Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.11 Thermodynamic vs kinetic stability-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.11 Thermodynamic vs kinetic stability- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.11 Thermodynamic vs kinetic stability- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
12.11 Thermodynamic Stability vs Kinetic Stability
The feasibility of a reaction and the rate at which it occurs are controlled by different factors. Thermodynamic stability relates to whether a reaction is energetically favourable, while kinetic stability relates to how fast the reaction occurs. These concepts must be clearly distinguished.
Thermodynamic Stability
Thermodynamic stability refers to how energetically favourable a substance is, determined by the total entropy change (or Gibbs free energy).
- A thermodynamically stable system has lower energy and is more favourable.
- A reaction is feasible if \( \mathrm{\Delta S_{total} > 0} \) or equivalently \( \mathrm{\Delta G < 0} \).
- Thermodynamics predicts the position of equilibrium.
- It does not provide information about how fast a reaction occurs.
Kinetic Stability
Kinetic stability refers to how slowly a reaction occurs, determined by the activation energy.
- A kinetically stable substance reacts very slowly.
- High activation energy means fewer particles have sufficient energy to react.
- The reaction rate is low, even if the reaction is thermodynamically feasible.
- Kinetics determines how quickly equilibrium is reached.
Key Differences
- Thermodynamic stability depends on energy changes (\( \mathrm{\Delta H, \Delta S, \Delta G} \)).
- Kinetic stability depends on activation energy (\( \mathrm{E_a} \)).
- A reaction can be thermodynamically feasible but kinetically slow.
- Thermodynamics predicts if a reaction can occur; kinetics predicts how fast it occurs.
Illustrative Example
- The conversion of diamond to graphite is thermodynamically favourable.
- However, it occurs extremely slowly due to a very high activation energy.
- Therefore, diamond is kinetically stable but thermodynamically unstable relative to graphite.
Key Features
- Thermodynamic stability relates to feasibility and equilibrium.
- Kinetic stability relates to rate of reaction.
- High activation energy leads to kinetic stability.
- Both concepts are needed to fully understand reaction behaviour.
Example 1:
Explain why a reaction can be thermodynamically feasible but kinetically slow.
▶️ Answer/Explanation
A reaction is thermodynamically feasible if it results in a decrease in Gibbs free energy or an increase in total entropy.
However, the rate of reaction depends on the activation energy.
If the activation energy is high, only a small fraction of particles have sufficient energy to react.
Therefore, the reaction occurs very slowly despite being energetically favourable.
Example 2:
Explain why diamond is described as kinetically stable but thermodynamically unstable.
▶️ Answer/Explanation
Graphite is the more thermodynamically stable form of carbon because it has lower energy.
Therefore, the conversion of diamond to graphite is thermodynamically favourable.
However, this conversion has a very high activation energy.
As a result, the reaction occurs extremely slowly, so diamond does not readily convert.
Therefore, diamond is kinetically stable but thermodynamically unstable.