You define a closed system in which a chemical reaction A+B?C is occurring, and write an energy balance. How does the presence of the reaction affect the writing of the energy balance?

Question

A. The ?Uterm is replaced by a ?G.
B. The potential energy terms take into account the electron shell changes of the valence electrons of the reacting species’ atoms.
C. The accumulation term in the energy balance is negative, because there are more moles of reactant than product.
D. The reaction doesn’t affect the terms themselves, it only influences the values of the terms.
E. The ?Htermsare replaced with ??H_R^0, or equivalent for the reaction temperature.

Answer 1

A. Incorrect. Internal energy appears in the “accumulation” term of any energy balance. This does not change when a reaction is present, though the reaction means the contents of the system at the end of the process may be different from the contents at the beginning.
B. Incorrect. You are perhaps mixing up “potential energy” and “reaction potential.” The energy balance “potential energy” terms are important when the system experiences a change in position within a gravitational field, and are not related to what is happening at molecular level during the reaction.
C. Incorrect. You are confusing the equations for the equilibrium constant (in which ?? is -1) with the energy balance. A reacting system can have a positive or negative change in stored energy—we cannot say without knowing whether work or heat is involved in the process.
D. Correct.
E. Incorrect. Enthalpy does not appear in closed system energy balances, only internal energy.