Rate of Reactions - Chemistry

Collision Theory

• a chemical system consists of particles

• the average kinetic energy of the particles is proportional to the temperature

• a chemical reaction must involve collisions of particles with each other

• no all collisions result in a chemical reaction

Effective and Ineffective Collisions

• effective collision: sufficicent energy and correct orientation of colliding particles, such that the bonds can be broken and new bonds formed

• ineffective collision: involve particles that do not have the qualities listed above

Dependency of the rate of reaction

• the rate of reaction depends on the frequency of collisions, and the fraction of those collisions that are effective

Activation Energy

• the minimum energy with which particles must collide before they can rearrange in structure

• only consider the largest bump, do not add the activation energy bumps together

• ONLY catalysts can impact activation energy

Activated Complex

• an unstable chemical species containing partially broken and partially formed bonds representing the maximum potential energy point

• also known as the transition state

Factors to adjust rates

• frequency of collisions:

  • concentration

  • pressure

  • surface area

  • temperature

• fraction of collisions that are effective

  • nature of reactant

  • catalyst

  • temperature

Factor of Rates: Concentration

• increasing concentration of the reacting species, more collisions, higher total number of effective collisions = faster rate

• pressure - increasing pressure of gaseous reactants has the same effect

Factor of Rates: Surface Area

• applies to reactions involving a solid (heterogenous reactions)

• more surface area, more opportunities for collisions, higher total number of effective collisions = faster rate

Factor of Rates: Nature of Chemical Reactant

• compounds contain bonds of specific energy, which must be broken and formed

• change the chemical, change the activation energy, lower the threshold, higher fraction of effective collisions = faster rate

• collision geometry - when molecules are not symmetrical, it is possible for them only to react when the collision occurs on one side

• change the chemical - remove the restriction of collision geometry, higher fraction of effective collisions = faster rate

Catalysts

• “speed up a reaction without being consumed”

• catalysts will speed up the reaction if the new pathway has a lower activation energy

• more particles meet the activation energy, higher fraction of effective collisions = faster rate

Rate Law:

• the rate of a reaction is exponentially proportional to the product of the initial concentrations of the reactants

• r α [χ]^m [Y]ⁿ

• r = k [χ]^m [Y]ⁿ

• r is expressed as mol / L*s

Rate Determining Step

• species that affect the rate are found in the rate determining step

• the number of that species in the rate determining step is the exponent of that species in the rate law

• rate determining step that which is the slowest step in the sequence of steps leading to the product formation

Composants of a Reaction Mechanism

• starting materials and products

• reaction intermediates - species that are made, tehn used up

• catalysts - species that are used up, then regenerated

Reaction Mechanism

• a series of simples reactions that represent the progress of the overall reaction at the molecular level

Molecularity

• the molecularity of a reaction is the # of molecules reacting in an elementary step

• can be the same or different species

• biomolecular = 2

• unimolecular = 1

• termolecular = 3 → rare due to the statistions of both the collisions and the geometry