Entropy Questions Q 20 video2046594162

Source: 2025 AP CHEM BARRON'S BOOK
Question:
For a certain reaction, ΔH° = +2.98 kJ and ΔS° = +12.3 J/K, what is ΔG° at 298 K, 200 K, and 400 K?
🔬 “Have you ever wondered why some reactions happen on their own while others need heat or energy to get started? What if I told you that just ONE equation can predict it all? Welcome to the magic of Gibbs Free Energy! 🚀 Stay with me, and in just 3 minutes, you’ll master it!”
⏳ PART 1: The Golden Equation
📝 The equation that rules it all:ΔG∘=ΔH∘−TΔS∘ΔG∘ =ΔH∘ −TΔS
∘
✔ ΔG° (Gibbs Free Energy) → The "GO" signal! If it’s negative, the reaction is spontaneous!
✔ ΔH° (Enthalpy change) → Heat energy. If positive, it absorbs heat; if negative, it releases heat.
✔ T (Temperature in Kelvin) → The control switch! Higher T means bigger impact of entropy.
✔ ΔS° (Entropy change) → The chaos factor! Positive means more disorder, negative means more order.
🚀 Key idea: Reactions favor high entropy (more disorder) and low enthalpy (less energy needed).
🔥 PART 2: Real Example Breakdown (0:45 - 1:45)
💡 Problem: We have ΔH° = +2.98 kJ and ΔS° = +12.3 J/K, and we need to find ΔG° at 298 K, 200 K, and 400 K!
Step 1: Convert units!
👉 Entropy (ΔS°) is in J, but enthalpy (ΔH°) is in kJ. Convert ΔS° into kJ:
12.3J/K=0.0123kJ/K
12.3J/K=0.0123kJ/K
Step 2: Plug into the equation!
For 298 K:ΔG∘=2.98−(298×0.0123)ΔG ∘ =2.98−(298×0.0123)
=2.98−3.67=−0.69kJ
=2.98−3.67=−0.69kJ
🟢 NEGATIVE! The reaction is spontaneous!
For 200 K:ΔG∘=2.98−(200×0.0123)ΔG∘ =2.98−(200×0.0123)
=2.98−2.46=+0.52kJ
=2.98−2.46=+0.52kJ
🔴 POSITIVE! The reaction is NOT spontaneous.
For 400 K:ΔG∘=2.98−(400×0.0123)ΔG∘ =2.98−(400×0.0123)
=2.98−4.92=−1.94kJ
=2.98−4.92=−1.94kJ
🟢 NEGATIVE! Spontaneous at high temperature!
⚡ PART 3: What Did We Learn? (1:45 - 2:30)
💡 Big Takeaways:
1️⃣ If ΔG° is negative, the reaction happens spontaneously!
2️⃣ If ΔS° is positive, higher temperature makes the reaction more spontaneous!
3️⃣ At low temperatures, enthalpy dominates → reaction may NOT happen.
4️⃣ At high temperatures, entropy dominates → reaction is MORE likely!
🔥 Summary:
Low temp (200 K) → Not spontaneous
Room temp (298 K) → Spontaneous
High temp (400 K) → More spontaneous!
🎯 FINAL CHALLENGE
🚀 Now it’s your turn!
What happens if we LOWER temperature even more? 🤔 Will the reaction ever be spontaneous at 100 K? Comment below! ⬇
👍 LIKE, SHARE & SUBSCRIBE for more quick chemistry tricks! See you in the next one! 🔥