TEMPERATURE AND REACTION RATESCH4U

Unit 3: Energy and Kinetics

Collision Theory

Recall that as temperature increases, the average kinetic energy increases. Therefore:

i) more collisions will take place

ii) collisions will be more effective

For example, this reaction is very fast.

Maxwell-Boltzmann Distribution

Recall that temperature is the average kinetic energy of the particles in matter.

A Maxwell-Boltzmann curve is a probability distribution representing the kinetic energy (speed) of particles in a gas.

Midterm Mark Distribution

Maxwell-Boltzmann(Kinetic Energy ) Distributions

• These distributions appear as smooth curves due to the number of particles involved.

Maxwell-Boltzmann Distribution• As temperature increases, the kinetic energy distributions

flatten & shifts to the right.

Activation Energy

• The activation energy (Ea) is the energy threshold required for a successful collision.

Particles here cannot react (< Ea)

Particles here can react (> Ea)

If the mark cut-off for a program is 86%, only students to the right of the mark barrier will qualify.

Temperature and Rate

• At high temperature, a greater proportion of the particles will possess the required activation energy:

As the area under the curve and to the right of Ea increases, the reaction rate will increase.

Potential Energy Diagrams

• These graphs show the relative potential energy (EP or P.E.) vs. reaction progress.

• The activation energy represents a high-energy barrier that is required to form an activated complex.

• The activated complex activated complex is defined by the International Union of Pure and Applied Chemistry as "that assembly of atoms which corresponds to an arbitrary infinitesimally small region at or near the col (saddle point) of a potential energy surface".[1] In other words, it refers to a collection of intermediate structures in a chemical reaction that persist while bonds are breaking and new bonds are forming. It therefore represents not one defined state, but rather a range of transient configurations that a collection of atoms passes through in between clearly defined products and reactants.

Endothermic Reactions

• convert kinetic energy into potential energy (Ep) (decrease temperature)

Exothermic Reactions• The reverse of an endothermic reaction is an exothermic

reaction.

Notes of P.E. Diagrams

• The greater the energy barrier (Ea) the lower the rate of reaction.

• Only the sign (+ or -) of the enthalpy change (ΔH) changes when a reaction is reversed, but the activation energy (Ea) is NOT the same for the forward and reverse reactions!

• Exothermic reactions will possess a lower Ea than the reverse (endothermic) reaction.