Imagine that 1.00 mol H2O(l) is placed in a piston with a constant external pres
ID: 630607 • Letter: I
Question
Imagine that 1.00 mol H2O(l) is placed in a piston with a constant external pressure Pext = 1 bar. The piston is then placed in a large thermal bath at a temperature of 270 K. Let water be the system. Using relationships derived in class, one can calculate that the freezing of water (to form ice, a crystal) is ?G = -65 J and ?S = -21 J/K. A student claims that this process cannot be spontaneous, because S must increase for a spontaneous process. Is the student correct? If so, why? If not, why not?Explanation / Answer
The student is in correct because : For a reaction at constant temperature and pressure, ?G in the Gibbs free energy is : ?G=?H-T?S When ?G is negative, a process or chemical reaction proceeds spontaneously in the forward direction. Examining the signs of the two terms on the right side of the equation. 1.When ?S is positive and ?H is negative, a process is always spontaneous 2.When ?S is positive and ?H is positive, a process is spontaneous at high temperatures, where exothermicity plays a small role in the balance. 3.When ?S is negative and ?H is negative, a process is spontaneous at low temperatures, where exothermicity is important. 4.When ?S is negative and ?H is positive, a process is not spontaneous at any temperature, but the reverse process is spontaneous. The above problem follows the third condition, i.e., at low temperature (=270K here) When ?S is negative and ?H is negative, a process can spontaneous also.