1. Explain why during a spontaneous process the entropy change of the system cou

Question

1. Explain why during a spontaneous process the entropy change of the system could be negative. (How could the entropy of the system decrease for a spontaneous process?)

2. Calculate the entropy change of the system (?Ssys), the entropy change of the surroundings (?Ssur), and the entropy change of the universe (?Suni) when 100 g of ice melt at (1) -10?C, (2) 0?C, and (3) 10?C. Predict the spontaneity of each process. The heat of fusion of ice is 6.01 kJ/mol.

3. Determine ?S rxn for H2(g) + I2(g) ? 2HI(g) given the following information. Can you predict the spontaneity of this reaction?

Substance

S? (J/mol

Substance

S? (J/mol

Explanation / Answer

The second law of thermodynamics states that for any spontaneous process the overall ?S must be greater than or equal to zero, yet a spontaneous chemical reaction can result in a negative change in entropy. This does not contradict the second law, however, since such a reaction must have a sufficiently large negative change in enthalpy (heat energy) that the increase in temperature of the reaction surroundings (considered to be part of the system in thermodynamic terms) results in a sufficiently large increase in entropy that overall the change in entropy is positive. That is, the ?S of the surroundings increases enough because of the exothermicity of the reaction that it overcompensates for the negative ?S of the system, and since the overall ?S = ?Ssurroundings + ?Ssystem, the overall change in entropy is still positive.

Another way to view the fact that some spontaneous chemical reactions can lead to products with lower entropy is to realize that the second law states that entropy of anisolated system must increase (or remain constant). Since a negative enthalpy change in a reaction means that energy is being released to the surroundings, then the 'isolated' system includes the chemical reaction plus its surroundings. This means that the heat release of the chemical reaction sufficiently increases the entropy of the surroundings such that the overall entropy of the isolated system increases in accordance with the second law of thermodynamics.

Spontaneity does not imply that the reaction proceeds with great speed. For example, the decay of diamonds into graphite is a spontaneous process that occurs very slowly, taking millions of years. The rate of a reaction is independent of its spontaneity, and instead depends on the chemical kinetics of the reaction. Every reactant in a spontaneous process has a tendency to form the corresponding product. This tendency is related to stability. Stability is gained by a substance if it is in a minimum energy state or is in maximum randomness. Only one of these can be applied at a time. e.g. Water converting to ice is a spontaneous process because ice is more stable since it is of lower energy. However, the formation of water is also a spontaneous process as water is the more random state.

Related Questions

Navigate

• Browse (All)
• Browse 1
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.