonds requires energy input, so the enthalpy change (\(\triangle\) H) for bond breaking is positive (\(\triangle\) H > 0).
Look at each given reaction and identify whether bonds are being formed or broken by analyzing the reactants and products.
For the reaction \( \mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)} \) with \( \triangle H < 0 \), note that the negative enthalpy indicates bond formation (energy released), not bond breaking.
For the reaction \( \mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)} \) with \( \triangle H < 0 \), the negative enthalpy suggests energy is released, indicating bond formation or solvation, not bond breaking.
For the reaction \( \mathrm{H_2(g) \rightarrow 2H(g)} \) with \( \triangle H > 0 \), recognize that the bond in \( \mathrm{H_2} \) is being broken into atoms, which requires energy absorption, matching the condition for bond breaking.
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