Chemistry Reaction Rates and Activation Energy

Reaction Rates and Collision Theory

• Chemical reactions occur when reactant atoms, molecules, or ions collide.
• Two conditions must be met for a collision to result in a reaction:
  • The colliding particles must have the correct orientation.
  • The colliding particles must have sufficient kinetic energy to break existing bonds and form new ones.
• Reaction rate = frequency of collisions × fraction of effective collisions

Activation Energy

• For a reaction to proceed, reactants must possess enough kinetic energy to overcome repulsive electrostatic forces between particles and weaken bonding in the reactants.
• Activation energy (Ea): The minimum kinetic energy needed for a reaction to occur.
• Activation energy is used to overcome the repulsive forces between particles and weaken bonds in reactants.
• Activation energy is represented in an energy profile diagram as the energy difference between reactants and the transition state/activated complex.

Potential Energy in Chemical Reactions

• Potential energy is stored in the bonds within and between chemical species.
• Kinetic energy is the energy of movement of the reacting entities.

Activated Complex

• An unstable arrangement of atoms formed during a chemical reaction.
• It's a temporary, high-energy state between reactants and products.
• It is characterized by partially formed and broken bonds.
• The activated complex represents the maximum potential energy point in a reaction.
• Another name for the activated complex is the transition state.

Energy Changes during Reactions

• Exothermic reaction: Reactions that release energy to the surroundings.
  • ΔH = negative value.
• Endothermic reaction: Reactions that absorb energy from the surroundings.
  • ΔH = positive value.

Factors Affecting Reaction Rate Using Collision Theory

• Temperature: Increasing temperature increases the average kinetic energy of the reactants. This results in more collisions with sufficient energy to overcome the activation energy barrier leading to a higher reaction rate.
• Chemical nature of the reactants: Comparing reactions involving the breaking of different bonds with different bond strengths. Fewer bonds per reactant usually leads to faster reactions. Weaker bonds break easier and lead to faster reactions.
• Concentration: Higher concentrations of reactants mean more particles in a given volume, leading to more collisions and a higher reaction rate.
• Surface area (for solids): Increasing the surface area of a solid reactant increases the reaction rate as more reactant particles are exposed and available for collisions.
• Catalyst: A catalyst provides an alternate reaction pathway with a lower activation energy. This allows more reacting particles to have enough energy for a reaction to occur. Consequently, the rate of reaction increases.

Maxwell-Boltzmann Distribution

• Illustrates the distribution of kinetic energies among molecules at a particular temperature.
• The area under the curve represents the total number of molecules.
• Only molecules with kinetic energy equal to or greater than the activation energy are involved in effective collisions, leading to reaction.
• A higher temperature results in a greater number of molecules exceeding the activation energy (and a greater reaction rate).

Further Reading

• Biocatalysts on page 373 (as noted in last page)

Explore the concepts of reaction rates and collision theory in this quiz. Understand the conditions necessary for chemical reactions and the role of activation energy in overcoming energetic barriers. Test your knowledge of these fundamental principles in chemistry.