AP Chemistry

Unit 5.5

COLLISION MODEL

Unit 5.5

Enduring Understanding:

• There is a relationship between the speed of a reaction and the collision frequency of particle collisions

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Learning Objective:

• Explain the relationship between the rate of an elementary reaction and the frequency, energy, and orientation of molecular collisions

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Collision Theory

• The collision theory explains why different reactions occur at different rates
• Particles are continually colliding but not all collisions are “successful”
• i.e. result in a reaction/formation of products
• A reaction occurs when bonds are broken and remade

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Collision Theory

• Collision theory states that in order for a reaction to occur the following must happen:
• Particles must collide
• The rate of the reaction is dependent on the frequency of collisions
• The colliding particles must have sufficient energy
• Minimum energy required: activation energy
• Corresponds to the amount of energy needed to break the requisite bonds
• The particles must be in the correct orientation
• Certain parts of one particle and certain parts on the other particle must line up correctly in order for the reaction to occur

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Collisions

• Collisions occur all the time but not all collisions result in a reaction
• Consider bimolecular vs. termolecular (or higher) reactions:
• Bimolecular (involving 2 molecules) reactions can occur because the chance of two molecules colliding is high
• Termolecular (involving 3 molecules) reactions are very rare and unlikely to occur because three particles colliding simultaneously is much more unlikely

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Collisions

• In order to increase the number of collisions, you must increase the concentration of the particles
• If your reactants are solids or immiscible liquids, increasing the surface area will increase the rate of a reaction

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Activation Energy

• Activation Energy: the minimum amount of energy that the reacting particles need in order to react
• If the colliding particles do not meet the minimum energy threshold they will simply bounce off of each other and no reaction will take place
• This energy is used to break bonds
• On a Maxwell-Boltzmann distribution curve we can show the activation energy (E_A) in relation to temperature (kinetic energy)
• The particles to the right of the E_A line have the minimum amount of energy required so as long as they achieve correct orientation they will react
• Particles to the left of the E_A line lack the energy required so will not react

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Activation Energy

These particles will react as long as they collide with the correct orientation

These particles will NOT react, even if they collide with the correct orientation

Activation Energy

In order to increase the number of particles with energy greater than the E_A the sample must be heated

Increase temp, increase kinetic energy

Activation Energy

• General rule of thumb:
• Increasing the temperature by 10℃ will double the reaction rate
• The reaction rate increases because more particles have energy greater than the activation energy
• Misconception:
• The reaction rate increases because the particles are moving faster and colliding more
• True, and this does increase the rate slightly, but not the doubling that we see
• Doubling of the rate is due to more particles having the E_A

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Activation Energy

Heating the sample changes the amount of particles that have the E_A, NOT the amount of energy needed (E_A)

Orientation

• Specific parts of one particle must collide with a specific part of another particle in order for a successful collision to occur

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Orientation

• Orientation factor: a number between 0 and 1 that indicates how difficult it is for the reaction to go to completion.
• Dependent on how specific the orientation of the reactants needs to be in order for products to form
• The smaller the orientation factor the more specific the orientation requirement is
• Example:
• H₂ + CH₂=CH₂ → CH₃-CH₃
• (= is a double bond, - is a single bond)
• Experimentally derived orientation factor: 1.7 x 10^-6
• This reaction will likely have a slow rate due to the very small orientation factor which means that the reactants must align in a very specific way

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Orientation

• Example:
• K (g) + Br₂ (g) → KBr (g) + Br (g)
• The orientation factor between individual atoms is usually ~1
• This reaction will likely have a fast rate due to the large orientation factor which means that the reactants’ required orientation is not specific at all
• Atoms are spherically symmetric and thus any orientation can lead to the formation of products

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Practice: I Do

1. Given the following reaction, which set of reaction conditions will result in the highest reaction rate? Explain in terms of collision theory

H₂ + Cl₂ → HCl

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Practice: I Do

• Given the following reaction, which set of reaction conditions will result in the highest reaction rate? Explain in terms of collision theory

H₂ + Cl₂ → HCl

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Experiment 3 will have the highest rate. The [reactants]--looking at moles vs. volume--is the highest and therefore the particles will be more likely to collide with one another