Consider the equilibrium dissociation of N2O4(g) to NO2(g).

N2O4(g) ⇌ 2NO2(g)

Using your knowledge of Lewis structures and bond energies (bond breaking vs. bond forming) to predict whether this reaction is

Question

Consider the equilibrium dissociation of N2O4(g) to NO2(g).

N2O4(g) ⇌ 2NO2(g)

Using your knowledge of Lewis structures and bond energies (bond breaking vs. bond forming) to predict whether this reaction is

BlackTom AI · Accepted Answer

Question restatement: The equilibrium involves N2O4(g) ⇌ 2 NO2(g), and you are asked to predict whether this reaction is endothermic or exothermic based on Lewis structures and bond energies.
Option analysis:
Option 1: endothermic.
- Consider the structural change: N2O4 consists of two NO2 units connected by an N–N single bond. When it dissociates to 2 NO2 molecules, the N–N bond must be broken to yield two separate NO2 radicals.
- Importantly, the N–O bonds that exist within each NO2 unit are not broken in the overall process; you just separate the two NO2 fragments. Therefore, the primary energetic requirement is the energy input to break the N–N single bond between the two halves of N2O4.
- Since bond breaking requires energy, and no compensating new bond formation occurs (beyond the existing intra-NO2 bonds which remain intact in the products), this reaction absorbs energy from the surroundings, making it endothermic.
Option 2: exothermic.
- If this reaction were exothermic, we would expect net energy release due to bond formation surpassing bond breaking. However, as explained above, there is no formation of stronger new bonds during the process; only a bond (N–N) is cleaved, while the existing N–O bonds within each NO2 unit persist in the products. Therefore, there is no energetically favorable bond-forming step to offset the bond breaking.
- A common source of confusion is counting the total number of bonds; here the change focuses on breaking the interunit N–N linkage while intraunit bonds remain, so the overall energy change is not driven by new bond formation.
In summary, the key idea is that dissociation requires breaking a covalent N–N bond without creating an equivalent or stronger new bond, which leads to energy absorption and an endothermic process.

Consider the equilibrium dissociation of N2O4(g) to NO2(g). N2O4(g) ⇌ 2NO2(g) Using your knowledge of Lewis structures and bond energies (bond breaking vs. bond forming) to predict whether this reaction is单项选择题

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