RG's E-Learning Centre · Book a free demo class · WhatsApp +91-9379416374

Gibbs energy and spontaneity: Class 11 practice

ΔG = ΔH − TΔS (T in kelvin, ΔS in kJ/K to match ΔH in kJ). Read the method, follow the three worked examples, then try the six questions.

What you should know first

  • Units
  • Signs

The idea behind gibbs energy and spontaneity

ΔG = ΔH − TΔS (T in kelvin, ΔS in kJ/K to match ΔH in kJ). ΔG < 0 spontaneous, > 0 non-spontaneous, = 0 equilibrium. The crossover temperature is T = ΔH/ΔS.

Step-by-step method

  1. Convert ΔS to kJ/K and T to K.
  2. ΔG = ΔH − TΔS.
  3. Sign of ΔG decides spontaneity; T = ΔH/ΔS for the switch.

Worked example 1 (Basic)

ΔH = -110 kJ, ΔS = -97 J/K, T = 306 K. Find ΔG and say if the reaction is spontaneous.

  1. ΔG = -110 − 306 × -0.097
  2. = -80.32 kJ
  3. Negative → spontaneous
  4. Answer: -80.32 kJ, spontaneous

Worked example 2 (Standard)

ΔH = 59 kJ, ΔS = -160 J/K, T = 825 K. Find ΔG and say if the reaction is spontaneous.

  1. ΔG = 59 − 825 × -0.16
  2. = 191 kJ
  3. Positive → non-spontaneous
  4. Answer: 191 kJ, non-spontaneous

Worked example 3 (Challenge)

For a reaction with ΔH = +191 kJ and ΔS = +82 J/K, above what temperature is it spontaneous?

  1. ΔG = 0 at T = ΔH/ΔS = 191000/82
  2. T = 2329.27 K
  3. Answer: Above 2329.27 K

Common mistakes and misconceptions

  • Mixing J and kJ
  • Using °C

Practice questions

  1. (Basic) ΔH = -35 kJ, ΔS = -118 J/K, T = 779 K. Find ΔG and say if the reaction is spontaneous.
  2. (Basic) ΔH = -44 kJ, ΔS = 144 J/K, T = 264 K. Find ΔG and say if the reaction is spontaneous.
  3. (Standard) ΔH = 81 kJ, ΔS = -130 J/K, T = 402 K. Find ΔG and say if the reaction is spontaneous.
  4. (Standard) ΔH = 159 kJ, ΔS = 184 J/K, T = 681 K. Find ΔG and say if the reaction is spontaneous.
  5. (Challenge) For a reaction with ΔH = +173 kJ and ΔS = +326 J/K, above what temperature is it spontaneous?
  6. (Challenge) For a reaction with ΔH = +116 kJ and ΔS = +129 J/K, above what temperature is it spontaneous?

Answer key with full working

  • Q1: ΔG = -35 − 779 × -0.118 → = 56.92 kJ → Positive → non-spontaneous → Answer: 56.92 kJ, non-spontaneous
  • Q2: ΔG = -44 − 264 × 0.144 → = -82.02 kJ → Negative → spontaneous → Answer: -82.02 kJ, spontaneous
  • Q3: ΔG = 81 − 402 × -0.13 → = 133.26 kJ → Positive → non-spontaneous → Answer: 133.26 kJ, non-spontaneous
  • Q4: ΔG = 159 − 681 × 0.184 → = 33.7 kJ → Positive → non-spontaneous → Answer: 33.7 kJ, non-spontaneous
  • Q5: ΔG = 0 at T = ΔH/ΔS = 173000/326 → T = 530.67 K → Answer: Above 530.67 K
  • Q6: ΔG = 0 at T = ΔH/ΔS = 116000/129 → T = 899.22 K → Answer: Above 899.22 K

Syllabus note

This skill is listed under "Thermodynamics" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Related practice and notes

Frequently asked questions

Which chapter is gibbs energy and spontaneity in?

"Thermodynamics", Class 11 Chemistry (NCERT/CBSE). Other boards may place it differently.

How were the answers checked?

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Book a free demo class