Which of the following statements is false?

Question

A. If a system is at steady state and has only one entering stream and only one exiting stream, the molar flow rates of the entering and exiting streams must be equal.
B. For a closed, stationary, constant-volume system with no moving parts, the 1st Law of Thermodynamics becomes ?U = Q.
C. A cyclic process starts and ends at a specific state (T1, P1). A different cyclic process that also starts and ends at the state (T1, P1) may produce different amounts of work.
D. The generalized energy balances are applicable in outer space.
E. We cannot generally write “volume balances,” but there are circumstances in which volume is constant.

Answer 1

A. Correct. For this statement to be true, we must also know that no chemical reactions are taking place or their product and reactant stoichiometric coefficients are equal. That is, for aA + bB ->cC +dD then (a + b) – (c+d) must equal zero. In the reaction A + B -> C the exiting molar flowrate will be half the entering flowrate if the reaction goes to completion.
B. Incorrect. This statement is true. Review Equation 3.35 and its terms.
C. Incorrect. This statement is true. Because work is not a state function, two processes with identical start and end states can have wildly different outputs.
D. Incorrect. This statement is true. The generalized energy balance is applicable to any system. Note that the generalized energy balance has the term g. If we substituted the value 9.81 m/s2 for g, then we have something that is only true on earth specifically.
E. Incorrect. This statement is true. Volume is not a conserved quantity so the idea of a “generalized volume balance” is completely invalid, but in some specific circumstances we will know or be safe in assuming a fluid has a constant fluid density(s).