Reactivity 2.2.2 – Kinetic Energy, Temperature (K), and Collision Geometry

1. Relationship Between Kinetic Energy and Temperature (Kelvin)

In the kinetic theory of matter, particles in all states of matter (solid, liquid, gas) are in constant motion. In gases, this motion is most significant and is measured as kinetic energy.

Definition: The average kinetic energy of particles in a substance is directly proportional to the absolute temperature (in Kelvin).

Mathematical Expression:

\( \text{Average Kinetic Energy} \propto T \)

• \( T \) = absolute temperature in Kelvin (K)
• This means: as temperature increases, the particles move faster on average
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Important Notes:

• At 0 K (absolute zero), the kinetic energy of particles would be zero – this is a theoretical point where particles stop moving.
• The Kelvin scale is used because it starts at absolute zero, providing a direct relationship with energy.
• If the temperature doubles in Kelvin, the average kinetic energy of the particles also doubles.

Implication for Reaction Rates:

• As temperature increases, more particles have higher energy.
• This increases the frequency and energy of collisions between reacting particles.
• Thus, reactions occur faster at higher temperatures.
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2. The Role of Collision Geometry in Reactions

Collision theory states that for a chemical reaction to occur:

• Reactant particles must collide
• They must collide with sufficient energy (activation energy)
• They must collide in the correct orientation (collision geometry)

Definition: Collision geometry refers to the relative orientation of the colliding particles at the moment of impact.

Why It Matters:

• Only certain orientations allow bonds to break and new bonds to form.
• Even if the particles collide with sufficient energy, if they don’t align properly, the reaction won’t occur.

3. Effective vs Ineffective Collisions

Effective Collision

An effective collision is one that results in the formation of products. This occurs only if:

• The particles collide with sufficient energy (equal to or greater than the activation energy)
• The particles collide in the correct orientation (collision geometry)

Ineffective Collision

An ineffective collision is one that does not result in a chemical reaction. This happens when:

• The colliding particles lack enough energy to overcome the activation energy barrier
• The orientation of the particles is incorrect for bond breaking and bond formation

Key Conditions for Effective Collisions:

• Activation energy: Minimum energy needed to start the reaction by breaking bonds in the reactants.
• Proper orientation: Particles must align in a way that allows new bonds to form.

Factors Influencing the Frequency of Effective Collisions:

• Temperature: Higher temperature increases kinetic energy, resulting in more frequent and more energetic collisions.
• Concentration/Pressure: Increases the number of particles in a given volume, increasing collision frequency.
• Surface Area: Greater surface area (e.g. powdered solids) allows more particles to be exposed for collisions.
• Catalysts: Lower the activation energy and provide an alternative reaction pathway, increasing the proportion of effective collisions.

Conclusion

• Only a small fraction of total collisions are effective.
• Reaction rate depends on the frequency of effective collisions.
• Controlling temperature, pressure, surface area, and adding catalysts helps maximize effective collisions and improve reaction efficiency.