Collision Theory and Factors Affecting Rates of Reaction

Collision Theory

• A reaction occurs when two particles (atoms, molecules or ions) collide at the correct orientation and with a certain amount of minimum energy.

• These reactions can happen very quickly, considering that molecules can move at a fast pace.

  • However, not all collisions will result in a reaction (depending on how many molecules collide- the more there is, the less likely all will be a reaction).

Effective Collisions
Q: What must happen for a collision between reactants to result in a reaction?

Two criteria must be met:

1. The orientation of the reactants must be favorable.

2. The collision must occur with sufficient energy.

Effective Collision: Criteria 1

• Reacting particles can only collide in the proper orientation. This is called collision geometry.

• Example: NO + NO3 🡪 2NO2 — Only 1 out of 5 possibilities will work.

Effective Collision- Criteria 2

• Reactant particles must collide with enough energy to generate a reaction.

  • Only a fraction of collisions have sufficient energy.

• The minimum amount of energy required is referred to as an activation energy, Ea.

• This type of reaction is dependent on kinetic energy of the colliding particles (not temperature).

• We can show this relationship through a Maxwell-Boltzmann distribution graph

Representing the Progress of a Chemical Reaction

• For a reaction to occur, the activation energy must be greater than the potential energy of the reactants.

  • Anything that doesn’t, will cause reactants to bounce back.

• The small fraction that do change will reach a transition state, where molecules can either go back to reactants or move forward to the products.

  • The difference between the potential energy of reactants and products is the enthalpy change, ∆H.

Activation Energy and Enthalpy

• The activation energy cannot be predicted from the enthalpy change.

• It is determined by the reaction rate at various temperatures.

  • Reactions with low activation energy happen faster, and reactions with high activation energy happen slower.

  • Enthalpy change is much lower than activation energy.

Activation Energy for Reversible Reactions

Many reactions can go forward or reverse. More energy is required for the reverse reaction. The activated complex is a temporary formation of particles during the transition state. Remember to change signs when determining ∆Hr.

What factors affect reaction rate?

Reaction rates are dependent on several

factors;

• Nature of the reactants

• Concentration of the solution

• Temperature

• Pressure

• Surface area

• Presence of a catalyst

Substance that increases the reaction rate without being consumed during the reaction

How does the nature of reactants affect reaction rate?

• The type of reactant involved in the reaction makes a difference for the reaction type.
• Ions tend to move faster than molecules.
- Negative/positive charges are more likely to be attracted to each other.
- Bonds are not required to break, as they easily dissolve in water.
• Larger molecules (or stronger bonds) require more energy to break, making the reaction slower.

Concentration

Increasing the concentration of reactants leads to more collisions.

• This can then increase the rate of reactions.

However, as more products are formed, the rate decreases.

• This is due to the reactants having less molecules to react to, since it’s already a product.

Temperature

• As temperature increases, the particles have more kinetic energy

  • This causes more collisions, which increase the rate of reaction

Pressure

Increasing the pressure increases the number of collisions.

• This increases the rate of reaction.

Pressure can be increased by adding more reactant gas particles to a fixed volume or by reducing the volume of the container.

Surface Area

• The smaller the surface area, the faster the rate of reaction is.
• If the surrounding (solution) can easily cover the entire solute, then it is more likely to engage in a reaction.
• Example: think of sugar dissolving in coffee.

Catalysts

• Some reactions have a very high activation energy, which can result in products taking years to form.
• In these cases, a catalyst is introduced to lower the activation energy to speed the reaction.