Entropy, Free Energy & Reaction Feasibility — Practice · Grade 11 Science

Chapter: Chemical Energy & Thermodynamics

What this practice covers

By the end of Entropy, Free Energy & Reaction Feasibility you should be able to:

8 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: entropy, Gibbs free energy, spontaneous.

Problem 1

ΔH=−50kJ/mol, ΔS=−.100kJ/mol·K, T=300 K. Find ΔG.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

-20 kJ/mol

−50−300(−.100)=−20.

Problem 2

Can a favorable reaction be slow?
Answer and explanation

Yes. A high activation-energy barrier can limit its rate.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 3

ΔH=+12 kJ/mol, ΔS=+.050 kJ/(mol·K), T=300 K. Find ΔG.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

-3 kJ/mol

12−300(.050)=−3.

Problem 4

ΔH=30 kJ/mol, ΔS=.050 kJ/mol·K. Find crossover T.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

600 K

30/.050=600.

Problem 5

For ΔH=−20 kJ/mol and ΔS=−0.040 kJ/(mol·K) at 300 K, find ΔG. Show the reasoning, not just the conclusion.
Hints

Units and absolute temperature must match.

Answer and explanation

−20−300×(−0.040)=−8 kJ/mol.

Problem 6

Why can an endothermic process be thermodynamically favorable at high temperature?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

A sufficiently positive entropy change makes TΔS exceed ΔH, producing negative ΔG under appropriate conditions.

Problem 7

Standard-state favorability and the actual mixture are differentStandard-state favorability and the actual mixture are differentΔG = ΔG° + RT ln QAt equilibrium: ΔG = 0 and Q = KΔG° = −RT ln KQ and K use compatible dimensionless activity conventions.
Original thermodynamic relationships; no logarithm of a dimensional raw concentration is intended.
Interpret the supplied visual. Why can a reaction with negative ΔG° fail to proceed net forward from a very product-rich starting mixture?
Answer and explanation

A sufficiently large Q can make RT ln Q positive enough to change the actual ΔG; the mixture composition matters.

The standard reaction free energy ΔG° refers to a defined reference-state account. The actual driving tendency also depends on mixture composition through Q, the reaction quotient, using consistent dimensionless activities. At equilibrium Q=K and the reaction’s ΔG is zero. A negative ΔG° does not mean every possible starting mixture moves forward, because the RT ln Q contribution can change the actual sign. Concentration-only classroom expressions approximate activities and require an explicitly consistent convention.

A sufficiently large Q can make RT ln Q positive enough to change the actual ΔG; the mixture composition matters.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

For a selected process use ΔH=+30.0 kJ/mol and ΔS=+0.150 kJ/(mol·K), treated constant. Find ΔG at 250 K, the modeled crossover temperature and one limit of the prediction.
Answer and explanation

At 250 K, ΔG=30.0−250(0.150)=−7.50 kJ/mol. The modeled crossover is 30.0/0.150=200 K. Actual ΔH/ΔS can vary with temperature and phases, and the calculation does not establish a rate.

At 250 K, ΔG=30.0−250(0.150)=−7.50 kJ/mol. The modeled crossover is 30.0/0.150=200 K. Actual ΔH/ΔS can vary with temperature and phases, and the calculation does not establish a rate.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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