CHEM 1312 Chapter Notes - Chapter 19: Elementary Reaction, Exothermic Reaction, Molecularity
Chapter 19: Chemical Kinetics
Focus of this section: Rates of reactions
Factors that affect reaction
Reaction Rates: rates
• reactions occur at different speeds
= some are fast photosynthesis, ignition of
gasoline
= some are slow hardening of cement, conversion
of diamond to graphite
• speed of a reaction reaction rate
• Kinetics = study of how fast reactions take place
• WHY IMPORTANT???
-- Speed up desirable reactions
-- Minimize damage by undesirable reactions
-- Useful in: = drug design and treating diseases
= pollution control
= food processing
= industrial processes
Collision Theory.
• given the following: reactants
→
products
• most reactions occur thru collisions between reactants
vast majority of collisions involve 2 particles ONLY
• the rate of reaction is proportional to the
number of effective collisions per second
rate α # effective collisions
second
-- effective collision one that yields product
• anything that increases the frequency of collisions will
increase the rate
-- increase concentration of reactants
• only a small fraction of all collisions are effective
1. Molecular Kinetic Energy:
-- molecules have kinetic energy (motion)
= the faster they move, the more kinetic energy
-- in a collision,
kinetic energy transferred into vibrational energy
(from internal motion)
-- if vibrations large enough, bonds can break
= 1st step to forming products
-- minimum combined energy required to initiate reaction
= Activation Energy (Ea)
-- if less than Ea, then ineffective collision and reactants
bounce off each other unchanged
2. also require correct Molecular Orientation:
-- molecules need to be oriented appropriate with respect
to each other, or else collision will be ineffective
= correct bonds must break/form between correct
atoms
Cl + NOCl
→
Cl2 + NO
-- only a fraction of all collisions have correct orientation
-- of those, only a small fraction have enough energy to
be effective (at ordinary temps)
-- temperature α average KE
-- as increase temperature, a larger fraction of the
molecules have a sufficient energy (KE) to
participate in productive collision
= overcome repulsion between electron clouds
-- Most reactions occur faster at a higher temperature
-- food spoilage slowed by refrigeration
• effective collision generates an activated complex
(transition state)
-- temporary species
effective
ineffective
Document Summary
Reaction rates: reactions occur at different speeds. = some are fast photosynthesis, ignition of. - useful in: = drug design and treating diseases. - effective collision one that yields product: anything that increases the frequency of collisions will increase the rate. - increase concentration of reactants: only a small fraction of all collisions are effective, molecular kinetic energy: = the faster they move, the more kinetic energy kinetic energy transferred into vibrational energy (from internal motion) - if vibrations large enough, bonds can break. - minimum combined energy required to initiate reaction. = activation energy (ea) bounce off each other unchanged. - if less than ea, then ineffective collision and reactants: also require correct molecular orientation: - molecules need to be oriented appropriate with respect to each other, or else collision will be ineffective. = correct bonds must break/form between correct atoms. - only a fraction of all collisions have correct orientation.
