Gibbs free energy for IB Chemistry R1.4 (HL): how to predict the sign of ΔS, calculate ΔS from standard entropies, use ΔG = ΔH − TΔS without mixing kJ and J, and find the temperature at which a reaction becomes spontaneous. One original six-mark question on the Haber process solved mark by mark, a ΔG vs temperature graph, and why calcium carbonate only decomposes when it is hot.
Questions this lesson answers:
– How do you predict the sign of the entropy change?
– How do you calculate ΔS from standard entropy values?
– Why do you divide ΔS by 1000 in ΔG = ΔH − TΔS?
– At what temperature does a reaction become spontaneous?
– Does spontaneous mean fast?
Practice (try them first, then check):
1. Predict the sign of ΔS° for 2SO₂(g) + O₂(g) → 2SO₃(g). Answer: negative: 3 mol of gas become 2 mol of gas.
2. A reaction has ΔH° = +50 kJ mol⁻¹ and ΔS° = +200 J K⁻¹ mol⁻¹. Above what temperature is it spontaneous? Answer: T = 50 ÷ 0.200 = 250 K, so above 250 K.
3. Calculate ΔG° at 298 K when ΔH° = −100 kJ mol⁻¹ and ΔS° = −150 J K⁻¹ mol⁻¹. Answer: −100 − 298 × (−0.150) = −55 kJ mol⁻¹ (spontaneous).
Watch the full playlist:
More IB Chemistry Year 2 lessons:
Chapters:
0:00 Gibbs free energy (R1.4): the kJ and J trap
0:34 Entropy: predict the sign of ΔS
1:24 Calculate ΔS from standard entropies
2:27 Spot the mistake: three ΔS answers
2:59 Gibbs free energy vs temperature graph: when is it spontaneous?
4:09 Flip the signs: CaCO3 and the Haber process
4:55 Quick check
5:07 Next: ΔG and K
The questions in this video are original, written in the style of the exam. Marks are estimates.
New IB lessons every week, plus an IB short every day. Subscribe in one tap:
Free practice questions and every IB Chemistry lesson by topic:
This channel is not affiliated with or endorsed by the International Baccalaureate Organization.
#IBChemistry #IBDiploma #GibbsFreeEnergy
source