Question

Select all of the following equations that would most likely give a positive value for ΔS. -- How do you determine the value of ΔS just by a reaction???

1. AgNO₃ (aq) + KCl (aq) → AgCl (s) + KNO₃ (aq)

2. 2 NOBr (g) → 2 NO (g) + Br₂ (g)

3. 2 Al (s) + 3 Cl₂ (g) → 2 AlCl₃ (s)

4. N₂ (g) + O₂ (g) → 2 NO (g)

5. 2 NO (g) + O₂ (g) → 2 NO₂ (g)

6. 2 NO₂ (g) → N₂O₄ (g)

7. 2 NH₃ (g) → N₂ (g) + 3 H₂ (g)

8. 2 CH₄ (g) → C₂H₄ (g) + 2 H₂ (g)

Answer 1

1. AgNO₃ (aq) + KCl (aq) → AgCl (s) + KNO₃ (aq)

This reaction will not give positive entropy. It is because, 2 moles solution produces 1 mol of solid and 1 mol of solution. Hence, the disorder of the system decreases.

2. 2 NOBr (g) → 2 NO (g) + Br₂ (g)

2 moles of gaseous reactants produces 3 moles of gaseous products. Since the number of gaseous products increases, hence entropy of the system increases. Hence entropy is positive

3. 2 Al (s) + 3 Cl₂ (g) → 2 AlCl₃ (s)

1 mol of solid and 3 moles of gases produces 2 moles solids. Hence, entropy is negative.

4. N₂ (g) + O₂ (g) → 2 NO (g)

2 moles of gases produces 2 moles of gases. So, there will be no change in entropy. Hence, the entropy is not positive.

5. 2 NO (g) + O₂ (g) → 2 NO₂ (g)

3 moles of gases produces 2 moles of gases. Hence, change in entropy is negative.

6. 2 NO₂ (g) → N₂O₄ (g)

2 moles of gases produces 1 mol of gas. Hence, change in entropy is negative.

7. 2 NH₃ (g) → N₂ (g) + 3 H₂ (g)

2 moles of gases produces 4 moles of gases. Hence, change in entropy is positive.

8. 2 CH₄ (g) → C₂H₄ (g) + 2 H₂ (g)

2 moles of gases produce 3 moles of gases. Hence, change in entropy is positive.