6.3 Collision Theory and Factors Affecting Rates of Reaction Section 6.2 6.3 Collision Theory and Factors Affecting Rates of Reaction According to collision theory, a chemical reaction occurs when the reacting particles collide with one another. Only a fraction of collisions between particles result in a chemical reaction because certain criteria must be met.

Effective Collision Criteria 1: The Correct Orientation of Reactants For a chemical reaction to occur, reactant molecules must collide with the correct orientation relative to each other (collision geometry). Image source: MHR, Chemistry 12 © 2011. ISBN 07-106010-3; page 365 Five of many possible ways that NO(g) can collide with NO3(g) are shown. Only one has the correct collision geometry for reaction to occur.

Effective Collision Criteria 2: Sufficient Activation Energy For a chemical reaction, reactant molecules must also collide with sufficient energy. Activation energy, Ea, is the minimum amount of collision energy required to initiate a chemical reaction. -Collision energy depends on the kinetic energy of the colliding particles. Based on two things: -overcoming the electrostatic repulsive forces between colliding entities -Weakening the bonds between the reactants (ie. ions vs. covalent bonds) Image source: MHR, Chemistry 12 © 2011. ISBN 07-106010-3; page 366

The shaded part of the Maxwell-Boltzmann distribution curve represents the fraction of particles that have enough collision energy for a reaction (ie the energy is ≥ Ea).

Representing the Progress of a Chemical Reaction From left to right on a potential energy curve for a reaction: potential energy increases as reactants become closer when collision energy is ≥ maximum potential energy, reactants will transform to a transition state products then form (or reactants re-form if ineffective) Image source: MHR, Chemistry 12 © 2011. ISBN 07-106010-3; page 366 Exothermic Endothermic

Activation Energy and Enthalpy The Ea for a reaction cannot be predicted from ∆H. ∆H is determined only by the difference in potential energy between reactants and products. Ea is determined by analyzing rates of reaction at differing temperatures. Reactions with low Ea occur quickly. Reactions with high Ea occur slowly. Image source: MHR, Chemistry 12 © 2011. ISBN 07-106010-3; page 367 Potential energy diagram for the combustion of octane.

Activation Energy for Reversible Reactions Potential energy diagrams can represent both forward and reverse reactions. follow left to right for the forward reaction follow right to left for the reverse reaction Image source: MHR, Chemistry 12 © 2011. ISBN 07-106010-3; page 368

Analyzing Reactions Using Potential Energy Diagrams Image source: MHR, Chemistry 12 © 2011. ISBN 07-106010-3; page 369 The BrCH3 molecule and OH- collide with the correct orientation and sufficient energy and an activated complex forms. When chemical bonds reform, potential energy decreases and kinetic energy increases as the particles move apart.

Temperature Increasing the temperature of the system increases the number of particles with sufficient energy to react. New particles with sufficient energy to effectively collide in the chemical reaction. ET Number of particles Particles with sufficient energy to effectively collide in the chemical reaction. Kinetic energy (kJ)

Increased concentration An increase in concentration results in more particles with the required threshold energy being added to the system. Increased concentration New particles with sufficient energy to effectively collide in the chemical reaction. ET Number of particles Particles with sufficient energy to effectively collide in the chemical reaction. Kinetic energy (kJ)

Catalysts A catalyst reduces the threshold energy and consequentially, activation energy . New particles with sufficient energy to effectively collide in the chemical reaction. ET Catalyzed ET Number of particles Particles with sufficient energy to effectively collide in the chemical reaction. Kinetic energy (kJ)