Multiple Choice
The following reaction is spontaneous at room temperature: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g). Which of the following is true about the enthalpy (ΔH) and entropy (ΔS) changes for this reaction?
19. Chemical Thermodynamics
Spontaneous vs Nonspontaneous Reactions
Multiple Choice
We do not observe the diamond converting to graphite under normal conditions. Which of the following is the correct explanation?
A
The reaction is nonspontaneous, and the equilibrium favors diamond.
B
The reaction is spontaneous, but the rate of the reaction is extremely slow.
C
The reaction is spontaneous, but the mechanism of the reaction is extremely complex.
D
The reaction is nonspontaneous, and the activation energy is too high.
Verified step by step guidance1
Understand the concept of spontaneity in chemical reactions. A spontaneous reaction is one that occurs naturally under given conditions without needing external energy input. However, spontaneity does not imply that the reaction happens quickly.
Consider the thermodynamics of the conversion of diamond to graphite. Graphite is thermodynamically more stable than diamond, meaning the reaction from diamond to graphite is spontaneous under standard conditions.
Examine the kinetics of the reaction. Even if a reaction is thermodynamically favorable (spontaneous), it may proceed at an extremely slow rate if the activation energy is high.
Recognize that the conversion of diamond to graphite involves breaking strong covalent bonds in the diamond structure, which requires significant energy input, resulting in a very slow reaction rate.
Conclude that the correct explanation is that the reaction is spontaneous, but the rate of the reaction is extremely slow due to the high activation energy required to break the diamond's strong covalent bonds.
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