Question

question 6 (4 points)
which reaction produces a decrease in the entropy of the system?
o a) 4 nh3 (g) + 5 o2 (g) = 4 no (g) + 6 h2o (g)
o b) na (s) + 1/2 cl2 (g) → nacl (s)
o c) 2 hgo (s) → 2 hg (l) + o2 (g)
o d) uf6 (s) → u (s) + 3f2 (g)
o e) h2o (s) → h2o (g)

Explanation:

To determine which reaction decreases entropy, we analyze the change in the number of moles of gas (or phase change to more ordered states). Entropy generally decreases when there's a reduction in the number of gas molecules or a transition to a more ordered phase (e.g., gas to liquid/solid).

• Option a: Reactants have \(4 + 5 = 9\) moles of gas; products have \(4 + 6 = 10\) moles of gas. Entropy increases.
• Option b: Reactants have \( \frac{1}{2} \) mole of gas (and 1 mole of solid); product is 1 mole of solid. The gas is consumed, and the system becomes more ordered (solid from solid + gas). Entropy decreases.
• Option c: Reactant is solid; products are liquid + gas. Entropy increases (gas is produced).
• Option d: Reactant is solid; products are liquid + gas. Entropy increases (gas is produced).
• Option e: Reactant is liquid; product is gas. Entropy increases (liquid to gas).

So, the reaction in option b decreases the entropy of the system.

Answer:

b. \( \text{Na (s)} + \frac{1}{2} \text{Cl}_2 \text{(g)}
ightarrow \text{NaCl (s)} \)