Explaining Reaction Rates 6.3

Collision Theory The rate of a reaction is proportional to the number of collisions per second among the reactant molecules Anything that can increase the frequency of collision should increase the rate

BUT…. for most reactions it is impossible for every one of the collisions between the reactants to actually result in a chemical change In a gas or a liquid, molecules of the reactants undergo an enormous number of collisions with each other each second. If each collision were effective, all reactions would be over in an instant Of all the collisions that occur, only a very small fraction actually results in a chemical change.

Orientation: movie

Orientation: When 2 reactants collide, their atoms must be oriented correctly in order for a reaction to occur

Activation Energy

Activation Energy Minimum kinetic energy (KE) that molecules require in order to be able to react

Temperature Increasing temperature increases reaction rate Temperature is a measure of the average kinetic energy of the particles in a substance Particles with higher kinetic energy will: Move faster (increasing the collision frequency) Collide with higher energy (more effective collisions) Temperature affects both collision frequency and fraction of effective collisions

The Maxwell-Boltzmann apparatus Maxwell and Boltzmann performed an experiment to determine the kinetic energy distribution of atoms Because all atoms of an element have roughly the same mass, the kinetic energy of identical atoms is determined by velocity (KE= ½mv2)

The Maxwell-Boltzmann apparatus Maxwell and Boltzmann performed an experiment to determine the kinetic energy distribution of atoms Because all atoms of an element have roughly the same mass, the kinetic energy of identical atoms is determined by velocity (KE= ½mv2)

The Maxwell-Boltzmann apparatus Maxwell and Boltzmann performed an experiment to determine the kinetic energy distribution of atoms Because all atoms of an element have roughly the same mass, the kinetic energy of identical atoms is determined by velocity (KE= ½mv2)

The Maxwell-Boltzmann distribution The resulting disk looks like this: Basically, if we plot the intensity of the dots on a graph we get a graph of fraction of atoms/molecules vs. kinetic energy: Fraction of molecules Kinetic energy  Molecules hit disk first Molecules hit disk last

Maxwell-Boltzmann Distribution

Temperature and Kinetic Energy

Temperature and reaction rate 4/28/2017 Demonstrations: Mg + O2, H2 + O2 By increasing the temperature, a small number of molecules reach Ea. The reaction is exothermic, further increasing temperature and causing more molecules to reach Ea, etc. Draw the M-B distribution for H2 + O2 before heat was applied. Show how heat affects the diagram. movie Shift due to higher temperature Fraction of molecules Kinetic energy  Ea

Concentration For solutions Increasing concentration will increase reaction rate More particles per unit volume Increases the collision frequency but not the fraction of effective collisions

Concentration

Pressure (same as concentration for gases) Increasing pressure will increase reaction rate More particles per unit volume Increases the collision frequency but not the fraction of effective collisions

Surface Area Increasing surface area increases reaction rate Increases reaction sites Affects collision frequency but not fraction of effective collisions

Surface Area

Surface Area

Catalyst For some reactions, the activation energy is so high that the reaction will either not occur or will occur very slowly A catalyst is regenerated in the process (i.e. not used up) A catalyst increases the rate of reaction by lowering the activation energy Can make the orientation of molecules more favourable, therefore stabilizing the activated complex A catalyst increases the fraction of effective collisions but not the collision frequency

Maxwell-Boltzman Distribution

Potential Energy Diagram

Catalyst

Catalyst movie

Nature of Reactant Atomic structure (periodic trends, activity series) Ions in solution tend to have a rapid reaction rate No bonds to be broken Positive and negative charges attract In molecular reactions, bonds must first be broken before new bonds can form If molecules are large, or have strong covalent bonds reaction rate will be slower

Nature of Reactant Can increase or decrease rate Affects the fraction of effective collisions but not the collision frequency